Claire Le Gall

Phase-locked indistinguishable photons with synthesized waveforms from a solid-state source

Resonance fluorescence in the Heitler regime provides access to single photons with coherence well beyond the Fourier transform limit of the transition, and holds the promise to circumvent environment-induced dephasing common to all solid-state systems. Here we demonstrate that the coherently generated single photons from a single self-assembled InAs quantum dot display mutual coherence with the excitation laser on a timescale exceeding 3 s. Exploiting this degree of mutual coherence, we synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field. In contrast to post-emission filtering, our technique avoids both photon loss and degradation of the single-photon nature for all synthesized waveforms. By engineering pulsed waveforms of single photons, we further demonstrate that separate photons generated coherently by the same laser field are fundamentally indistinguishable, lending themselves to the creation of distant entanglement through quantum interference.

Environment-assisted quantum control of a solid-state spin via coherent dark states

The interaction of a quantum system with its surroundings is usually detrimental, introducing decoherence. Experiments now show how such interactions can be harnessed to provide all-optical control of the spin state of a quantum dot.

Quadrature squeezed photons from a two-level system

Resonance fluorescence arises from the interaction of an optical field with a two-level system, and has played a fundamental role in the development of quantum optics and its applications. Despite its conceptual simplicity, it entails a wide range of intriguing phenomena, such as the Mollow-triplet emission spectrum, photon antibunching and coherent photon emission. One fundamental aspect of resonance fluorescence—squeezing in the form of reduced quantum fluctuations in the single photon stream from an atom in free space—was predicted more than 30 years ago. However, the requirement to operate in the weak excitation regime, together with the combination of modest oscillator strength of atoms and low collection efficiencies, has continued to necessitate stringent experimental conditions for the observation of squeezing with atoms. Attempts to circumvent these issues had to sacrifice antibunching, owing to either stimulated forward scattering from atomic ensembles or multi-photon transitions inside optical cavities. Here, we use an artificial atom with a large optical dipole enabling 100-fold improvement of the photon detection rate over the natural atom counterpart and reach the necessary conditions for the observation of quadrature squeezing in single resonance-fluorescence photons. By implementing phase-dependent homodyne intensity-correlation detection, we demonstrate that the electric field quadrature variance of resonance fluorescence is three per cent below the fundamental limit set by vacuum fluctuations, while the photon statistics remain antibunched. The presence of squeezing and antibunching simultaneously is a fully non-classical outcome of the wave–particle duality of photons.

Extension of electron spin coherence in a quantum dot via CPT-locking of the Overhauser field

Semiconductor quantum dots (QD) offer the current state-of-the-art in intensity and coherence for single photon generation, as well as serving as a testbed for various novel approaches to quantum networking. Their high brightness provides a means to produce high-bandwidth spin-photon entanglement [1]. However, the purity and usefulness of the generated entangled state is severely limited by the coherence time of the QD charge carrier spin, typically on the order of 2ns for electrons in a self-assembled InGaAs QD. This limit is imposed by low-frequency fluctuations of the Overhauser field generated by host nuclear spins, affecting the electron spin resonance frequency.

Phase-tuned entangled state generation between distant spin qubits

Entanglement between distant nodes is essential for the successful realisation of a distributed quantum network. Photon-mediated entanglement has recently been demonstrated in atomic [1] and diamond defect systems [2] and, very recently, using heavy hole spins in quantum dots [3]. However, until now controlled creation of entangled states with arbitrary phase had yet to be demonstrated in these systems. Here we demonstrate the high frequency creation of Bell states with arbitrary phase using electron-spin qubits confined to distant self-assembled InGaAs quantum dots (QD).

Measuring the local environment of a quantum dot

We present a survey of the solid state environment of a quantum dot utilizing resonance fluorescence as a sensitive probe. Nuclear field fluctuations are identified with 10 μs correlation times by comparison to a theoretical model.

Reservoir-assisted coherent control of a quantum dot spin

We demonstrate all-optical coherent manipulation of a quantum dot spin through coherent population trapping with a sub-linewidth spin splitting, enabled by the hyperfine interaction with a mesoscopic nuclear spin ensemble.