M. B. Ward

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.

On-demand single-photon source for 1.3μm telecom fiber

We demonstrate an on-demand single-photon source that is compatible with standard telecom optical fiber. Through careful control of the critical strains of InAs∕GaAs self-assembled quantum dots, we produce a microcavity sample with a low density of large dots emitting into the fiber-optic transmission band at 1.3μm. The second-order correlation function of the source reveals a strong suppression in the rate of multiphoton pulses at both 5K and above 30K. The source may be useful for fiber-optic-based single-photon applications, such as quantum metrology, quantum communications, and distributed quantum computing.

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.

Narrow emission linewidths of positioned InAs quantum dots grown on pre-patterned GaAs(100) substrates

We report photoluminescence measurements on a single layer of site-controlled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on pre-patterned GaAs(100) substrates with a 15 nm re-growth buffer separating the dots from the re-growth interface. A process for cleaning the re-growth interface allows us to measure single dot emission linewidths of 80 µeV under non-resonant optical excitation, similar to that observed for self-assembled QDs. The dots reveal excitonic transitions confirmed by power dependence and fine structure splitting measurements. The emission wavelengths are stable, which indicates the absence of a fluctuating charge background in the sample and confirms the cleanliness of the re-growth interface.

Quantum key distribution using a triggered quantum dot source emitting near 1.3μm

We report the distribution of a cryptographic key, secure from photon number splitting attacks, over 35km of optical fiber using single photons from an InAs quantum dot emitting ∼1.3μm in a pillar microcavity. Using below GaAs-bandgap optical excitation, we demonstrate suppression of multiphoton emission to 10% of the Poissonian level without detector dark count subtraction. The source is incorporated into a phase encoded interferometric scheme implementing the BB84 protocol for key distribution over standard telecommunication optical fiber. We show a transmission distance advantage over that possible with (length-optimized) uniform intensity weak coherent pulses at 1310nm in the same system.

Site-Control of InAs Quantum Dots using Ex-Situ Electron-Beam Lithographic Patterning of GaAs Substrates

Conventional e-beam lithography followed by either dry or wet etching of small holes in GaAs substrates has been used to control the position of InAs self-assembled quantum dots. The dependence of hole occupancy on both hole area and hole depth has been investigated. We show a range of hole sizes where greater than 30% of sites contain a single dot with up to 60% single dot occupancy seen for dry-etched holes ~60 nm wide, ~35 nm deep and for wet-etched holes ~90 nm wide, ~20 nm deep. Single dot luminescence from these placed dots is demonstrated despite only a 10 nm GaAs buffer between dots and regrowth interface.

Modulation of single quantum dot energy levels by a surface-acoustic-wave

This letter presents an experimental investigation into the effect of a surface-acoustic-wave (SAW) on the emission of a single InAs quantum dot. The SAW causes the energy of the transitions within the dot to oscillate at the frequency of the SAW, producing a characteristic broadening of the emission lines in their time-averaged spectra. This periodic tuning of the transition energy is used as a method to regulate the output of a device containing a single quantum dot and we study the system as a high-frequency periodic source of single photons.

High-performance planar light-emitting diodes

High-speed planar light-emitting diodes fabricated within a single high-mobility quantum well are demonstrated. Devices were fabricated by photolithography and wet chemical etching starting from p-type modulation-doped Al0.5Ga0.5As/GaAs heterostructures grown by molecular beam epitaxy. Electrical and optical measurements from room temperature down to 1.8 K show high spectral purity, high external efficiency, and extremely short recombination times of the order of 50 ps. Time-resolved electroluminescence measurements demonstrate subnanosecond modulation time scale.

Single electron-spin memory with a semiconductor quantum dot

We show storage of the circular polarization of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently read out through the electronically-triggered emission of a single photon. The emitted photon shares the same polarization as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 T, spin memory is preserved for at least 1000 times more than the excitons radiative lifetime.

Single-photon-emitting diodes: a review

Compact and reliable sources of non-classical light could find many applications in emerging technologies such as quantum cryptography, quantum imaging and also in fundamental tests of quantum physics. Single self-assembled quantum dots have been widely studied for this reason, but the vast majority of reported work has been limited to optically excited sources. Here we discuss the progress made so far, and prospects for, electrically driven single-photon-emitting diodes (SPEDs). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)