arXiv subjects

G. A. D. Briggs

Publications and source records attributed to G. A. D. Briggs.

At least 37 records · Page 2Linked to original sources

arXiv2014

A wide-band tunable phase shifter for radio-frequency reflectometry

Radio-frequency reflectometry of nanodevices requires careful separation of signal quadratures to distinguish dissipative and dispersive contributions to the device impedance. A tunable phase shifter for this purpose is described and characterized. The phase shifter, consisting of a varactor-loaded transmission line, has the necessary tuning range combined with acceptable insertion loss across a frequency band 100 MHz - 1 GHz spanning most radio-frequency experiments. Its operation is demonstrated by demodulating separately the signals due to resistance and capacitance changes in a model device.

physics.ins-det↗

arXiv2013

The Oxford Questions on the foundations of quantum physics

The twentieth century saw two fundamental revolutions in physics -- relativity and quantum. Daily use of these theories can numb the sense of wonder at their immense empirical success. Does their instrumental effectiveness stand on the rock of secure concepts or the sand of unresolved fundamentals? Does measuring a quantum system probe, or even create, reality, or merely change belief? Must relativity and quantum theory just co-exist or might we find a new theory which unifies the two? To bring such questions into sharper focus, we convened a conference on Quantum Physics and the Nature of Reality. Some issues remain as controversial as ever, but some are being nudged by theory's secret weapon of experiment.

quant-ph↗

arXiv2011

Quantum control in spintronics

Superposition and entanglement are uniquely quantum phenomena. Superposition incorporates a phase which contains information surpassing any classical mixture. Entanglement offers correlations between measurements in quantum systems that are stronger than any which would be possible classically. These give quantum computing its spectacular potential, but the implications extend far beyond quantum information processing. Early applications may be found in entanglement enhanced sensing and metrology. Quantum spins in condensed matter offer promising candidates for investigating and exploiting superposition and entanglement, and enormous progress is being made in quantum control of such systems. In GaAs, individual electron spins can be manipulated and measured, and singlet-triplet states can be controlled in double-dot structures. In silicon, individual electron spins can be detected by ionisation of phosphorous donors, and information can be transferred from electron spins to nuclear spins to provide long memory times. Electron and nuclear spins can be manipulated in nitrogen atoms incarcerated in fullerene molecules, which in turn can be assembled in ordered arrays. Spin states of charged nitrogen vacancy centres in diamond can be manipulated and read optically. Collective spin states in a range of materials systems offer scope for holographic storage of information. Conditions are now excellent for implementing superposition and entanglement in spintronic devices, thereby opening up a new era of quantum technologies.

quant-ph↗

arXiv2010

Cryogenic instrumentation for fast current measurement in a silicon single electron transistor

We present a realisation of high bandwidth instrumentation at cryogenic temperatures and for dilution refrigerator operation that possesses advantages over methods using radio-frequency single electron transistor or transimpedance amplifiers. The ability for the low temperature electronics to carry out faster measurements than with room temperature electronics is investigated by the use of a phosphorous-doped single-electron transistor. A single-shot technique is successfully implemented and used to observe the real time decay of a quantum state. A discussion on various measurement strategies is presented and the consequences on electron heating and noise are analysed.

cond-mat.mes-hall↗

arXiv2010

High cooperativity coupling of electron-spin ensembles to superconducting cavities

Electron spins in solids are promising candidates for quantum memories for superconducting qubits because they can have long coherence times, large collective couplings, and many quantum bits can be encoded into the spin-waves of a single ensemble. We demonstrate the coupling of electron spin ensembles to a superconducting transmission-line resonator at coupling strengths greatly exceeding the cavity decay rate and comparable to spin linewidth. We also use the enhanced coupling afforded by the small cross-section of the transmission line to perform broadband spectroscopy of ruby at millikelvin temperatures at low powers. In addition, we observe hyperfine structure in diamond P1 centers and time domain saturation-relaxation of the spins.

cond-mat.mes-hall↗

arXiv2010

Single Spin Detection with a Carbon Nanotube Double Quantum Dot

Spin qubits defined in carbon nanotube quantum dots are of considerable interest for encoding and manipulating quantum information because of the long electron spin coherence times expected. However, before carbon nanotubes can find applications in quantum information processing schemes, we need to understand and control the coupling between individual electron spins and the interaction between the electron spins and their environment. Here we make use of spin selection rules to directly measure - and demonstrate control of - the singlet-triplet exchange coupling between two carbon nanotube quantum dots. We furthermore elucidate the effects of spin-orbit interaction on the electron transitions and investigate the interaction of the quantum dot system with a single impurity spin - the ultimate limit in spin sensitivity.

cond-mat.mes-hall↗

arXiv2010

Spin detection at elevated temperatures using a driven double quantum dot

We consider a double quantum dot in the Pauli blockade regime interacting with a nearby single spin. We show that under microwave irradiation the average electron occupations of the dots exhibit resonances that are sensitive to the state of the nearby spin. The system thus acts as a spin meter for the nearby spin. We investigate the conditions for a non-demolition read-out of the spin and find that the meter works at temperatures comparable to the dot charging energy and sensitivity is mainly limited by the intradot spin relaxation.

cond-mat.mes-hall↗

arXiv2009

Quantum computing with an electron spin ensemble

We propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper Pair Box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations.

quant-ph↗

arXiv2009

Magnetic field sensing beyond the standard quantum limit using 10-spin NOON states

The concept of entanglement, in which coherent quantum states become inextricably correlated, has evolved from one of the most startling and controversial outcomes of quantum mechanics to the enabling principle of emerging technologies such as quantum computation and quantum sensors. The use of entangled particles in measurement permits the transcendence of the standard quantum limit in sensitivity, which scales as N^1/2 for N particles, to the Heisenberg limit, which scales as N. This approach has been applied to optical interferometry using entangled photons and spin pairs for the measurement of magnetic fields and improvements on atomic clocks. Here, we demonstrate experimentally an 9.4-fold increase in sensitivity to an external magnetic field of a 10-spin entangled state, compared with an isolated spin, using nuclear spins in a highly symmetric molecule. This approach scales in a favourable way compared to systems where qubit loss is prevalent, and paves the way for enhanced precision in magnetic field sensing

quant-ph↗

arXiv2008

Single shot measurement of a silicon single electron transistor

We have fabricated a custom cryogenic Complementary Metal-Oxide-Semiconductor (CMOS) integrated circuit that has a higher measurement bandwidth compared with conventional room temperature electronics. This allowed implementing single shot operations and observe the real-time evolution of the current of a phosphorous-doped silicon single electron transistor that was irradiated with a microwave pulse. Relaxation times up to 90 us are observed, suggesting the presence of well isolated electron excitations within the device. It is expected that these are associated with long decoherence time and the device may be suitable for quantum information processing.

cond-mat.str-el↗

arXiv2008

Hyperfine structure of Sc@C82 from ESR and DFT

The electron spin g- and hyperfine tensors of the endohedral metallofullerene Sc@C82 are anisotropic. Using electron spin resonance (ESR) and density functional theory (DFT), we can relate their principal axes to the coordinate frame of the molecule, finding that the g-tensor is not axially symmetric. The Sc bond with the cage is partly covalent and partly ionic. Most of the electron spin density is distributed around the carbon cage, but 5% is associated with the scandium d_yz orbital, and this drives the observed anisotropy.

cond-mat.mtrl-sci↗

arXiv2008

Spin Lifetimes in Quantum Dots from Noise Measurements

We present a method of obtaining information about spin lifetimes in quantum dots from measurements of electrical transport. The dot is under resonant microwave irradiation and at temperatures comparable to or larger than the Zeeman energy. We find that the ratio of the spin coherence times T_{1}/T_{2} can be deduced from a measurement of current through the quantum dot as a function the applied magnetic field. We calculate the noise power spectrum of the dot current and show that a dip occurs at the Rabi frequency with a line width given by 1/T_{1}+1/T_{2}.

cond-mat.mes-hall↗

arXiv2008

Pauli spin blockade in carbon nanotube double quantum dots

We report Pauli spin blockade in an impurity defined carbon nanotube double quantum dot. We observe a pronounced current suppression for negative source-drain bias voltages which is investigated for both symmetric and asymmetric coupling of the quantum dots to the leads. The measured differential conductance agrees well with a theoretical model of a double quantum dot system in the spin-blockade regime which allows us to estimate the occupation probabilities of the relevant singlet and triplet states. This work shows that effective spin-to-charge conversion in nanotube quantum dots is feasible and opens the possibility of single-spin readout in a material that is not limited by hyperfine interaction with nuclear spins.

cond-mat.mes-hall↗

arXiv2008

Deriving molecular bonding from macromolecular self-assembly

Macromolecules can form regular structures on inert surfaces. We have developed a combined empirical and modeling approach to derive the bonding. From experimental scanning tunneling microscopy (STM) images of structures formed on Au(111) by melamine, by PTCDA, and by a 2:3 mixture of the two, we determine the molecular bonding morphologies. Within these bonding morphologies and recognizing the distinction between cohesive and adhesive molecular interactions we simultaneously simulated different molecular structures using a lattice Monte Carlo method. Within these bonding morphologies there is a distinction between cohesive and adhesive molecular interactions. We have simulated different molecular structures using a lattice Monte Carlo method.

cond-mat.mtrl-sci↗

arXiv2006

Zener quantum dot spin filter in a carbon nanotube

We predict and analyze a novel spin filter in semiconducting carbon nanotubes. By using local electrostatic gates, the conduction and valence bands can be modulated to form a double-barrier structure. The confined region below the valence band defines a Zener quantum dot, which exhibits resonant tunneling. The resonances split in a magnetic field to make a bipolar spin filter for applications in spintronics and quantum information processing. We model this using k*p envelope function theory and show that this is in excellent agreement with a corresponding tight-binding calculation.

cond-mat.mes-hall↗

arXiv2006

Entanglement between static and flying qubits in a semiconducting carbon nanotube

Entanglement can be generated by two electrons in a spin-zero state on a semiconducting single-walled carbon nanotube. The two electrons, one weakly bound in a shallow well in the conduction band, and the other injected into the conduction band, are coupled by the Coulomb interaction. Both transmission and entanglement are dependent on the well characteristics, which can be controlled by a local gate, and on the kinetic energy of the injected electron. Regimes with different degrees of electron correlation exhibit full or partial entanglement. In the latter case, the maximum entanglement can be estimated as a function of width and separation of a pair of singlet-triplet resonances.

cond-mat.mes-hall↗

arXiv2006

Bandgap modulation of narrow-gap carbon nanotubes in a transverse electric field

We propose a method to modulate the bandgaps in quasi-metallic carbon nanotubes using a transverse electric field. Unlike previous investigations, we include curvature effects of the nanotubes by incorporating both $π$- and $σ$-orbitals in our tight-binding calculations. The calculations show that the small curvature-induced bandgaps decrease quadratically with electric field amplitude to zero. As the electric field amplitude continues to increase, the bandgap then expands in a similar manner to that presented in earlier studies. The bandgap dependence is verified by analytical calculations, which also agree with preceding analyses for the limit of no curvature.

cond-mat.mes-hall↗

arXiv2005

Coherence of Spin Qubits in Silicon

Given the effectiveness of semiconductor devices for classical computation one is naturally led to consider semiconductor systems for solid state quantum information processing. Semiconductors are particularly suitable where local control of electric fields and charge transport are required. Conventional semiconductor electronics is built upon these capabilities and has demonstrated scaling to large complicated arrays of interconnected devices. However, the requirements for a quantum computer are very different from those for classical computation, and it is not immediately obvious how best to build one in a semiconductor. One possible approach is to use spins as qubits: of nuclei, of electrons, or both in combination. Long qubit coherence times are a prerequisite for quantum computing, and in this paper we will discuss measurements of spin coherence in silicon. The results are encouraging - both electrons bound to donors and the donor nuclei exhibit low decoherence under the right circumstances. Doped silicon thus appears to pass the first test on the road to a quantum computer.

cond-mat.mtrl-sci↗