Single photon source employing micro-fibre-coupled photonic crystal cavities

Abstract

Single photons in optical fibre are trusted to play an active role of messenger intermediating resisters of extensive quantum networks. This ultimate role relies on how efficiently converting energy from a quantum emitter to a photon of fibre and in reverse direction are performed. Micro-fibre-coupled photonic crystal cavities are attractive to this demand for following two reasons. First, photonic crystal cavities have been effectually employed for strong light matter interactions and to accelerate spontaneous emission. At last, efficient coupling of electric fields of a cavity mode with a fundamental mode of fibre avails of transferring photons into optical fibre with high probability without causing severe loss of the cavity mode.

Single photon source is a milestone of techniques not only because it is a central device for quantum key distribution or further application of linear optics quantum computation, but also it is a manifestation of an efficiency of interaction, at least for one-way from quantum dot to photon in optical fibre. We strongly propose micro-fibre coupled photonic crystal cavity with quantum dot for single photon source as well as an extendable basic element for obtain and exploiting strong matter-photon interaction. It is a composite of every singleness whence an intensified cavity mode midst in very large free spectral region (photonic band-gap), a single quantum dot and a fundamental fibre mode are only participants of this elementary system. For this reason, the efficient coupling of reverse direction from a photon in optical fibre to a quantum dot is expectable as well.

We demonstrated a single photon source based on a micro-fibre-coupled photonic crystal L3 cavity in which a semiconductor quantum dot (QD, InAs/GaAs) is embedded. Field coupling efficiency is the share of electromagnetic flux coming out through optical fibre mode in total flux, and was 41% signified by a depth of transmission dip in optical transmission measurements. Total extraction efficiency $\xi$, which is a ratio of the number of single photons (18MHz) to the number of a ultrashort carrier injection pulses (80MHz), was 23% measured by photon counting experiments and corrected by photon correlation measurements to rule out the contribution of multi-photon states. Spontaneous emission factor $\Beta$ was at least 0.82, by measuring dependence of spontaneous emission time on spectral detuning between the QD and the cavity, sorting out contribution from the cavity mode and one from others including non-radiative process. The spontaneous emission rate on the cavity mode at resonance was 4.1GHz, which corresponds to 10GHz of vacuum Rabi frequency regarding with the mode volume of $0.8(\lambda /n)^3$ cubic wavelength, and measured Q of 4,000. Fibre-coupled L3 cavity is chosen because of its large mode spacing, to which the prediction and result that total efficiency is kept > 25% attribute even when QD-cavity detuning reached $\pm 1nm$. The last singleness I had to challenge is about carrier reservoir. Resonant excitation is regarded as an only way to operate quantum dot quietly without interruptions caused by the turbulent reservoir. Resonant excitation required us to understand an internal electronic structure of each individual quantum dot. With efficient collection of our cavity, we established a method to draw two dimensional map directing what energy of photons should be taken to excite a selected transition of quantum dot. I theoretically examined resonant Rayleigh scattering in a fibre-coupled cavity structure envisaging fast and coherent single photon generation.