Photon-pair source with controllable delay based on shaped inhomogeneous broadening of rare-earth-metal-doped solids

Pavel Sekatski, Nicolas Sangouard, Nicolas Gisin, Hugues de Riedmatten, and Mikael Afzelius

PHYSICAL REVIEW A 83, 053840 (2011)

Spontaneous Raman emission in atomic gases provides an attractive source of photon pairs with a controllable delay. We show how this technique can be implemented in solid state systems by appropriately shaping the inhomogeneous broadening. Our proposal is eminently feasible with current technology and provides a realistic solution to entangle remote rare-earth-metal-doped solids in a heralded way.

Atomic frequency comb memory with spin wave storage in ¹⁵³Eu³⁺:Y₂SiO₅

N. Timoney, B. Lauritzen, I. Usmani, M. Afzelius, and N. Gisin

Submitted to Journal of Physics B - Special Issue on Quantum Memory

Spectroscopic investigations of Eu³⁺:Y₂SiO₅ for quantum memory applications

B. Lauritzen, N. Timoney, N. Gisin, M. Afzelius, H. de Riedmatten, Y. Sun, R. M. Macfarlane, and R. L. Cone

Submitted to Phys Rev B

Heralded quantum entanglement between two crystals

Imam Usmani, Christoph Clausen, Félix Bussières, Nicolas Sangouard, Mikael Afzelius, and Nicolas Gisin

arXive:1109.0440

Quantum networks require the crucial ability to entangle quantum nodes. A prominent example is the quantum repeater which allows overcoming the distance barrier of direct transmission of single photons, provided remote quantum memories can be entangled in a heralded fashion. Here we report the observation of heralded entanglement between two ensembles of rare-earth-ions doped into separate crystals. A heralded single photon is sent through a 50/50 beamsplitter, creating a single-photon entangled state delocalized between two spatial modes. The quantum state of each mode is subsequently mapped onto a crystal, leading to an entangled state consisting of a single collective excitation delocalized between two crystals. This entanglement is revealed by mapping it back to optical modes and by estimating the concurrence of the retrieved light state. Our results highlight the potential of rare-earth-ions doped crystals for entangled quantum nodes and bring quantum networks based on solid-state resources one step closer.

Quantum storage of photonic entanglement in a crystal

Christoph Clausen, Imam Usmani, Félix Bussières, Nicolas Sangouard, Mikael Afzelius, Hugues de Riedmatten, Nicolas Gisin

Nature 469, 508 (2011)

Entanglement is the fundamental characteristic of quantum physics. Large experimental efforts are devoted to harness entanglement between various physical systems. In particular, entanglement between light and material systems is interesting due to their prospective roles as “flying” and stationary qubits in future quantum information technologies, such as quantum repeaters and quantum networks. Here we report the first demonstration of entanglement between a photon at telecommunication wavelength and a single collective atomic excitation stored in a crystal. One photon from an energy-time entangled pair is mapped onto a crystal and then released into a well-defined spatial mode after a predetermined storage time. The other photon is at telecommunication wavelength and is sent directly through a 50 m fiber link to an analyzer. Successful transfer of entanglement to the crystal and back is proven by a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality by almost three standard deviations (S=2.64+/-0.23). These results represent an important step towards quantum communication technologies based on solid-state devices. In particular, our resources pave the way for building efficient multiplexed quantum repeaters for long-distance quantum networks.

Long-term performance of the SwissQuantum quantum key distribution network in a field environment

D Stucki, M Legré, F Buntschu, B Clausen, N Felber, N Gisin, L Henzen, P Junod, G Litzistorf, P Monbaron, L Monat, J-B Page, D Perroud, G Ribordy, A Rochas, S Robyr, J Tavares, R Thew, P Trinkler, S Ventura, R Voirol, N Walenta, H Zbinden

New Journal of Physics 13, 123001 (2011)

In this paper, we report on the performance of the SwissQuantum quantum key distribution (QKD) network. The network was installed in the Geneva metropolitan area and ran for more than one-and-a-half years, from the end of March 2009 to the beginning of January 2011. The main goal of this experiment was to test the reliability of the quantum layer over a long period of time in a production environment. A key management layer has been developed to manage the key between the three nodes of the network. This QKD-secure network was utilized by end-users through an application layer.

Efficient solid state memories for quantum cryptography

F. Beaudoux, R. Marino, J. Lejay, A. Ferrier, B.Tumino, O. Guillot-Noel, P. Goldner

Journal of Luminescence 131 469-472 (2011)

Long distance quantum cryptography requires quantum repeaters which use quantum memories. The latter are designed to store and retrieve photon quantum states on demand. Although quantum memories have been demonstrated in atomic vapors and ultra cold gases, a solid state alternative may better fulfill quantum memories requirements. Rare earth based crystals, which exhibit long coherence lifetimes, are actively studied for this purpose. Memory efficiency, i.e. the probability to retrieve a photon after storage, should be close to unity for practical applications. This can be achieved in highly doped crystals. Although Pr–Pr interactions could be detrimental in this case, we show that in a 3% Pr3+ doped La2(WO4)3 crystal ground state hyperfine coherence lifetime is still close to that measured at low Pr concentration. Since the latter determines the memory storage time, this result suggests that highly doped crystals may be useful for efficient quantum memories.

Hyperfine characterization and coherence lifetime extension in Pr³⁺:La₂(WO₄)₃

Marko Lovric, Philipp Glasenapp, Dieter Suter, Biagio Tumino, Alban Ferrier, Philippe Goldner, Mahmood Sabooni, Lars Rippe, and Stefan Kröll

Phys. Rev. B 84, 104417 (2011)

Rare-earth ions in dielectric crystals are interesting candidates for storing quantum states of photons. A limiting factor on the optical density and thus the conversion efficiency is the distortion introduced in the crystal by doping elements of one type into a crystal matrix of another type. Here we investigate the system Pr³⁺:La₂(WO₄)₃, where the similarity of the ionic radii of Pr and La minimizes distortions due to doping. We characterize the praseodymium hyperfine interaction of the ground-state (³H₄) and one excited state (¹D₂) and determine the spin Hamiltonian parameters by numerical analysis of Raman-heterodyne spectra, which were collected for a range of static external magnetic-field strengths and orientations. On the basis of a crystal-field analysis, we discuss the physical origin of the experimentally determined quadrupole and Zeeman tensor characteristics. We show the potential for quantum memory applications by measuring the spin coherence lifetime in a magnetic field that is chosen such that additional magnetic fields do not shift the transition frequency in first order. Experimental results demonstrate a spin coherence lifetime of 158 ms — almost 3 orders of magnitude longer than in zero field.

Engineering integrated pure narrow-band photon sources

E. Pomarico, B. Sanguinetti, C. I. Osorio, H. Herrmann, R. T. Thew

arXive:1108.5542

Engineering and controlling well defined states of light for quantum information applications is of increasing importance as the complexity of quantum systems grows. For example, in quantum networks high multi-photon interference visibility requires properly devised single mode sources. In this paper we propose a spontaneous parametric down conversion source based on an integrated cavity- waveguide, where single narrow-band, possibly distinct, spectral modes for the idler and the signal fields can be generated. This mode selection takes advantage of the clustering effect, due to the intrinsic dispersion of the nonlinear material. In combination with a CW laser and fast detection, our approach provides a means to engineer a source that can efficiently generate pure photons, without filtering, that is compatible with long distance quantum communication. Furthermore, it is extremely flexible and could easily be adapted to a wide variety of wavelengths and applications.

Revival of Silenced Echo and Quantum Memory for Light

V. Damon, M. Bonarota, A. Louchet-Chauvet, T. Chanelière, J.-L. Le Gouët

arXive:1104.4875

We propose an original quantum memory protocol. It belongs to the class of rephasing processes and is closely related to two-pulse photon echo. It is known that the strong population inversion produced by the rephasing pulse prevents the plain two-pulse photon echo from serving as a quantum memory scheme. Indeed gain and spontaneous emission generate prohibitive noise. A second -pulse can be used to simultaneously reverse the atomic phase and bring the atoms back into the ground state. Then a secondary echo is radiated from a non-inverted medium, avoiding contamination by gain and spontaneous emission noise. However, one must kill the primary echo, in order to preserve all the information for the secondary signal. In the present work, spatial phase mismatching is used to silence the standard two-pulse echo. An experimental demonstration is presented.

Absorption of a pulse by an optically dense medium: An argument for field quantization

P. R. Berman, J.-L. Le Gouët

Am. J. Phys. 79 5, May 2011

The theory of the absorption of a weak optical pulse by an optically dense medium is shown to lead to unphysical results unless the radiation field is quantized. In contrast to the photoelectric effect, the atom-field dynamics for pulsed field absorption can be obtained using elementary quantum mechanics without imposing any assumptions on the nature of the detection process. As such, pulsed field absorption offers distinct advantages over the photoelectric effect as a proving ground for field quantization. If the classical field pulse is replaced by a quantized, multimode field state, many classical field results are recovered without the inconsistencies that arise in the classical field calculation.

Quantum storage of photonic entanglement in a crystal

Christoph Clausen, Imam Usmani, Félix Bussières, Nicolas Sangouard, Mikael Afzelius, Hugues de Riedmatten, Nicolas Gisin

http://dx.doi.org/10.1038/nature09662

Entanglement is the fundamental characteristic of quantum physics. Large experimental efforts are devoted to harness entanglement between various physical systems. In particular, entanglement between light and material systems is interesting due to their prospective roles as "flying" and stationary qubits in future quantum information technologies, such as quantum repeaters and quantum networks. Here we report the first demonstration of entanglement between a photon at telecommunication wavelength and a single collective atomic excitation stored in a crystal. One photon from an energy-time entangled pair is mapped onto a crystal and then released into a well-defined spatial mode after a predetermined storage time. The other photon is at telecommunication wavelength and is sent directly through a 50 m fiber link to an analyzer. Successful transfer of entanglement to the crystal and back is proven by a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality by almost three standard deviations (S=2.64+/-0.23). These results represent an important step towards quantum communication technologies based on solid-state devices. In particular, our resources pave the way for building efficient multiplexed quantum repeaters for long-distance quantum networks.

Spin-wave storage using chirped control fields in atomic frequency comb-based quantum memory

Jiri Minar, Nicolas Sangouard, Mikael Afzelius, Hugues de Riedmatten, and Nicolas Gisin

Phys. Rev. A, 82, 042309 (2010)

It has been shown that an inhomogeneously broadened optical transition shaped into an atomic frequency comb can store a large number of temporal modes of the electromagnetic field at the single-photon level without the need to increase the optical depth of the storage material. The readout of light modes is made efficient thanks to the rephasing of the optical-wavelength coherence similar to photon-echo-type techniques, and the reemission time is given by the comb structure. For on-demand readout and long storage times, two control fields are used to transfer the optical coherence back and forth into a spin wave. Here, we present a detailed analysis of the spin-wave storage based on chirped adiabatic control fields. In particular, we verify that chirped fields require significantly weaker intensities than π pulses. The price to pay is a reduction of the multimode storage capacity that we quantify for realistic material parameters associated with solids doped with rare-earth-metal ions.

Impossibility of faithfully storing single photons with the three-pulse photon echo

Nicolas Sangouard, Christoph Simon, Jiri Minar, Mikael Afzelius, Thierry Chanelière, Nicolas Gisin, Jean-Louis Le Gouët, Hugues de Riedmatten, and Wolfgang Tittel

Phys. Rev. A, 81, 062333 (2010)

The three-pulse photon echo is a well-known technique to store intense light pulses in an inhomogeneously broadened atomic ensemble. This protocol is attractive because it is relatively simple and it is well suited for the storage of multiple temporal modes. Furthermore, it offers very long storage times, greater than the phase relaxation time. Here, we consider the three-pulse photon echo in both two- and three-level systems as a potential technique for the storage of light at the single-photon level. By explicit calculations, we show that the ratio between the echo signal corresponding to a single-photon input and the noise is smaller than one. This severely limits the achievable fidelity of the quantum state storage, making the three-pulse photon echo unsuitable for single-photon quantum memory.

Impedance-matched cavity quantum memory

Mikael Afzelius and Christoph Simon

Phys.  Rev. A, A 82, 022310 (2010)

We consider an atomic frequency comb based quantum memory inside an asymmetric optical cavity. In this configuration it is possible to absorb the input light completely in a system with an effective optical depth of one, provided that the absorption per cavity round trip exactly matches the transmission of the coupling mirror (“impedance matching”). We show that the impedance matching results in a readout efficiency only limited by irreversible atomic dephasing, whose effect can be made very small in systems with large inhomogeneous broadening. Our proposal opens up an attractive route toward quantum memories with close to unit efficiency.

Efficiency optimization for Atomic Frequency Comb storage

M. Bonarota, J. Ruggiero, J.-L. Le Gouët, T. Chanelière

Phys Rev A 81 (2010) 033803

We study the efficiency of the Atomic Frequency Comb storage protocol. We show that for a given optical depth, the preparation procedure can be optimize to significantly improve the retrieval. Our prediction is well supported by the experimental implementation of the protocol in a Tm3+:YAG crystal. We observe a net gain in efficiency from 10% to 17% by applying the optimized preparation procedure. In the perspective of high bandwidth storage, we investigate the protocol under different magnetic fields. We analyze the effect of the Zeeman and superhyperfine interaction.

Highly multimode storage in a crystal

M Bonarota, J-L Le Gouët and T Chanelière

New J. of Physics 13 013013 (2010)

We experimentally demonstrate the storage of 1060 temporal modes onto a thulium-doped crystal using an atomic frequency comb (AFC). The comb covers 0.93 GHz defining the storage bandwidth. As compared to previous AFC preparation methods (pulse sequences, i.e. amplitude modulation), we only use frequency modulation to produce the desired optical pumping spectrum. To ensure an accurate spectrally selective optical pumping, the frequency- modulated laser is self-locked on the atomic comb. Our approach is general and should be applicable to a wide range of rare-earth-doped materials in the context of multimode quantum memory.

Adiabatic refocusing of nuclear spins in Tm³⁺:YAG

R. Lauro, T. Chanelière and J.- L. Le Gouët

Phys Rev B 83 (2011) 035124

We show that the optical absorbance detection of nuclear spin echo gives direct access to the spin concentration, unlike most coherent signal detection techniques where the signal intensity/amplitude is difficult to connect experimentally with the spin concentration. This way we measure the spin refocusing efficiency in a crystal of Tm3+:YAG. Given the large inhomogeneous broadening of the spin transition in this material, rephasing the spins with the usual hard pulse procedure would require excessively high radiofrequency power. Instead we resort to an adiabatic pulse sequence that perfectly returns the spins to their initial common orientation, at low power cost.

Efficient optical pumping of Zeeman spin levels in Nd³⁺: YVO₄

Mikael Afzelius, Matthias U.Staudt, Hugues de Riedmattena, Nicolas Gisin, Olivier Guillot-Noël, Philippe Goldner, Robert Marino, Pierre Porcher, Enrico Cavalli, Marco Bettinelli

Journal of Luminescence 130 1566 (2010)

We demonstrate that Zeeman ground-state spin levels in Nd3þ: YVO4 provides the possibility to create an efficient L- system for optical pumping experiments. The branching ratio R in the L-system is measured experimentally via absorption spectroscopy and is compared to a theoretical model. We show that R can be tuned by changing the orientation of the magnetic field. These results are applied to optical pumping experiments, where significant improvement is obtained compared to previous experiments in this system. The tunability of the branching ratio in combination with its good coherence properties and the high oscillatorstrength makes Nd3þ: YVO4 an interesting candidate for various quantum information protocols.

Emission of photon echoes in a strongly scattering medium

F. Beaudoux, B. Tumino, A. Ferrier, R. Marino, J. Lejay, O. Guillot-Noël,  T. Chanelière, J.-L. Le Gouët and Ph. Goldner

Opt. Express 19 15236-15243 2011

We observe the two- and three-pulse photon echo emission from a scattering powder, obtained by grinding a Pr3+:Y2SiO5 rare earth doped single crystal. We show that the collective emission is coherently constructed over several grains. A well defined atomic coherence can therefore be created between randomly placed particles. Observation of photon echo on powders as opposed to bulk materials opens the way to faster material development. More generally, time-domain resonant four-wave mixing offers an attractive approach to investigate coherent propagation in scattering media.

Quantum communication technology

Nicolas Gisin & Rob Thew

Electronic Letters,  46 995 (2010)

Quantum communication is built on a set of disruptive concepts and technologies. It is driven by fascinating physics and by promising applications. It requires a new mix of competencies, from telecom engineering to theoretical physics, from theoretical computer science to mechanical and electronic engineering. First applications have already found their way into niche markets, and university labs are working on futuristic quantum networks, but most of the surprises are still ahead of us. Quantum communication, and more generally quantum information science and technologies, are here to stay and will have a profound impact on the 21st century.

Storage and recall of weak coherent optical pulses with an efficiency of 25%

M. Sabooni, F. Beaudoin, A. Walther, Lin Nan, A. Amari, M. Huang, S. Kröll

arXiv:0912.2525v1 [quant-ph]

We demonstrate experimentally a quantum memory scheme for the storage of weak coherent light pulses in an inhomogeneously broadened optical transition in a Pr3+: YSO crystal at 2.1 K. Precise optical pumping using a frequency stable (≈1kHz linewidth) laser is employed to create a highly controllable Atomic Frequency Comb (AFC) structure. We report single photon storage and retrieval efficiencies of 25%, based on coherent photon echo type re-emission in the forward direction. The coherence property of the quantum memory is proved through interference between a super Gaussian pulse and the emitted echo. Backward retrieval of thephoton echo emission has potential for increasing storage and recall efficiency.