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Gonzalez-Zalba M.F.,Hitachi Cambridge Laboratory | Barraud S.,CEA Grenoble | Ferguson A.J.,University of Cambridge | Betz A.C.,Hitachi Cambridge Laboratory
Nature Communications | Year: 2015

Quantum computation requires a qubit-specific measurement capability to readout the final state of individual qubits. Promising solid-state architectures use external readout electrometers but these can be replaced by a more compact readout element, an in situ gate sensor. Gate-sensing couples the qubit to a resonant circuit via a gate and probes the qubit's radiofrequency polarizability. Here we investigate the ultimate performance of such a resonant readout scheme and the noise sources that limit its operation. We find a charge sensitivity of 37 μe Hz-1/2, the best value reported for this technique, using the example of a gate sensor strongly coupled to a double quantum dot at the corner states of a silicon nanowire transistor. We discuss the experimental factors limiting gate detection and highlight ways to optimize its sensitivity. In total, resonant gate-based readout has advantages over external electrometers both in terms of reduction of circuit elements as well as absolute charge sensitivity. © 2015 Macmillan Publishers Limited. All rights reserved.

Sinova J.,Johannes Gutenberg University Mainz | Sinova J.,ASCR Institute of Physics Prague | Valenzuela S.O.,Catalan Institute of Nanoscience and Nanotechnology | Valenzuela S.O.,Catalan Institution for Research and Advanced Studies | And 5 more authors.
Reviews of Modern Physics | Year: 2015

Spin Hall effects are a collection of relativistic spin-orbit coupling phenomena in which electrical currents can generate transverse spin currents and vice versa. Despite being observed only a decade ago, these effects are already ubiquitous within spintronics, as standard spin-current generators and detectors. Here the theoretical and experimental results that have established this subfield of spintronics are reviewed. The focus is on the results that have converged to give us the current understanding of the phenomena, which has evolved from a qualitative to a more quantitative measurement of spin currents and their associated spin accumulation. Within the experimental framework, optical-, transport-, and magnetization-dynamics-based measurements are reviewed and linked to both phenomenological and microscopic theories of the effect. Within the theoretical framework, the basic mechanisms in both the extrinsic and intrinsic regimes are reviewed, which are linked to the mechanisms present in their closely related phenomenon in ferromagnets, the anomalous Hall effect. Also reviewed is the connection to the phenomenological treatment based on spin-diffusion equations applicable to certain regimes, as well as the spin-pumping theory of spin generation used in many measurements of the spin Hall angle. A further connection to the spin-current-generating spin Hall effect to the inverse spin galvanic effect is given, in which an electrical current induces a nonequilibrium spin polarization. This effect often accompanies the spin Hall effect since they share common microscopic origins. Both can exhibit the same symmetries when present in structures comprising ferromagnetic and nonmagnetic layers through their induced current-driven spin torques or induced voltages. Although a short chronological overview of the evolution of the spin Hall effect field and the resolution of some early controversies is given, the main body of this review is structured from a pedagogical point of view, focusing on well-established and accepted physics. In such a young field, there remains much to be understood and explored, hence some of the future challenges and opportunities of this rapidly evolving area of spintronics are outlined. © 2015 American Physical Society.

Agency: GTR | Branch: EPSRC | Program: | Phase: Training Grant | Award Amount: 5.00M | Year: 2014

Quantum technologies promise a transformation of measurement, communication and computation by using ideas originating from quantum physics. The UK was the birthplace of many of the seminal ideas and techniques; the technologies are now ready to translate from the laboratory into industrial applications. Since international companies are already moving in this area, there is a critical need across the UK for highly-skilled researchers who will be the future leaders in quantum technology. Our proposal is driven by the need to train this new generation of leaders. They will need to be equipped to function in a complex research and engineering landscape where quantum physics meets cryptography, complexity and information theory, devices, materials, software and hardware engineering. We propose to train a cohort of leaders to meet these challenges within the highly interdisciplinary research environment provided by UCL, its commercial and governmental laboratory partners. In their first year the students will obtain a background in devices, information and computational sciences through three concentrated modules organized around current research issues. They will complete a team project and a longer individual research project, preparing them for their choice of main research doctoral topic at the end of the year. Cross-cohort training in communication skills, technology transfer, enterprise, teamwork and career planning will continue throughout the four years. Peer to peer learning will be continually facilitated not only by organized cross-cohort activities, but also by the day to day social interaction among the members of the cohort thanks to their co-location at UCL.

Agency: GTR | Branch: EPSRC | Program: | Phase: Training Grant | Award Amount: 3.99M | Year: 2014

The Scottish Doctoral Training Centre in Condensed Matter Physics, known as the CM-DTC, is an EPSRC-funded Centre for Doctoral Training (CDT) addressing the broad field of Condensed Matter Physics (CMP). CMP is a core discipline that underpins many other areas of science, and is one of the Priority Areas for this CDT call. Renewal funding for the CM-DTC will allow five more annual cohorts of PhD students to be recruited, trained and released onto the market. They will be highly educated professionals with a knowledge of the field, in depth and in breadth, that will equip them for future leadership in a variety of academic and industrial careers. Condensed Matter Physics research impacts on many other fields of science including engineering, biophysics, photonics, chemistry, and materials science. It is a significant engine for innovation and drives new technologies. Recent examples include the use of liquid crystals for displays including flat-screen and 3D television, and the use of solid-state or polymeric LEDs for power-saving high-illumination lighting systems. Future examples may involve harnessing the potential of graphene (the worlds thinnest and strongest sheet-like material), or the creation of exotic low-temperature materials whose properties may enable the design of radically new types of (quantum) computer with which to solve some of the hardest problems of mathematics. The UKs continued ability to deliver transformative technologies of this character requires highly trained CMP researchers such as those the Centre will produce. The proposed training approach is built on a strong framework of taught lecture courses, with core components and a wide choice of electives. This spans the first two years so that PhD research begins alongside the coursework from the outset. It is complemented by hands-on training in areas such as computer-intensive physics and instrument building (including workshop skills and 3D printing). Some lecture courses are delivered in residential schools but most are videoconferenced live, using the well-established infrastructure of SUPA (the Scottish Universities Physics Alliance). Students meet face to face frequently, often for more than one day, at cohort-building events that emphasise teamwork in science, outreach, transferable skills and careers training. National demand for our graduates is demonstrated by the large number of companies and organisations who have chosen to be formally affiliated with our CDT as Industrial Associates. The range of sectors spanned by these Associates is notable. Some, such as e2v and Oxford Instruments, are scientific consultancies and manufacturers of scientific equipment, whom one would expect to be among our core stakeholders. Less obviously, the list also represents scientific publishers, software houses, companies small and large from the energy sector, large multinationals such as Solvay-Rhodia and Siemens, and finance and patent law firms. This demonstrates a key attraction of our graduates: their high levels of core skills, and a hands-on approach to problem solving. These impart a discipline-hopping ability which more focussed training for specific sectors can complement, but not replace. This breadth is prized by employers in a fast-changing environment where years of vocational training can sometimes be undermined very rapidly by unexpected innovation in an apparently unrelated sector. As the UK builds its technological future by funding new CDTs across a range of priority areas, it is vital to include some that focus on core discipline skills, specifically Condensed Matter Physics, rather than the interdisciplinary or semi-vocational training that features in many other CDTs. As well as complementing those important activities today, our highly trained PhD graduates will be equipped to lay the foundations for the research fields (and perhaps some of the industrial sectors) of tomorrow.

Jungwirth T.,ASCR Institute of Physics Prague | Jungwirth T.,University of Nottingham | Wunderlich J.,ASCR Institute of Physics Prague | Wunderlich J.,Hitachi Cambridge Laboratory | And 8 more authors.
Reviews of Modern Physics | Year: 2014

Over the past two decades, the research of (Ga,Mn)As has led to a deeper understanding of relativistic spin-dependent phenomena in magnetic systems. It has also led to discoveries of new effects and demonstrations of unprecedented functionalities of experimental spintronic devices with general applicability to a wide range of materials. This is a review of the basic material properties that make (Ga,Mn)As a favorable test-bed system for spintronics research and a discussion of contributions of (Ga,Mn)As studies in the general context of the spin-dependent phenomena and device concepts. Special focus is on the spin-orbit coupling induced effects and the reviewed topics include the interaction of spin with electrical current, light, and heat. © 2014 American Physical Society.

Roy P.E.,Hitachi Cambridge Laboratory | Wunderlich J.,Hitachi Cambridge Laboratory | Wunderlich J.,ASCR Institute of Physics Prague
Applied Physics Letters | Year: 2011

Current and field-induced domain wall motion in a perpendicular material system having a tuneable magnetocrystalline in-plane anisotropy constant is micromagnetically simulated. It is shown that it is possible to significantly modulate critical current densities, Walker fields, and thus the domain wall velocity. In addition, suggestions that may help in determining the non-adiabatic spin transfer torque term are shown. © 2011 American Institute of Physics.

Jungwirth T.,ASCR Institute of Physics Prague | Jungwirth T.,University of Nottingham | Wunderlich J.,ASCR Institute of Physics Prague | Wunderlich J.,Hitachi Cambridge Laboratory | And 2 more authors.
Nature Materials | Year: 2012

The spin Hall effect is a relativistic spin-orbit coupling phenomenon that can be used to electrically generate or detect spin currents in non-magnetic systems. Here we review the experimental results that, since the first experimental observation of the spin Hall effect less than 10 years ago, have established the basic physical understanding of the phenomenon, and the role that several of the spin Hall devices have had in the demonstration of spintronic functionalities and physical phenomena. We have attempted to organize the experiments in a chronological order, while simultaneously dividing the Review into sections on semiconductor or metal spin Hall devices, and on optical or electrical spin Hall experiments. The spin Hall device studies are placed in a broader context of the field of spin injection, manipulation, and detection in non-magnetic conductors. © 2012 Macmillan Publishers Limited. All rights reserved.

Agency: GTR | Branch: EPSRC | Program: | Phase: Research Grant | Award Amount: 533.48K | Year: 2013

If a quantum computer can be realised over the next couple of decades it would adversely affect secure communications which are vital in the financial, military and diplomatic sectors, since current encryption methods will be vulnerable to hacking. Totally secure systems, based on quantum mechanics and in particular the properties of single photons, have been investigated for some years and a few working systems demonstrated. These systems operate at wavelengths which took advantage of available single photon detectors but unfortunately coincide with moderately high absorption in conventional optical fibres. If such quantum key distribution systems based on single photon sources are to be commercially successful then the operating wavelength needs to move to the 1300nm or 1500nm range. This is certainly possible using quantum dots as the photon emitter and recent advances in the quality of single photon detectors at these wavelengths means that detection of the single photons should no longer be an impediment. A major problem is efficient extraction of the single photons produced by the dot. Placing the dot in a photonic crystal cavity (PhC) structure overcomes this difficulty allowing only photons that coincide with an optical mode of the PhC to be emitted along the normal to the surface of the device. This is far from trivial since the dot must be located at the centre of the PhC (to within a few nanometers) and the dot photons must be nearly identical in wavelength to the mode of the cavity or at least be able to be tuned to the same wavelength. Here we use methods familiar to the electronics industry to identify the dot position with metal film markers and use these to fabricate the PhC by drilling a particular pattern of holes in the sample. When this is done correctly the dot and cavity are said to be strongly coupled, meaning that a photon emitted by the dot will always be emitted in the normal direction and can subsequently be collected by an optical fibre. Single photon sources are also crucial components of quantum information networks. Here photons are used like electrons in electrical circuits and are guided along waveguides (which can easily be incorporated into the PhC structures mentioned above) and used to perform logic operations. There are many possible schemes for such networks but in this proposal we hope to demonstrate some basic manipulations of photons using only a few PhC cavities connected by a waveguide and strongly coupled to one or perhaps two dots. This would represent the very first steps towards a possible photonics network. Drawing an analogy with electrical circuits again this would represent the very early stages of an integrated circuit.

Roy P.E.,Hitachi Cambridge Laboratory | Trypiniotis T.,University of Cambridge | Barnes C.H.W.,University of Cambridge
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Spin normal modes of a 360° domain wall trapped in a stripe are investigated by micromagnetic simulations, for two in-plane field excitation directions. Within the substructure of the 360° domain wall, we identify highly localized, low-frequency spin-wave well-type modes on the lateral boundaries and in the surrounding subdomain walls. At intermediate frequencies, oscillations distributed over a range of modes located in different regions of the substructure's domain walls are found. Higher-frequency oscillations such as quasiuniform and multinodal spin-wave modes are identified inside each of the constituent subdomains. Mode splitting due to interaction with traveling spin waves is found. Indications are that domain mode quantization due to confinement effects induced by the constituent domain walls are relatively weak. Strong hybridization between domain and domain-wall modes are likely to occur within a certain frequency interval. Finally, we resonantly drive the system at the four lowest frequency modes, whereby the in-plane behavior of the 360° structure displays translational, breathing, and wobbling motions. © 2010 The American Physical Society.

Roy P.E.,Hitachi Cambridge Laboratory
Applied Physics Letters | Year: 2013

The effect of an in-plane induced uniaxial anisotropy on the magnetic vortex gyrotropic frequency is investigated by micromagnetic simulations, exemplified by the inverse magnetostriction in a uniaxially stressed circular dot. It is found that the gyrotropic frequency decreases with increasing magnitude of the induced uniaxial anisotropy. The results are analyzed by extracting the restoring forces from the vortex dynamical potential-well. The dominant contribution to the decreasing trend in frequency is found to be due to a softening of both the restoring force spring constants. This work offers an alternative method to control the gyrotropic frequency of a magnetic vortex. © 2013 AIP Publishing LLC.

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