National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

On-demand single-electron transfer between distant quantum dots

Abstract: Electrons strongly interact with other electrons and their environment, making it extremely difficult to isolate and detect a single moving electron in a similar way to single photons in quantum optics experiments. But now, in two unrelated reports, Hermelin et al. and McNeil et al. demonstrate that it is possible to emit a single electron from one quantum dot and detect it again with high efficiency after longevity propagation over several micrometres to another quantum dot. The single electron is isolated from other electrons as it is sent into a one-dimensional channel, where it is carried along on a surface acoustic wave induced by microwave excitation. McNeil et al. also show that the same electron can be transferred back and forth up to 60 times, a total distance of 0.25 millimetres. This work demonstrates a new way of transporting a single quantum particle over a long distance in nanostructures, and could pave the way for a range of quantum optics experiments and for quantum information circuits based on single electrons. Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer1 decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit2. Highly controlled tunnelling between neighbouring dots has been demonstrated3,4, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly5,6,7,8. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times—a cumulative distance of 0.25 mm—without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

Compensation of Quadrature Imbalance in an Optical QPSK Coherent Receiver

Abstract: This letter explores the Gram-Schmidt orthogonalization procedure (GSOP) for compensation of quadrature imbalance in an optical 90deg hybrid. We present computer simulations for an optical QPSK communication system using a digital coherent receiver and investigate the impact of quadrature imbalance on the required optical signal-to-noise ratio for the receiver and the frequency estimation algorithm. We then demonstrate the improvement which can be achieved using the GSOP, including the impact of quantization in the digital coherent receiver. Finally, we show that the GSOP can equally be applied to polarization-division multiplexed systems, applying the GSOP in conjunction with the constant modulus algorithm to demultiplex a PDM-QPSK signal.

Frequency of the hydrogen maser.

Abstract: THE accuracy of the unperturbed frequency of the hydrogen maser is limited by the uncertainty of the wall shift caused by the storage bulb coating1. Different samples of nominally the same material have been found to give different shifts2 and so it seems that a frequency determination should include an independent measurement of wall shift. The results of such determinations3–7, however, have a spread of 0.018 Hz which is large compared with the potential accuracy of the maser. The wall shift depends on the number of bounces of the atoms on the wall of the bulb and for a spherical bulb can be expressed as K/D where K is a constant for a particular coating material and D is the diameter of the bulb. If measurements are made with bulbs of different size and the frequency is plotted against I/D), the true frequency is that corresponding to infinite bulb size, that is 1/D = 0, and the slope of the line gives the constant K. The range of bulb sizes that can be used is limited by practical considerations and there is therefore an extrapolation error in addition to the error of measurement. This extrapolation error can be reduced by making measurements with two different coating materials and making the assumption that the values should coincide at 1/D = 0.

AES: Energy calibration of electron spectrometers. I—an absolute, traceable energy calibration and the provision of atomic reference line energies

Abstract: Atomic standards of binding energies are presented for the calibration of X-ray photoelectron spectrometers. The binding energies are measured using a VG Scientific ESCA 3 Mk II for which the photoelectron peak positions can be established with a measurement precision standard deviation of 5 meV. The absolute calibration of the voltage scale is established with an accuracy of 11 ppm over the binding energy range 0–1250 eV, using a measurement chain traceable to the NPL primary voltage standards. The zero of energy is set to ±11 meV on the differential of the Ni Fermi edge using Mg Kα12 radiation and absolute binding energies, for the Cu 2p3/2, Cu L3MM, Cu 3p, Ag 3d5/2, Ag M4NN and Au 4f7/2 peaks, are established with errors of 0.01 to 0.02 eV. An analysis is made of literature calibrations to assess their zero setting and voltage scaling errors. Preliminary tests have been made to show that the calibrations may be reproduced on instruments outside NPL using a simple work procedure.