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  • Quantum advantage in postselected metrology


    In every parameter-estimation experiment, the final measurement or the postprocessing incurs a cost. Postselection can improve the rate of Fisher information (the average information learned about an unknown parameter from a trial) to cost. We show that this improvement stems from the negativity of a particular quasiprobability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, our quasiprobability distribution is real and nonnegative. In a quantum-mechanically noncommuting theory, nonclassicality manifests in negative or nonreal quasiprobabilities. Negative quasiprobabilities enable postselected experiments to outperform optimal postselection-free experiments: postselected quantum experiments can yield anomalously large information-cost rates. This advantage, we prove, is unrealizable in any classically commuting theory. Finally, we construct a preparation-and-postselection procedure that yields an arbitrarily large Fisher information. Our results establish the nonclassicality of a metrological advantage, leveraging our quasiprobability distribution as a mathematical tool. Article


  • Optical Gating of Photoluminescence from Color Centers in Hexagonal Boron Nitride


    We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2 eV. Resonant, or quasi-resonant, excitation of photoluminescence also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Repumping back into the bright state has a step-like spectrum with a defect-dependent threshold between 2.25 and 2.6 eV. This behavior is consistent with photoionization and charging between optically bright and dark states of the defect. Furthermore, a second zero phonon line, detuned by +0.4 eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center. Article


  • Imaging and writing magnetic domains in the non-collinear antiferromagnet Mn3Sn


    Non-collinear antiferromagnets are revealing many unexpected phenomena and they became crucial for the field of antiferromagnetic spintronics. To visualize and prepare a well-defined domain structure is of key importance. The spatial magnetic contrast, however, remains extraordinarily difficult to be observed experimentally. Here, we demonstrate a magnetic imaging technique based on a laser induced local thermal gradient combined with detection of the anomalous Nernst effect. We employ this method in one the most actively studied representatives of this class of materials Mn3Sn. We demonstrate that the observed contrast is of magnetic origin. We further show an algorithm to prepare a well-defined domain pattern at room temperature based on heat assisted recording principle. Our study opens up a prospect to study spintronics phenomena in non-collinear antiferromagnets with spatial resolution. Article


  • Polaron spin dynamics in high-mobility polymeric semiconductors


    Polymeric semiconductors exhibit exceptionally long spin lifetimes, and recently observed micrometre spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin-orbit and hyperfine interactions lie at the origin of their long spin lifetimes, but the coupling mechanism of a spin to its environment remains elusive. Here, we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges' hopping motion at low temperatures, whereas an Elliott-Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime, charge, spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains. Article


  • A CMOS dynamic random access architecture for radio-frequency readout of quantum devices


    As quantum processors become more complex, they will require efficient interfaces to deliver signals for control and readout while keeping the number of inputs manageable. Complementary metal-oxide-semiconductor (CMOS) electronics offers established solutions to signal routing and dynamic access, and the use of a CMOS platform for the qubits themselves offers the attractive proposition of integrating classical and quantum devices on-chip. Here, we report a CMOS dynamic random access architecture for readout of multiple quantum devices operating at millikelvin temperatures. Our circuit is divided into cells, each containing a control field-effect transistor and a quantum dot device, formed in the channel of a nanowire transistor. This set-up allows selective readout of the quantum dot and charge storage on the quantum dot gate, similar to one-transistor-one-capacitor (1T-1C) dynamic random access technology. We demonstrate dynamic readout of two cells by interfacing them with a single radio-frequency resonator. Our approach provides a path to reduce the number of input lines per qubit and allow large-scale device arrays to be addressed. Article


  • Long spin diffusion lengths in doped conjugated polymers due to enhanced exchange coupling


    Carbon-based semiconductors such as conjugated organic polymers are of potential use in the development of spintronic devices and spin-based information processing. In particular, these materials offer a low spin-orbit coupling strength due to their relatively light constituent chemical elements, which should, in principle, favour long spin diffusion lengths. However, organic polymers are relatively disordered materials and typically have a carrier mobility that is orders of magnitude lower than crystalline inorganic materials. As a result, small spin diffusion lengths of around 50nm have typically been measured using vertical organic spin valves. Here, we report measuring spin diffusion lengths in doped conjugated polymers using a lateral spin transport device architecture, which is based on spin pumping injection and inverse spin Hall detection. The approach suggests that long spin diffusion lengths of more than 1μm and fast spin transit times of around 10ns are possible in conjugated polymer systems when they have a sufficiently high spin density (around 10^20 cm-3). We explain these results in terms of an exchange-based spin diffusion regime in which the exchange interactions decouple spin and charge transport. Article


  • Gate-based single-shot readout of spins in silicon


    Electron spins in silicon quantum dots provide a promising route towards realizing the large number of coupled qubits required for a useful quantum processor. For the implementation of quantum algorithms and error detection, qubit measurements are ideally performed in a single shot, which is presently achieved using on-chip charge sensors, capacitively coupled to the quantum dots. However, as the number of qubits is increased, this approach becomes impractical due to the footprint and complexity of the charge sensors, combined with the required proximity to the quantum dots. Alternatively, the spin state can be measured directly by detecting the complex impedance of spin-dependent electron tunnelling between quantum dots. This can be achieved using radiofrequency reflectometry on a single gate electrode defining the quantum dot itself, significantly reducing the gate count and architectural complexity, but thus far it has not been possible to achieve single-shot spin readout using this technique. Here, we detect single electron tunnelling in a double quantum dot and demonstrate that gate-based sensing can be used to read out the electron spin state in a single shot, with an average readout fidelity of 73%. The result demonstrates a key step towards the readout of many spin qubits in parallel, using a compact gate design that will be needed for a large-scale semiconductor quantum processor. Article


  • Electrically induced and detected Néel vector reversal in a collinear antiferromagnet


    Antiferromagnets are enriching spintronics research by many favorable properties that include insensitivity to magnetic fields, neuromorphic memory characteristics, and ultra-fast spin dynamics. Designing memory devices with electrical writing and reading is one of the central topics of antiferromagnetic spintronics. So far, such a combined functionality has been demonstrated via 90° reorientations of the Néel vector generated by the current-induced spin orbit torque and sensed by the linear-response anisotropic magnetoresistance. Here we show that in the same antiferromagnetic CuMnAs films as used in these earlier experiments we can also control 180° Néel vector reversals by switching the polarity of the writing current. Moreover, the two stable states with opposite Néel vector orientations in this collinear antiferromagnet can be electrically distinguished by measuring a second-order magnetoresistance effect. We discuss the general magnetic point group symmetries allowing for this electrical readout effect and its specific microscopic origin in CuMnAs. Article


  • Japanese companies in the UK


    Introductory videos aiming at the UK parliament have been realised and describes the role of Japanese companies in the UK economy. The following videos also introduce interviews realised at the Hitachi Cambridge Laboratory. Youtube video Youtube video


  • Conditional Dispersive Readout of a CMOS Single-Electron Memory Cell


    Quantum computers require interfaces with classical electronics for efficient qubit control, measurement, and fast data processing. Fabricating the qubit and the classical control layer using the same technology is appealing because it will facilitate the integration process, improving feedback speeds and offering potential solutions to wiring and layout challenges. Integrating classical and quantum devices monolithically, using complementary metal-oxide-semiconductor (CMOS) processes, enables the processor to profit from the most mature industrial technology for the fabrication of large-scale circuits. We demonstrate a CMOS single-electron memory cell composed of a single quantum dot and a transistor that locks charge on the quantum-dot gate. The single-electron memory cell is conditionally read out by gate-based dispersive sensing using a lumped-element LC resonator. The control field-effect transistor (FET) and quantum dot are fabricated on the same chip using fully depleted silicon-on-insulator technology. We obtain a charge sensitivity of dq=95x10-6eHz-1/2 when the quantum-dot readout is enabled by the control FET, comparable to results without the control FET. Additionally, we observe a single-electron retention time on the order of a second when storing a single-electron charge on the quantum dot at millikelvin temperatures. These results demonstrate first steps towards time-based multiplexing of gate-based dispersive readout in CMOS quantum devices opening the path for the development of an all-silicon quantum-classical processor. Article


  • Current polarity-dependent manipulation of antiferromagnetic domains


    Antiferromagnets have several favourable properties as active elements in spintronic devices, including ultra-fast dynamics, zero stray fields and insensitivity to external magnetic fields. Tetragonal CuMnAs is a testbed system in which the antiferromagnetic order parameter can be switched reversibly at ambient conditions using electrical currents. In previous experiments, orthogonal in-plane current pulses were used to induce 90 deg rotations of antiferromagnetic domains and demonstrate the operation of all-electrical memory bits in a multi-terminal geometry. Here, we demonstrate that antiferromagnetic domain walls can be manipulated to realize stable and reproducible domain changes using only two electrical contacts. This is achieved by using the polarity of the current to switch the sign of the current-induced effective field acting on the antiferromagnetic sublattices. The resulting reversible domain and domain wall reconfigurations are imaged using X-ray magnetic linear dichroism microscopy, and can also be detected electrically. Switching by domain-wall motion can occur at much lower current densities than those needed for coherent domain switching. Article


  • Terahertz electrical writing speed in an antiferromagnetic memory


    The speed of writing of state-of-the-art ferromagnetic memories is physically limited by an intrinsic gigahertz threshold. Recently, realization of memory devices based on antiferromagnets, in which spin directions periodically alternate from one atomic lattice site to the next has moved research in an alternative direction. We experimentally demonstrate at room temperature that the speed of reversible electrical writing in a memory device can be scaled up to terahertz using an antiferromagnet. A current-induced spin-torque mechanism is responsible for the switching in our memory devices throughout the 12-order-of-magnitude range of writing speeds from hertz to terahertz. Our work opens the path toward the development of memory-logic technology reaching the elusive terahertz band. Article


  • The multiple directions of antiferromagnetic spintronics


    Spintronics has revolutionized the field of magnetic recording and it is hoped that it will complement semiconductor-based microelectronics in 'beyond Moore's law' information technologies. Whether designed for sensing, memory or logic applications, spintronic devices have been traditionally based on ferromagnetic materials. Here we present a brief overview of an emerging subfield of spintronics research in which antiferromagnets take the central role. We focus on the latest developments, which range from demonstrations of experimental microelectronic memory devices and optical control of antiferromagnetic spins to the interplay of antiferromagnetic spintronics with topological phenomena, noncollinear antiferromagnets, antiferromagnet/ferromagnet interfaces and synthetic antiferromagnets. We illustrate that the envisaged applications of antiferromagnetic spintronics may expand to areas as diverse as terahertz information technologies or artificial neural networks. Article


  • Inertial displacement of a domain wall excited by ultra-short circularly polarized laser pulses


    Domain wall motion driven by ultra-short laser pulses is a pre-requisite for envisaged low-power spintronics combining storage of information in magnetoelectronic devices with high speed and long distance transmission of information encoded in circularly polarized light. Here we demonstrate the conversion of the circular polarization of incident femtosecond laser pulses into inertial displacement of a domain wall in a ferromagnetic semiconductor. In our study, we combine electrical measurements and magneto-optical imaging of the domain wall displacement with micromagnetic simulations. The optical spin-transfer torque acts over a picosecond recombination time of the spin-polarized photo-carriers that only leads to a deformation of the initial domain wall structure. We show that subsequent depinning and micrometre-distance displacement without an applied magnetic field or any other external stimuli can only occur due to the inertia of the domain wall. Article


  • Spintronics Current-switched magnetic insulator


    Exploiting the spin Hall effect acting on magnetic oxides outlines an unprecedented path towards low-power, non-volatile spintronics devices. Article


  • Quantum information processing in a silicon-based system


    For the first time, long coherence times (T2) of several microseconds were observed in a silicon-based charge quantum bit (qubit) device at 4.2 K. The coherence times demonstrated in this paper are two orders of magnitude longer, and the operating temperature is two orders of magnitude higher than the reported semiconductor charge qubit systems. In contrast to other approaches, in this work the qubits are formed by trench isolation instead of surface gate-defined. The qubits were fabricated on P-doped silicon-on-insulator (SOI) wafers through current industrial semiconductor manufacturing technology. We have demonstrated the accurate readout of the qubits' electronic states by using a single electron transistor (SET) as an electrometer. The first observation of the interaction between two sets of capacitively coupled charge movements was achieved by using our charge detection technique. Article


  • Gate-Sensing Coherent Charge Oscillations in a Silicon Field-Effect Transistor


    Quantum mechanical effects induced by the miniaturization of complementary metal-oxide-semiconductor (CMOS) technology hamper the performance and scalability prospects of field-effect transistors. However, those quantum effects, such as tunneling and coherence, can be harnessed to use existing CMOS technology for quantum information processing. Here, we report the observation of coherent charge oscillations in a double quantum dot formed in a silicon nanowire transistor detected via its dispersive interaction with a radio frequency resonant circuit coupled via the gate. Differential capacitance changes at the interdot charge transitions allow us to monitor the state of the system in the strong-driving regime where we observe the emergence of Landau-Zener-Stückelberg-Majorana interference on the phase response of the resonator. A theoretical analysis of the dispersive signal demonstrates that quantum and tunneling capacitance changes must be included to describe the qubit-resonator interaction. Furthermore, a Fourier analysis of the interference pattern reveals a charge coherence time, T2~100 ps. Our results demonstrate charge coherent control and readout in a simple silicon transistor and open up the possibility to implement charge and spin qubits in existing CMOS technology. Article


  • Electrical switching of an antiferromagnet


    Antiferromagnets are hard to control by external magnetic fields because of the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization. However, relativistic quantum mechanics allows for generating current-induced internal fields whose sign alternates with the periodicity of the antiferromagnetic lattice. Using these fields, which couple strongly to the antiferromagnetic order, we demonstrate room-temperature electrical switching between stable configurations in antiferromagnetic CuMnAs thin-film devices by applied current with magnitudes of order 10^6 ampere per square centimeter. Electrical writing is combined in our solid-state memory with electrical readout and the stored magnetic state is insensitive to and produces no external magnetic field perturbations, which illustrates the unique merits of antiferromagnets for spintronics. Article


  • Antiferromagnetic spintronics


    Antiferromagnetic materials are internally magnetic, but the direction of their ordered microscopic moments alternates between individual atomic sites. The resulting zero net magnetic moment makes magnetism in antiferromagnets externally invisible. This implies that information stored in antiferromagnetic moments would be invisible to common magnetic probes, insensitive to disturbing magnetic fields, and the antiferromagnetic element would not magnetically affect its neighbours, regardless of how densely the elements are arranged in the device. The intrinsic high frequencies of antiferromagnetic dynamics represent another property that makes antiferromagnets distinct from ferromagnets. Among the outstanding questions is how to manipulate and detect the magnetic state of an antiferromagnet efficiently. In this Review we focus on recent works that have addressed this question. The field of antiferromagnetic spintronics can also be viewed from the general perspectives of spin transport, magnetic textures and dynamics, and materials research. We briefly mention this broader context, together with an outlook of future research and applications of antiferromagnetic spintronics. Article


  • Optical-electrical detection of donor electron spins


    Electrical detection of spins is an essential tool for understanding the dynamics of spins, with applications ranging from optoelectronics and spintronics, to quantum information processing. For electron spins bound to donors in silicon, bulk electrically detected magnetic resonance has relied on coupling to spin readout partners such as paramagnetic defects or conduction electrons, which fundamentally limits spin coherence times. Here we demonstrate electrical detection of donor electron spin resonance in an ensemble by transport through a silicon device, using optically driven donor-bound exciton transitions. We measure electron spin Rabi oscillations, and obtain long electron spin coherence times, limited only by the donor concentration. We also experimentally address critical issues such as non-resonant excitation, strain, and electric fields, laying the foundations for realising a single-spin readout method with relaxed magnetic field and temperature requirements compared with spin-dependent tunnelling, enabling donor-based technologies such as quantum sensing. Article


  • Gate-based charge sensing


    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 37µe 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. Article


  • Mapping electrically the strength and direction of spin-orbit fields


    Electrons in solid-state systems can behave like waves, and, in both metals and semiconductors, quantum-relativistic effects can influence the motion of the charge carriers. One of these effects is the spin-orbit interaction, which describes the coupling of the orbital and spin degrees of freedom of the electrons, and manifests itself as an effective momentum-dependent magnetic field seen by the carriers. The interference of relativistic, spin-orbit-coupled electrons has in the past been considered primarily in terms of destructive interference. Furthermore, previous measurements of the strength of the spin-orbit coupling by phase coherent transport could only return approximate values. Writing in Nature Nanotechnology, Junsaku Nitta and colleagues at Tohoku University and the University of Regensburg now report observing constructive interference effects in relativistic electrons in semiconductor quantum structures, and use these effects to precisely map the relativistic field, including its momentum-dependent direction. Article


  • Celebrating Germany victory at the Football World Cup : 17th July 2014


    We had a small celebration at HCL for the victory of the German team at the World Cup. A nice occasion for a very nice cake !!! Pictures here.


  • Antidamping spin-orbit torque


    Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin-orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin-orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic-paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin-orbit torque as well as its microscopic modelling. We expect the Berry curvature spin-orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect. Article


  • Wireless control of electrons in quantum dots


    We discuss the effects of gigahertz photon irradiation on a degenerately phosphorus-doped silicon quantum dot, in particular, the creation of voltage offsets on gate leads and the tunneling of one or two electrons via Coulomb blockade lifting at 4.2 K. A semi-analytical model is derived that explains the main features observed experimentally. Ultimately both effects may provide an efficient way to optically control and operate electrically isolated structures by microwave pulses. In quantum computing architectures, these results may lead to the use of microwave multiplexing to manipulate quantum states in a multi-qubit configuration. Article


  • Successful piezoelectric control of domain walls


    The rich internal degrees of freedom of magnetic domain walls make them an attractive complement to electron charge for exploring new concepts of storage, transport and processing of information. Here we use the tunable internal structure of a domain wall in a perpendicularly magnetized GaMnAsP/GaAs ferromagnetic semiconductor and demonstrate devices in which piezoelectrically controlled magnetic anisotropy yields up to 500% mobility variations for an electrical-current-driven domain wall. We observe current-induced domain wall motion over a wide range of current-pulse amplitudes and report a direct observation and the piezoelectric control of the Walker breakdown separating two regimes with different mobilities. Our work demonstrates that in spin-orbit-coupled ferromagnets with weak extrinsic domain wall pinning, the piezoelectric control allows one to experimentally assess the upper and lower boundaries of the characteristic ratio of adiabatic and non-adiabatic spin-transfer torques in the current-driven domain wall motion. Article


Latest News

Quantum advantage in postselected metrology

July 2020

Article published in Nature Communications

Optical Gating in Hexagonal Boron Nitride

May 2020

Article published in Nanoletters

A new method for imaging and writing magnetic domains

November 2019

Article published in Nature Communications

Polaron spin dynamics

June 2019

Article published in Nature Physics

Quantum devices read by a dynamic random access architecture

June 2019

Article published in Nature Electronics

An observation of μm long spin diffusion length in organic polymers

May 2019

Article published in Nature Electronics

Single shot readout of electron spins

March 2019

Article published in Nature Nanotechnology

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