SCIENTIFIC PROGRAMS AND ACTIVITIES

July 3, 2015

RM 210

Stephen Hughes (Queen's University)

How to "fix" Purcell's formula for leaky optical cavities and plasmonic nanoresonators

Two of the most common and useful metrics for characterizing the properties of optical cavities are the Q value (quality factor) and the effective mode volume V. The Purcell effect is a beautiful example of a situation in which a cavity with a large Q/V ratio enhances the spontaneous emission rate of an atom or quantum dot. In Purcell's original paper, a modest abstract published in the proceedings of the American Physical Society meeting at Cambridge in 1946, Purcell formulated the enhanced spontaneous emission factor in a very elegant way as scaling with Q/V. It is no exaggeration to say that Purcell's formula has been the workhorse for cavity physics for decades, but it turns out to be wrong! At least it turns out to be wrong in general with the way that the modes and effective mode volume are obtained for open and dissipative resonators. In this talk, I will argue that most, if not all, confusion about cavity modes can be removed by a proper treatment within the framework of quasinormal modes (QNMs), defined as the frequency domain solutions to the wave equation with open boundary conditions. Using these QNMs, I will describe a newly developed mode expansion technique that can be used to evaluate the electric field from a dipole emitter at arbitrary positions outside and within optical cavities and plasmonic resonators. I will then introduce a rigorous definition of the Purcell factor and enhanced spontaneous emission factor and point out why the usual expression for effective mode volume is wrong. Several applications of the theory for modelling hybrid plasmonic-coupled emitter systems will be exemplified, including metal-dimer single photon sources and plasmon-mediated entanglement between two quantum dots.

June 19, 2015

RM 210

Aharon Brodutch (Institute for Quantum Computing, University of Waterloo)
"Should we care about quantum discord?"

Quantum bipartite systems can be correlated in various ways. For pure states, the total correlation can be quantified using mutual information while the entanglement entropy can be used to quantify the 'quantum correlation'. For mixed states the situation is not as clear cut. First, there is no unique measure of entanglement. Second, there are some tasks involving bipartite separable state that have no classical analogue. The 'quantum part' of the correlation can therefore be quantified in different ways depending on the task, these can be entanglement monotones or more general quantum correlation measures that do not vanish for separable states, e.g. discord.

In this talk I will begin with a brief overview of the mathematical properties of quantum discord and similar quantities. I will then give some physical examples of where these quantities can be useful and some intuition on where they are probably useless.

May 29, 2015

*ROOM 332-Fields Institute*

Marc Dignam (Queen's University)
Nonlinear Dynamics of Photons in Lossy, Coupled Photonic Crystal Cavities

There has been a great dealt of interest in recent years in developing nanophotonic systems for use as single-photon and entangled photon sources. One promising class of such systems is semiconductor photonic crystal slabs that contain line and/or point defects. In this talk I will discuss the formalism that we have developed to model nonlinear photon generation and propagation in coupled-defect cavities in photonic crystal slabs. The key to our approach is the non-hermitian projection of the photon dynamics onto a set of lossy and potentially non-orthogonal cavity quasi-modes. I will apply our approach to single quantum dots coupled to multiple cavities and to photon pair generation due to spontaneous four-wave mixing in coupled optical resonator waveguides.

May 1, 2015

RM 210

Paul Barclay (University of Calgary)
Diamond nanophotonic optomechanics: towards hybrid quantum systems

Nanophotonic optomechanical devices allow highly sensitive optical coupling to nanomechanical resonances, providing opportunities for probing their quantum properties, and for creating sensing and information processing technologies. Owing its desirable optical and mechanical properties, single crystal diamond is an attractive material for implementing optomechanical devices. In addition, diamond hosts color centers whose highly coherent electronic and nuclear spins are promising for quantum information science. We have recently demonstrated a diamond chip-based optomechanical system which allows sensitive readout of mechanical resonances with ultrahigh mechanical quality factor. This waveguide optomechanical device exhibits striking nonlinear nanomechanical behaviour, can be optically cooled to mK temperatures, and may allow coupling between nanomechanical resonances and single diamond spins. We have also created high quality factor diamond optical microcavities, which are promising devices for high-frequency optomechanics. This talk will review this work, together with related optomechanics results obtained in more conventional semiconductor materials such as silicon and GaP.

April 24, 2015

RM 210

Richard Haglund (Vanderbilt University)
Is Vanadium Dioxide credible for ultrafast photonic switching?

Ever since picosecond optical switching of the semiconductor-to-metal transition in vanadium dioxide (VO2) was discovered two decades ago, there has been growing interest in the potential for photonic applications of this deceptively simple oxide. But until recently, it appeared that the bottleneck in switching times for the rutile-to-monoclinic structural transition - of order hundreds of picoseconds at the very least - counseled a skeptical, if suspended, judgment. Recent demonstrations of a short-lived metallic monoclinic state in VO2 suggest that the potential indeed real, provided that high-quality VO2 thin films can integrated in silico. After motivating the idea of all-optical photonic switching in silicon from recent results, I will describe direct and indirect evidence for the monoclinic metallic state, and then consider the materials issues that will have to be solved to realize hybrid silicon-VO2 photonics in practice.

April 10, 2015 POSTPONED

RM 210

Girish Agarwal (Oklahoma State University)
Dicke Superradiance, Entanglement and Quantum Interference

As the superposition principle is basic to quantum mechanics, the interference effects occur very widely in quantum systems. Interferences are especially important in quantum optics and one of the most remarkable developments has been the possibility of interference of independent, but indistinguishable particles. These interferences, besides being very fundamental, have important applications in the context of quantum entanglement and superradiance. An important component of superradiance is the initial preparation of the system in Dicke states which can be shown to be multiparticle entangled states. Thus the production of Dicke states with higher number of excitations remains a challenge and a new option is the repeated measurements of photons starting from a fully excited system of quantum emitters. These ideas are quite generic and applicable to a wide variety of quantum sources. Our studies suggest new kind of enhancement in the efficiency of nonlinear optical processes in systems prepared in entangled states.

March 20, 2015
11:00am

RM 210

Joel Yuen (Massachusetts Institute of Technology)
Spectroscopy and topological phases for organic excitons

The understanding and control of energy flow at the nanoscale via exciton dynamics in organic materials is of fundamental chemical and physical interest, but is also technologically relevant for the design of novel photovoltaic materials. In the first part of my talk, I will explain some of our work designing spectroscopic protocols to understand exciton dynamics under coherent illumination via ultrafast Quantum Process Tomography (QPT), a technique which retrieves the time evolution of the quantum state of the excitons via nonlinear spectroscopy (1,2). As an application, I will describe the first ultrafast QPT experiment carried out with the Nelson and Bawendi groups at MIT on a nanotubular J-aggregate system at room temperature.

In the second part, I will describe current work (3) designing topologically nontrivial phases that robustly and selectively move excitons in particular spatial directions of a molecular crystal, simulating solid state "topologically protected" phenomena like the Quantum Hall Effect, which are robust against material imperfections and static disorder.

(1) J. Yuen-Zhou, Jacob J. Krich, Masoud Mohseni, and A. Aspuru-Guzik, Quantum state and process tomography of energy transfer systems via ultrafast spectroscopy, Proc. Nat. Acad. Sci. USA. 108, 43, 17615 (2011).
(2) J. Yuen-Zhou, D. Arias, D. Eisele, J. J. Krich, C. Steiner, K. A. Nelson, and A. Aspuru. Guzik, Coherent exciton dynamics in supramolecular light-harvesting nanotubes revealed by ultrafast quantum process tomography, ACS Nano 8 (6) 5527 (2014).
(3) J. Yuen-Zhou, S. Saikin, N. Yao, and A. Aspuru-Guzik, Topologically protected excitons in porphyrin thin films, in press, Nature Materials 13, 1026 (2014).

Host: Paul Brumer (CQIQC) pbrumer@chem.utoronto.ca

Feb 25, 2015
12:00pm

RM 210

Guoxing Miao (University of Waterloo)
Spin Manipulation through Tunable Magnetic Semiconductors

Magnetic semiconductor materials are best known for their spin-filtering properties which can effectively create highly polarized spin currents from nonmagnetic electrodes. One can readily generate and analyze spin information through quantum tunneling across these materials. Magnetic semiconductors have another less explored property: interfacial exchange fields onto neighboring electronic systems. Through indirect exchange interaction between the localized magnetic moments and the adjacent free electrons, the electron spins experience an effective Zeeman field on the order of tens of Tesla. This effect is especially pronounced on low-dimensional systems such as 2DEG and topological materials. We first probe its strength on a cluster of Al nano dots under Coulomb confinement, then proceed to show a number of material and device concepts for spintronic and quantum information applications originating from such spin filtering and interfacial exchange effects.

Feb 13, 2015
11:00am

RM 210

Vlad Pribiag (University of Minnesota)
Superconducting Edge-Mode Transport in InAs/GaSb Double Quantum Wells

Topological insulators are characterized by boundary modes with very strong spin-momentum coupling. In proximity to a conventional superconductor, these modes are predicted to host topological superconductivity, an exotic state of matter that supports Majorana zero-modes [1]. Localized Majorana modes obey non-Abelian exchange statistics, making them interesting building blocks for topological quantum computing. In this talk, I will describe our current efforts to realize topological superconductivity in nanostructures based on InAs/GaSb quantum wells, a two-dimensional topological insulator (2D TI). By electrostatically-gating the devices we observe superconducting transport in all three regimes of the 2D TI: bulk electrons, edge modes and bulk holes. We use superconducting quantum interference measurements as a sensitive method for determining the spatial distribution of the supercurrent in each regime. These measurements reveal a clear transition from bulk- to edge-dominated supercurrent under conditions of high bulk resistivity, which we associate with the 2D topological phase. These experiments establish InAs/GaSb as a promising platform for observing Majoranas modes and probing their exchange statistics. [1] /J. Alicea,//Rep. Prog. Phys./ *75* 076501 (2012). [2] /V.S. Pribiag et al/., /arXiv/:1408.1701.

Jan 30, 2015
11:10am

RM 210

Edward Taylor (University of Toronto)
Decoherence immunity using Majorana fermions: state of play and possible challenges

The central challenge to harnessing the power of quantum entanglement to do useful tasks is figuring out how to maintain coherences for long periods of time. As realized by Kitaev a little over a decade ago, one way to do this is to encode quantum information in the wavefunction of particles—so-called non-Abelian anyons—which obey neither Bose nor Fermi statistics. Since then, there has been an enormous amount of activity devoted to trying to find such particles in the form of Majorana surface states (quasiparticles) of topological superconductors. In this talk, I will review the theoretical and experimental state of play before raising questions about the current theory orthodoxy, which is based on the mean-field BCS theory of superconductivity and excludes fluctuations—Goldstone modes—which can interact with the Majoranas and modify their properties.

Jan 9, 2015
11:00am

RM 210

Ole Steuernagel (STRI, University of Hertfordshire)
Wigner flow reveals non-classical features in quantum phase space

The behaviour of classical mechanical systems is characterised by their phase portraits, the collections of their trajectories. Heisenberg's uncertainty principle precludes the existence of sharply defined trajectories, which is why traditionally only the time evolution of wave functions is studied in quantum dynamics. These studies are quite insensitive to the underlying structure of quantum phase space dynamics. We identify the flow that is the quantum analog of classical particle flow along phase portrait lines. It reveals hidden features of quantum dynamics and extra complexity. Being constrained by conserved flow winding numbers, it also introduces topological order into quantum dynamics.

Dec 5, 2014
11:00am

RM 210

Hamed Majedi (University of Waterloo)
Photon Detection and Generation by Superconductor and Semiconductor Nanostructures

The generation, manipulation, control and detection of quantum states of lights such as single and entangled photons are at the heart of quantum photonics. The integration and combination of single photon sources, passive optical circuits and single photon detectors are key enabling technology for two main reasons; first, it provides a feasible route toward scalable quantum photonic processors that are genuinely useful in practical applications and form "quantum-optics-lab-on-a-chip" and second, it enables building more complicated devices such as quantum amplifiers, repeaters and transceivers that are necessary for some applications such as quantum communication networking.

After a brief introduction to various technologies for single photon detectors and sources, I will focus on our research work on two key-elements of integrated quantum photonics, namely Superconducting Nanowire Single Photon Detector (SNSPD) and III-V NanoWire Quantum Dot (NWQD) single and entangled photon sources.

In SNSPD part, I will focus on quantum tomographic characterization of SNSPD and introduce our original contribution on how gated SNSPD increase the detection speed by an order of magnitude. In NWQD part, I will present the results for the first demonstration of polarization-entangled photon generation from a single InAsP quantum dot embedded in an InP nanowire in collaboration with Philip Poole's group at NRC and Gregor Weihs's group at University of Innsbruck. At the end, I will introduce the challenges and our ongoing attempts to integrate these two devices on a single chip.

Nov 21, 2014
11:00am

RM 210

Yaoyun Shi (University of Michigan)
How to generate the first secret, then as many as you like

Secrecy is randomness. A perfect secret is one for which all the alternatives are equally likely to the adversary. For secrecy to be possible, we have to assume that the world is not deterministic. Here we show how this necessary assumption, together with the validity of quantum mechanics and relativity, will allow us to generate the first and almost perfect secret, and then to expand it to be arbitrarily long. Unlike all existing solutions, the security of our construction is provable, unconditional (as opposed to computational), and verifiable. Our method can also be used for the important task of distributing cryptographic keys.

Technically speaking, we formulate a precise model of extracting randomness from quantum devices whose inner-workings may be imperfect or even malicious. We then construct such a "physical extractor" that needs only a single and arbitrarily weak classical source, and the output randomness can be arbitrarily long and almost optimally close to uniform. This is impossible to achieve for classical randomness extractors, which cannot increase entropy and requires two or more *independent* sources.

Our construction also differs from quantum-based random number generators in the market, as they all require that the users trust their quantum inner-workings. Such a trust threatens security when the devices are defective or were procured from an untrusted vendor. Several features of our construction, such as maximum noise tolerance and unit quantum memory requirement, have fundamentally lowered the implementation requirements.

Nov 14, 2014
11:00am

RM 210

Nancy Makri (University of Illinois)
Quantum-Classical Path Integral

The path integral formulation of time-dependent quantum mechanics provides the ideal framework for rigorous quantum-classical or quantum-semiclassical treatments, as the spatially localized, trajectory-like nature of the quantum paths circumvents the need for mean-field-type assumptions. However, the number of system paths grows exponentially with the number of propagation steps. In addition, each path of the quantum system generally gives rise to a distinct classical solvent trajectory. This exponential proliferation of trajectories with propagation time is the quantum-classical manifestation of time nonlocality, familiar from influence functional approaches.

A quantum-classical path integral (QCPI) methodology has been developed. The starting point is the identification of two components in the effects induced on a quantum system by a polyatomic environment. The first, “classical decoherence mechanism” dominates completely at high temperature/low-frequency solvents and/or when the system-environment interaction is weak. Within the QCPI framework, the memory associated with classical decoherence is removable. A second, nonlocal in time, “quantum decoherence process” is also operative at low temperatures, although the contribution of the classical decoherence mechanism continues to play the most prominent role. The classical decoherence is analogous to the treatment of light absorption via an oscillating dipole, while quantum decoherence is primarily associated with spontaneous emission, whose description requires quantization of the radiation field. The QCPI methodology takes advantage of the memory-free nature of system-independent solvent trajectories to account for all classical decoherence effects on the dynamics of the quantum system in an inexpensive fashion. Inclusion of the residual quantum decoherence is accomplished via phase factors in the path integral expression, which is amenable to large time steps and iterative decompositions. Preliminary tests on dissipative two-level systems and fully atomistic simulations of charge transfer in solution suggest that the QCPI methodology is realistically applicable to many processes of chemical and biological interest.

Josh Combes (Perimeter Institute for Theoretical Physics)
Perimeter Institute and Institute for Quantum Computing

In 1988 Yakir Aharonov, David Albert, and Lev Vaidman wrote a paper provocatively titled "How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100". In this paper they defined a quantity, similar to the expectation value of an operator, called the "weak value" of an operator. The weak value of an operator has many weird properties which has lead some researchers to: (1) think that quantum paradoxes are solved by this defined quantity, and (2) suggest that the weak value can be used to perform sensitive measurements. In this talk I will address both points. First, I argue that the phenomenon of anomalous weak values is not limited to quantum theory. In particular, I show that the same features occur in a simple model of a coin subject to a form of classical backaction with pre- and post-selection. Second, I will explain how rigorous estimation and detection theory imply that weak values do not aid quantum metrology. This is joint work with Chris Ferrie of the University of New Mexico.

Paola Cappellaro (Massachusetts Institute of Technology)
Quantum control strategies for imaging and spectroscopy

Quantum control techniques have proven effective to extend the coherence of qubit sensors, thus allowing quantum-enhanced sensitivity at the nano-scale. The key challenge is to decouple the qubit sensors from undesired sources of noise, while preserving the interaction with the system or field that one wishes to measure. In addition, tailoring the sensor dynamics can help reveal temporal and spatial information about the target.

In this talk I will show how we can use coherent control of quantum sensors to reconstruct the arbitrary profile of time-varying fields, while correcting the effects of unwanted noise sources. These control techniques can be further used to reveal information about classical and quantum noise sources. For example, they can achieve high frequency resolution, thus allowing precise spectroscopy and imaging of the spatial configuration of a spin bath.

I will illustrate applications of these strategies in experimental implementations based on the Nitrogen-Vacancy center in diamond.

Man-Duen Choi (University of Toronto)
The Principle of Locality made simple

In physics, the Principle of Locality states that an object is influenced directly only by its immediate surroundings. This could be transformed to a simple mathematical statement of NO wisdom at all. Nevertheless, with extravagent assumption (on the obvious truth) and fascinating explanation (of the ultimate nonsense), the Principle may become a big Law/Theory/Theorem or a tremendous Paradox to shake your heart/body.
This is an expository talk of my own adventure in the quantum wonderland (concerning the structure problems of direct sums and tensor products). No working knowledge of quantum information is required in this talk.

Sept 19, 2014
12:30 p.m
Fields RM 210

Boris Braverman (Massachusetts Institute of Technology)
Progress toward a spin squeezed optical atomic clock beyond the standard quantum limit

State of the art optical lattice atomic clocks have reached a relative inaccuracy level of $10^{-18}$, making them the most stable time references in existence. One limitation to the precision of these clocks is the projection noise caused by the measurement of the atomic state. This limit, known as the standard quantum limit (SQL), can be overcome by entangling the atoms. By performing spin squeezing, it is possible to robustly generate such entanglement and therefore surpass the SQL of precision in optical atomic clocks. I will report on recent experimental progress toward realizing spin squeezing in an ${}^{171}$Yb optical lattice clock. A high-finesse micromirror-based optical cavity mediates the atom-atom interaction necessary for generating the entanglement. By exceeding the SQL in this state of the art system, we are aiming to advance precision time metrology and expand the boundaries of quantum control and measurement.

Raphael Pooser (Oak Ridge National Labs)
Quantum Sensors: Data at the information frontier of physics

Quantum information processing has a host of applications, including quantum key distribution and quantum computing as some of the most prominent. In all of these applications, sensing and control are needed in order to maintain the fidelity of quantum information. In quantum sensors, information stored in quantum mechanical systems is extracted and put to use, either in subsequent control signals, or in general information processing applications. Some famous examples of quantum sensors include atomic clocks, cold atom interferometers, or Bose-Einstein condensates used in gravitometers, accelerometers, etc. Some of the original proposals for quantum sensors involved optical fields. In particular, sensors that exploit continuously variable degrees of freedom have been of interest since the discovery of quantum noise reduction. One of the first examples proposed by Caves is the use quantum noise reduction to achieve interferometric sensitivity in the quantum regime. Advanced LIGO is an example of an upcoming application. In addition to LIGO, in recent years continuous variables have seen renewed interest. In this talk we will discuss quantum sensors and their applications with a focus on the sensors developed at ORNL. We use quantum noise reduction to produce sub-shot-noise limited sensing devices, particularly in quantum plasmonic sensors and displacement sensors using MEMS cantilevers. Some applications for these devices include trace detection or quantum information applications, such as removing bias from QRNGs through adaptive control. We will also discuss other sensing types that use both discrete and continuous variables, such as quantum compressive imaging, and single photon detection applications.

Robert Boyd (University of Ottawa)
Menzel's Experiment: Violation of Complementarity?

In 2012, the group of Ralf Menzel in Potsdam, Germany published an article in PNAS that appeared to violate the accepted quantum mechanical notion of complementarity. Specifically, they observed interference with good fringe visibility in a Young's two-slit experiment, even though, through use of a quantum protocol, they were able to deduce through which slit each photon had passed. Our group has recently articulated an explanation for these unexpected results (Bolduc et al., PNAS 2014). Our explanation is that the Potsdam group had inadvertently violated a fair-sampling assumption by means of the manner in which they collected and analyzed their data.

Ioannis Thanapoulos (National Hellenic Research Foundation)
Quantum dynamics by the Effective Modes Differential Equations method

We show that the non-Markovian quantum dynamics of a system comprised of a subspace Q coupled to a much larger subspace P can be described by a set of Effective Modes Differential Equations (EMDE). The computational efficiency of the method is demonstrated by investigating the 24-mode decay dynamics and laser control of the radiationless transitions from the second to the first singlet electronic excited state of the pyrazine molecule.

Thomas Monz (University of Innsbruck)
Topological qubits

Arbitrarily long quantum computation requires techniques to overcome errors accumulated during the operation. Here, different approaches have been proposed, with topological quantum computation yielding one of the highest thresholds against errors. In this talk I will first provide a brief introduction into topological quantum computation, in particular the color code. Subsequently I will show how, for the first time, a qubit has been topologically encoded using an ion-trap based quantum computer. The presented experimental data illustrates how we can detect all physical single-qubit errors, perform the entire set of Clifford operations on this logical qubit and investigate its coherence properties. The presentation is concluded by an outline on upcoming milestones and their experimental as well as theoretical challenges.

Prof. Charlie Ironside (Curtin University)
A surface-patterned chip as a strong source of ultra-cold atoms for quantum technologies

Aug 1, 2014
11:00 am

Prof. Lianao Wu (University of Basque Country)
One Component Dynamical Equation and a Universal Control Theory

We use a Feshbach P-Q partitioning technique to derive a closed one- component integro-differential equation. The resultant equation properly traces the footprint of the target state in quantum control theory. The physical significance of the derived dynamical equation is illustrated by both general analysis and concrete examples. We show that control can be realized by fast-changing external fields, even fast noises. We illustrate the results by quantum memory and controlled adiabatic paths.

Prof. Gershon Kurizki, Weizmann Institute of Science
A thermal bath: more friend than foe?

Traditionally, the interaction of quantum systems with a thermal bath is viewed as detrimental to their quantumness. Yet this is not always the case, as the bath may actually promote quantumness, particularly when system-bath interactions are subject to control. I will review our recent results concerning different types of control capable of generating or enhancing quantum processes via the bath:
1. Control by modulation: By periodically modulating the energy of two-level or multilevel systems we may purify the state of the systems or the bath they couple to, upon tailoring the modulation to the bath spectrum. An intriguing consequence of such purification is the possibility to cool a bath consisting of coupled spins down to absolute zero, in apparent violation of Nernst's third law of thermodynamics. The thermal bath may also mediate the transfer of quantum information between distant systems, at a rate and fidelity controllable by the modulation.
2. Control by state preparation: The quantum state of an oscillator coupled to a thermalized qubit determines the amount and efficiency of work extractable from the thermal bath, thereby retaining its quantum features over surprisingly long time scales. Remarkably, certain quantum states yield higher efficiency than allowed by the Carnot bound, yet in full compliance with the second law of thermodynamics. In N-level systems, appropriate state preparation allows for N-fold enhancement of work extractable from the bath at steady state.
3. Control by bath engineering: The ability to control the coupling of quantum systems to appropriately designed, axially-guided modes of the bath, may drastically enhance the range of entanglement mediated by the bath, or lead to giant enhancement of bath-induced dispersion forces, colloquially known as van der Waals and Casimir forces.

July 11, 2014
Stewart Library

Matthew Broome, University of New South Wales
My Quantum Optics Show and Tell: Topology, complexity and biology

Progress in optical quantum computation has started to slow in recent times due to the problems associated with probabilistic quantum gates, lack of good single photon sources and poor non-linear optical materials. However, by looking at other applications besides a fully scalable quantum computer, we see that linear optics alone (beam splitters and phase shifters) is a powerful tool for simulation or emulation of interesting physical systems. In this talk I will discuss some recent results from the University of Queensland's Quantum Technology Lab that employ purely linear optical schemes for this purpose. In particular, I will focus the talk around single- and multi-particle quantum walks for investigating areas from condensed matter science to complexity theory.

Katja Ried, Perimeter Institute, Waterloo
How drug trials are simpler if your subjects are quantum (and other applications of quantum causal models)

A fundamental question in trying to understand the world -- be it classical or quantum -- is why things happen. We seek a causal account of events, and merely noting correlations between them does not provide a satisfactory answer. In classical statistics, a better alternative exists: the framework of causal models has proven useful for studying causal relations in a range of disciplines. We try to adapt this formalism to allow for quantum variables, and in the process discover a new perspective on how causality is different in the quantum world. One of the peculiarities that arise in this context can be harnessed to solve a task of causal inference -- inferring the causal relation between variables based on observed statistics -- that is impossible for classical variables. I will report on a recent experimental realization of this scheme.

Time permitting, I will also discuss a more realistic approach to the problem of characterizing quantum processes in the presence of initial
correlations with an environment, viz non-Markovian dynamics. Another application of quantum causal inference arises in the context of quantum field theory: if one couples two detectors to a quantum field at different points throughout space-time, this may allow one of them to causally influence the other, via the field. We explore how different variables of the model, such as the acceleration of the detectors and the ultraviolet cutoff of the field theory, are reflected in the strength and quality of the causal influence.