Large-scale quantum computers must be built upon quantum bits that are both highly coherent and locally controllable. We demonstrate the quantum control of the electron and the nuclear spin of a single 31P atom in silicon, using a continuous microwav
e magnetic field together with nanoscale electrostatic gates. The qubits are tuned into resonance with the microwave field by a local change in electric field, which induces a Stark shift of the qubit energies. This method, known as A-gate control, preserves the excellent coherence times and gate fidelities of isolated spins, and can be extended to arbitrarily many qubits without requiring multiple microwave sources.
We experimentally investigate the non-resonant feeding of photons into the optical mode of a zero dimensional nanocavity by quantum dot multiexciton transitions. Power dependent photoluminescence measurements reveal a super-linear power dependence of
the mode emission, indicating that the emission stems from multiexcitons. By monitoring the temporal evolution of the photoluminescence spectrum, we observe a clear anticorrelation of the mode and single exciton emission; the mode emission quenches as the population in the system reduces towards the single exciton level whilst the intensity of the mode emission tracks the multi-exciton transitions. Our results lend strong support to a recently proposed mechanism mediating the strongly non-resonant feeding of photons into the cavity mode.
We report the design, fabrication and optical investigation of electrically tunable single quantum dot - photonic crystal defect nanocavities operating in both the weak and strong coupling regimes of the light matter interaction. Unlike previous stud
ies where the dot-cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert-gases at low temperatures, we demonstrate that the quantum confined Stark effect can be employed to quickly and reversibly switch the dot-cavity coupling simply by varying a gate voltage. Our results show that exciton transitions from individual dots can be tuned by ~4 meV relative to the nanocavity mode before the emission quenches due to carrier tunneling escape. This range is much larger than the typical linewidth of the high-Q cavity modes (~0.10 meV) allowing us to explore and contrast regimes where the dots couple to the cavity or decay by spontaneous emission into the 2D photonic bandgap. In the weak coupling regime, we show that the dot spontaneous emission rate can be tuned using a gate voltage, with Purcell factors >=7. New information is obtained on the nature of the dot-cavity coupling in the weak coupling regime and electrical control of zero dimensional polaritons is demonstrated for the highest-Q cavities (Q>=12000). Vacuum Rabi splittings up to ~0.13 meV are observed, much larger than the linewidths of either the decoupled exciton or cavity mode. These observations represent a voltage switchable optical non-linearity at the single photon level, paving the way towards on-chip dot based nano-photonic devices that can be integrated with passive optical components.
