Jean-Philippe Poizat

Single Photon Quantum Cryptography

We report the full implementation of a quantum cryptography protocol using a stream of single photon pulses generated by a stable and efficient source operating at room temperature. The single photon pulses are emitted on demand by a single nitrogen-vacancy color center in a diamond nanocrystal. The quantum bit error rate is less that 4.6% and the secure bit rate is 7700 bits/s. The overall performances of our system reaches a domain where single photons have a measurable advantage over an equivalent system based on attenuated light pulses.

Photon antibunching in the fluorescence of individual color centers in diamond.

We observed photon antibunching in the fluorescent light emitted from a single nitrogen-vacancy center in diamond at room temperature. The possibility of generating triggerable single photons with such a solid-state system is discussed.

Nonclassical radiation from diamond nanocrystals

The quantum properties of the fluorescence light emitted by diamond nanocrystals containing a single nitrogen-vacancy (NV) colored center are investigated. We have observed photon antibunching with very low background light. This system is therefore a very good candidate for the production of single photon on demand. In addition, we have measured a larger NV center lifetime in nanocrystals than in the bulk, in good agreement with a simple quantum electrodynamical model.

Quantum non-demolition measurements in optics

Quantum non-demolition measurements are designed to circumvent the limitations imposed by Heisenbergs uncertainty principle when performing repeated measurements of quantum states. Recent progress in quantum optics has enabled the experimental realization of quantum non-demolition measurements of the photon flux of a light beam. This achievement bears on fundamental issues about the ultimate sensitivity of measurements, and may open the way for applications such as noise-free information tapping in optical telecommunications.

Giant optical nonlinearity induced by a single two-level system interacting with a cavity in the Purcell regime

A two-level system that is coupled to a high-finesse cavity in the Purcell regime exhibits a giant optical nonlinearity due to the saturation of the two-level system at very low intensities, of the order of one photon per lifetime. We perform a detailed analysis of this effect, taking into account the most important practical imperfections. Our conclusion is that an experimental demonstration of the giant nonlinearity is feasible using semiconductor micropillar cavities containing a single quantum dot in resonance with the cavity mode.

Energy loss by MeV carbon clusters and fullerene ions in solids

Abstract Energy losses by carbon clusters C p ( p ≤ 8) and fullerene ions C 60 in thin carbon foils have been measured in the energy range from 0.3 to 5.65 MeV/atom. A small enhancement in energy loss is obtained for carbon atoms in the clusters relative to single carbon ions at the same velocity. Very large pulse height defects have been observed in the energy response of a silicon detector bombarded by C 60 ions with energies ranging from 6 to 30 MeV.

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.

Strain-mediated coupling in a quantum dot–mechanical oscillator hybrid system

Recent progress in nanotechnology has allowed the fabrication of new hybrid systems in which a single two-level system is coupled to a mechanical nanoresonator. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies, and for the exploration of the quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain: a quantum dot is embedded within a nanowire that features discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light-extraction efficiency and a large exciton-phonon coupling strength g0. By means of optical and mechanical spectroscopy, we find that g0/2 π is nearly as large as the mechanical frequency, a criterion that defines the ultrastrong coupling regime.

A high-temperature single-photon source from nanowire quantum dots.

We present a high-temperature single-photon source based on a quantum dot inside a nanowire. The nanowires were grown by molecular beam epitaxy in the vapor-liquid-solid growth mode. We utilize a two-step process that allows a thin, defect-free ZnSe nanowire to grow on top of a broader, cone-shaped nanowire. Quantum dots are formed by incorporating a narrow zone of CdSe into the nanowire. We observe intense and highly polarized photoluminescence even from a single emitter. Efficient photon antibunching is observed up to 220 K, while conserving a normalized antibunching dip of at most 36%. This is the highest reported temperature for single-photon emission from a nonblinking quantum-dot source and principally allows compact and cheap operation by using Peltier cooling.

Pure emitter dephasing: A resource for advanced solid-state single-photon sources

We have computed the spectrum emitted spontaneously by a quantum dot coupled to an arbitrarily detuned single mode cavity, taking into account pure dephasing processes. We show that if the emitter is incoherent, the cavity can efficiently emit photons with its own spectral characteristics. This effect opens unique opportunities for the development of devices exploiting both cavity quantum electrodynamic effects and pure dephasing, such as wavelength-stabilized single-photon sources robust against spectral diffusion.