P. See

Ultrafast voltage sampling using single-electron wavepackets

We demonstrate an ultrafast voltage sampling technique using a stream of electron wavepackets. Electrons are emitted from a single-electron pump and travel through electron waveguides towards a detector potential barrier. Our electrons sample an instantaneous voltage on the gate upon arrival at the detector barrier. Fast sampling is achieved by minimising the duration that the electrons interact with the barrier, which can be made as small as a few picoseconds. The value of the instantaneous voltage can be determined by varying the gate voltage to match the barrier height to the electron energy, which is used as a stable reference. The test waveform can be reconstructed by shifting the electron arrival time against it. Although we find that the our current system is limited by the experimental line bandwidth to 12–18 GHz, we argue that this method has scope to increase the bandwidth of voltage sampling to 100 GHz and beyond.

Towards a quantum representation of the ampere using single electron pumps

Electron pumps generate a macroscopic electric current by controlled manipulation of single electrons. Despite intensive research towards a quantum current standard over the last 25 years, making a fast and accurate quantized electron pump has proved extremely difficult. Here we demonstrate that the accuracy of a semiconductor quantum dot pump can be dramatically improved by using specially designed gate drive waveforms. Our pump can generate a current of up to 150 pA, corresponding to almost a billion electrons per second, with an experimentally demonstrated current accuracy better than 1.2 parts per million (p.p.m.) and strong evidence, based on fitting data to a model, that the true accuracy is approaching 0.01 p.p.m. This type of pump is a promising candidate for further development as a realization of the SI base unit ampere, following a redefinition of the ampere in terms of a fixed value of the elementary charge.

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.

Magnetic-field-induced reduction of the exciton polarization splitting in InAs quantum dots

By the application of an in-plane magnetic field, we demonstrate control of the fine structure polarization splitting of the exciton emission lines in individual InAs quantum dots. The selection of quantum dots with certain barrier composition and confinement energies is found to determine the magnetic field dependent increase or decrease of the separation of the bright exciton emission lines, and has enabled the splitting to be tuned to zero within the resolution of our experiments. Observed behavior allows us to determine g factors and exchange splittings for different types of dots.

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.

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.

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.

Quantum Dot Resonant Tunneling Diode for Telecommunication Wavelength Single Photon Detection

The authors present a quantum dot (QD) based single photon detector operating at a fiber optic telecommunication wavelength. The detector is based on an AlAs∕In0.53Ga0.47As∕AlAs double-barrier resonant tunneling diode containing a layer of self-assembled InAs QDs grown on an InP substrate. The device shows an internal efficiency of about 6.3% with a dark count rate of 1.58×10−6ns−1 for 1310nm photons.

Clock-controlled emission of single-electron wave packets in a solid-state circuit.

We demonstrate the energy- and time-resolved detection of single-electron wave packets from a clock-controlled source transmitted through a high-energy quantum Hall edge channel. A quantum dot source is loaded with single electrons which are then emitted ~150 meV above the Fermi energy. The energy spectroscopy of emitted electrons indicates that at high magnetic field these electrons can be transported over several microns without inelastic electron-electron or electron-phonon scattering. Using a time-resolved spectroscopic technique, we deduce the wave packet size at picosecond resolution. We also show how this technique can be used to switch individual electrons into different electron waveguides (edge channels).

Cryogenic on-chip multiplexer for the study of quantum transport in 256 split-gate devices

We present a multiplexing scheme for the measurement of large numbers of mesoscopic devices in cryogenic systems. The multiplexer is used to contact an array of 256 split gates on a GaAs/AlGaAs heterostructure, in which each split gate can be measured individually. The low-temperature conductance of split-gate devices is governed by quantum mechanics, leading to the appearance of conductance plateaux at intervals of 2e2/h. A fabrication-limited yield of 94% is achieved for the array, and a “quantum yield” is also defined, to account for disorder affecting the quantum behaviour of the devices. The quantum yield rose from 55% to 86% after illuminating the sample, explained by the corresponding increase in carrier density and mobility of the two-dimensional electron gas. The multiplexer is a scalable architecture, and can be extended to other forms of mesoscopic devices. It overcomes previous limits on the number of devices that can be fabricated on a single chip due to the number of electrical contacts avai...