A. J. Bennett

Two-photon interference of the emission from electrically tunable remote quantum dots

Single semiconductor quantum dots have been widely studied within devices that can apply an electric field. In the most common system, the low energy offset between the InGaAs quantum dot and the surrounding GaAs material limits the magnitude of field that can be applied to tens of kVcm−1, before carriers tunnel out of the dot. The Stark shift experienced by the emission line is typically ∼ 1 meV. We report that by embedding the quantum dots in a quantum well heterostructure the vertical field that can be applied is increased by over an order of magnitude whilst preserving the narrow linewidths, high internal quantum efficiencies and familiar emission spectra. Individual dots can then be continuously tuned to the same energy allowing for two-photon interference between remote, independent, quantum dots. PACS numbers: 78.67.-n, 85.35.Ds ∗Electronic address: anthony.bennett@crl.toshiba.co.uk

Electric-field-induced coherent coupling of the exciton states in a single quantum dot

The ability to generate entangled photon pairs from a quantum dot critically depends on the size of the fine-structure splitting of its exciton states. A demonstration of the ability to tune this splitting with an electric field represents a promising step in the use of quantum dots to generate entangled photon pairs on demand.

Microcavity single-photon-emitting diode

We show that a planar semiconductor cavity can be used to enhance by a factor of ten the efficiency with which photons are collected from an electrically driven single In As ∕ Ga As quantum dot. Under a fixed bias we observe that the photon statistics change when the injection current is modified. The observed bunching of photons from the biexciton state can be explained by the presence of charged states or dark states within the quantum dot with lifetimes greater than 4 ns. Single-photon emission from both the exciton and biexciton states is demonstrated under pulsed electrical injection.

Coherence of an entangled exciton-photon state.

We study the effect of the exciton fine-structure splitting on the polarization entanglement of photon pairs produced by the biexciton cascade in a quantum dot. Entanglement persists despite separations between the intermediate energy levels of up to 4 microeV. Measurements show that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first-order coherence time of either single photon. We present a theoretical framework incorporating the effects of spin scattering, background light, and dephasing. We distinguish between the first-order coherence time, and a parameter which we measure for the first time and define as the cross-coherence time.

Electrically driven telecommunication wavelength single-photon source

An electrically driven ∼1.3μm single-photon source is demonstrated. The source contains InAs quantum dots within a planar cavity light-emitting diode. Electroluminescence (EL) spectra show clear emission lines and from time resolved EL we estimate a primary decay time of ∼1ns. Time-varying Stark shifts are studied and proposed for truncating the emission in jitter-sensitive applications (optimization for 2ns detector gate width demonstrated) and for relaxing excitation pulse-length requirements. A correlation measurement demonstrates suppression of multiphoton emission to below 28% of the Poissonian level before correction for detector dark counts, suggesting g(2)(0)∼0.19 for the source itself.

Giant Stark effect in the emission of single semiconductor quantum dots

We study the quantum-confined Stark effect in single InAs/GaAs quantum dots embedded within a AlGaAs/GaAs/AlGaAs quantum well. By significantly increasing the barrier height we can observe emission from a dot at electric fields of 500 kV cm −1, leading to Stark shifts of up to 25 meV. Our results suggest this technique may enable future applications that require self-assembled dots with transitions at the same energy.

Ultrashort dead time of photon-counting InGaAs avalanche photodiodes

We report a 1.036 GHz gated Geiger mode InGaAs avalanche photodiode with a detection dead time of just 1.93 ns. This is demonstrated by full recovery of the detection efficiency two gate cycles after a detection event, as well as a measured maximum detection rate of 497 MHz. As an application, we measure the second order correlation function g(2) of the emission from a diode laser with a single detector that works reliably at high speed owing to the extremely short dead time of the detector. The device is ideal for high bit rate fiber wavelength quantum key distribution and photonic quantum computing.

Evolution of entanglement between distinguishable light states.

We investigate the evolution of quantum correlations over the lifetime of a multiphoton state. Measurements reveal time-dependent oscillations of the entanglement fidelity for photon pairs created by a single semiconductor quantum dot. The oscillations are attributed to the phase acquired in the intermediate, nondegenerate, exciton-photon state and are consistent with simulations. We conclude that emission of photon pairs by a typical quantum dot with finite polarization splitting is in fact entangled in a time-evolving state, and not classically correlated as previously regarded.

Enhancement and suppression of spontaneous emission by temperature tuning InAs quantum dots to photonic crystal cavities

We report on the control of the spontaneous emission rates in InAs self-assembled quantum dots weakly coupled to the mode of a modified H1 defect cavity in a two-dimensional photonic crystal slab. Changes in sample temperature are used to spectrally tune the exciton emission from a single quantum dot to the monopole mode of the microcavity. A Purcell enhancement of the spontaneous emission rate of up to a factor of 11.4 is seen on-resonance, while suppression by up to a factor of 4.4 is seen off-resonance. Also, a two orders of magnitude increase in the intensity of light detected from the exciton is measured when compared to a quantum dot in bulk GaAs.

Cavity-enhanced radiative emission rate in a single-photon-emitting diode operating at 0.5 GHz

We report the observation of a Purcell enhancement in the electroluminescence decay rate of a single quantum dot, embedded in a microcavity light-emitting-diode structure. Lateral confinement of the optical mode was achieved using an annulus of low-refractive-index aluminium oxide, formed by wet oxidation. The same layer acts as a current aperture, reducing the active area of the device without impeding the electrical properties of the p-i-n diode. This allowed single photon electroluminescence to be demonstrated at repetition rates up to 0.5 GHz.