Gaetan Messin

Quantum interference between two single photons emitted by independently trapped atoms.

When two indistinguishable single photons are fed into the two input ports of a beam splitter, the photons will coalesce and leave together from the same output port. This is a quantum interference effect, which occurs because two possible paths—in which the photons leave by different output ports—interfere destructively. This effect was first observed in parametric downconversion (in which a nonlinear crystal splits a single photon into two photons of lower energy), then from two separate downconversion crystals, as well as with single photons produced one after the other by the same quantum emitter. With the recent developments in quantum information research, much attention has been devoted to this interference effect as a resource for quantum data processing using linear optics techniques. To ensure the scalability of schemes based on these ideas, it is crucial that indistinguishable photons are emitted by a collection of synchronized, but otherwise independent sources. Here we demonstrate the quantum interference of two single photons emitted by two independently trapped single atoms, bridging the gap towards the simultaneous emission of many indistinguishable single photons by different emitters. Our data analysis shows that the observed coalescence is mainly limited by wavefront matching of the light emitted by the two atoms, and to a lesser extent by the motion of each atom in its own trap.

Controlled single-photon emission from a single trapped two-level atom

By illuminating an individual rubidium atom stored in a tight optical tweezer with short resonant light pulses, we created an efficient triggered source of single photons with a well-defined polarization. The measured intensity correlation of the emitted light pulses exhibits almost perfect antibunching. Such a source of high-rate, fully controlled single-photon pulses has many potential applications for quantum information processing.

Experimental open air quantum key distribution with a single photon source

We describe the implementation of a quantum key distribution (QKD) system using a single-photon source, operating at night in open air. The single- photon source at the heart of the functional and reliable set-up relies on the pulsed excitation of a single nitrogen-vacancy colour centre in a diamond nanocrystal. We tested the effect of attenuation on the polarized encoded photons for inferring the longer distance performance of our system. For strong attenuation, the use of pure single-photon states gives measurable advantage over systems relying on weak attenuated laser pulses. The results are in good agreement with theoretical models developed to assess QKD security.

Two-dimensional transport and transfer of a single atomic qubit in optical tweezers

Quantum computers have the capability of out-performing their classical counterparts for certain computational problems. Several scalable quantum-computing architectures have been proposed. An attractive architecture is a large set of physically independent qubits arranged in three spatial regions where (1) the initialized qubits are stored in a register, (2) two qubits are brought together to realize a gate and (3) the readout of the qubits is carried out. For a neutral-atom-based architecture, a natural way to connect these regions is to use optical tweezers to move qubits within the system. In this letter we demonstrate the coherent transport of a qubit, encoded on an atom trapped in a submicrometre tweezer, over a distance typical of the separation between atoms in an array of optical traps. Furthermore, we transfer a qubit between two tweezers, and show that this manipulation also preserves the coherence of the qubit.

Diffraction-limited optics for single-atom manipulation

We present an optical system designed to capture and observe a single neutral atom in an optical dipole trap, created by focusing a laser beam using a large-numerical-aperture (NA= 0.5) aspheric lens. We experimentally evaluate the performance of the optical system and show that it is diffraction limited over a broad spectral range (~200 nm) with a large transverse field (±25 µm) The optical tweezer created at the focal point of the lens is able to trap single atoms of 87 Rb and to detect them individually with a large collection efficiency. We measure the oscillation frequency of the atom in the dipole trap and use this value as an independent deter-mination of the waist of the optical tweezer. Finally, we produce with the same lens two dipole traps separated by 2.2 µm and show that the imaging system can resolve the two atoms.

A frequency-doubled laser system producing ns pulses for rubidium manipulation

We have constructed a pulsed laser system for the manipulation of cold Rb atoms. The system combines optical telecommunications components and frequency doubling to generate light at 780 nm. Using a fast, fibre-coupled intensity modulator, output from a continuous laser diode is sliced into pulses with a length between 1.3 and 6.1 ns and a repetition frequency of 5 MHz. These pulses are amplified using an erbium-doped fibre amplifier, and frequency-doubled in a periodically poled lithium niobate crystal, yielding a peak power up to 12 W. Using the resulting light at 780 nm, we demonstrate Rabi oscillations on the F = 2<->F=3-transition of a single 87Rb atom.

Remote preparation of single-plasmon states

Quantum entanglement is a stunning consequence of the superposition principle. This universal property of quantum systems has been intensively explored with photons, atoms, ions and electrons. Collective excitations such as surface plasmons exhibit quantum behaviors. For the first time, we report an experimental evidence of non-local control of single plasmon interferences through entanglement of a single plasmon with a single photon. We achieved photon-plasmon entanglement by converting one photon of an entangled photon pair into a surface plasmon. The plasmon is tested onto a plasmonic platform in a Mach-Zehnder interferometer. A projective measurement on the polarization of the photon allows the non-local control of the interference state of the plasmon. Entanglement between particles of various natures paves the way to the design of hybrid systems in quantum information networks.

Revisiting quantum optics with single plasmons

The growing field of quantum plasmonics lies at the intersection between nanophotonics and quantum optics. QUantum plasmonics investigate the quantum properties of single surface plasmons, trying to reproduce fundamental and landmark quantum optics experiment that would benefit from the light-confinement properties of nanophotonic systems, thus paving the way towards the design of basic components dedicated to quantum experiments with sizes inferior to the diffraction limit. Several groups have recently reproduced fundamental quantum optics experiments with single surface plasmons polaritons (SPPs). We have investigated two situations of quantum interference of single SPPs on lossy beamsplitters : a plasmonic version of the Hong-Ou-Mandel experiment, and the observation of plasmonic N00N states interferences. We numerically designed and fabricated several beamsplitters that reveal new quantum interference scenarios, such as the coalescence and the anti-coalescence of SPPs, or quantum non-linear absorption. Our work show that losses can be seen as a new degree of freedom in the design of plasmonic devices.

Wave-Particle duality of single surface plasmon polaritons

We use a plasmonic beamsplitter in an interferometer to demonstrate the wave-particle duality of surface plasmon polaritons lying on a plane gold surface. The particle or wave behavior is chosen with the orientation of a half waveplate.

Time-resolved detection of relative intensity squeezed nanosecond pulses in a 87 Rb vapor

Squeezed light is a valuable resource in the fields of continuous-variable quantum information, quantum communication, and quantum optics [1]. In this talk, we demonstrate a system capable of producing pulsed squeezed light via four-wave mixing in a rubidium vapor [2]. By employing a pulsed input [3], we produce nanosecond relative-intensity squeezed pulses and employ time-resolved detection to measure the degree of squeezing obtained. With respect to recent noise-spectrum squeezing experiments in atomic vapors [2], the present work is based on time-domain detection. The basic idea behind the generation of relative-intensity squeezed light as presented in this work relies on off-resonant four-wave mixing in a double lambda-system [2]. A strong pump, ω p , interacts with a pulsed probe beam, ω s , which is offset from the pump by approximately the hyperfine ground state separation. Under suitable conditions, this energy level structure allows for the parametric amplification of the probe beam while simultaneously creating its quantum-correlated conjugate.