arXiv subjects

Gavin W. Morley

Publications and source records attributed to Gavin W. Morley.

26 records · Page 2Linked to original sources

arXiv2017

A Spin Entanglement Witness for Quantum Gravity

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, a prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. It is fair to say that there are no feasible ideas yet to test the quantum coherent behaviour of gravity directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes.

quant-ph↗

arXiv2017

Medical applications of diamond magnetometry: commercial viability

The sensing of magnetic fields has important applications in medicine, particularly to the sensing of signals in the heart and brain. The fields associated with biomagnetism are exceptionally weak, being many orders of magnitude smaller than the Earth's magnetic field. To measure them requires that we use the most sensitive detection techniques, however, to be commercially viable this must be done at an affordable cost. The current state of the art uses costly SQUID magnetometers, although they will likely be superseded by less costly, but otherwise limited, alkali vapour magnetometers. Here, we discuss the application of diamond magnetometers to medical applications. Diamond magnetometers are robust, solid state devices that work in a broad range of environments, with the potential for sensitivity comparable to the leading technologies.

physics.app-ph↗

arXiv2016

Laser writing of coherent colour centres in diamond

Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies.

quant-ph↗

arXiv2014

Review: Towards Spintronic Quantum Technologies with Dopants in Silicon

Dopants in crystalline silicon such as phosphorus (Si:P) have electronic and nuclear spins with exceptionally long coherence times making them promising platforms for quantum computing and quantum sensing. The demonstration of single-spin single-shot readout brings these ideas closer to implementation. Progress in fabricating atomic-scale Si:P structures with scanning tunnelling microscopes offers a powerful route to scale up this work, taking advantage of techniques developed by the computing industry. The experimental and theoretical sides of this emerging quantum technology are reviewed with a focus on the period from 2009 to mid-2014.

quant-ph↗

arXiv2011

Quantum control of hybrid nuclear-electronic qubits

Pulsed magnetic resonance is a wide-reaching technology allowing the quantum state of electronic and nuclear spins to be controlled on the timescale of nanoseconds and microseconds respectively. The time required to flip either dilute electronic or nuclear spins is orders of magnitude shorter than their decoherence times, leading to several schemes for quantum information processing with spin qubits. We investigate instead the novel regime where the eigenstates approximate 50:50 superpositions of the electronic and nuclear spin states forming "hybrid nuclear-electronic" qubits. Here we demonstrate quantum control of these states for the first time, using bismuth-doped silicon, in just 32 ns: this is orders of magnitude faster than previous experiments where pure nuclear states were used. The coherence times of our states are five orders of magnitude longer, reaching 4 ms, and are limited by the naturally-occurring 29Si nuclear spin impurities. There is quantitative agreement between our experiments and no-free-parameter analytical theory for the resonance positions, as well as their relative intensities and relative Rabi oscillation frequencies. In experiments where the slow manipulation of some of the qubits is the rate limiting step, quantum computations would benefit from faster operation in the hybrid regime.

quant-ph↗

arXiv2010

Initializing, manipulating and storing quantum information with bismuth dopants in silicon

A prerequisite for exploiting spins for quantum data storage and processing is long spin coherence times. Phosphorus dopants in silicon (Si:P) have been favoured as hosts for such spins because of measured electron spin coherence times (T2) longer than any other electron spin in the solid state: 14 ms at 7 K. Heavier impurities such as bismuth in silicon (Si:Bi) could be used in conjunction with Si:P for quantum information proposals that require two separately addressable spin species. However, the question of whether the incorporation of the much less soluble Bi into Si leads to defect species that destroy coherence has not been addressed. Here we show that schemes involving Si:Bi are indeed feasible as the electron spin coherence time T2 exceeds 1 ms at 10 K. We polarized the Si:Bi electrons and hyperpolarized the I=9/2 nuclear spin of 209Bi, manipulating both with pulsed magnetic resonance. The larger nuclear spin means that a Si:Bi dopant provides a 20-dimensional Hilbert space rather than the four dimensional Hilbert space of an I=1/2 Si:P dopant.

cond-mat.mtrl-sci↗

arXiv2008

A multi-frequency high-field pulsed EPR / ENDOR spectrometer

We describe a pulsed multi-frequency electron paramagnetic resonance spectrometer operating at several frequencies in the range of 110-336 GHz. The microwave source at all frequencies consists of a multiplier chain starting from a solid state synthesizer in the 12-15 GHz range. A fast PIN-switch at the base frequency creates the pulses. At all frequencies a Fabry-Perot resonator is employed and the pi/2 pulse length ranges from ~100 ns at 110 GHz to ~600 ns at 334 GHz. Measurements of a single crystal containing dilute Mn2+ impurities at 12 T illustrate the effects of large electron spin polarizations. The capabilities also allow for pulsed electron nuclear double resonance experiments as demonstrated by Mims ENDOR of 39K nuclei in Cr:K3NbO8.

quant-ph↗

arXiv2006

Efficient Dynamic Nuclear Polarization at High Magnetic Fields

By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in 15N@C60 by a factor of 1000 at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting N@C60 molecules as components of a quantum information processing device.

quant-ph↗