Controllable single photon time-bin encoded qubits from a quantum dot

Semiconductor quantum dots are a popular choice for single photon sources as they have unparalleled brightness and may be embedded in a variety of solid-state devices. Recently, the coherent excitation of charged excitons has... [ view full abstract ]

Semiconductor quantum dots are a popular choice for single photon sources as they have unparalleled brightness and may be embedded in a variety of solid-state devices. Recently, the coherent excitation of charged excitons has been used for a range of different experiments including the generation photons coherently superposed across multiple time-bins.  Interestingly, the wavelength of the photons which make up these time-bin encoded qubits may be tuned by tuning the wavelength of the excitation laser.  The photons also have longer coherence times, as this is determined by the coherence of the ground state charge rather than the exciton.  This means the qubits are well suited to use in quantum logic operations, where long coherence times are critical. 

Here, cavity-enhanced Raman scattering in a charged InAs quantum dot in a magnetic field (Voigt geometry) is used to generate a single photon time-bin encoded qubit superposed across two time-bins [1].   We begin by showing that modulating the phase of the laser pulses driving a spin-flip Raman transition results in the modulation of the phase difference between the time-bins of the generated single photon state.  We achieve phase shifts greater than 2π (shown in the Bloch sphere in Figure 1), showing that we have control the phase of the time-bin qubit. Hence, we demonstrate the possibility of complete control over the time bin qubit without the need for an interferometer.  

Finally, we demonstrate that the cavity-enhanced Raman transition may be used to control the wavelength of the generated photons by using two driving lasers which have been detuned to either side of the Raman transition.  Each laser encodes a different state of the time-bin qubit, and spectral filtering enables recovery of the encoded state for each frequency.  This is direct evidence of the possibility of achieving wavelength division multiplexing at the single photon level.

[1] J.P. Lee, L. M. Wells, B. Villa, S. Kalliakos, R. M. Stevenson, D. J. P. Ellis, I. Farrer, D. A. Ritchie, A. J. Bennett, and A. J. Shields. (2018). Controllable Photonic Time-Bin Qubits from a Quantum Dot. Physical Review X, 8(2).