Session A30: Focus Session: Qubits in Diamond I

Stark tuning spin qubits in diamond for quantum optical networks

Integrated diamond networks based on cavity-coupled spin impurities offer a promising platform for scalable quantum computing. A key ingredient for this technology involves heralding entanglement by interfering indistinguishable photons emitted by pairs of identical spin qubits. Here we demonstrate the required control over the internal level structure of nitrogen-vacancy (NV) centers located within 100 nm of the diamond surface using the DC Stark effect. By varying the voltages applied to lithographically-defined metal electrodes, we tune the zero-phonon emission wavelength of a single NV center over a range of $\sim $0.5 nm. Using high-resolution emission spectroscopy, we directly observe electrical tuning of the relative strengths of spin-altering lambda transitions to arbitrary values. Under resonant excitation, we apply dynamic feedback to stabilize the optical transition against spectral diffusion. Progress on application of gated control to single NV centers coupled to single-crystal diamond photonic crystal cavities and other nanophotonic structures will be presented. This work was supported by DARPA and the UC Regents.   

Decoherence-protected quantum gates for a hybrid spin register in diamond

Protecting the dynamics of coupled quantum systems from decoherence by the environment is a key challenge for solid-state quantum information processing. An idle qubit can be efficiently insulated from the environment via dynamical decoupling, but quantum gate operations are, in general, disrupted by the decoupling. This problem is particularly salient for hybrid systems, where different types of qubits evolve and decohere at vastly different rates. Here we present an efficient scheme for combining the dynamical decoupling with the quantum gate operation, using the internal resonance in the coupled-spin system. We theoretically demonstrate and experimentally achieve high-fidelity operation of a two-spin register made of the   

Quantum Optics with Spins and Photons in Diamond

Quantum control of interactions between photons and solid-state systems has important applications in quantum information, metrology, and the study of material properties. The nitrogen-vacancy (NV) color center in diamond is one such solid-state system that has shown great promise as an optically addressable spin qubit and highly sensitive magnetometer. We present recent work on coherent control of spin-photon interactions in a complex solid-state environment using coherent population trapping (CPT). The intrinsic magnetic field sensitivity of our CPT scheme allows us to measure the instantaneous Overhauser field associated with the $^{13}$C bath present in the diamond crystal. We show that this quantum measurement technique can be used to prepare a state of the $^{13}$C bath that has much reduced uncertainty in the associated Overhauser field. Such a state is verified by observing a modification and narrowing of the transmission window. The preparation of a more well-defined configuration of the nuclear spin environment could lead to an increase in the coherence times of the NV electronic spin qubit, which in turn has applications in increasing the sensitivity of NV-based magnetometers.   

Substitutional nickel impurities in diamond: decoherence-free subspace for quantum information processing

Magnetic color centers in diamond have received interest as qubits for quantum information processing due to diamond's wide band gap and long spin lifetimes which offer the possibility to initialize, manipulate and readout the quantum state of the qubit. Ni-related impurities have been known to form various color centers in diamond and here we propose the use of a substitutional Ni impurity as a qubit. The electronic and magnetic properties of a neutral substitutional nickel impurity in diamond are studied using density functional theory in the generalized gradient approximation. The spin-one ground state consists of two electrons with parallel spins, one located on the nickel ion in the 3d9 configuration and the other distributed among the nearest-neighbor carbons. The exchange interaction between these spins is due to p-d hybridization and is controllable with compressive hydrostatic or uniaxial strain. For sufficient strain the antiparallel spin configuration becomes the ground state. Hence, the Ni impurity forms a controllable two-electron exchange-coupled system that should be a robust qubit for solid-state quantum information processing.   

Cooling Nuclear Spins in Diamond via Dark State Spectroscopy

Optical cooling methods in atomic physics, developed over the last half century, enable reaching temperatures as low as a few nK. Some of these methods can be applied for cooling spin ensembles in solid state systems. We describe a method for cooling the nuclear spins of$^{ 13}$C impurities in diamond, via optical manipulation of the electronic spin associated with an NV$^{-}$~center. We present the physical mechanism which leads to optical pumping of the nuclear spin ensemble into particular nuclear states. The method relies on optically driving three electronic levels in the $\Lambda $ configuration, and on using the formation of dark states under the conditions of Coherent Population Trapping, (CPT). The dynamics of the nuclear ensemble during this cooling process can be described analytically by using statistical tools, including anomalous random walk models and Levy flights. I survey the theoretical results of the model and discuss some predictions for experimental signatures of Levy flights in this system.   

Decoherence imaging of spin ensembles by a scannable single nitrogen-vacancy center in diamond

Measuring the decoherence of the spin state of a single nitrogen-vacancy (NV) center in diamond has been proposed as a sensitive method for detecting ensembles of electron or nuclear spins. Using a scanning NV center magnetometer with a single NV residing about 10 nm from the scanning device surface, we explore the effect of T$_{2}$ as the device is brought in close proximity to a sample surface. We observe that the spin coherence of the NV center is strongly reduced when it comes into contact with the sample. We are able to restore the coherence by performing dynamic decoupling schemes on the NV spin, suggesting that the sample-induced decoherence originates from the fluctuating magnetic field of a surface spin ensemble. The decoherence diminishes when we increase the NV to sample surface distance by 10nm either vertically or laterally. These experiments demonstrate the potential for using the coherence of a single NV spin to locally detect and spatially map spin ensembles.   

Investigating spin decoherence in nanodiamonds using a multi-frequency electron spin resonance

Nitrogen-vacancy (NV) centers in nanodiamonds (NDs) are extremely useful for applications of nanoscale magnetic sensing as well as for conducting fundamental science because of their unique spin properties including capability to initialize the NV spin state and their long decoherence time even at room temperature. Various sizes of nanodiamonds are commercially available. Spin properties of NV centers in NDs are often quite different from those of bulk diamonds. Possible reasons for the difference are surface defects and surface distortions and strains. In this presentation, we will discuss spin properties and spin decoherence in various NDs studied by X-band (10 GHz) and high field (230 and 115 GHz) spectrometers. We will also study properties of environmental noises in NDs using dynamical decoupling techniques.   

Deep Level Tight-Binding Model for Transition Metal Dopant States in Diamond

Diamond is a promising system for quantum information processing [1], providing the possibility of single-spin-photon entanglement, as well as the potential for high-speed spin manipulation at room temperature (such as has been demonstrated for the electronic spin associated with an NV center [2]). Ion implantation has been demonstrated for controllable positioning of NV centers; in principle other dopants could be so implanted as well. For example, transition-metal dopants could potentially be used as optically and electrically active single spin qubits [3]. Here we use a deep level tight binding model to study the electronic trends and defect wave functions of transition-metal dopants in diamond. Starting with the Green's functions of homogeneous diamond (within an spds* tight-binding model), a Koster-Slater approach is used to evaluate the defect state. This work is supported by an AFOSR MURI.\\[4pt] [1] A. M. Stoneham, A. H. Harker and G. W. Morley, J. Phys.: Condns. Matter 21, 364222 (2009).\\[0pt] [2] R. Hanson, O. Gywat and D. D. Awschalom, Phys. Rev. B. 74, 161203(r) (2006).\\[0pt] [3] R. Larico, et. al., Phys. Rev. B. 79, 115202 (2009).   

A method to measure hyperfine interaction beyond standard statistical limit

We propose a method to measure the hyperfine interaction between a single electron spin and a nuclear spin and apply it to the trapped electron and the $^{15}$N nuclear spin in a diamond nitrogen-vacancy (NV) center. The electron spin is prepared in a pure quantum state and the nuclear spin acquires an unknown partial polarization as a consequence of the preparation of the electron state. The proposed quantum measurement protocol is independent of the incoherence of the initial nuclear spin state. The model utilizes the time $\tau$ of a sequence of quantum operations as well as the number $N$ of repeated sequences to increase the accuracy and the precision of the estimation. The deleterious effect of the electron spin decoherence on the precision during time $\tau$ is included in the simulation. While in the statistical limit the standard deviation (measure of imprecision) of the estimation is proportional to $1/\sqrt{N}$, the quantum operations in time $\tau$ enables the imprecision to decrease faster as $1/\tau$ instead of the equivalent statistical limit of $1/\sqrt{\tau}$. Thus, the net imprecision of the estimation dips below the statistical limit. The robustness test of our simulation shows that experimental implementation of such a precision measurement is possible.   

Towards Probing Living Cell Function with NV Centers in Nanodiamonds

We report on recent progress in using the nitrogen-vacancy (NV) center in nanodiamonds as a local probe of paramagnetic free radical concentrations in living cells. The ability to monitor the local magnetic environment within the cell provides us a new tool to study organelle function during normal operation or in response to applied stimuli. Our approach involving biologically inert, robust sensor of local magnetic fields with nanoscale resolution opens up a new interface between quantum and biological sciences.   

Anomalous decoherence of a nitrogen-vacancy center in a nuclear spin bath

A spin loses it coherence in its noisy environment. It is generally believed that stronger noise causes faster decoherence. Here we show that in a quantum bath, the case can be the opposite. We predict that the double-transition of a nitrogen-vacancy center spin-1 in diamond can have longer coherence time than the single transitions, even though the former suffers twice stronger noise from the nuclear spin bath than the latter. This anomalous decoherence effect is due to manipulation of the bath evolution via flips of the center spin.   

Manipulation of $^{13}$C nuclear spins in diamond via dynamical decoupling control of the electron spin

Utilizing the anisotropic nature of the hyperfine coupling between a negatively charged nitrogen-vacancy (NV) center spin and a moderately separated $^{13} $C nuclear spin, we present a scheme to efficiently control the $^{13}$C spin in 3 to 4 pulse cycles. This scheme uses only microwave pulses tuned to swap the NV center spin between the $m_s = 0$ and $m_s = 1$ states. With a strong magnetic field of the order of $10^{3}$ G along the NV center symmetry axis, the nuclear spin can be flipped in approximately $10~\mu s$. We also numerically study the effect of various sources of errors in realistic scenario and demonstrate that the fidelity of the scheme is satisfactory. The pulse sequences can be readily generalized to perform any single qubit operation on the nuclear spin.   

Narrowing Overhauser field by coherent population trapping

By optical coupling to an excited state, an electron spin may be trapped to a coherent dark state when the Overhauser field has a proper value. At the same time, the fluctuation of nuclear spins is suppressed. Although the electron may jump out of the dark state due to the Fermi-contact hyperfine interaction between the electron and the nuclei, it still has a predominant probability to stay at the dark state. The narrowing of the Overhauser field prolongs the coherent time of the electron spin. The Overhauser field is always suppressed as the external magnetic field varies in a large range. A window with a long coherent time forms when we scan the external magnetic field. This work is supported by Hong Kong RGC/GRF Project CUHK402209 and NSFC/RGC Joint Project N{\_}CUHK403/11.   

Spin echo probe of quantum criticality in the one-dimensional transverse-field Ising model

We study the detection of quantum phase transition in the one dimensional transverse-field Ising model. In the model, we attach a spin-1/2 to the Ising spin chain as a probe and study the decoherence behavior of the probing spin with spin echo techniques. We evaluate the effective spectrum density of the local field that the Ising spin chain impart to the probing spin and identify the contributions from the thermal fluctuations and quantum fluctuations to the probe spin decoherence. The output of our work provides a better understanding of the quantum phase transition of this model. This work is supported by Hong Kong RGC/GRF Project CUHK 402410 and NSFC/RGC Joint Project N{\_}CUHK403/11.