Single Spins, Single Photons

We apply the concepts of quantum optics to solid-state emitters. The overriding goal is to create useful hardware for quantum information applications: a single photon source and a spin qubit. The single photon source should be a fast and bright source of indistinguishable photons on demand. The spin qubit should retain its coherence over many quantum operations. These are challenging goals. Rich and varied solid-state physics enters into the work via our design work – control of charge states, photonics, and phononics environments – and via the complex dephasing mechanisms. In fact, by quantum control of the solid-state emitter, we are able to probe the underlying interactions in the solid-state – examples include the central spin problem, the electron-phonon interaction and Kondo-like interactions – with unprecedented resolution. The goals are to understand what limits these devices and to come up with creative ideas to circumvent the main problems. An outstanding goal is to develop a scalable approach to couple multiple qubits.

A Tunable Micro-Cavity

A microcavity enhances the light-matter interaction. In quantum photonics, a microcavity is a versatile tool. Radiative recombination of a solid-state emitter can be accelerated and photon extraction efficiency enhanced by exploiting the weak coupling regime of cavity-QED. The strong coupling regime allows two emitters to be coupled together even when the microcavity is not populated by a real photon. A microcavity is particularly valuable in solid-state systems: the microcavity can accelerate the photonic interaction, effectively weakening the effects of the solid-state-related dephasing mechanisms. Solid-state monolithic micro-cavities, micro-pillars, photonic crystal cavities, for instance, offer limited tuning. The emitter position is typically fixed; there are limited in situ possibilities of tuning the emitter and cavity mode into resonance. This lack of tuning represents a problem, particularly in the present development phase where it is important to quantify the effects of the micro-cavity via the detuning dependence.

We have developed a microcavity that is fully tunable. It is essentially a highly miniaturized Fabry-Perot cavity: the bottom mirror is a plane mirror; the top mirror is curved to confine the light. The radius of curvature of the top mirror is typically 10 microns, the distance between the two mirrors is at most a few microns: this results in a microcavity mode with an extent just above the diffraction limit (λ/2).

The curved mirror is fabricated in silica, either in a silica substrate or in the end facet of an optical glass fibre, by laser ablation. In the ablation process, local melting takes place resulting in an atomically flat surface. The substrate is then coated with a high-quality dielectric Bragg mirror. The micro-cavity itself consists of a bottom and top mirrors with position control: 3-axes to determine the micro-cavity properties (lateral mode location with respect to the sample, mode frequency); 3-axes to position the mode with respect to the focus of a fixed lens.

Two-dimensional semiconductors

Two-dimensional semiconductors such as monolayer MoS2 have a direct bandgap in the red part of the spectrum. The bandgap is located not at the centre of the Brillouin zone (as for GaAs) but at the edges, the +K and –K points. Electrons can be injected optically into the +K-valley or in the –K-valley simply by choosing the circular polarisation. A monolayer of MoS2 can be embedded in a so-called van der Waals heterostructure using h-BN as an insulator, few-layer graphene as a gate. A striking feature is that electrons interact strongly with each other in MoS2 and related materials. Quantum nanosystems can be built by exploiting these features.