Quantum Spin Dynamics
- Vacancies/PhD positions
- UCL Quantum
- London Centre for Nanotechnology
- Dept. of Electronic and Electrical Engineering
Recent publicationsHyperﬁne Stark effect of shallow donors in silicon G Pica et al. Phys. Rev. B 90 195204 (2014)
Conditional Control of Donor Nuclear Spins in Silicon Using Stark Shifts G Wolfowicz et al. Phys. Rev. Lett 113 157601 (2014)
Uncovering many-body correlations in nanoscale nuclear spin baths by central spin decoherence W-L Ma et al. Nature Communications 5 4822 (2014)
Microwave Manipulation of Electrically Injected Spin-Polarized Electrons in Silicon CC Lo et al. Phys. Rev. Applied 1 014006 (2014)
Quantum bath-driven decoherence of mixed spin systems S Balian et al. Phys. Rev. B 89 045403 (2014)
Room-temperature quantum bit storage exceeding 39 minutes using ionized donors in silicon-28 K Saeedi et al. Science (2013)
Atomic clock transitions in silicon-based spin qubits G Wolfowicz et al., Nature Nano (2013)
High-fidelity readout and control of a nuclear spin qubit in silicon JJ Pla et al., Nature (2013)
Electron nuclear double resonance (ENDOR)
In the field of EPR spectroscopy, ENDOR is a powerful polarization transfer technique which permits the measurement of small energy (nuclear spin) transitions at the much enhanced sensitivity of higher energy (electron spin) transitions. ENDOR is therefore an alternative to NMR methods, with the benefits of improved spin number sensitivity and a specific focus on NMR transitions of nuclei coupled to paramagnetic species.
In the context of electron and nuclear spin quantum information processing, ENDOR is the essential technique for executing gates between electron and nuclear spin qubits.
Understanding spin relaxation mechanisms can provide a wealth of information regarding time-dependent perturbations to a spin environement, such as spectral diffusion, molecular diffusion, fluctuating zero-field splittings, hyperfine couplings etc., and the associated correlation times. In order to embody quantum information within spins, it is essential to understand spin relaxation and investigate ways in which it may be mitigated.
Endohedral fullerene spins
Fullerenes have been shown to provide remarkable nanovoids in which atoms, ions and molecules can exhibit nearly-free behaviour within a solid state environment. For example, the protection afforded by the carbon cage has led to the longest decoherence times of any molecular electron spin - in the case of the nitrogen-doped C60 molecule, termed N@C60. This class of molecule has been identified as an attractive candidate for the embodiment of quantum information, and benefits from a rich fullerene chemistry which permits the synthesis of higher order structures such as fullerene dimers, and chains in carbon nanotubes: 'peapods'.
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 become 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 sqrt(N) for N particles, to the Heisenberg limit, which scales as N. This approach can be applied to optical interferometry by using entangled photons or the measurement of magnetic fields using entangled spins.
Historically most materials in magnetic applications are based on inorganic materials. Recently, however, organic and molecular materials have begun to show increasing promise. We collaborate with the correlated electron systems group at Oxford to perform research on molecular magnets using spin resonance.
Quantum information processing
Magnetic domains, each consisting of vast numbers of magnetic atoms, have been used for decades for information storage, for example in hard disk drives. New models for computing have been put forth in which information is encoded at a much deeper level, within individual electron spins, and calculations performed through the interactions between spins. The nature of this information inherits the quantum mechanical properties of the spin states, such as superposition, allowing calculations to take place at a rate unthinkable using conventional computers. Potential applications range from challenges in code breaking and pattern recognition to simulations of complex processes in molecular systems.
Quantum computing has provided a new way of thinking about the interactions and evolution of quantum states in terms of the information they represent and algorithms performed. Spins represent natural two-state systems for the embodiment of a quantum bit, existing in a range of solid state environments and often benefiting from long coherence times.
Electrically detected magnetic resonance
Electrically detected magnetic resonance (EDMR) is a very sensitive spectroscopic technique, which can be used for modern materials characterisation as well as quantum information processing in order to readout few to single electron spins. We have investigated the EDMR effect in silicon field-effect transistors (FETs) and demonstrate the readout of arsenic and phosphorus donor spins in a resonant microwave cavity at 3.36 T and 94 GHz (W-band) for the first time worldwide. A comparison between conventional low- and high-field EDMR on the same devices shows that bolometric heating as well as spin-dependent scattering can be ruled out as the underlying mechanism giving rise to the spin resonance signals in FETs. Our signals are rather understood in terms of a polarisation transfer from the donor to the two-dimensional electron gas forming in the device channel.
The idea that objects can be in two places at the same time is difficult to reconcile with our intuition, and even our logic; this is especially true when one considers proposals to place larger objects into quantum superpositions. Establishing quantum coherence in the macroscopic world remains a considerable technical challenge, but if it could be demonstrated it would represent a significant conceptual advance in our understanding. It could confirm that in principle there are no reasons why macroscopic objects (such as cats, or the moon) cannot exist in paradoxical quantum states. Two ingredients are necessary for this ambitious goal. Firstly, one requires sufficient control over a quantum system to isolate it from the environment and set up the delicate quantum state. Secondly, a rigorous experimental method is needed - one which can exclude alternative theories that describe the system as being either here or there (rather than both here and there, which is what quantum theory tells us). By using an electron spin to 'measure' the nuclear spin in a non-invasive way, it was possible to show inescapable evidence that the nuclear spin state was indeed in a superposition of both up and down at the same time. This was possible thanks to a protocol invented by Leggett and Garg, which was extended to apply to imperfect measurement procedures. Most would consider this system microscopic, and therefore not be terribly surprised by the results - but the experiment will inspire future work probing larger and more complex objects.
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