National Physical Laboratory

Quantum Current Standards Quantum Current Standards

NPL is developing nano-scale devices for moving electrons one at a time around an electrical circuit. These devices may form the foundation of a future redefinition of the SI base unit for current, the ampere.

Scientists in the Quantum Detection Group are attempting to perfect the control of electrical charge using nano-devices to trap and manipulate single electrons. These techniques can be used to generate very precise electrical currents, which one day might be used to represent the unit of current, the ampere. QCSSingle electron devices may also be a building block in future quantum circuitry and can be used to test our understanding of the laws of quantum mechanics.


People working on project

Masaya Kataoka
Stephen Giblin
Jonathan Fletcher
JT Janssen
Patrick See
Ateeq Nasir
Malcolm Connolly
Nathan Johnson (LCN, UCL)
Pardis Sahafi (RHUL)


Recent results


Clock-controlled emission of single-electron wavepackets in a solid-state circuit

Single-electron partitioning (artist's impression)
Single-electron partitioning (artist's
impression)

The electron ejection process is critical for the of electron pumps in the field of Fermionic Quantum Optics (FQO) - the demonstration of quantum-optics type phenomenon in solid state systems. The suitability of electron pumps for FQO depends on exactly how they are ejected and how they propagate through the system. The electron energy distribution, ejection time and propogation length are the important parameters which determine whether quantum effects can be observed.

In a paper published in Physical Review Letters (see also Physics Focus), we have developed a time and energy resolved technique to probe and interact with the ejected electrons. We find that the individual electrons travel through quantum hall edge channels carrying a remarkably large amount of energy (more than 100 meV). Dissipation via phonon emission can be seen in some circumstances, but these processes are very weak at higher magnetic fields. We also probe the arriving single-electron 'pulse' in the time domain, establishing an upper limit on the time distribution of the electrons of ~80ps. The high temporal resolution of our detection system is neatly demonstrated by the dynamical partitioning of two electrons ejected from the pump during the same pumping cycle; we detect the first and second electron separately and redirect these into different output channels based on the small ~350ps difference in ejection time.

Clock-Controlled Emission of Single-Electron Wave Packets in a Solid-State Circuit
J. D. Fletcher, P. See, H. Howe, M. Pepper, S.P. Giblin, J.P. Griffiths, G.A.C. Jones, I. Farrer, D.A. Ritchie, T.J.B.M. Janssen, and M. Kataoka
Phys. Rev. Lett., 111, 216807 (2013)

See also:


One part per million measurements of a high speed electron pump, driven with a customised waveform

This customised waveform speeds up electron ejection, allowing more time to be spent on accurate pump loading
This customised waveform speeds up electron ejection,
allowing more time to be spent on accurate pump loading

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 ppm. 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.

Towards a quantum representation of the ampere using single electron pumps
S.P. Giblin, M. Kataoka, J.D. Fletcher, P. See, T.J.B.M. Janssen, J.P. Griffiths, G.A.C. Jones, I. Farrer and D.A. Ritchie
Nature Communications, 3, 930 (2012)



Explaining the stabilisation of electron pumps in high magnetic fields

Magnetic-field-driven accuracy improvements in a pump operating at 400 MHz (data are offset vertically for clarity)
Magnetic-field-driven accuracy improvements in a pump
operating at 400 MHz (data are offset vertically for clarity)

It was has been observed experimentally that the quantized current produced by GaAs devices can be massively enhanced by applying very large magnetic fields. While this enhancement has been critical in the levels of accuracy significant results, the origin of this effect has yet to be explained.

We have studied the way in which magnetic fields influence the tunneling dynamics in these and show how large magnetic fields can reduce back-tunneling errors by more than five orders of magnitude.

Numerical calculations, performed in collaboration with scientists at KAIST, show that the back-tunnelling rates in the pump, which determine how many electrons are trapped, are sensitive to magnetic field in an interesting way - magnetic fields enhance the sensitivity of tunnelling rates to the confinement barriers, stabilizing the number of electrons trapped.

We also find that the 'quantum spillage' of electrons through non-adiabatic processes has a distinctive magnetic field dependence, with a strong suppression of excitations at high field. This is may be an important observation on the interplay between magnetic and electrostatic effects in confined systems subject to rapid perturbations

Stabilization of single-electron pumps by high magnetic fields
J.D. Fletcher, M. Kataoka, S.P. Giblin, Sunghun Park, H.-S. Sim, P. See, D.A. Ritchie, J.P. Griffiths, G.A.C. Jones, H.E. Beere, and T.J.B.M. Janssen
Phys. Rev. B, 86, 155311 (2012)

See also: Effect of magnetic fields on high accuracy single-electron pumps


Quantum spillage in single electron pump

At higher frequency the rapid squeezing of electrons in the trap causes excitations, which can be detected by the step-like features (vertical stripes) which destroy quantization accuracy
At higher frequency the rapid squeezing of electrons
in the trap causes excitations, which can be detected
by the step-like features (vertical stripes) which destroy
quantization accuracy

Our single-electron pumps generate a current by repeatedly trapping and releasing individual electrons. For use in quantum current standards, operation at high speed is desirable. However, the fundamental laws of quantum mechanics (first predicted in the 1920s) dictate how fast an electronic wavefunction can be perturbed without disturbing the system from its ground state.

We have discovered that these 'nonadiabatic' effects can lead to 'quantum spillage' of electrons from our electron pumps. These electrons escape from our trap like water spilled from a coffee cup, which has a detrimental effect on the accuracy of the pump.

Interestingly, this actually gives us a novel way of looking at the electronic states in the dot. Fortunately, we have also found that high magnetic fields can be used to control the spillage. At high magnetic fields we can effectively 'stiffen' the electronic wavefunction, which protects it from the external perturbation.

Tunable Nonadiabatic Excitation in a Single-Electron Quantum Dot
M. Kataoka, J. D. Fletcher, P. See, S.P. Giblin, T.J.B.M. Janssen, J.P. Griffiths, G.A.C. Jones, I. Farrer, and D.A. Ritchie
Phys. Rev. Lett., 106, 126801 (2011)

See also: Quantum Spillage in a Single Electron Pump

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