Carmen Gomez Carbonell

Integrated AlGaAs source of highly indistinguishable and energy-time entangled photons

The generation of nonclassical states of light in miniature chips is a crucial step toward practical implementations of future quantum technologies. Semiconductor materials are ideal for achieving extremely compact and massively parallel systems and several platforms are currently under development. In this context, spontaneous parametric downconversion in AlGaAs devices combines the advantages of room temperature operation, possibility of electrical injection, and emission in the telecom band. Here we report on a chip-based AlGaAs source, producing indistinguishable and energy-time entangled photons with a brightness of 7.2×106  pairs/s and a signal-to-noise ratio of 141±12. Indistinguishability between the photons is demonstrated via a Hong–Ou–Mandel experiment with a visibility of 89±3%, mainly limited by the reflectivity of the chip facets, while energy-time entanglement is tested via a Franson interferometer leading to a visibility of 96±4.

Long-lived spin coherence of indirect excitons in GaAs coupled quantum wells

We study spatially indirect excitons confined in a symmetric GaAs double quantum well. We show that the spin polarisation of very dilute gases can be optically imprinted, in both pure and superposition states. In the regime where indirect excitons can be localized, we observe at 350 mK that the excitons spin degree of freedom is frozen with a relaxation time comparable to the coherence time and to the radiative lifetime .

Resonance fluorescence revival in a voltage-controlled semiconductor quantum dot

We demonstrate systematic resonance fluorescence recovery with near-unity emission efficiency in single quantum dots embedded in a charge-tunable device in a wave-guiding geometry. The quantum dot charge state is controlled by a gate voltage, through carrier tunneling from a close-lying Fermi sea, stabilizing the resonantly photocreated electron-hole pair. The electric field cancels out the charging/discharging mechanisms from nearby traps toward the quantum dots, responsible for the usually observed inhibition of the resonant fluorescence. Fourier transform spectroscopy as a function of the applied voltage shows a strong increase of the coherence time though not reaching the radiative limit. These charge controlled quantum dots act as quasi-perfect deterministic single-photon emitters, with one laser pulse converted into one emitted single photon.

Multi-user quantum key distribution with a semiconductor source of entangled photon pairs (Conference Presentation)

In view of real world applications of quantum information technologies, the combination of miniature quantum resources with existing fibre networks is a crucial issue. Among such resources, on-chip entangled photon sources play a central role for applications spanning quantum communications, computing and metrology. Here, we use a semiconductor source of entangled photons operating at room temperature in conjunction with standard telecom components to demonstrate multi-user quantum key distribution, a core protocol for securing communications in quantum networks. The source consists of an AlGaAs chip emitting polarization entangled photon pairs over a large bandwidth in the main telecom band around 1550 nm without the use of any off-chip compensation or interferometric scheme; the photon pairs are directly launched into a dense wavelength division multiplexer (DWDM) and secret keys are distributed between several pairs of users communicating through different channels. We achieve a visibility measured after the DWDM of 87% and show long-distance key distribution using a 50-km standard telecom fibre link between two network users. These results illustrate a promising route to practical, resource-efficient implementations adapted to quantum network infrastructures.

Publisher’s Note: Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons [Phys. Rev. Lett. 118 , 127402 (2017)]

This corrects the article DOI: 10.1103/PhysRevLett.118.127402.

Spectroscopic signatures for the dark Bose-Einstein condensation of spatially indirect excitons

We study semiconductor excitons confined in an electrostatic trap of a GaAs bilayer heterostructure. We evidence that optically bright excitonic states are strongly depleted while cooling to sub-Kelvin temperatures. In return, the other accessible and optically dark states become macroscopically occupied so that the overall exciton population in the trap is conserved. These combined behaviours constitute the spectroscopic signature for the mostly dark Bose-Einstein condensation of excitons, which in our experiments is restricted to a dilute regime within a narrow range of densities, below a critical temperature of about 1K.