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The addition and subtraction of single photons to/from arbitrary
light fields have been recently demonstrated, and sequences or coherent
superpositions of these operations have been used to test fundamental
quantum rules. Now, an interesting new application of these tools
has been proven: the noiseless linear amplification of quantum light
states. We have experimentally shown that a non-deterministic noiseless
amplifier based on a sequence of photon addition and subtraction
can greatly outperform other approaches and might be used to distill
and concentrate entanglement, form part of a quantum repeater, improve
the performance of phase-estimation schemes, and enable high-fidelity
probabilistic cloning and discrimination of coherent states. Information is Quantum---how physics has
helped us understand what information is and what can be done with
it -------------------------------------------------------------------------------- Tomography for fault-tolerance: confidence
regions for quantum hardware Fault tolerant quantum computation will depend upon highly reliable characterization of individual components - "states and gates". This requires region (a.k.a. interval) estimators, rather than the point estimators provided by current tomographic schemes. In this talk, I introduce likelihood-ratio (LR) confidence regions for quantum states and processes. I prove that LR regions are optimally powerful (i.e., have minimum expected volume). I also derive the distribution of the LR statistic for tomographic data (a necessary step in constructing LR regions), and show how it differs from a canonical c2 distribution. Finally, I demonstrate LR regions in action and confirm that they work. Back
to top Gas-phase quantum memory A practical optical quantum memory must be able to store and recall quantum states on demand with high efficiency and low noise. Ideally, the platform for the memory would also be simple and inexpensive. In this talk we present tomographic reconstruction of quantum states that have been stored in an off-the-shelf rubidium vapour cell operating at around 80 degrees Celsius. Our analysis demonstrates an optical memory with quantum fidelity as high as 98% and recall efficiency up to 87%. In order to unambiguously verify that our memory beats the quantum no-cloning limit we employ state independent verification using conditional variance and signal transfer coefficients. Back
to top Optimization of coherent energy transfer in
light-harvesting systems Energy transfer in photosynthesis is the initial step in the conversion
of solar energy into chemical energy for human consumption. This
talk will discuss the optimal conditions under which photosynthetic
light-harvesting systems can achieve maximal energy transfer efficiency,
i.e., the maximal exciton mobility. A simple scaling theory is developed
to explain the optimal energy transfer efficiency, as a function
of temperature, noise level, and solvent relaxation time-scale,
and the dependence on the initial state preparation due to photo-excitation.
A perturbation technique is then developed on the basis of NIBA
(non-interaction blip approximation) to systematically map a quantum
network to a kinetic network, where the leading order is hopping
and higher order corrections are non-local quantum effects. . In
addition, the influence of intrinsic symmetry of the exciton system
on the efficient and robust light-harvesting energy transfer is
demonstrated for LH2 B850 rings. These results provide useful insights
to the natural selection in light-harvesting systems and optimal
design principles of artificial energy devices. Back
to top Suddenly, there arises the new era of REAL quantum computers.As
the time runs backwards in an alternating world through the looking Now, I am ready to give an expository talk, about my personal adventure,
in the Quantum Wonderland. In particular, I shall give a MODERN
report Back
to top Weak Values and Precision Measurements Weak values were originally introduced by Aharanov, Albert and Vaidman for understanding the arrow of time in quantum mechanics. Among other things, weak values have recently proved useful in amplifying very small effects. I will introduce the ideas of weak values and discuss some of our recent experimental results in which we were able to observe a deflection of a laser beam of less than 1 picoradian (equivalent to measuring a deflection of the width of a hair at the distance of the moon). Perhaps more interestingly, the noise properties lead to a suppression of technical noise and an amplification of the signal to noise ratio to the optimal value for coherent states for standard beam deflection techniques. I will also discuss the use of weak values and precision deflection measurements for precision spectral and phase measurements. Back
to top Correlation Functions of 1 D anyons One dimensional anyons describes edge state of fqhe. The edge state is used to perform gates in topological quantum computation. I will consider a model of 1D anyons with local interaction and show how correlation functions depend on statistics. Back
to top A family of norms with applications in quantum
information In this talk I will discuss recent work with Nathaniel Johnston in which we consider a family of operator norms that quantify the degree of entanglement in quantum states. The norms are defined by the Schmidt decomposition theorem for quantum states, and they can be used to tackle two fundamental problems in quantum information: the classification problem for k-positive linear maps and entanglement witnesses, and the existence problem for non-positive partial transpose bound entangled states. I’ll begin by giving an overview, then discuss some properties of the norms and their applications. Back
to top Quantum thermodynamics via measurements
on non-Markovian time scales The anomalies of work, heating or cooling induced by frequent perturbations of open quantum systems are intimately related to the little-explored quantum correlations (entanglement or discord) that arise between the system (e.g., a qubit) and a thermal bath because of their coupling. Such correlations, which have previously eluded attention, have been shown by us both theoretically [1-4] and experimentally [5] to profoundly change the dynamics of the bath and the system once we perturb the system within the bath-memory (non-Markovian) time scales.The required perturbations can either be effected by frequent projective measurements of the qubit energy, or by its frequent modulation (ultrafast driving), giving rise to novel , anomalous regimes: a) Quantum heat engines (QHE): We present a hitherto unexplored QHE design, based on anomalies that arise from frequent quantum nondemolition (QND) measurements or phase flips of a qubit in contact with a non-Markovian bath [1,4]. Either operation results in a non-equilibrium state that starts evolving and can close a cycle via qubit-modulation by a piston, e.g., a coherently-driven oscillator mode. An intriguing anticipated consequence of such QND operations is the ability to extract net work (from the qubit to the piston) using a single bath, although such operations do not acquire information that can be converted into work, as opposed to Maxwell's demon. This anomaly may appear to contradict the second law, but in fact it does not, once the measurement or phase-flip cost in energy and entropy is accounted for. b) Entanglement-based QHE: Two or more qubits coupled to the same bath mode have recently been predicted by us to be inevitably entangled via the bath[6]. This entanglement is expected to principally affect the QHE performance. c) Non-Markovian quantum refrigerator (QR):Ultrafast cooling (purification) of qubits, may be attained at non-Markovian time-scales by frequent quantum measurements or phase shifts [3,5].It allows us to put forward a novel, highly-compact, QR design which consists of a single qubit simultaneously coupled to hot and cold non-Markovian baths. Phase flips of the qubit at high rates are shown to cause refrigeration: Heat may then flow from the cold to the hot bath via the qubit. The third law is upheld: under no circumstances can the bath refrigeration attain absolute zero. Back
to top Quantum gates for superconducting qubits
with fixed coupling At low temperature, nanoscale electrical circuits based on superconductors behave as artificial atoms. Their dynamics is described by a small number of degrees of freedom, and their properties can be tailored by circuit design. Superconducting artificial atoms have potential applications in quantum information processing and provide a testbed for the study of light-matter interaction in a new regime of ultra-strong interactions. I will discuss a new type of two-qubit gate applicable to qubits with fixed coupling. This type of gate requires the same resources as needed for single-qubit control (ie independent driving pulses on two qubits). I will discuss the experimental implementation using flux-type superconducting qubits. I will also present experimental work in progress on the application of this method to a system formed by a few qubits.
Remote preparation of arbitrary states
of an atomic collective In the Duan-Lukin-Cirac-Zoller protocol, single collective excitations
of atomic ensembles and, subsequently, heralded single photons,
can be prepared by conditional photon number measurements on Raman
scattered light. More complex measurements, such as projections
onto displaced Fock states, permit preparation of these collective
excitations in arbitrary quantum states. These states can be characterized
by converting them into the optical form and applying homodyne tomography. Photoinduced dynamics in photosynthetic
complexes under incoherent light excitation Back
to top Quantum coherence in biology:facts, fiction
and challenges The idea that quantum superpositions can survive in molecular componenets of living organisms, in conditions of biological relevance and for long enough to be exploited in life processes is fascinating -at least for lovers of the quantum world. Light-initiated reactions in biological systems are some of the extraordinary phenomena that have for long been suspected to benefit from coherent quantum effects. In this talk I would like to discuss facts supporting these ideas, the importance (or not) of such quantum phenomena in biology, the fictional stories that these ideas have inspired but most importantly the scienfic opportunities that are being opened. Back
to top Virtual qubits, virtual temperatures,
and the foundations of thermodynamics In my talk I will present a new view on the foundations of thermodynamics. Back
to top Pulsed quantum states of light for
multi-mode quantum systems Optical networks, which comprise multiple optical modes as well
as highly non-classical states of light, have been investigated
intensively over the last two decades in various theoretical proposals.
They can serve as an ideal test-bed for different application in
quantum information science. Most recently the role of coherence
and quantum properties in quantum walk architectures has attracted
attention. However, the implementation of experi¬mental setups
with increasing complexity in terms of number of modes and input
states with distinct quantum char¬acteristics implicates several
challenges. These are related to the controlled interaction amongst
different channels, the detection of a large number of modes, their
stabilization as well as the synchronization with interferometric
precision. For pulsed light the preparation of pure photonic quantum
states in combination with appropriate state characterization constitutes
an additional challenge for the implementation of practical systems. Back
to top Photonic quantum circuits and their application In this talk, we report our recent effort for making photonic quantum circuits and discuss their possible applications. The first example is we `an entanglement filter’[1]. The ability to filter quantum states is a key capability in quantum information science and technology, where one-qubit filters, or polarizers, have found wide application. Filtering on the basis of entanglement requires extension to multi-qubit filters with qubit-qubit interactions. We demonstrate an optical entanglement filter that passes a pair of photons if they have the desired correlations of their polarization. Such a device has been proposed for photonic qubits[2], however, the technical requirements to build such a device, an optical circuit with two ancillary photons and multiple quantum gates, requiring both quantum interference and classical interference in several nested interferomters, have been lacking. We demonstrate an entanglement filter by combining two key recent technological approaches---a displaced-Sagnac architecture[3] and partially polarizing beam splitters[4]. The entangling capability of the filter was verified, distinguishing it from classical ones. The second example is the optical quantum circuit of a Knill-Laflamme-Milburn (KLM) CNOT gate[5]. This photonic quantum circuit combines two efficient `artificial’ nonlinear elements. We developed a stable architecture to realize the required four-photon network of nested multiple interferometers, and found that the average gate fidelity of our experimental quantum CNOT gate is 0.82 ± 0.01[6]. This result confirms the first step in the KLM `recipe' for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification. We will also briefly introduce our recent activities on the realization of solid state quantum using diamond nano-crystals coupled to tapered optical fibers and microsphere resonators [7-9]. This work was supported in part by Grant-in-Aid from JSPS, Quantum
Cybernetics project, JST-CREST project, FIRST Program of JSPS, Special
Coordination Funds for Promoting Science and Technology, and the
GCOE program, and Research Foundation for Opto-Science and Technology. Back
to top Affleck-Kennedy-Lieb-Tasaki states as a resource
for universal quantum computation The study of quantum spin systems dates back to the early twentieth century and has been an active research field. Quantum computation, on the other hand, is a relatively new research field, of less than three decades of age. Here, we investigate the question of whether quantum computational resource states can arise as ground states of two-body interacting Hamiltonians in spin systems. It is known that unfortunately cluster states, the first known resource state, cannot be the unique ground states of such Hamiltonians. We shall investigate a few examples that are constructed from Affleck-Kennedy-Lieb-Tasaki (AKLT) models of quantum antiferromagnets and show an intriguing connection between these AKLT-type states and cluster states. In particular the spin-3/2 two-dimensional AKLT state on the hexagonal lattice, as well as the Cai-Miyake-Dur-Briegel AKLT-like state on the decorated hexagonal lattice, can be locally converted to a cluster state and hence is a universal resource for measurement-based quantum computation. Back
to top
On the Ambainis-Bach-Nayak-Vishwanath-Watrous
Conjecture We show the flaw in "N. Konno, T. Namiki, T. Soshi and A. Sudbury, Absorption problems for quantum walks in one dimension, J. Phys. A: Math. Gen. 36 (2003), 241-253" and provide the necessary correction in the case of the finite Hadamard Walk and use it to show that conjecture 11 of Ambainis-Bach-Nayak- Vishwanath-Watrous in "A. Ambainis, E. Bach, A. Nayak, A. Vishwanath and A. Watrous, One-dimensional Quantum Walks, Proceedings of the 33rd Annual ACM Symposium on the Theory of Computing (2001), 37-49." is false. Back
to top Critical current noise and junction resonators
in Josephson junction from interacting trap states We analyze the impact of trap states in the oxide layer of superconducting tunnel junctions on the fluctuation of the Josephson current and thus on coherence in superconducting qubits. We are extending previous studies of noninteracting traps to the case where the traps have on-site electron repulsion. We use second order perturbation theory which allows to obtain analytical results limited to small and intermediate repulsion. Remarkably, it still reproduces the main features of the model as identified from the Numerical Renormalization group. We present analytical formulations for the subgap bound state energies, the singlet-doublet phase boundary, and the spectral weights, which are in agreement with recent numerical renormalization group analysis. We show that interactions can reverse the supercurrent across the trap. We finally work out the spectrum of junction resonators for qubits in the presence of on-site repulsive electrons and analyze its dependence on microscopic parameters that may be controlled by fabrication. Back
to top Direct Measurement of the Quantum Wavefunction Central to quantum theory, the wavefunction is a complex distribution associated with a quantum system. Despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, we present a method to directly measure the wavefunction so that its real and imaginary components appear on our measurement apparatus. We will describe an experimental example by directly measuring the transverse spatial wavefunction of a single photon. This method gives the wavefunction a plain and general meaning in terms of a specific set of operations in the lab. Back
to top Analysis of multiqubit entanglement and
nonlocality for optimal quantum communication Quantum entanglement can be used as a resource for efficient information transfer between different parties via protocols such as dense coding or teleportation. For two-party communication, the maximally entangled Bell states serve as a resource for efficient and secure communication. In networks of three or more parties, multiqubit entanglement allows the possibility of new protocols and flexible communication between different members of a network. Our goal is to analyze important classes of multiqubit entangled states and develop novel schemes for quantum communication in multiparty networks. We have explored the nonlocal properties of different N-qubit partially entangled states and generalized the well-known relationship between 2-qubit entanglement and violation of the Bell-CHSH inequality to the case of N-qubit entanglement and violation of an N-qubit Bell inequality. Based on these results, we discuss the use of partially entangled states for optimal communication in a network. We also discuss new protocols for quantum voting and password sharing. Back
to top Multiple-copy state discrimination: Thinking
globally, acting locally The degree to which pairs of nonorthogonal qubit states can be
discriminated is an important topic for gaining insight into practical
and fundamental questions about quantum measurement. In this context,
it is known that, when a sequence of multiple copies of a state
are available, discrimination schemes that employ adaptive local
measurements that are locally optimized (i.e. optimized for each
copy measured) can exhibit higher error rates than schemes in which
the measurement bases are fixed. In this case, a scheme employing
optimization over the entire set of measurements (global optimization)
is necessary to attain the least error with conclusive outcomes. Back
to top Isotropic and squeezed fluctuations in
n-qubit system We show that 2n coherent states of an n-qubit system, generated by application of the discrete displacement operators to a symmetric fiducial state have isotropic fluctuations, with á DS2 ñ = n, in a specific tangent plane, which in general is not orthogonal to the mean spin direction. This allows to use them as reference states to define a discrete squeezing for non-symmetric n -qubit states. Examples of states with reduced fluctuations, obtained after application of XOR gates to correlate (partially entangle) qubits are analyzed. We also extend the idea of the isotropic fluctuation plane to n-qubit states with different types of symmetry, which allows us to characterize quantum correlations (in particular squeezing) in terms of specific second-order moments. Back
to top Optimal Trajectories for Quantum Adiabatic
Processing We show how any classical logic circuit (eg, multiplication) can be expressed as an optimisation problem. The resulting circuit is effectively omnidirectional; for example, output states can be fixed and the corresponding input states computed; the multiplication circuit can thereby be used for factoring, with the optimisation cast as a problem in adiabatic quantum computing. We have developed general heuristics to find explicit trajectories from initial to final model Hamiltonians that minimise the overall computation time. Particularly for NP-type problems, where high-accuracy is not essential, trajectories have been found whose efficacy vastly exceeds that of the usual linear trajectory, and may even change the scaling behaviour of the algorithm. Explicit examples will be given, including the factoring of up to 6-bit integers. Back
to top A variational master equation approach to
dissipative energy transfer dynamics Recent experiments demonstrating signatures of quantum coherence
in the energy transfer dynamics of a variety of light-harvesting
systems [1] have sparked renewed interest in the theoretical modelling
of energy transfer processes. A major challenge remains the development
of techniques which allow one to probe the diverse parameter regimes
relevant to such systems. Master equation methods provide useful
tools with which to efficiently analyse energy transfer dynamics
in the presence of an external environment. However, they are often
valid only in rather restrictive parameter regimes, limiting their
applicability in the present context. Back
to top All non-classical correlations can be activated
into distillable entanglement We devise a protocol in which general non-classical multipartite correlations produce a physically relevant effect, leading to the creation of bipartite entanglement. In particular, we show that the relative entropy of quantumness, which measures all non-classical correlations among subsystems of a quantum system, is equivalent to and can be operationally interpreted as the minimum distillable entanglement generated between the system and local ancillae in our protocol. We emphasize the key role of state mixedness in maximizing non-classicality: Mixed entangled states can be arbitrarily more non-classical than separable and pure entangled states. Back
to top Integrated Quantum Photonics Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Encoding quantum information in photons is appealing thanks to their low-noise properties and ease of manipulation at the single qubit level, and their promise in the fields of quantum communication, metrology and other quantum technologies. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability. Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter. We have used this architecture to implement a small-scale compiled version of Shor’s quantum factoring algorithm and demonstrated heralded generation of tunable four photon entangled states from a six photon input. We have combined waveguide photonic circuits with superconducting single photon detectors. Finally, we describe complex quantum interference behaviour in multi-mode interference devices with up to eight inputs and outputs, and quantum walks of correlated particles in arrays of coupled waveguides. Back
to top Scavenging quantum information: Multiple
observations of quantum systems Back
to top Quantum tomography of photonic time -energy
entanglement by photon bunching with short-time reference pulses Characterizing time-energy entanglement of photons is particularly challenging because of the difficulty of realizing time-resolved quantum measurements. Hence, we analyse the probability of obtaining the full quantum states of photons in their time-energy degree of freedom by bunching with short-time reference pulses. We show that the complete quantum coherence in time can be obtained using reference pulses in a superposition of two short time pulses. The application to entanglement shows that the method allows an efficient detection of temporal entanglement using entanglement witness criterion obtainable with only a minimal number of measurement settings. The Power of Many Settings or Many Outcomes
in Experimental Demonstrations of EPR-Steering Einstein Podolsky and Rosen (EPR) first highlighted the fact that ``as a consequence of two different measurements performed upon the first system, the second system may be left in states with two different [kinds of] wave functions'' [1]. In the same year, Schrödinger introduced the term steering to describe the EPR paradox, and discussed the possibility of using more than two kinds of measurements [2]. Surprisingly, it is only very recently that general EPR-steering inequalities, allowing for measurements of an arbitrary number of different observables by the two parties, have been developed [3], following the first formal definition of EPR-steering [4]. This proved that demonstrating EPR-steering is strictly easier than demonstrating Bell-nonlocality, but strictly harder than demonstrating entanglement (that is, nonseparability). I will describe two recent experimental demonstrations of this hierarchy. In [5] we implemented more than two settings so as to be able to show, for the first time, that EPR-steering occurs for mixed entangled states that cannot possibly demonstrate Bell-nonlocality. Increasing the number of measurement settings beyond two – we use up to six – dramatically increases the robustness of EPR-steering to noise. In [6] we implemented the maximally parsimonious demonstrations of the three types of nonlocality, involving 16, 12, and 9 different possible joint outcomes in the cases of Bell-nonlocality, EPR-steering, and entanglement respectively. In the latter two cases, this involved using a non-projective 3-outcome measurement (the “trine”). REFERENCES
On the efficiency of excitonic energy
transport What is the role of quantum coherence for the mechanisms underlying
efficient energy transport though photosynthetic light-harvesting
complexes? To explore this question, we conduct a large-scale statistical
survey of excitation transport in ensembles of spatially disordered,
finitely sized molecular networks with dipolar interactions in the
presence of tunable dephasing noise, and we compare the efficiency
of noise-assisted transport with that achievable by means of constructive
quantum interference. In contrast to the common presumption that
coherent effects generally lead to localization and thus to suppression
of transport, we prove the existence of certain rare optimal molecular
configurations that mediate highly efficient coherent excitation
transport. Although dephasing noise---which gradually destroys interference
and thereby gives rise to essentially classical transport---enhances
the efficiency of most configurations in our statistical ensemble,
the detected optimal configurations yield systematically higher
transport efficiencies and attain the maximum efficiency in the
absence of noise. These insights---combined with recent experimental
demonstrations [1] of long-lived coherence in certain light-harvesting
structures---provide a strong hint that nature takes advantage of
quantum mechanical coherent dynamics in order to enhance the efficiency
of principal tasks. [2] T. Scholak et al., arXiv:1103.2944v1 Back
to top Practical characterization of quantum devices
without tomography The complexity of quantum tomography experiments presents a major obstacle for the characterization of even moderately large quantum information devices. Part of the problem is that tomography generates much more data than is actually sought. We describe how a more targeted approach allows for (i) the verification of the fidelity of an experiment to a theoretical state or process, and for (ii) the estimation of which state or process from a reduced subset best matches the experimental data. Both these cases lead to a significant reduction in quantum and classical resources when compared against tomography - in general this is a quadratic reduction, but for some cases of practical interest we obtain an exponential reduction. In particular, we show that for fidelity estimation a constant number of different local experimental settings always suffices, and that this number of settings is always smaller than what is required for tomography. Back
to top Broadband waveguide quantum memory for
entangled photons Reversible mapping of quantum states, particularly entangled states,
between light and matter is important for advanced applications
of quantum information science. This mapping, i.e. operation of
a quantum memory [1], is imperative for realizing quantum repeaters
[2] and quantum networks [3]. Here we report the reversible transfer
of photon–photon entanglement into entanglement between a photon
and a collective atomic excitation in a solid-state device [4] (see
also [5]). Specifically, we generate time-bin enangled pairs of
photons [6] at the low-loss 795 nm (in free-space) and 1532 nm (in
fibre) wavelengths. The 795 nm photons are sent into a thulium-doped
lithium niobate waveguide cooled to 3K, absorbed by the Tm ions,
and retrieved after 7 ns by means of a photon-echo quantum memory
protocol employing an atomic frequency comb [7]. The acceptance
bandwidth of the memory has been expanded to 5 GHz, more than one
order of magnitude larger than the previous state-of-the-art [8],
to match the spectral width of the filtered 795 nm photons. The
entanglement-preserving nature of our storage device is assessed
through quantum state tomography before and after storage. Within
statistical error, we find a perfect mapping process. Furthermore,
by violating the CHSH inequality [9], we directly verify the nonlocal
nature of the generated and stored entangled photons. [2] N. Sangouard et al., Quantum repeaters based on atomic ensembles and linear optics, Rev. Mod. Phys. 83, 33-80 (2011). [3] H. J. Kimble, The quantum internet, Nature 453, 1023-1030 (2008). [4] E. Saglamyurek et al., Broadband waveguide quantum memory for entangled photons, Nature 469, 512-515 (2011). [5] C. Clausen et al., Quantum storage of photonic entanglement in a crystal, Nature 469, 508-511 (2011). [6] I. Marcikic et al., Distribution of time-bin entangled qubits over 50 km of optical fiber, Phys. Rev. Lett. 93, 180502 (2004). [7] M. Afzelius et al., Multimode quantum memory based on atomic frequency combs, Phys. Rev. A 79, 052329 (2009). [8] I. Usmani et al., Mapping multiple photonic qubits into and out of one solid-state atomic ensemble, Nat. Comm. 1 (12), 1-7 (2010). [9] J. F. Clauser et al., Proposed experiment to test local hidden-variable theories, Phys. Rev. Lett. 23, 880-884 (1969). Back
to top Spectral Manipulation of Optical Pulses
Using the Gradient Echo Memory Scheme The burgeoning fields of quantum computing and quantum key distribution have created a demand for a quantum memory. The gradient echo memory (GEM) is one such scheme that can boast efficiencies approaching unity. Here we investigate the ability of GEM to spectrally manipulate light pulses stored in the memory. Spectral manipulation is important for pulse compression sideband extraction, and matching of pulse spectra to resonant and spectroscopic systems, as well as the potential to increase qubit rates in quantum communications networks. We present both theoretical and experimental results demonstrating the ability to shift the frequency, as well as spectrally compress or expand a pulse. Also the ability of GEM to recall different frequency components of a pulse at different times, and interfere two initially time separated pulses that are stored in the memory, are shown. Back
to top Coherent Control of Intramolecular
Energy Transfer in 24-mode Pyrazine We study the intramolecular energy transfer from the S2 excited electronic state of pyrazine into the energetically lower-lying first singlet state S1 due to internal conversion, during and after femtosecond laser irradiation. The dynamics is studied within an highly efficient methodology for computing quantum dynamics for radiationless transitions in multi-dimensional configurational spaces. We further investigate the use of bichromatic laser pulses with simple analytical pulse profiles in order to control the population transfer from S0 to the S2 and S1 excited electronic states. We find that 80% of the total initial population on S0 can be transferred to the S1 state within 100 fs by such pulses. We also find that the population in S2 can be 60% of the total initial population of S0 within 60 fs, and about 30% at the end of a 100 fs pulse. Back
to top The curious nonexistence of Gaussian 2-designs Quantum t-designs –-- ensembles of quantum pure states whose
t-th (and lower) moments mimic those of the uniform distribution
of states in Hilbert space -–- have found a variety of applications
in quantum information science and the foundations of quantum theory.
They have primarily been studied in finite-dimensional Hilbert spaces,
although some continuous-variable 1-designs (such as the coherent
states) have a long and illustrious history. While 1-designs are
nice, 2-designs seem to be the most useful and interesting. They
are far superior to 1-designs –-- often optimal --– for
a variety of tasks, including quantum state and process tomography. Back
to top Quantum Cryptography Approaching the
Classical Limit We consider the security of continuous-variable quantum cryptography as we approach the classical limit, i.e., when the unknown preparation noise at the sender’s station becomes significantly noisy or thermal (even by as much as 10, 000 times greater than the variance of the vacuum mode). We show that, provided the channel transmission losses do not exceed 50on the channel transmission, and is therefore incredibly robust against significant amounts of excess preparation noise. We extend these results to consider for the first time quantum cryptography at wavelengths considerably longer than optical and find that regions of security still exist all the way down to the microwave. Back
to top Nondeterministic fast ground state cooling
of a mechanical resonator We present an ultrafast feasible scheme for ground state cooling of a mechanical resonator via repeated random time-interval measurements on an auxiliary flux qubit. We find that the ground state cooling can be achieved with several such measurements. The cooling efficiency hardly depends on the time-intervals between any two consecutive measurements. The scheme is also robust against environmental noises Back
to top A general framework of weak measurement and
its application to optical non-linearlity We extend the original idea of weak measurement to the case of a general preselection (mixed state) and a general postselection (a projection onto a subspace), we provide a complete treatment for both the regime when the preselection and the postselection (PP) are almost orthogonal and the regime when they are exactly orthogonal. We surprisingly find that for a fixed interaction strength, there may exist a maximum signal amplification and a corresponding optimum choice of PP to achieve it. We also find interesting quantities, the orthogonal weak values, which play the role of weak values for the case when the PP are exactly orthogonal. We also study how weak measurement can amplify the nonlinearity effect in a nonlinear crystal. --------------------------------------------------------------------------------
Properties of the three qubits entangled
state generated by the application of the Relative Phase Gate In this contribution we study the properties of a new entangled state. This state is generated by the application of the Relative Phase Gate (with an arbitrary phase #966;) on three qubits. The Relative Phase gate was recently defined (Quan. Inf. Compt. 10, 190 (2010)) for two and three qubits and it represents a slightly different conditional quantum evolution to that which appears in the application of the usual quantum gates. We calculate some entangled measures, like the residual tangle, defined for three qubits to measure the degree of entanglement of this states. Back
to top Two-Photon Parity and Analytical Approximations
to the Two-Photon Rabi Hamiltonian We study a close relative of the well-known spin-boson/Rabi Hamiltonian,
the two-photon Rabi Hamiltonian (TPRH). This Hamiltonian describes
a two-level system interacting with a quantum harmonic oscillator
via quadratic coupling. As opposed to a displacement in position
in the case of the Rabi Hamiltonian, the coupling in the two-photon
Rabi Hamiltonian is through frequency displacement or “squeezing.”
This Hamiltonian arose from describing two-photon processes in quantum
optics and can potentially model any two-level system for which
the two levels are at different frequencies. Back
to top Security of high speed quantum key distribution
with finite detector dead time The security of a high speed quantum key distribution system with finite detector dead time t is analyzed. When the transmission rate becomes higher than the maximum count rate of the individual detectors (1/t), security issues affect the algorithm for sifting bits. Analytical calculations and numerical simulations of the Bennett-Brassard BB84 protocol are performed. We study Rogers et al.'s protocol (introduced in "Detector dead-time effects and paralyzability in high-speed quantum key distribution, " New J. Phys. 9 (2007) 319) in the presence of an active eavesdropper Eve who has the power to perform an intercept-resend attack. It is shown that Rogers et al.'s protocol is no longer secure. More specifically, Eve can induce a basis-dependent detection efficiency at the receiver's end. Modified key sifting schemes that are secure in the presence of dead time and an active eavesdropper are then introduced. We analyze and compare these secure sifting schemes for this active Eve scenario, and calculate and simulate their key generation rate. It is shown that the maximum key generation rate is 1/(2t) for passive basis selection, and 1/t for active basis selection. The security analysis for finite detector dead time is also extended to the decoy state BB84 protocol. Back
to top Attainability of Chernoff bound by
LOCC Hypothesis testing is a fundamental problem in statistical inference and has been a crucial element in the development of information sciences. The Chernoff bound gives the minimal average probability of error when discriminating two hypothesis given a large number of i.i.d. observations. We have addressed the quantum counterpart of this problem, i.e., discriminate between two known states of a system given a large number of copies. The Quantum Chernoff bound gives an (asymptotically attainable) upper-bound for the error probability of discriminating many copies of two possible states using the most general collective measurement. We showed that in general this bound cannot be reached by repeating the same fixed measurements on every copy, but it remained an open question whether adaptive measurements schemes, which do use classical communication, can saturate the bound. We have shown how to efficiently compute bounds on the LOCC discrimination between two mixed states. In contrast with the pure--state case, these experimentally feasible protocols perform strictly worse than the general collective ones. We find that in order for LOCC and collective protocols to achieve the same accuracy, the former can require up to twice the number of copies than the latter. This gap in the error rates takes its largest value in the region of nearly pure, but strictly mixed, states. Excluding this region, there are no significant differences in performance between the simplest (repeated) and optimal LOCC strategies. This shows that while the performance of collective pure-state discrimination is not very much affected by the presence of noise, the optimal LOCC protocols change drastically if the states become slightly mixed. Similar approaches can be used to bound the power of separable strategies in other similar settings, which is still one of the most elusive questions in quantum communication. Back
to top The Number of Measurement and Broadcast
Rounds Needed to Perform Certain LOCC Operations Despite its importance to quantum information, the class of Local Operations with Classical Communication (LOCC) is still not satisfactorily understood. For instance, very little is known about what new operational possibilities become available using LOCC as more rounds of measurement and communication are performed. In this talk, I will present surprising new results concerning the round dependence of certain LOCC tasks. In particular, we will see how the class of LOCC operations becomes strictly more powerful as additional rounds of classical communication are permitted. More precisely, for every n, there always exists an n round protocol that is impossible to implement in n-2 rounds. Furthermore, certain tasks become possible if and only if the protocol uses an infinite (unbounded) number of rounds. The LOCC process examined is the conversion of the state |W> = v{1/3}(|100>+|010>+|001>) into bipartite pure entanglement shared between any two of three parties. Such a task is known as random distillation, having been first studied by Fortescue and Lo [Phys. Rev. Lett. 98, 260501 (2007)]. We will additionally find that, for the random distillation of |W> to succeed with probability one, the required number of rounds discontinuously jumps from four to an unbounded number when the amount of distilled entanglement gets too large. Back
to top Mutually unbiased bases in Majorana
representation The Majorana representation, which was firstly shown in 1932, still
remains an interesting problem and many of its aspects are so far
unknown. The representation allows to present pure state of spin-J
system as 2J points on Bloch-Riemann sphere . Moreover, when the
action of unitary operator on the state is reflected in rotation
of the corresponding points as rigid solid. Back
to top Violation of Heisenberg’s Precision
Limit by Weak Measurements Using a Composite Circuit One-Way model
of Quantum Computing Along with the uncertainty principle that relates simultaneous statistics of non-commuting observables for a quantum state, Heisenberg postulated another set of relations which set a lower limit on the disturbance to an observable caused by a second measurement of another possibly non-commuting observable[1]. These relations, though previously accepted, were shown to be inaccurate [2] shedding doubt on various widely-accepted limitations of high-precision microscopy, spectroscopy and other metrology concepts and offering new insights into foundations of quantum physics. A theoretical scheme for testing the precision-disturbance relation of Ozawa based on quantum information concepts was proposed in [3]. In this proposal the hurdle of destructive measurements, which previously impeded such tests, is addressed by the weak value approach of [4]. This scheme is based on a 3-qubit quantum circuit that requires two controlled-NOT gates of variable strength with a common control qubit. Here, we present an experimental realization of Heisenberg’s precision limit violation based on weak value measurements. We implement a one-way quantum computing technique, using entanglement as the substrate for quantum gates. A pair of polarization entangled photons is used to carry out two consecutive CNOT operations on one qubit, where the outcome of the first CNOT is teleported to the second photon of the pair. [1] Heisenberg W 1983 Quantum Theory and Measurement ed J A Wheeler and W H Zurek (Princeton, NJ:Princeton University Press) pp 62–84 (originally published in 1927 Z. Phys. 43 172) [2] Ozawa M 2004 Ann. Phys. NY 311 350 [3] Lund A P and Wiseman H 2010 New J. Phys. 12 093011 [4] Aharonov Y, Albert D Z and Vaidman L 1988 Phys. Rev. Lett. 60 1351 Back
to top Qutrit squeezing via semiclassical evolution Recently quantum systems more complex than the qubit have been studied in the context of possible applications to quantum information processes. In particular, the qutrit-like systems with symmetry group SU(3) naturally appear in three-level system. In this talk we will discuss how a particular type of squeezing, derived by comparing the minimum fluctuation of an observable and the fluctuation of this observable in a coherent state, can be understood using an approximate form of SU(3) Wigner functions. The short time dynamics required to generate this squeezing is obtained from the classical evolution generated by a simple Hamiltonian quadratic in the generators of su(3). Some numerical comparison between the dynamics for the fully quantum, exact Wigner functions and its approximation will be presented. Back
to top Towards single-photon cross-phase modulation
in cold atoms Single photons have long been thought of as ideal quantum information
carriers but due to their very weak interactions with one another,
have encountered obstacles as an architecture for quantum computation
itself. The inability to efficiently and deterministically perform
universal quantum logic gates using photons has prevented their
usage in such contexts. Back
to top Quantum coherence and optimal electronic
energy transfer in light-harvesting antenna proteins PE545 and PC645
of cryptophyte algae We study electronic energy transfer (EET) in the light-harvesting
antenna proteins PE545 and PC645 isolated from marine cryptophyte
algae, where long-lived quantum coherence has been reported at room
temperature [1]. To do so, we apply a recently developed non-perturbative
method, based on the reduced hierarchical equation approach [2-4].
This method is capable of interpolating between the regimes of weak
and strong system-bath coupling, thus allowing an analysis of EET
under a wide range of environmental parameters. The optimal conditions
for fast and efficient EET, as well as the role of quantum coherence
are investigated in this work. Back
to top On the Possibility of Amplifying Single-Photon
Nonlinearity Using Weak Measurement One of the most important challenges of optical quantum information
processing has been to create and detect optical non-linearities
at few-photon level. There has been many new proposals for engineering
larger non-linear effects, eg by using atomic coherence effects
such as electromagnetically induced transparency. However a great
deal of conceptual and technical progress is needed before obtaining
the required non-linearities. Back
to top Universal Squash Model For Optical Communications
Using Linear Optics And Threshold Detectors Quantum communications often rely on single photons as information carriers in order to exploit their quantum mechanical properties. However, practical detectors are often threshold detectors that are incapable of resolving the number of photons received. This apparently subtle issue has surprisingly immense implication to many quantum communications protocols. In fact, it has been shown that this issue leads to many problems including fake violation of Bell's inequality, insecurity of quantum key distribution, and false entanglement verification. The source of these problems is the theoretical consideration of the incoming signals for detection being single-photon signals; but in practice they may be multi-photon signals. We report a universal solution that is protocol-independent to bridge this gap between theory and experiments. Back
to top Analysis of exceptional points in open
quantum systems and QPT analogy for the appearance of the resonant
state We propose an analysis technique for the exceptional points (EPs) occurring in the discrete spectrum of open quantum systems, relying on a semi-infinite chain coupled to an endpoint impurity as a prototype model. We outline our method to locate the EPs in such systems and carry this out for our prototype, further obtaining an eigenvalue expansion in the vicinity of the EPs that gives rise to characteristic exponents. Finally, we offer a heuristic QPT analogy for the emergence of the resonance (giving rise to broken time symmetry via exponential decay) in which the decay width plays the role of the order parameter; the associated critical exponent is determined by the eigenvalue expansion in the vicinity of the EP. Back
to top Coherent Quantum Control in a System of
Overlapping Resonances: Simultaneous Excitation and Decay to the
Continuum The coherent control of the simultaneous (weak field) preparation and decay of a system of overlapping resonances coupled to a continuum of states has been investigated. The current approach is a generalization of a previous theory for the post-preparation coherent control of internal conversion in the presence of overlapping resonances. The relation between the previous and present theories has been exposed. Two control objectives with different constraints have been investigated numerically in a simple one-dimensional iodine bromide model involving two or more overlapping resonances. Back
to top Multidimensional Quantum Communication by
Temporal Phase Manipulation In contrast to photon internal polarization degree of freedom limited
only to two dimensions, external degrees of freedom related to space
and time can be used to construct infinite dimensional Hilbert spaces.
Spatial degree of freedom has been recently employed to encode multidimensional
quantum information using photon orbital angular momentum, however
this approach is not suitable for the single-mode fiber optical
communication. The temporal degree of freedom has been employed
for both quantum communication and entanglement. However the in
these Franson-type interferometer approaches, the Hilbert space
is limited to only a few discrete time bins. Recently, single-photon
sates generated in cavity parametric downconversion (PDC) with extremely
long coherence time have become available, allowing the full advantage
of the infinite-dimensional temporal degree of freedom. Back
to top Delocalization-Enhanced Long-Range Energy
Transfer between Cryptophyte Algae PE545 Antenna Proteins We study the dynamics of interprotein energy transfer in a cluster, consisting of four units of phycoerythrin 545 (PE545) antenna proteins via a hybrid quantum-classical approach. Our results indicate that persistent exciton delocalization is an important implication of the quantum nature of energy transfer on a multiprotein length scale, and that a hybrid quantum-classical approach is a viable starting point in studies of long-range energy transfer in condensed phase biological systems. Back
to top Playing the Aharon-Vaidman quantum
game with a photonic qutrit. We present a simple experimental scheme, which allows to encode and measure the quantum state of a qutrit and simulate the Aharon-Vaidman quantum game. The three level system is encoded in a spatial mode of a single photon passing through a system of slits. Within this scenario one can prepare the class of qutrit states by controlling the direction of a photon propagation and the number of slits that are open. The rank 7 POVM was implemented by placing a single photon detector in the respective positions related to "near and far field". This allowed us to topographically reconstruction of a pure state and play the quantum game. Back
to top Simulation of chemical isomerization reaction
dynamics on an NMR quantum simulator Quantum simulation can beat current classical computers with minimally a few tens of qubits. We report an experimental demonstration that a small nuclear-magnetic-resonance (NMR) quantum simulator is already able to simulate the dynamics of a prototype laser-driven isomerization reaction using engineered quantum control pulses. The experimental results agree well with classical simulations. We conclude that the quantum simulation of chemical reaction dynamics not computable on current classical computers is feasible in the near future. Back
to top Finding Decoherence Free Subspaces Without
Quantum Process Tomography It is well known that one of the greatest challenges facing quantum computation and communication today is quantum decoherence. Quantum decoherence destroys information contained in quantum superposition states, and effectively creates classical mixtures. There have been several strategies for minimizing quantum deoherence, or even circumventing it, but these methods do not get rid of decoherence. Rather, they construct the system in such a way that decoherence can be corrected for. Another method for circumventing decoherence exists that allows decoherence to be minimized at the cost of information: decoherence free subspaces. In order to characterize a process completely, quantum process tomography must be performed. Quantum process tomography is exponentially expensive, in that it scales as 24n, where n is the number of qubits in the system. This task becomes rapidly infeasible as the size of the system increases. As a result, it is essential that we ask whether we can learn something about the process without doing full process tomography. In this poster, we will show that by only making a maximum of O(23n) measurements, we can identify all the decoherence free subspaces for a given process. If the process possesses certain properties, the number of measurements needed can be as few as O(22n) – on the order of state tomography. We also present an experiment in which a 2-qubit process containing a decoherence free subspace is characterized in only 32 measurements – instead of the 256 measurements required for full process tomography. Back
to top A generalization of Noether's theorem
and the information-theoretic approach to the study of symmetric
dynamics Information theory provides a novel approach to study of the consequences of symmetry of dynamics which goes far beyond the traditional conservation laws and Noether's theorem. The conservation laws are not applicable to the dissipative and open systems. In fact, as we will show, even in the case of closed system dynamics if the state of system is not pure the conservation laws do not capture all the consequences of symmetry. Using information theoretic approach to this problem we introduce new quantities called asymmetry monotones, that if the system is closed they are constant of motion and otherwise, if the system is open, they are always non-increasing. We also explain how different results in quantum information theory can have non-trivial consequences about the symmetric dynamics of quantum systems. Back
to top Thermalization of Open Quantum Systems In generic isolated systems, non-equilibrium dynamics is expected to result in thermalization: a relaxation to states in which the values of macroscopic quantities are stationary, universal with respect to widely differing initial conditions, and predictable using statistical mechanics. From a classical point of view, the thermalization of a system S coupled to a thermal bath TB is an ergodic process. However, it not obvious, at the quantum level, to what extend this process can be considered as an incoherent situation. The situation is more involving when the system S+TB, already thermalized, is in contact with a second thermal bath BB. Here we show that second scenario, the thermalization of an open quantum system, is accompanied by the presence of quantum features. Back
to top Entanglement dynamics in coupled harmonic
oscillators I study the dynamics of the entanglement of two harmonic oscillators
linearly coupled and undergoing decoherence by interacting with
memoryless independent reservoirs. A Markovian master equation of
the form [1]: dr/dt=i [r, g(a1 a2f+a1fa2)]+ 2 [1] F. P. Laussy, E. del Valle, and C. Tejedor, “Luminescence
spectra of quantum dots in microcavities. i. bosons, ” Phys.
Rev. B, vol. 79, no. 23, p. 235325, 2009. Back
to top Decoherence in quantum walks on one-dimensional
regular networks Back
to top Knitting distributed cluster states with
spin chains Cluster states are one of the fundamental resources of quantum computing and crucial for a vast number of quantum protocols. In this contribution, we present a protocol for producing distributed cluster state ladders of arbitrary length using only a single spin chain. Spin chains have recently been the subject of many studies and are a promising candidate for quantum information transfer. The proposed protocol makes use of a spin chain set up for perfect state transfer and requires access for injection and extraction of excitations at the two end spins only. An outline of potential sources of errors and their effect on the fabrication of cluster states will also be given. Back
to top On the Choice of Input States for Process
Tomography One of the roadblocks to quantum computation and cryptography is
decoherence. In order bypass the problem of decoherence it must
first be characterized. By performing quantum process tomography
(QPT) the decohering effects of the system can be discovered. In
standard QPT one sends a complete set of states through the process
and characterizes each state at the output. From this information
the process and the decoherence can be reconstructed. In order to
best characterize the decoherence the set of input states must be
sensitive to it. Back
to top Adaptive qubit Hamiltonian parameter
estimation in presence of dephasing applied to double quantum dots. We present an optimized two-electron double-quantum-dot qubit Hamiltonian parameter estimation algorithms based on Bayesian reasoning. Qubit evolution in such a system is driven by magnetic field difference between the electrons in each part of the dot. This field is in part due to the nuclei of surrounding atoms, and because of its instability it is important to estimate its value quickly. We use a procedure consisting of repeated measurements in same fixed basis after different time intervals, which is motivated by experimental limitations on preparing and measuring qubits in a general basis (as required by most parameter estimation algorithms). Based on the set of outcomes one can make Bayesian inferences about the unknown qubit evolution frequency. The precision of estimation depends on the number of measurements performed and intervals between these measurements. Another important issue is the measurement procedure itself which gives higher fidelity for longer measurement times which are in turn much longer than the characteristic evolution times for solid state systems. Thus, in order to optimize the estimation it is crucial to minimize the number of measurement steps keeping in mind the trade-off between the measurement fidelity and measurement time as well. We present the optimal adaptive (each following evolution time interval depends on the outcomes of the previous measurements) and non-adaptive algorithms which can be used to estimate the qubit evolution frequency and show how the effects of measurement fidelity and dephasing will affect the results. Back
to top Control in classical limit: Robustness against
decoherence in an optical lattice Recently it has been shown that control, measured as non-zero average momentum in an 1D optical lattice, survives in classical limit and is robust against moderate decoherence. In addition to the earlier work, the same system is studied here to understand the influence of the degree and type of decoherence on the control in the classical and quantum regime. The survival of transport in the presence of strong decoherence induced via spontaneous emission is observed. We demonstrate that even large number of jumps do not destroy the transport significantly as distinct from spatial jitter which does alter the dynamics and destroy transport. The underlying theory for such observations will be discussed in the poster. Back
to top Measures and Implications of Electronic
Coherence in Photosynthetic Light-Harvesting We present the various different methods employed in measuring delocalization in light harvesting complexes, and focus on deriving direct relations between traditional inverse participation ratios and entanglement measures. The B850 ring from the LH2 complex in Rhodopseudomonas acidophila acts as our model system. By analysing the behaviour of these metrics under Electronic Energy Transfer (EET) dynamics in the B850 ring, we conclude that measures of entanglement are far more robust (in terms of timescale, temperature and level of decoherence) than inverse participation ratios, and are therefore more appropriate for the purpose of studying the time evolution of coherence in a system. Back
to top A Photonic Loop-Graph State for One-way
Quantum Computing We report on the experimental realization of a 4-qubit loop-graph state that uses polarization and path degrees of freedom of photons to implement the logical qubits. The loop in this graph allows one to use the so called ‘generalized flow’, which is shown to optimize the complexity depth of the computation and possibly reduce the required number of qubits compared to a cluster. In addition, this graph corresponds to a circuit with a time-like loop, hence it provides an operational method for better understanding such loops. For the experiment we start with a symmetric maximally entangled polarization state. The path qubits are then added to each photon using 50:50 beam splitters. We use a novel method that takes advantage of the symmetry in polarization states to do a controlled-Z operation between the polarization of one photon and the path of the other only by using a half-wave plate. This is the key step that enables us to realize this loop graph that has never been implemented before. Using 4-qubit state tomography we completely characterize the loop graph state. To show the equivalency of this graph state to the circuit, that is found using the generalized flow, we carry out various computations with this state and contrast the outputs to what we expect to get from an equivalent circuit. Back
to top Improved Accuracy for Adiabatic Quantum
State Transfer We present a technique that dramatically improves the accuracy of adiabatic state transfer for a broad class of realistic Hamiltonians. For some systems, the total error scaling can be quadratically reduced at a fixed maximum transfer rate and these improvements rely only on the judicious choice of the total evolution time. Our technique may be immediately applicable to existing experiments utilizing adiabatic passage. We give two examples as proofs-of-principle, showing quadratic error reductions for an adiabatic search algorithm and a tunable two-qubit quantum logic gate. Back
to top Quantum trade-off coding for bosonic communication Recent work has precisely characterized the achievable trade-offs between three key information processing tasks---classical communication (generation or consumption), quantum communication (generation or consumption), and shared entanglement (distribution or consumption), measured in bits, qubits, and ebits per channel use respectively. Slices and corner points of this three-dimensional region reduce to several well-known quantum communication protocols over noisy channels. A single trade-off coding technique can attain any point in the region and can outperform time-sharing between the best known protocols for accomplishing each information processing task alone. Previously, the benefits of trade-off coding that had been found were too small to be of much practical value (for the dephasing and the universal cloning machine channels, for instance). In this article, we demonstrate to the contrary that the associated performance gains are remarkably high for several physically relevant bosonic channels that model free-space / fiber-optic links, thermalizing channels, or amplifiers (or even relativistic communication). We show that significant performance gains from trade-off coding also apply when trading photon-number resources between transmitting public and private classical information simultaneously over secret-key-assisted bosonic channels. Back
to top Slowing single photons with cold Rb atoms Spontaneous Parametric Down-Conversion (SPDC) has been widely used
to generate single photons and entanglement in many quantum information
applications. Using a far-below-threshold Optical Parametric Oscillator
(OPO), we sucessfully modified the output spectrum of the SPDC,
which allows us to generate long-coherence-time single photons whose
bandwidth is matched with the Rb natrual linewidth. Back
to top High speed quantum random number generation
with quantum phase noise Random number generators(RNG) are important in many felds of science
and technology [1-3]. Currently, fast random numbers can easily
be generated from either computer algorithms or the chaotic behavior
of complex systems [4]. However, both of the two schemes are deterministic
in nature and, thus cannot generate true random numbers with information-theoretically
provable randomness. Quantum RNG (QRNG), on the other hand, is able
to generate perfect random numbers from the truly probabilistic
nature of fundamental quantum processes [5]. Up to now, QRNGs are
limited by system implementation complexities and generation rates
[5-7] owing to the strict requirements on measuring the quantum
properties. The highest generation rate for the commercial QRNGs
is only 16 Mbits/s [8]. Back
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