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Wolfowicz G.,University College London | Wolfowicz G.,University of Oxford | Urdampilleta M.,University College London | Thewalt M.L.W.,Simon Fraser University | And 5 more authors.
Physical Review Letters | Year: 2014

Electric fields can be used to tune donor spins in silicon using the Stark shift, whereby the donor electron wave function is displaced by an electric field, modifying the hyperfine coupling between the electron spin and the donor nuclear spin. We present a technique based on dynamic decoupling of the electron spin to accurately determine the Stark shift, and illustrate this using antimony donors in isotopically purified silicon-28. We then demonstrate two different methods to use a dc electric field combined with an applied resonant radio-frequency (rf) field to conditionally control donor nuclear spins. The first method combines an electric-field induced conditional phase gate with standard rf pulses, and the second one simply detunes the spins off resonance. Finally, we consider different strategies to reduce the effect of electric field inhomogeneities and obtain above 90% process fidelities. © 2014 American Physical Society.

Wolfowicz G.,University College London | Wolfowicz G.,University of Oxford | Tyryshkin A.M.,Princeton University | George R.E.,University College London | And 7 more authors.
Nature Nanotechnology | Year: 2013

A major challenge in using spins in the solid state for quantum technologies is protecting them from sources of decoherence. This is particularly important in nanodevices where the proximity of material interfaces, and their associated defects, can play a limiting role. Spin decoherence can be addressed to varying degrees by improving material purity or isotopic composition, for example, or active error correction methods such as dynamic decoupling (or even combinations of the two). However, a powerful method applied to trapped ions in the context of atomic clocks is the use of particular spin transitions that are inherently robust to external perturbations. Here, we show that such 'clock transitions' can be observed for electron spins in the solid state, in particular using bismuth donors in silicon. This leads to dramatic enhancements in the electron spin coherence time, exceeding seconds. We find that electron spin qubits based on clock transitions become less sensitive to the local magnetic environment, including the presence of 29 Si nuclear spins as found in natural silicon. We expect the use of such clock transitions will be of additional significance for donor spins in nanodevices, mitigating the effects of magnetic or electric field noise arising from nearby interfaces and gates. © 2013 Macmillan Publishers Limited. All rights reserved.

Tyryshkin A.M.,Princeton University | Tojo S.,Keio University | Morton J.J.L.,University of Oxford | Riemann H.,Institute For Kristallzuchtung | And 7 more authors.
Nature Materials | Year: 2012

Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a 28Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure 28Si material (residual 29Si concentration <50 ppm) with donor densities of 10 14-10 15 cm 3. We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T 2 up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ∼200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T 2 of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits. © 2012 Macmillan Publishers Limited. All rights reserved.

Sigillito A.J.,Princeton University | Malissa H.,Princeton University | Malissa H.,University of Utah | Tyryshkin A.M.,Princeton University | And 10 more authors.
Applied Physics Letters | Year: 2014

We demonstrate the use of high-Q superconducting coplanar waveguide (CPW) microresonators to perform rapid manipulations on a randomly distributed spin ensemble using very low microwave power (400 nW). This power is compatible with dilution refrigerators, making microwave manipulation of spin ensembles feasible for quantum computing applications. We also describe the use of adiabatic microwave pulses to overcome microwave magnetic field (B1) inhomogeneities inherent to CPW resonators. This allows for uniform control over a randomly distributed spin ensemble. Sensitivity data are reported showing a single shot (no signal averaging) sensitivity to 107 spins or 3×104spins/√Hz with averaging. © 2014 AIP Publishing LLC.

Wu H.,University of Oxford | Gauger E.M.,University of Oxford | Gauger E.M.,National University of Singapore | George R.E.,University of Oxford | And 8 more authors.
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

High-fidelity quantum operations are a key requirement for fault-tolerant quantum information processing. Manipulation of electron spins is usually achieved with time-dependent microwave fields. In contrast to the conventional dynamic approach, adiabatic geometric phase operations are expected to be less sensitive to certain kinds of noise and field inhomogeneities. Here, we introduce an adiabatic geometric phase gate for the electron spin. Benchmarking it against existing dynamic and nonadiabatic geometric gates through simulations and experiments, we show that it is indeed inherently robust against inhomogeneity in the applied microwave field strength. While only little advantage is offered over error-correcting composite pulses for modest inhomogeneities 10%, the adiabatic approach reveals its potential for situations where field inhomogeneities are unavoidably large. © 2013 American Physical Society.

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