arXiv subjects

Nicolas Sangouard

Publications and source records attributed to Nicolas Sangouard.

At least 55 records · Page 3Linked to original sources

arXiv2017

Bell correlations in a many-body system with finite statistics

A recent experiment reported the first violation of a Bell correlation witness in a many-body system [Science 352, 441 (2016)]. Following discussions in this paper, we address here the question of the statistics required to witness Bell correlated states, i.e. states violating a Bell inequality, in such experiments. We start by deriving multipartite Bell inequalities involving an arbitrary number of measurement settings, two outcomes per party and one- and two-body correlators only. Based on these inequalities, we then build up improved witnesses able to detect Bell-correlated states in many-body systems using two collective measurements only. These witnesses can potentially detect Bell correlations in states with an arbitrarily low amount of spin squeezing. We then establish an upper bound on the statistics needed to convincingly conclude that a measured state is Bell-correlated.

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arXiv2016

Bounding quantum gravity inspired decoherence using atom interferometry

Hypothetical models have been proposed in which explicit collapse mechanisms prevent the superposition principle to hold at large scales. In particular, the model introduced by Ellis and co-workers [Phys. Lett. B ${\bf 221}$, 113 (1989)] suggests that quantum gravity might be responsible for the collapse of the wavefunction of massive objects in spatial superpositions. We here consider a recent experiment reporting on interferometry with atoms delocalized over half a meter for timescale of a second [Nature ${\bf 528}$, 530 (2015)] and show that the corresponding data strongly bound quantum gravity induced decoherence and rule it out in the parameter regime considered originally.

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arXiv2016

Bell Correlations in a Bose-Einstein Condensate

Characterizing many-body systems through the quantum correlations between their constituent particles is a major goal of quantum physics. Although entanglement is routinely observed in many systems, we report here the detection of stronger correlations - Bell correlations - between the spins of about 480 atoms in a Bose-Einstein condensate. We derive a Bell correlation witness from a many-particle Bell inequality involving only one- and two-body correlation functions. Our measurement on a spin-squeezed state exceeds the threshold for Bell correlations by 3.8 standard deviations. Our work shows that the strongest possible non-classical correlations are experimentally accessible in many-body systems, and that they can be revealed by collective measurements.

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arXiv2016

Probing wave function collapse models with a classically driven mechanical oscillator

We show that the interaction of a pulsed laser light with a mechanical oscillator through the radiation pressure results in an opto-mechanical entangled state in which the photon number is correlated with the oscillator position. Interestingly, the mechanical oscillator can be delocalized over a large range of positions when driven by an intense laser light. This provides a simple yet sensitive method to probe hypothetic post-quantum theories including an explicit wave function collapse model, like the Diosi and Penrose model. We propose an entanglement witness to reveal the quantum nature of this opto-mechanical state as well as an optical technique to record the decoherence of the mechanical oscillator. We also report on a detailed feasibility study giving the experimental challenges that need to be overcome to confirm or rule out predictions from explicit wave function collapse models.

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arXiv2016

Light-matter micro-macro entanglement

Quantum mechanics predicts microscopic phenomena with undeniable success. Nevertheless, current theoretical and experimental efforts still do not yield conclusive evidence that there is, or not, a fundamental limitation on the possibility to observe quantum phenomena at the macroscopic scale. This question prompted several experimental efforts producing quantum superpositions of large quantum states in light or matter. Here we report on the observation of entanglement between a single photon and an atomic ensemble. The certified entanglement stems from a light-matter micro-macro entangled state that involves the superposition of two macroscopically distinguishable solid-state components composed of several tens of atomic excitations. Our approach leverages from quantum memory techniques and could be used in other systems to expand the size of quantum superpositions in matter.

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arXiv2016

What does it take to see entanglement?

Tremendous progress has been realized in quantum optics for engineering and detecting the quantum properties of light. Today, photon pairs are routinely created in entangled states. Entanglement is revealed using single-photon detectors in which a single photon triggers an avalanche current. The resulting signal is then processed and stored in a computer. Here, we propose an approach to get rid of all the electronic devices between the photons and the experimentalist i.e. to use the experimentalist's eye to detect entanglement. We show in particular, that the micro entanglement that is produced by sending a single photon into a beam-splitter can be detected with the eye using the magnifying glass of a displacement in phase space. The feasibility study convincingly demonstrates the possibility to realize the first experiment where entanglement is observed with the eye.

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arXiv2016

Generation of single photons with highly tunable wave shape from a cold atomic quantum memory

We report on a single photon source with highly tunable photon shape based on a cold ensemble of Rubidium atoms. We follow the DLCZ scheme to implement an emissive quantum memory, which can be operated as a photon pair source with controllable delay. We find that the temporal wave shape of the emitted read photon can be precisely controlled by changing the shape of the driving read pulse. We generate photons with temporal durations varying over three orders of magnitude up to 10 μs without a significant change of the read-out efficiency. We prove the non-classicality of the emitted photons by measuring their antibunching, showing near single photon behavior at low excitation probabilities. We also show that the photons are emitted in a pure state by measuring unconditional autocorrelation functions. Finally, to demonstrate the usability of the source for realistic applications, we create ultra-long single photons with a rising exponential or doubly peaked wave shape which are important for several quantum information tasks.

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arXiv2016

Testing unconventional decoherence models with atoms in optical lattices

Various models have been proposed in which the Schrödinger equation is modified to account for a decay of spatial coherences of massive objects. While optomechanical systems and matter-wave interferometry with large clusters are promising candidates to test these models, we here show that using available techniques for atoms in optical lattices, some of these models can be efficiently tested. In particular, we compare unconventional decoherence due to quantum gravity as introduced by Ellis and co-workers [Phys. Lett. B ${\bf 221}$, 113 (1989)] and conventional decoherence due to scattering of the lattice photons and conclude that optimal performances are achieved with a few atoms in the realistic case where product atomic states are prepared. A detailed analysis shows that a single atom delocalized on a scale of 10 cm for about a second can be used to test efficiently the hypothetical quantum gravity induced decoherence.

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arXiv2015

Heralded amplification of photonic qubits

We demonstrate heralded qubit amplification for Time-Bin and Fock-state qubits in an all-fibre, telecom-wavelength, scheme that highlights the simplicity, the stability and potential for fully integrated photonic solutions. Exploiting high-efficiency superconducting detectors, the gain, the fidelity and the performance of the amplifier are studied as a function of loss. We also demonstrate the first heralded Fock-state qubit amplifier without post-selection. This provides a significant advance towards demonstrating Device-Independent Quantum Key Distribution as well as fundamental tests of quantum mechanics over extended distances.

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arXiv2014

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

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. Phys. Rev. Lett. 110, 130401 (2013)]. 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.

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arXiv2014

Heralded single phonon preparation, storage and readout in cavity optomechanics

We analyze theoretically how to use the radiation pressure coupling between a mechanical oscillator and an optical cavity field to generate in a heralded way a single quantum of mechanical motion (a Fock state), and release on-demand the stored excitation as a single photon. Starting with the oscillator close to its ground state, a laser pumping the upper motional sideband leads to dynamical backaction amplification and to the creation of correlated photon-phonon pairs. The detection of one Stokes photon thus projects the macroscopic oscillator into a single-phonon Fock state. The non-classical nature of this mechanical state can be demonstrated by applying a readout laser on the lower sideband (i.e. optical cooling) to map the phononic state to a photonic mode, and by performing an autocorrelation measurement on the anti-Stokes photons. We discuss the relevance of our proposal for the future of cavity optomechanics as an enabling quantum technology.

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arXiv2014

How difficult it is to prove the quantumness of macroscropic states?

General wisdom tells us that if two quantum states are ``macroscopically distinguishable'' then their superposition should be hard to observe. We make this intuition precise and general by quantifying the difficulty to observe the quantum nature of a superposition of two states that can be distinguished without microscopic accuracy. First, we quantify the distinguishability of any given pair of quantum states with measurement devices lacking microscopic accuracy, i.e. measurements suffering from limited resolution or limited sensitivity. Next, we quantify the required stability that have to be fulfilled by any measurement setup able to distinguish their superposition from a mere mixture. Finally, by establishing a relationship between the stability requirement and the ``macroscopic distinguishability'' of the two superposed states, we demonstrate that indeed, the more distinguishable the states are, the more demanding are the stability requirements.

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arXiv2014

Macroscopic optomechanics from displaced single-photon entanglement

Displaced single-photon entanglement is a simple form of optical entanglement, obtained by sending a photon on a beamsplitter and subsequently applying a displacement operation. We show that it can generate, through a momentum transfer in the pulsed regime, an optomechanical entangled state involving macroscopically distinct mechanical components, even if the optomechanical system operates in the single-photon weak coupling regime. We discuss the experimental feasibility of this approach and show that it might open up a way for testing unconventional decoherence models.

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arXiv2013

Displacing entanglement back and forth between the micro and macro domains

Quantum theory is often presented as the theory describing the microscopic world, and admittedly, it has done this extremely well for decades. Nonetheless, the question of whether it applies at all scales and in particular at human scales remains open, despite considerable experimental effort. Here, we report on the displacement of quantum entanglement into the domain where it involves two macroscopically distinct states, i.e. two states characterised by a large enough number of photons to be seen, at least in principle, with our eyes and that could be distinguished using mere linear - coarse-grained - detectors with a high probability. Specifically, we start by the generation of entanglement between two spatially separated optical modes at the single photon level and subsequently displace one of these modes up to almost a thousand photons. To reliably check whether entanglement is preserved, the mode is re-displaced back to the single photon level and a well established entanglement measure, based on single photon detection, is performed. The ability to displace an entangled state from the micro to the macro domain and back again provides a fascinating tool to probe fundamental questions about quantum theory and holds potential for more applied problems such as quantum sensing.

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arXiv2013

Heralded Mapping of Photonic Entanglement into Single Atoms in Free Space: Proposal for a Loophole-Free Bell Test

An obvious way to entangle two atoms located at remote locations is to produce a pair of entangled photons half-way between the two atoms, to sent one photon to each location and to subsequently map the photonic entanglement into the atoms. The efficiency of this process is, however, fundamentally limited due to overall transmission losses. We propose a method to herald the success of the mapping operation in free space without destroying nor revealing the stored quantum state. Interestingly for a Bell test, the heralding signal does not open the detection loophole provided the measurement choice is performed once the heralding is obtained only. We show through a detailed feasibility study that this approach could provide an attractive alternative to Bell tests where the atom--atom entanglement is created from atom--photon entanglement using an entanglement swapping operation.

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arXiv2013

High-bandwidth quantum memory protocol for storing single photons in rare-earth doped crystals

We present a detailed analysis of a high-bandwidth quantum memory protocol for storing single photons in a rare-earth-ion doped crystal. The basic idea is to benefit from a coherent free-induced decay type re-emission which occurs naturally when a photon with a broadband spectrum is absorbed by a narrow atomic transition in an optically dense ensemble. This allows for a high-bandwidth memory for realistic material parameters. Long storage time and on-demand readout are obtained by means of spins states in a lambda-type configuration, through the transfer of the optical coherence to a spin coherence (so-called spin-wave storage). We give explicit formulas and show numerical results which make it possible to gain insight into the dependence of the memory efficiency on the optical depth and on the width and the shape of stored photons. We present a feasibility study in rare-earth doped crystals and show that high efficiencies and high bandwidth can be obtained with realistic parameters. High-bandwidth memories using spin-wave storage offers the possibility of very high time-bandwidth products, which is important for experiment where high repetition rates are needed.

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arXiv2013

Analysis of a photon number resolving detector based on an ion Coulomb crystal inside an optical cavity

The ability to detect single photons with high efficiency is a crucial requirement for various quantum information applications. By combining the storage process of a quantum memory for photons with fluorescence-based quantum state measurement, it is in principle possible to achieve high efficiency photon counting in large ensembles of atoms. The large number of atoms can, however, pose significant problems in terms of noise stemming from imperfect initial state preparation and off-resonant fluorescence. We propose a concrete implementation of a photon number resolving detector based on an ion Coulomb crystal inside a moderately high-finesse optical cavity. The cavity enhancement leads to an effective optical depth of 15 for a finesse of 3000 with only about 1500 ions interacting with the light field. We show that these values allow for essentially noiseless detection with an efficiency larger than 90%. Moderate experimental parameters allow for repetition rates of about 5 kHz, limited by the time needed for fluorescence collection. Potential applications are discussed.

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arXiv2013

Exploring Macroscopic Entanglement with a Single Photon and Coherent States

Entanglement between macroscopically populated states can easily be created by combining a single photon and a bright coherent state on a beam-splitter. Motivated by the simplicity of this technique, we report on a method using displacement operations in the phase space and basic photon detections to reveal such an entanglement. We demonstrate through preliminary experimental results, that this eminently feasible approach provides an attractive way for exploring entanglement at various scales, ranging from one to a thousand photons. This offers an instructive viewpoint to gain insight into the reasons that make it hard to observe quantum features in our macroscopic world.

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