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Single-Hole Pumping in Silicon

Periodic Reporting for period 1 - SINHOPSI (Single-Hole Pumping in Silicon)

Reporting period: 2016-01-11 to 2018-01-10

In order to perform measurements of physical quantities, record and compare them in a consistent way, systems of units and standards have been historically developed and agreed upon. At first, the reference standards were based on artefacts or prototypes, but over the last few decades the need for increasingly high accuracy and stability has determined a shift towards standards based on physical phenomena and true constants of nature. Nanoscale systems based on the principles of quantum mechanics are today acknowledged as the most stable and reliable metrological tools, as they can be strongly intertwined with fundamental constants. Exquisitely quantum-mechanical phenomena such as the Josephson Effect and the Quantum Hall Effect have paved the way toward new and more reliable reference standards for the units of voltage and resistance, respectively. Presently, a major research challenge is to find a realization of the electric current in terms of elementary charge, which is the charge of an electron. This will lead to a quantum-based standard for the unit of current, i.e. the ampere.
At present, vast research efforts exploit the discreteness of single electrons to transfer a controlled number of elementary charges and, thus, achieve highly accurate currents. Electronic nanodevices that allow single-electron clocking are known as charge pumps and are routinely used as stable sources of small currents. The most successful generation of these devices makes use of electrostatically-defined quantum dots, which are small regions of semiconductor material where individual electrons can be trapped and released at will. The main identified challenges of this approach are the deterministic initialization of the number of electrons in the dot, and the suppression of pumping errors as the operational speed rises to increase the level of the generated current. Both instances are ultimately related to the electron confinement in the quantum dot. In practice, it has been shown that the tighter the charge confinement within the dot, the better the pump’s performance can be.

The aim of this project is to develop novel nanotechnology to address the issue of confinement in single-charge pumps and, therefore, advance the state-of-the-art of highly accurate current generation. The main aim is to realise ambipolar silicon-based pumps with industrially-compatible metal-oxide-semiconductor technology. Tunable confinement will be attained by means of purposely designed gate electrodes, as well as by implementing controlled transfers of single electrons and single holes at will. By comparing the device performance in different operation modes, detailed information on the effect of confinement will be gathered. Furthermore, conventional transport-based measurement techniques will be validated via a nascent remote sensing technique called radiofrequency reflectometry.

The project has been successful in producing novel nanotechnology and improve the state-of-the-art of single-charge electronics. In particular, a factor of 3 improvement in the accuracy of current generation has been demonstrated via silicon single-electron pumps. Furthermore, the state-of-the-art of charge detection sensitivity with gate-based reflectometry techniques has been improved by a factor of about 30.
Main results:
SINHOPSI was a collaborative effort among researchers at Cambridge University, the National Physical Laboratory (NPL) and Hitachi Ltd. Experimental demonstrations were made that advanced the state-of-the-art of both accurate electric current generation and single-charge remote sensing.
The high-accuracy experiments were performed with silicon single-charge pumps specifically tailored to achieve unprecedented levels of electron confinement. In fact, nanodevices were designed and manufactured with a quantum dot charging energy in excess of 25 meV. The underlying reason for the realisation of devices with these specifications was the possibility of increasing the driving speed of the pump due to an improved transfer error rejection. The experiments showed that, by driving a quantum dot at the frequency of 1.0 GHz, electrons can be individually pumped with an uncertainty as low as 0.27 parts per million. This was a record performance for quantised current generation in silicon, and a landmark achievement in the field of quantum electrical metrology. Notably, such a high accuracy was for the first time obtained with a device operating in the thermal regime, which is compatible with high temperature operation. This augurs well for the future realisation of a cheap and practical current standard, which will not need to rely on sophisticated cryogenic apparatuses to cool the sample down to millikelvin temperature.
Another important experiment demonstrated the enhancement of charge readout sensitivity for gate-based reflectometry techniques. It was shown that, by reducing the losses in the resonant circuit coupled to the quantum dot under test, one can achieve sensitivity levels of about 1.3 e/√Hz (nearly a factor of 30 better than the state-of-the-art). This outcome may pave the way for the implementation of time-resolved gate-based reflectometry readout.

Exploitation and Dissemination:
The results of SINHOPSI have been exploited for producing peer-reviewed scientific publications, as well as for collaborative research grant proposals.
Three scientific papers have already been published, and two more manuscripts are currently under review. The results of the project have also been disseminated through presentations at international conferences, as well as via outreach activities. Finally, a number of research proposals have been put forward with British and European agencies. Some of these have already led to the generation of research income.
Impact produced:
1) Impact on people’s skills:
The Fellow benefitted from the training he received during the secondment at NPL and through interactions with scientists at the host institution, as well as at the Hitachi labs. Furthermore, two postgraduate students were directly involved in the activities of SINHOPSI. The Fellow contributed toward training and transfer of knowledge to the students.
2) Impact on European priorities:
The main objective of this project was the development of novel technology relevant to quantum systems for metrological applications. In this respect, the Action contributed to the long-term strategy of the EURAMET to “develop a joint, coherent and efficient European metrology landscape at internationally competitive level by 2025”. A European accredited calibration laboratory has benefitted from the nanoelectronic device technology generated in this project and used it to improve the uncertainty of traceability measurements.
The technology developed in this project is also relevant to the area of quantum computing. As such, this project’s activities are in line with the strategic priorities of the European Cloud Initiative “to unlock the full potential of quantum technologies and accelerate their development and take-up into commercial products in Europe”. In particular, this Action has directly contributed to advance the state-of-the-art of quantum sensing measurement techniques.
3) Impact on industrial needs:
As a leading manufacturer of consumer electronics, Hitachi had a strategic commercial and research interest in the silicon technology developed as a result of SINHOPSI.
Illustration of a single-charge pump realised with multi-gate metal-oxide-semiconductor technology