Matthew Sellars

Nitrogen-vacancy center in diamond: Model of the electronic structure and associated dynamics

Symmetry considerations are used in presenting a model of the electronic structure and the associated dynamics of the nitrogen-vacancy center in diamond. The model accounts for the occurrence of optically induced spin polarization, for the change of emission level with spin polarization and for new measurements of transient emission. The rate constants given are in variance to those reported previously.

Observation and control of blinking nitrogen-vacancy centres in discrete nanodiamonds

Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics, nanoscale magnetometry and biolabelling. Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds and milled nanodiamonds, they have not been observed in very small isolated nanodiamonds. Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.

Efficient quantum memory for light

Storing and retrieving a quantum state of light on demand, without corrupting the information it carries, is an important challenge in the field of quantum information processing. Classical measurement and reconstruction strategies for storing light must necessarily destroy quantum information as a consequence of the Heisenberg uncertainty principle. There has been significant effort directed towards the development of devices—so-called quantum memories—capable of avoiding this penalty. So far, successful demonstrations of non-classical storage and on-demand recall have used atomic vapours and have been limited to low efficiencies, of less than 17 per cent, using weak quantum states with an average photon number of around one. Here we report a low-noise, highly efficient (up to 69 per cent) quantum memory for light that uses a solid-state medium. The device allows the storage and recall of light more faithfully than is possible using a classical memory, for weak coherent states at the single-photon level through to bright states of up to 500 photons. For input coherent states containing on average 30 photons or fewer, the performance exceeded the no-cloning limit. This guaranteed that more information about the inputs was retrieved from the memory than was left behind or destroyed, a feature that will provide security in communications applications.

Photon Echoes Produced by Switching Electric Fields

We demonstrate photon echoes in Eu3+:Y2SiO5 by controlling the inhomogeneous broadening of the Eu3+ 7F0<-->5D0 optical transition. This transition has a linear Stark shift, and we induce inhomogeneous broadening by applying an external electric field gradient. After optical excitation, reversing the polarity of the field rephases the ensemble, resulting in a photon echo. This is the first demonstration of such a photon echo, and its application as a quantum memory is discussed.

Electro-Optic Quantum Memory for Light Using Two-Level Atoms

We present a simple quantum memory scheme that allows for the storage of a light field in an ensemble of two-level atoms. The technique is analogous to the NMR gradient echo for which the imprinting and recalling of the input field are performed by controlling a linearly varying broadening. Our protocol is perfectly efficient in the limit of high optical depths and the output pulse is emitted in the forward direction. We provide a numerical analysis of the protocol together with an experiment performed in a solid state system. In close agreement with our model, the experiment shows a total efficiency of up to 15%, and a recall efficiency of 26%. We suggest simple realizable improvements for the experiment to surpass the no-cloning limit.

Dynamic decoherence control of a solid-state nuclear-quadrupole qubit.

We report on the application of a dynamic decoherence control pulse sequence on a nuclear-quadrupole transition in Pr3+:Y(2)SiO(5). Process tomography is used to analyze the effect of the pulse sequence. The pulse sequence was found to increase the decoherence time of the transition to over 30 seconds. Although the decoherence time was significantly increased, the population terms were found to rapidly decay on the application of the pulse sequence. The increase of this decay rate is attributed to inhomogeneity in the ensemble. Methods to circumvent this limit are discussed.

Method of Extending Hyperfine coherence Times in Pr3+:Y2SiO5

In this Letter, we present a method for increasing the coherence time of praseodymium hyperfine ground state transitions in Pr(3+):Y(2)SiO5 by the application of a specific external magnetic field. The magnitude and angle of the external field is applied such that the Zeeman splitting of a hyperfine transition is at a critical point in three dimensions, making the first order Zeeman shift vanishingly small for the transition. This reduces the influence of the magnetic interactions between the praseodymium ions and the spins in the host lattice on the transition frequency. Using this method a phase memory time of 82 ms was observed, a value 2 orders of magnitude greater than previously reported. It is shown that the residual dephasing is amenable to quantum error correction.

Infrared emission of the NV centre in diamond: Zeeman and uniaxial stress studies

An emission band in the infrared (IR) is shown to be associated with a transition within the negative nitrogen-vacancy centre in diamond. The band has a zero-phonon line at 1046?nm, and uniaxial stress and magnetic field measurements indicate that the emission is associated with a transition between 1E and 1A1 singlet levels. Inter-system crossing to these singlets causes the spin polarization that makes the NV- centre attractive for quantum information processing, and the IR emission band provides a new avenue for using the centre in such applications.

Optical addressing of an individual erbium ion in silicon

The detection of electron spins associated with single defects in solids is a critical operation for a range of quantum information and measurement applications under development. So far, it has been accomplished for only two defect centres in crystalline solids: phosphorus dopants in silicon, for which electrical read-out based on a single-electron transistor is used, and nitrogen–vacancy centres in diamond, for which optical read-out is used. A spin read-out fidelity of about 90 per cent has been demonstrated with both electrical read-out and optical read-out; however, the thermal limitations of the former and the poor photon collection efficiency of the latter make it difficult to achieve the higher fidelities required for quantum information applications. Here we demonstrate a hybrid approach in which optical excitation is used to change the charge state (conditional on its spin state) of an erbium defect centre in a silicon-based single-electron transistor, and this change is then detected electrically. The high spectral resolution of the optical frequency-addressing step overcomes the thermal broadening limitation of the previous electrical read-out scheme, and the charge-sensing step avoids the difficulties of efficient photon collection. This approach could lead to new architectures for quantum information processing devices and could drastically increase the range of defect centres that can be exploited. Furthermore, the efficient electrical detection of the optical excitation of single sites in silicon represents a significant step towards developing interconnects between optical-based quantum computing and silicon technologies.

Demonstration of conditional quantum phase shift between ions in a solid

Because of their long coherence times, dopant ions have been considered promising candidates for scalable solid state quantum computing. Here we demonstrate a conditional phase shift between two qubits based on an optical transition of europium ions. The demonstration uses ensembles that have been selected from a randomly doped sample using spectral hole burning techniques. The electron dipole-dipole interaction between the ions that usually causes instantaneous spectral diffusion is used to generate the conditional phase shift.