Speaker: Nicolas Gisin ( University of Geneva)

Title: Solid-state quantum memories for quantum repeaters

Today’s quantum key servers must be connected by one or two uninterrupted optical fibers. The ultimate limit of such direct point to point quantum key distribution is around 300-500 km.

Future fiber-based quantum networks, able to connect many quantum key servers over arbitrary long distances, require both high-fidelity entanglement swapping and multi-mode quantum memories.

We first discuss the general vision of a global quantum network. Next, we present progress in:

Long distance point-to-point Quantum Key Distribution.

A semi-classical Quantum Key Distribution network.

Entanglement distribution using the Swisscom network between two villages 18 km apart with a continuous violation of Bell’s inequality during 24 hours.

A new protocol for an efficient multimode quantum memory based on atomic ensembles. The atomic ensemble, rare-earth ions, is “frozen” in a crystal inside a cryostat. The protocol is inspired from photon echoes, but avoids any control light pulse after the single-photon(s) is (are) stored in the medium, thus avoiding any noise due to fluorescence.

First demonstrations of the new protocol for multimode quantum memories. The coherence of the re-emitted photons is investigated in an interference experiment showing net visibilities above 95%.

In summary, many hundreds of km long quantum communication is a long term objective. Many of the necessary building blocks have been demonstrated, but usually in independent experiments and with insufficient specifications to meet the challenge. Nevertheless, today the roadmap is relatively clear and a lot of interesting physics shall be found along the journey.

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Speaker: P.K. Pathak (Queen's University)

Title: Merging photonic crystal cavities and single quantum dots: a practical source of entangled photon pairs

There has been considerable progress for developing chip-based, and scalable, sources of entangled photons using single quantum dots (QDs)[1-3]. In semiconductor QDs, entangled photons are typically generated in a biexciton-exciton cascade decay. However, the entanglement between the generated photons is limited by inherent cylindrical asymmetries and various dephasing processes. The cylindrical asymmetries produce fine structure splitting (FSS) in the exciton states; as a result, the emitted x-polarized and y-polarized photon pairs become distinguishable in frequencies, and the entanglement between the photons is largely destroyed. Several methods have been employed to minimize the detrimental effects of FSS on the generated photons, for example, by spectrally filtering the indistinguishable photon pairs [1] and by suppressing the FSS using external fields [2] or thermal annealing [3]. Thus, the photons of different polarizations, generated within the same generations, are forced to match in their frequencies. An interesting alternate approach, insensitive to FSS, has been proposed recently [4], which suppressing the binding energy of the biexciton [5]. For a zero binding energy of the biexciton, photons of different polarizations match in frequencies in "across generations". Because of the different ordering in the emission for x-polarized and y-polarized photon pairs, the photons are distinguishable temporarily and remain unentangled, but the entanglement can be restored using a time delay between photons of different generations.

The effects of dephasing in the generated entangled state of photons can be minimized significantly by enhancing the emission rates of photons through the Purcell effect in a system comprised of a QD coupled with a microcavity; and various experiments have now demonstrated single QD strong coupling to miniaturized semiconductor cavities [6]. Recently, several cavity-QED schemes for generating entangled photons have also been proposed whereby the excitons are strongly coupled with the cavity modes and form degenerate polariton states [7, 8]. However, one major drawback of these proposed schemes is that because of the large biexciton binding energy, the biexciton remains uncoupled with cavity modes and thus the first generation of photons has a long life time, relative to the life time of the exciton-emitted photons. In this work, we will introduce a new scheme for the fast generation of entangled photons from a single QD coupled to a planar photonic crystal that supports two orthogonally polarized cavity modes [9]. We discuss, and develop a rigorous theory for, "within generation" and "across generation" of entangled photons when both biexciton to exciton, and exciton to ground state transitions, are coupled through cavity modes by manipulating the binding energy of the biexciton such that both biexciton to excitons and excitons to ground state are coupled with two cavity modes of orthogonal polarization; experimentally, manipulation of the binding energies of the biexciton can be realized by applying lateral electric field and by thermal annealing. The two photon concurrence is calculated to be greater than 0.7 and 0.8 using experimentally achievable parameters in across generation and within generation, respectively. We also show that the entanglement can be distilled in both cases using a simple spectral filter.

This work was supported by the National Sciences and Engineering Research Council of Canada and the Canadian Foundation for Innovation.

[1] N. Akopian et. al., Phys. Rev. Lett. 96, 130501 (2006).

[2] R. M. Stevenson et. al., Nature (London) 439, 179 (2006).

[3] R. Seguin et. al., Appl. Phys. Lett. 89, 263109 (2006); D. J. P. Ellis et. al., ibid. 90, 011907 (2007).

[4] J. E. Avron et. al., Phys. Rev. Lett. 100, 120501 (2008); see also P. K. Pathak and S. Hughes, Phys. Rev. Lett., In Press (2009) [arXiv:0905.4420v1].

[5] M. E. Reimer et. al., Phys. Rev. B 78, 195301 (2008).

[6] See, e.g., J. P. Reithmaier et. al., Nature (London) 432, 197 (2004); T. Yoshie et. al., ibid. 432, 200 (2004).

[7] R. Johne et. al., Phys. Rev. Lett. 100, 240404 (2008).

[8] P. K. Pathak and S. Hughes, Phys. Rev. B 79, 205416 (2009).

[9] P. K. Pathak and S. Hughes, arXiv:0906.3035 (1999).

Paper reference: P. K. Pathak and S. Hughes, arXiv:0906.3035

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Speaker: Jordan Kyriakidis (Dalhousie University)

Title: Non-adiabatic quantum control of multiple quantum dots embedded in cavities with global femtosecond optical pulses

Coauthors: Wayan Sudiarta

There are several proposals utilising quantum dots embedded in optical cavities as physical or logical qubits. The advantage of these systems is that distant qubits can be controllably coupled through virtual cavity modes. While there has been recent experimental progress in coherent manipulation in such systems, further progress is hampered by at least two limitations. One, is that transitions are accomplished adiabatically; since the light-qubit coupling is typically rather weak, switching times are not appreciably shorter than the relaxation time of these systems. Second, lasers typically need to address individual dots in the cavity, which is exceedingly difficult. We present results of our work showing how both these limitations can be simultaneously overcome. In the experimentally relevant case of dots of varying size, nonadiabatic transitions can be achieved using chirped pulses applied globally to the cavity. The nonadiabaticity enables switching times far quicker than either the relaxation time or (one over) the Rabi frequency. These global pulses further eliminate the need to address a single dot with a single pulse. We show results showing fast entangling operations on distant qubits even for arbitrarily closely-spaced energy levels. This level of quantum control has not yet been been demonstrated for multiple quantum dots embedded in cavities. Our scheme can be implemented with present-day experimental capabilities.