Qscale Quantum technologies for extending the range of quantum communications


Witnessing single-photon entanglement with local homodyne measurements : analytical bounds and robustness to losses

Melvyn Ho, Olivier Morin, Jean-Daniel Bancal, Nicolas Gisin, Nicolas Sangouard, and Julien Laurat.

New J. Phys. 16 103035

(Submitted : June, 2nd 2014, published : October 24th 2014).

Single-photon entanglement is one of the primary resources for quantum networks, including quantum repeater architectures. Such entanglement can be revealed with only local homodyne measurements through the entanglement witness presented in Morin et al (2013 Phys. Rev. Lett. 110 130401). Here, we provide an extended analysis of this witness by introducing analytical bounds and by reporting measurements confirming its great robustness with regard to losses. This study highlights the potential of optical hybrid methods, where discrete entanglement is characterized through continuous-variable measurements.

Preprint available at : http://arxiv.org/abs/1406.0381

Nonlinear Interaction between Single Photons

T. Guerreiro, A. Martin, B. Sanguinetti, J. S. Pelc, C. Langrock, M. M. Fejer, N. Gisin, H. Zbinden, N. Sangouard, R. T. Thew

Phys. Rev. Lett., 113, 173601

(Submitted : May, 27th 2014, published : October 22nd 2014).

Harnessing nonlinearities strong enough to allow two single photons to interact with one another is not only a fascinating challenge but is central to numerous advanced applications in quantum information science. Currently, all known approaches are extremely challenging although a few have led to experimental realisations with attenuated classical laser light. This has included cross-phase modulation with weak classical light in atomic ensembles and optical fibres, converting incident laser light into a non-classical stream of photon or Rydberg blockades as well as all-optical switches with attenuated classical light in various atomic systems. Here we report the observation of a nonlinear parametric interaction between two true single photons. Single photons are initially generated by heralding one photon from each of two independent spontaneous parametric downconversion sources. The two heralded single photons are subsequently combined in a nonlinear waveguide where they are converted into a single photon with a higher energy. Our approach highlights the potential for quantum nonlinear optics with integrated devices, and as the photons are at telecom wavelengths, it is well adapted to applications in quantum communication.

Preprint available at : http://arxiv.org/abs/1403.2084

Prospective applications of optical quantum memories

Felix Bussieres, Nicolas Sangouard, Mikael Afzelius, Hugues de Riedmatten, Christoph Simon

and Wolfgang Tittel

Journal of Modern Optics, 60, 1519

(Submitted : July, 18th 2013, published : October 13th 2013).

An optical quantum memory can be broadly defined as a system capable of storing a useful quantum state through interaction with light at optical frequencies. During the last decade, intense research was devoted to their development, mostly with the aim of fulfilling the requirements of their first two applications, namely quantum repeaters and linear-optical quantum computation. A better understanding of those requirements then motivated several different experimental approaches. Along the way, other exciting applications emerged, such as as quantum metrology, single-photon detection, tests of the foundations of quantum physics, device-independent quantum information processing and nonlinear processing of quantum information. Here we review several prospective applications of optical quantum memories with a focus on recent experimental achievements pertaining to these applications. This review highlights that optical quantum memories have become essential for the development of optical quantum information processing.

Preprint available at : http://arxiv.org/abs/1306.6904

Heralded noiseless amplification and attenuation of non-gaussian states of light

C. N. Gagatsos, J. Fiurasek, A. Zavatta, M. Bellini and N. J. Cerf.

Phys. Rev. A, 89, 062311

(Submitted : March, 26th 2014, published : June 9th 2014).

We examine the behavior of non-Gaussian states of light under the action of probabilistic noiseless amplification and attenuation. Surprisingly, we find that the mean field amplitude may decrease in the process of noiseless amplification — or increase in the process of noiseless attenuation, a counterintuitive effect that Gaussian states cannot exhibit. This striking phenomenon could be tested with experimentally accessible non-Gaussian states, such as single-photon added coherent states. We propose an experimental scheme, which is robust with respect to the major experimental imperfections such as inefficient single-photon detection and imperfect photon addition. In particular, we argue that the observation of mean field amplification by noiseless attenuation should be feasible with current technology.

Preprint available at : http://arxiv.org/abs/1403.6054

Quantum interference in the absorption and emission of single photons by a single ion

M. Schug, C. Kurz, P. Eich, J. Huwer, P. Müller, and J. Eschner.

Phys. Rev. A, 90 023829

(Submitted : June, 17th 2014, published : August 15th 2014).

We investigate quantum beats in the arrival-time distribution of single photons from a single trapped Ca40+ ion, revealing their fundamentally different physical origins in two distinct experimental situations : In a Λ-type level scheme the interference of two 854-nm absorption amplitudes suppresses and enhances the emission process of Raman-scattered 393-nm photons ; in a V-type level scheme the interference of two 393-nm emission amplitudes causes a rotation of their dipole emission pattern, resulting in a temporal modulation of the detected photons. For both cases we demonstrate coherent control over the quantum-beat phase through the phases of the atomic and photonic input states, which also allows controlled adjustment of the total photon detection efficiency.

Preprint available at : http://arxiv.org/abs/1312.5995.

Single calcium-40 ion as quantum memory for photon polarization : a case study

Philipp Müller and Jürgen Eschner.

Applied Physics B, 114, 303

(Dated : 01 November 2013).

We present several schemes for heralded storage of polarization states of single photons in single ions, using the calcium-40 ion and photons at 854 nm wavelength as specific example. We compare the efficiencies of the schemes and the requirements for their implementation with respect to the preparation of the initial state of the ion, the absorption process and its analysis. These schemes may be used to create and herald entanglement of two distant ions through entanglement swapping ; they are easily adapted to other atomic systems and wavelengths.

Preprint available at : http://arxiv.org/abs/1309.7863.

Efficient quantum state engineering of light

Engineering non-classical states of light is a central quest for quantum optics. Beyond their fundamental significance, such states are the resources for implementing a variety of protocols, ranging from enhanced metrology to quantum communication and computing.

The team led by J. Laurat at Laboratoire Kastler Brossel has recently demonstrated very efficient techniques to generate such states with optical parametric oscillators. These sources are based on non-linear crystals inserted inside optical cavities, and can generate pair of photons or squeezed light. Noise reduction by more than 90% below the shot noise level has been obtained. By adding to these initial resources single-photon detections operated by superconducting detectors, Fock states (single photon or two-photon) and optical Schrödinger cat states have also been generated with unprecedented fidelities. The well-controlled spatiotemporal mode in which they are produced will facilitate their use in subsequent protocols where such states need to interfere with other optical resources, e.g. in quantum gate implementations or cascaded state-engineering procedures.

These experiments are summarized in a video article, published in JoVE - Journal of Visualized Experiments.

Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

A quantum memory for orbital angular momentum photonic qubits

A. Nicolas, L. Veissier, L. Giner, E. Giacobino, D. Maxein, and J. Laurat.

Nature Photonics, 8, 234

(Submitted 22 July 2013, Published online 26 January 2014).

Among the optical degrees of freedom, the orbital angular momentum of light provides unique properties, including mechanical torque action with applications for light manipulation, enhanced sensitivity in imaging techniques and potential high-density information coding for optical communication systems. Recent years have also seen a tremendous interest in exploiting orbital angular momentum at the single-photon level in quantum information technologies. In this endeavor, here we demonstrate the implementation of a quantum memory for quantum bits encoded in this optical degree of freedom. We generate various qubits with computer-controlled holograms, store and retrieve them on demand. We further analyse the retrieved states by quantum tomography and thereby demonstrate fidelities exceeding the classical benchmark, confirming the quantum functioning of our storage process. Our results provide an essential capability for future networks exploring the promises of orbital angular momentum of photons for quantum information applications.

Preprint available at : http://arxiv.org/abs/1308.0238.

Quantum Storage of Heralded Single Photons in a Praseodymium-Doped Crystal

Daniel Rieländer, Kutlu Kutluer, Patrick M. Ledingham, Mustafa Gündoğan, Julia Fekete, Margherita Mazzera, and Hugues de Riedmatten.

Phys. Rev. Lett., 112 040504

(Submitted 17 October 2013, Published 31 January 2014).

We report on experiments demonstrating the reversible mapping of heralded single photons to long-lived collective optical atomic excitations stored in a Pr3+:Y2SiO5 crystal. A cavity-enhanced spontaneous down-conversion source is employed to produce widely nondegenerate narrow-band (≈2 MHz) photon pairs. The idler photons, whose frequency is compatible with telecommunication optical fibers, are used to herald the creation of the signal photons, compatible with the Pr3+ transition. The signal photons are stored and retrieved using the atomic frequency comb protocol. We demonstrate storage times up to 4.5 μs while preserving nonclassical correlations between the heralding and the retrieved photon. This is more than 20 times longer than in previous realizations in solid state devices, and implemented in a system ideally suited for the extension to spin-wave storage.

Preprint available at : http://arxiv.org/abs/1310.8261.


Experimentally accessing the optimal temporal mode of traveling quantum light states

Olivier Morin, Claude Fabre and Julien Laurat.

Phys. Rev. Lett., 111 213602

(Submitted 14 June 2013 ; published 19 November 2013).

The characterization or subsequent use of propagating optical quantum state require the knowledge of its precise temporal mode. Defining this mode structure very often relies on a detailed a priori knowledge of the used resources, when available, and can additionally call for an involved theoretical modeling. In contrast, here we investigate a practical method enabling to infer the optimal temporal mode directly from experimental data acquired via homodyne detection, without any assumptions on the state. The approach is based on a multimode analysis using eigenfunction expansion of the autocorrelation function. This capability is illustrated by experimental data from Fock states and Schrodinger cat-like state preparation.

Preprint available at : http://arxiv.org/abs/1307.6364.

Remote creation of hybrid entanglement between particle-like and wave-like optical qubits

Olivier Morin, Kun Huang, Jianli Liu, Hanna Le Jeannic, Claude Fabre, Julien Laurat.

http://arxiv.org/abs/1309.6191 (Submitted 24 September 2013).

The wave-particle duality of light has led to two different encodings for optical quantum information processing. Several approaches have emerged based either on particle-like discrete-variable states, e.g. finite-dimensional quantum systems, or on wave-like continuous-variable states, e.g. infinite-dimensional systems. Here, we demonstrate the first measurement-induced generation of entanglement between optical qubits of these different types, located at distant places and connected by a lossy channel. Such hybrid entanglement, which is a key resource for a variety of recently proposed schemes, including quantum cryptography and computing, enables to convert information from one Hilbert space to the other via teleportation and therefore connect remote quantum processors based upon different encodings. Beyond its fundamental significance for the exploration of entanglement and its possible instantiations, our optical circuit opens the promises for heterogeneous network implementations, where discrete and continuous-variable operations and techniques can be efficiently combined.

Entangling quantum and classical states of light

Hyunseok Jeong, Alessandro Zavatta, Minsu Kang, Seung-Woo Lee, Luca S. Costanzo, Samuele Grandi, Timothy C. Ralph, Marco Bellini.

http://arxiv.org/abs/1309.6192 (Submitted 24 September 2013).

Entanglement between quantum and classical objects is of special interest in the context of fundamental studies of quantum mechanics and potential applications to quantum information processing. In quantum optics, single photons are treated as light quanta while coherent states are considered the most classical among all pure states. Recently, entanglement between a single photon and a coherent state in a free-traveling field was identified to be a useful resource for optical quantum information processing. However, it was pointed out to be extremely difficult to generate such states since it requires a clean cross-Kerr nonlinear interaction. Here, we devise and experimentally demonstrate a scheme to generate such hybrid entanglement by implementing a coherent superposition of two distinct quantum operations. The generated states clearly show entanglement between the two different types of states. Our work opens a way to generate hybrid entanglement of a larger size and to develop efficient quantum information processing using such a new type of qubits.


Heralded photonic interaction between distant single ions

M. Schug, J. Huwer, C. Kurz, P. Müller, and J. Eschner .

Phys. Rev. Lett., 110, 213603

(Submitted 27 February 2013, Published 21 May 2013).

We establish a heralded interaction between two remotely trapped single Ca+40 ions through the exchange of single photons. In the sender ion, we release single photons with a controlled temporal shape on the P3/2 to D5/2 transition and transmit them to the distant receiver ion. Individual absorption events in the receiver ion are detected by quantum jumps. For continuously generated photons, the absorption reduces significantly the lifetime of the long-lived D5/2 state. For triggered single-photon transmission, we observe a coincidence between the emission at the sender and quantum jump events at the receiver.

Preprint available at : http://arxiv.org/abs/1302.1801.

A high rate source for single photons in a pure quantum state

C. Kurz, J. Huwer, M. Schug, P Müller and J Eschner.

New J. Phys. 15 055005

(Submitted 29 November 2012, Published 8 May 2013).

We report on the efficient generation of single photons, making use of spontaneous Raman scattering in a single trapped ion. The photons are collected through in-vacuum high-numerical-aperture objectives. Photon frequency, polarization and temporal shape are controlled through appropriate laser parameters, allowing for photons in a pure quantum state. These photons are suitable heralds for single-photon absorption in a single-ion quantum memory.

Preprint available at : http://arxiv.org/abs/1211.5922v1.


Single-photon-added coherent states : estimation of parameters and fidelity of the optical homodyne detection

S. N. Filippov, V. I. Man’ko, A. S. Coelho, A. Zavatta, and M. Bellini.

Phys. Scr. 2013, 014025

(Submitted 1 November 2012, published 28 March 2013).

The travelling modes of single-photon-added coherent states (SPACS) are characterized by using optical homodyne tomography. Given a set of experimentally measured quadrature distributions, we estimate parameters of the state and also extract information about the detector efficiency. The method used is minimal distance estimation between theoretical and experimental quantities, which additionally allows to evaluate the precision of the estimated parameters. Given the experimental data, we also estimate the lower and upper bounds on fidelity. The results are believed to encourage a more precise engineering and detection of SPACS.

Preprint available at : http://arxiv.org/abs/1301.2084.


Ultranarrow-Band Photon-Pair Source Compatible with Solid State Quantum Memories and Telecommunication Networks

Julia Fekete, Daniel Rieländer, Matteo Cristiani, and Hugues de Riedmatten.

Phys. Rev. Lett., 110 220502

(Submitted 26 February 2013, Published 28 May 2013).

We report on a source of ultranarrow-band photon pairs generated by widely nondegenerate cavity- enhanced spontaneous down-conversion. The source is designed to be compatible with Pr3+ solid state quantum memories and telecommunication optical fibers, with signal and idler photons close to 606 nm and 1436 nm, respectively. Both photons have a spectral bandwidth around 2 MHz, matching the bandwidth of Pr3+ doped quantum memories. This source is ideally suited for long distance quantum communication architectures involving solid state quantum memories.

ArXiv preprint available at : http://arxiv.org/abs/1304.6861


Quantum Process Nonclassicality

Rahimi-Keshari, T. Kiesel, W. Vogel, S. Grandi, A. Zavatta and M. Bellini.

Phys. Rev. Lett. 110, 160401

(Submitted 4 February 2013, Published 19 April 2013).

We propose a definition of nonclassicality for a single-mode quantum-optical process based on its action on coherent states. If a quantum process transforms a coherent state to a nonclassical state, it is verified to be nonclassical. To identify onclassical processes, we introduce a representation for quantum processes, called the process-nonclassicality quasiprobability distribution, whose negativities indicate nonclassicality of the process. Using this distribution, we derive a relation for predicting nonclassicality of the output states for a given input state. We experimentally demonstrate our method by considering the single-photon addition as a nonclassical process, and predicting nonclassicality of the output state for an input thermal state.

Preprint available at : http://arxiv.org/abs/1302.0934


Coherent Storage of Temporally Multimode Light Using an Atomic Frequency Comb Memory

M. Gündogan, M. Mazzera, P. M. Ledingham, M. Cristiani and H. de Riedmatten.

N. J. Phys. 15, 045012

(Submitted 11 January 2013, Published 19 April 2013).

We report on the coherent and multi-temporal mode storage of light using the full atomic frequency comb memory scheme. The scheme involves the transfer of optical atomic excitations in Pr3+ : Y2SiO5 to spin waves in hyperfine levels using strong single-frequency transfer pulses. Using this scheme, a total of five temporal modes are stored and recalled on-demand from the memory. The coherence of the storage and retrieval is characterized using a time-bin interference measurement resulting in visibilities higher than 80%, independent of the storage time. This coherent and multimode spin-wave memory is promising as a quantum memory for light.

Preprint available at :



Witnessing single-photon entanglement with local homodyne measurements

O. Morin, J-D. Bancal, M. Ho, P. Sekatski, V. D’Auria, N. Gisin, J. Laurat and N. Sangouard

Phys. Rev. Lett. 110, 130401

(Received 2 July 2012 ; published 25 March 2013)

Single-photon entangled states, i.e., states describing two optical paths sharing a single photon, constitute the simplest form of entanglement. Yet they provide a valuable resource in quantum information science. Specifically, they lie at the heart of quantum networks, as they can be used for quantum teleportation, swapped, and purified with linear optics. The main drawback of such entanglement is the difficulty in measuring it. Here, we present and experimentally test an entanglement witness allowing one to say whether a given state is path entangled and also that entanglement lies in the subspace, where the optical paths are each filled with one photon at most, i.e., refers to single-photon entanglement. It uses local homodyning only and relies on no assumption about the Hilbert space dimension of the measured system. Our work provides a simple and trustworthy method for verifying the proper functioning of future quantum networks.

Pripring available at : http://arxiv.org/abs/1206.5734.


Reversible optical memory for twisted photons

L. Veissier, A. Nicolas, L. Giner, D. Maxein, A.S. Sheremet, E. Giacobino and J. Laurat.

Optics Letters, Vol. 38, 712-714 (2013)

(Submitted December 10, 2012 ; revised manuscript received January 23, 2013 ; published February 25, 2013)

We report on an experiment in which orbital angular momentum (OAM) of light is mapped at the single-photon level into and out of a cold atomic ensemble. Based on the dynamic electromagnetically induced transparency protocol, the demonstrated optical memory enables the reversible mapping of Laguerre–Gaussian modes with preserved handedness of the helical phase structure. The demonstrated capability opens the possibility to the storage of qubits encoded as superpositions of OAM states and to multidimensional light matter interfacing.

Preprint available at : A reversible optical memory for twisted photons


Experimental investigation of the transition between Autler-Townes splitting and electromagnetically-induced-transparency models

L. Giner, L. Veissier, B. Sparkes, A.S. Sheremet, A. Nicolas, O.S. Mishina, M. Scherman, S. Burks, I. Shomroni, D.V. Kupriyanov, P.K. Lam, E. Giacobino and J. Laurat.

PHYSICAL REVIEW A 87, 013823 (2013) (Submitted 25 June 2012 ; published 22 January 2013)

Two phenomena can affect the transmission of a probe field through an absorbing medium in the presence of an additional field : electromagnetically induced transparency (EIT) andAutler-Townes splitting (ATS). Being able to discriminate between the two is important for various practical applications. Here we present an experimental investigation into a method that allows for such a disambiguation as proposed by Anisimov,Dowling, and Sanders in Phys. Rev. Lett. 107, 163604 (2011). We apply the proposed test based on Akaike’s information criterion to a coherently driven ensemble of cold cesium atoms and find a good agreementwith theoretical predictions, therefore demonstrating the suitability of the method. Beyond the applicability of the test, our results demonstrate that the transition features are highly sensitive to the properties of the medium under study, potentially providing a practical characterizing tool for complex systems.

Preprint available at : Experimental investigation of the transition between Autler-Townes splitting and electromagnetically-induced-transparency models.


Adaptive Detection of Arbitrarily Shaped Ultrashort Quantum Light States

C. Polycarpou, K. N. Cassemiro, G. Venturi, A. Zavatta, and M. Bellini

Phys. Rev. Lett. 109, 053602 (2012) (Received 4 May 2012 ; revised manuscript received 28 June 2012 ; published 3 August 2012)

A quantum state of light is the excitation of a particular spatiotemporal mode of the electromagnetic field. A precise control of the mode structure is therefore essential for processing, detecting, and using photonic states in novel quantum technologies. Here we demonstrate an adaptive scheme, combining techniques from the fields of ultrafast coherent control and quantum optics, for probing the arbitrary complex spectrotemporal profile of an ultrashort quantum light pulse. The ability to access the modal structure of a quantum light state could boost the capacity of current quantum information protocols.

Preprint available at : http://arxiv.org/abs/1111.7161


A high-fidelity single-photon source based on a type-II optical parametric oscillator

O. Morin, V. D’Auria, C. Fabre, J. Laurat

Optics Letters, Vol. 37, Issue 17, pp. 3738-3740 (2012) (Received June 25, 2012, accepted July 30, 2012, published August 31, 2012)

Using a continuous-wave type-II optical parametric oscillator below threshold, we have demonstrated a novel source of heralded single-photons with high-fidelity. The generated state is characterized by homodyne detection and exhibits a 79% fidelity with a single-photon Fock state (91% after correction of detection loss). The low admixture of vacuum and the well-define spatiotemporal mode are critical requirements for their subsequent use in quantum information processing.

Preprint available at : http://arxiv.org/abs/1207.6462v1


Effect of the heralding detector properties on the conditional generation of single-photon states

V. D’Auria, O. Morin, C. Fabre, and J. Laurat

Eur. Phys. J D, Vol 66, Num. 10, 249 (Submitted 04 June 2012, Published 04 October 2012)

Single-photons play an important role in emerging quantum technologies and information processing. An efficient generation technique consists in preparing such states via a conditional measurement on photon-number correlated beams : the detection of a single-photon on one of the beam can herald the generation of a single-photon state on the other one. Such scheme strongly depends on the heralding detector properties, such as its quantum efficiency, noise or photon-number resolution ability. These parameters affect the preparation rate and the fidelity of the generated state. After reviewing the theoretical description of optical detectors and conditional measurements, and how both are here connected, we evaluate the effects of these properties and compare two kind of devices, a conventional on/off detector and a two-channel detector with photon-number resolution ability.

Preprint available at : Effect of the heralding detector properties on the conditional generation of single-photon states


Towards higher precision and operational use of optical homodyne tomograms

M. Bellini, A.S. Coelho, S.N. Filippov, V.I. Man’ko, and A. Zavatta

Phys. Rev. A 85, 052129 (2012), (Received 13 March 2012 ; published 31 May 2012)

We present the results of an operational use of experimentally measured optical tomograms to determine state characteristics (purity) avoiding any reconstruction of quasiprobabilities. We also develop a natural way how to estimate the errors (including both statistical and systematic ones) by an analysis of the experimental data themselves. Precision of the experiment can be increased by postselecting the data with minimal (systematic) errors. We demonstrate those techniques by considering coherent and photon-added coherent states measured via the time-domain improved homodyne detection. The operational use and precision of the data allowed us to check purity-dependent uncertainty relations and uncertainty relations for Shannon and Rényi entropies.

Preprint available at : Towards higher precision and operational use of optical homodyne tomograms


What are single photons good for ?

N. Sangouard and Hugo Zbinden

(Dated : February 2nd, 2012)

In a long-held preconception, photons play a central role in present-day quantum technologies. But what are sources producing photons one by one good for precisely ? Well, in opposition to what many suggest, we show that single-photon sources are not helpful for point to point quantum key distribution because faint laser pulses do the job comfortably. However, there is no doubt about the usefulness of sources producing single photons for future quantum technologies. In particular, we show how single-photon sources could become the seed of a revolution in the framework of quantum communication, making the security of quantum key distribution device independent or extending quantum communication over many hundreds of kilometers. Hopefully, these promising applications will provide a guideline for researchers to develop more and more efficient sources, producing narrowband, pure and indistinguishable photons at appropriate wavelengths.


Quantum Storage of a Photonic Polarization Qubit in a Solid

Mustafa Gündoğan, Patrick M. Ledingham, Attaallah Almasi, Matteo Cristiani, Hugues de Riedmatten

PRL 108, 190504 (2012) (Received 18 January 2012 ; published 10 May 2012)

We report on the quantum storage and retrieval of photonic polarization quantum bits onto and out of a solid state storage device. The qubits are implemented with weak coherent states at the single photon level, and are stored for a predetermined time of 500 ns in a praseodymium doped crystal with a storage and retrieval efficiency of 10%, using the atomic frequency comb scheme. We characterize the storage by using quantum state tomography, and find that the average conditional fidelity of the retrieved qubits exceeds 95% for a mean photon number - µ=0,4. This is significantly higher than a classical benchmark, taking into account the Poissonian statistics and finite memory efficiency, which proves that our crystal functions as a quantum storage device for polarization qubits. These results extend the storage capabilities of solid state quantum light matter interfaces to polarization encoding, which is widely used in quantum information science.

Preprint available at : Quantum Storage of a Photonic Polarization Qubit in a Solid


Quantum repeaters based on heralded qubit amplifiers

Jirí Minár, Hugues de Riedmatten, and Nicolas Sangouard

(Dated : November 23rd, 2011)

We present a quantum repeater scheme based on the recently proposed qubit amplifier [N. Gisin, S. Pironio and N. Sangouard, Phys. Rev. Lett. 105, 070501 (2010)]. It relies on an on-demand entangled-photon pair source which uses on-demand single-photon sources, linear optical elements and atomic ensembles. Interestingly, the imperfections affecting the states created from this source, caused e.g. by detectors with non-unit efficiencies, are systematically purified from an entanglement swapping operation based on a two-photon detection. This allows the distribution of entanglement over very long distances with a high fidelity, i.e. without vacuum components and multiphoton errors. Therefore, the resulting quantum repeater architecture does not necessitate final postselections and thus achieves high entanglement distribution rates. This also provides unique opportunities for device independent quantum key distribution over long distances with linear optics and atomic ensembles.


Detector imperfections in photon-pair source characterisation

P. Sekatski, N. Sangouard, F. Bussieres, C. Clausen, N. Gisin and H. Zbinden

(Dated : September 20th, 2011)

We analyze how imperfections in single-photon detectors impact the characterization of photon-pair sources. We perform exact calculations to reveal the effects of multi-pair emissions and of noisy, non-unit efficiency, non photon-number resolving detections on the Cauchy-Schwarz parameter, on the second order auto-correlation and cross-correlation functions, and on the visibilities of both Hong-Ou-Mandel and Bell-like interferences. We consider sources producing either two-mode squeezed states or states with a Poissonian photon distribution. The proposed formulas are useful in practice to determine the impacts of multi-pair emissions and dark counts in standard tests used in quantum optics.