Nanoelectronic Based Quantum Physics- Technology and Applications.

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology

Abstract

Electrons flowing through semiconductor devices are of immense importance in modern life. When devices are made sufficiently small, such that one of the dimensions is in the nanometre regime, the quantum nature of the electron comes to the fore and must be considered in detail. Working at very low temperatures reduces the mutual electron-electron scattering and results in the wave nature of electron transport becoming observable over distances which can exceed the size of the device. Experiments using devices which are smaller than the coherence length of the wavefunction, or the distance between impurity scattering events, have allowed observation of a range of quantum effects.
In recent years theories have proposed that a "quantum computer" has certain advantages over conventional computers as they allow a massively parallel mode of operation. This is based on quantum principles, thus if two electrons are in a quantum state then their total spin wavefunction reflects the range of possible states that can be present. It is this superposition of states which is the basis of a quantum computer. It is a purely quantum phenomenon and has given rise to concepts such as "Schrodinger's Cat" which exemplify the non-intuitive nature of quantum mechanics. Another property which could give rise to new technological applications is the remarkable entanglement. This purely quantum effect results in two electrons being in the same quantum state and "knowing" about each other's existence, consequently if the spin of one is rotated then the spin of the other is affected despite there being a considerable distance between them.
In this work we propose to utilise semiconductor nanostructures to find new quantum effects and combine them to create integrated quantum circuits for practical exploitation. The project integrates theory, semiconductor growth/fabrication and measurements in three different centres, it has as initial targets the design and fabrication of key quantum components forsubsequent integration. A principal component is the Quantum Pump which can transmit controlled numbers of electrons at high frequencies with very high accuracy. This device can be used for the generation of entangled electrons which can then be investigated and put to use. Another component which is of importance is the electronic analogy of the polarising beam splitter in optics, here by using localised electron spins an incoming electron is either transmitted or reflected depending on its spin direction. We also propose to exploit the spin-orbit coupling which allows a spin polarised current to be established in a nanostructure which can then be utilised in a quantum device.
It is further proposed to build on the use of an indirect electron interaction mechanism to transmit spin information between different devices. A system which may have novel properties in this regard is the incipient Wigner lattice which can form when a line of electrons is weakly confined and minimisation of the electron-electron repulsion forces the electrons to form two separate rows. Here they can be entangled and constitute a continuous supply of entangled electrons in a manner which is complementary to the pump.
New types of quantum components will be developed. They will then be integrated to form an early type of circuit in which quantum effects dominate the properties. It is intended to develop basic quantum processors in particular a CNOT gate in which the spin of an electron is rotated depending on the direction of the spin of another. In addition to these objectives a number of spin-off achievements will have an impact on other fields. For example it will be necessary to develop techniques of measurement of electronic properties at ultra low temperatures, 1 milliKelvin, and the spin polarised currents to be developed will have applications in the important field of spintronics.

Planned Impact

Knowledge, the work will impact on the quantum information and spintronics communities as both are seeking new concepts and results. Demonstrating how quantum integration can bring new rules of electron transport in nanostructures will be of great interest to the user community of theoretical physicists. Our growth programme aimed at higher quality of interfaces and the use of undoped layers will be of significance to Material Scientists and Device Engineers, as improvements in semiconductor growth can be implemented in the fabrication of high frequency GaAs devices used in mobile phones and other applications. Other communities who will benefit from successful development of quantum gates and computation include those involved in cryptography,(companies and government establishments), and the medical/biological community who wish to search large amounts of data very quickly, for example crystallographic data bases.
The work has an influence on the Economy, success in the experimental programme will stimulate sales of low temperature equipment and associated instrumentation. Sales of cryogenic equipment have resulted from our earlier work which was taken up internationally, (see letter of support), and successful development of the concepts outlined in this proposal could result in furthering this trend. The growth in the study of low dimensional systems in recent years has fuelled growth of MBE and processing equipment as well as cryogenic systems. Quantum computational facilities could be centrally based, for example within a university department, government facilities, bank headquarter, etc.
The industrial partners in the project have had long standing and successful collaborations with us and we will maximise opportunities by involving other companies as the project progresses. Close interactions with the commercial departments of our institutions will ensure that every opportunity is taken to file patents and protect IPR in general. An Opportunities Committee will involve industrial collaborators and academics in maximising commercial value and the PDRA's and research students will be given training in Intellectual Property. Between them the investigators have established four companies and filed over 25 patents, they intend to implement a policy of industrial involvement which depends on the nature of the collaboration and could include joint patents and the academic institutions setting up spin-off companies.
People are crucial to success. The extent of this project justifies a policy of moving the experimental PDRA's between the centres to broaden their expertise and maximise their contribution. This is very important in the context of training given the global competition for skilled personnel and the increasing international competition in new technologies. There will be spin-off and further grants as a result of this work with further opportunity for training and broadening expertise.
Within Society as a whole, here is considerable interest in sections of the public about the implications of relativity and quantum theory such as entanglement, as well as the significance for understanding the universe given by concepts such as superposition of states. Recent books on Dirac and quantum concepts have enjoyed success and the work in this project will be of considerable interest to this community. Investigators have given general lectures on their research and will continue to seek every opportunity of doing so in the future as part of an outreach policy. As discussed in the proposal, investigators have appeared in the media and will collaborate with a former physicist, now a well-known media figure, in producing a science programme for TV which will seek to explain quantum concepts to the public. In the past members of the consortium have been on committees giving advice to Government and one of us (MJK) has acted as a Chief Scientific Adviser. We would be pleased to continue this assistance

Publications

10 25 50
 
Description The research programme is based around the use of semiconductor nanostructures for investigating new quantum effects. We have made considerable although the milestones are presently being redefined but at the time of writing it appears as if a very significant discovery has been made in the last few weeks after the EPSRC sponsored review of our work. Essentially we have found that in a narrow, one dimensional system the two spin directions repel each other and so separate in energy. This is known to occur in other systems but the novel features are that we have found a length dependence to the spin separation and that the observation of it may depend on the shape of the injector. This was not anticipated and could be of great significance to quantum information schemes.
We are also developing electron pumps which may well be important in defining an important standard - the fundamental electron charge.In addition to previous entries we have further investigated the crossover in energy levels due to the electron-electron interaction and fully characterised the interaction and dependence on the form of the confinement potential.We have also discovered fractional quantisation in the absence of a magnetic field which was unanticipated and is being pursued.
We have also been able to show how to control spin rotations by injecting electrons between one and two dimensional regions. This spin transistor which makes use of the spin-orbit interaction in semiconductors has been predicted but never observed before.
Despite a slow start for administrative reasons the project is now producing important results and most recently we have discovered that the interaction time of electrons in a quasi one-dimensional channel determines the degree of spin polarisation in the electron gas. By non-adiabatically injecting electrons into a 2D electron gas polarisation of nuclei can be accomplished. This development was unexpected and its potential for quantum information is being assessed.In addition to previous entries we have further investigated the crossover in energy levels due to the electron-electron interaction and fully characterised the interaction and dependence on the form of the confinement potential.We have also discovered fractional quantisation in the absence of a magnetic field which was unanticipated and is being pursued.
Having found fractional quantisaation of conductance in the absence of a magnetic field exhibited by holes in Ge we have now gone on to find after a search the same effect exhibited by electrons in GaAs.
Exploitation Route We are of the opinion that the novelty of the effects here presage their observation in other semiconductor systems and may also be exploited in schemes of quantum computation not yet considered. The fractional charge which we have found could be of importance in future topological quantum computation.
As a result of the previous findings we extended our search for novel fractional behaviour and found a range of fractions exhibited by electrons in the absence of a magnetic field in GaAs. We anticipate that this may give rise to a new area of strong electron interactions and perhaps lead to new schemes of quantum computation based on topological considerations.
We are assessing data and will apply for a new grant to further study these effects.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics

URL https://physicsworld.com/a/holes-reveal-first-fractional-quantization/
 
Description As a result of progress a new CASE award has been agreed with Toshiba Research Europe. Publicity is being prepared by the Institute of Physics on the discovery of Fractional Quantisation. This publicity appeared, https://physicsworld.com/a/holes-reveal-first-fractional-quantization/ As a result of this work we launched an investigation into the properties of electrons in high mobility GaAs and found that new fractional effects appeared here. We think that these findings will open a new area of investigation into strong interactions with a possible application in quantum computation.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics
Impact Types Economic

 
Title Research data supporting "Reappearance of linear hole transport in an ambipolar undoped GaAs/AlGaAs quantum well" 
Description Low-temperature electrical transport data for an ambipolar field-effect-transistor based on an undoped GaAs/AlGaAs quantum well. The data supports the Journal of Physics: Condensed Matter article, 'Reappearance of linear hole transport in an ambipolar undoped GaAs/AlGaAs quantum well.' The data were collected at the Cavendish Laboratory, University of Cambridge, in the period February 2015 to September 2015. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Description CASE award with National Physical Laboratory, (NPL). Commencing 2016 
Organisation National Physical Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution Sample design, fabrication facilities, measurements, concepts
Collaborator Contribution Approximately £60K over the course of the Ph.D studentship
Impact Award only just commenced.
Start Year 2016
 
Description PhD Studentship Oxford NanoScience 
Organisation Oxford Instruments
Department Oxford Nanoscience
Country United Kingdom 
Sector Private 
PI Contribution Development of ULT methods and noise thermometry for possible products.
Collaborator Contribution 50% contribution to three year PhD studentship (HvdV) at RHUL.
Impact Visits by PhD student to company factory at Tubney Woods.
Start Year 2012
 
Description Theory of Electron Transport in Semiconductor Nanostructures and Collaborative Experiments 
Organisation University of Liverpool
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration and explanation of experiments as well as suggestions of new experiments. These collaborations started before this award but are ongoing and have contributed to the work in the award.
Collaborator Contribution Many ideas and experimental contributions. Allocation of a Dilution Refrigerator in Cape Town for the experimental programme. Dr Blumenthal who is now in CapeTown took a Ph.D in Cambridge and was supervised by Pepper, he is now a lecturer there and has allocated a dilution refrigerator to our collaboration on electron pumps. We have an excellent collaboration with the Taiwan group which has provided new insights into spin-orbit coupling. The Bar Ilan and CUNY collaborations are in the area of theory and have produced some important insights into the behaviour of electrons in nanostructures which is continuing.
Impact 4 good publications
 
Description Theory of Electron Transport in Semiconductor Nanostructures and Collaborative Experiments 
Organisation University of Liverpool
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration and explanation of experiments as well as suggestions of new experiments. These collaborations started before this award but are ongoing and have contributed to the work in the award.
Collaborator Contribution Many ideas and experimental contributions. Allocation of a Dilution Refrigerator in Cape Town for the experimental programme. Dr Blumenthal who is now in CapeTown took a Ph.D in Cambridge and was supervised by Pepper, he is now a lecturer there and has allocated a dilution refrigerator to our collaboration on electron pumps. We have an excellent collaboration with the Taiwan group which has provided new insights into spin-orbit coupling. The Bar Ilan and CUNY collaborations are in the area of theory and have produced some important insights into the behaviour of electrons in nanostructures which is continuing.
Impact 4 good publications
 
Description Theory of Electron Transport in Semiconductor Nanostructures and Collaborative Experiments 
Organisation University of the Balearic Islands
Department Physics
Country Spain 
Sector Academic/University 
PI Contribution Collaboration and explanation of experiments as well as suggestions of new experiments. These collaborations started before this award but are ongoing and have contributed to the work in the award.
Collaborator Contribution Many ideas and experimental contributions. Allocation of a Dilution Refrigerator in Cape Town for the experimental programme. Dr Blumenthal who is now in CapeTown took a Ph.D in Cambridge and was supervised by Pepper, he is now a lecturer there and has allocated a dilution refrigerator to our collaboration on electron pumps. We have an excellent collaboration with the Taiwan group which has provided new insights into spin-orbit coupling. The Bar Ilan and CUNY collaborations are in the area of theory and have produced some important insights into the behaviour of electrons in nanostructures which is continuing.
Impact 4 good publications
 
Description Theory of Electron Transport in Semiconductor Nanostructures and Collaborative Experiments 
Organisation University of the Balearic Islands
Department Physics
Country Spain 
Sector Academic/University 
PI Contribution Collaboration and explanation of experiments as well as suggestions of new experiments. These collaborations started before this award but are ongoing and have contributed to the work in the award.
Collaborator Contribution Many ideas and experimental contributions. Allocation of a Dilution Refrigerator in Cape Town for the experimental programme. Dr Blumenthal who is now in CapeTown took a Ph.D in Cambridge and was supervised by Pepper, he is now a lecturer there and has allocated a dilution refrigerator to our collaboration on electron pumps. We have an excellent collaboration with the Taiwan group which has provided new insights into spin-orbit coupling. The Bar Ilan and CUNY collaborations are in the area of theory and have produced some important insights into the behaviour of electrons in nanostructures which is continuing.
Impact 4 good publications
 
Description Schools lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact A one hour evening lecture was given in the main lecture hall on campus, pitched at the level of a secondary school student.
Most of the attendees were from the local area, and included student of secondary school age.
Year(s) Of Engagement Activity 2015