Session Q46: Invited Session: Quantum Information Processing in Diamond

Single spins in diamond: scalable quantum registers and nanoscale sensors

Ability to detect single atoms is a key element of several key technologies of including quantum information processing, communications as well as nanoscale imaging and sensing. Usually single atom control techniques are limited to low temperature operation. In this talk we will show that single spins associated with nitrogen-vacancy defects diamond (NV centers) can be used as nanoscale magnetic field sensor allowing detecting magnetic moment associated with single electrons under ambient conditions. We also show that coupled arrays of spins can be used as building blocks for scalable quantum registers.   

Towards large-scale quantum computing using spins and photons on a chip

Nitrogen-vacancy (NV) centers in diamond are attractive candidates for quantum bits for quantum information processing. Theoretically it should be possible to build large-scale quantum optical networks with the NV-diamond system. We report HP Labs' recent results toward coupling chip-based optical cavities to negatively charged NV centers in two systems: all diamond micro-ring cavities coupled to native NV centers and GaP micro-ring cavities coupled to near surface centers formed by implantation and annealing. In both systems we observe an enhancement of the spontaneous emission rate into the NV zero-phonon line. Additionally we will discuss recent results in engineering the optical properties of near-surface NV centers suitable for photonic coupling.   

Probing the motion of a mechanical resonator via coherent coupling to a single spin qubit

Mechanical systems can be inuenced by a wide variety of extremely small forces, ranging from gravitational to optical, electrical, and magnetic. If the mechanical resonator is scaled down to nanometer-scale dimensions, these couplings can be harnessed to monitor and control individual quantum systems. In this talk, I will describe experiments in which the coherent evolution of a single electronic spin associated with a Nitrogen Vacancy (NV) center in diamond is coupled to the motion of a magnetized mechanical resonator. Specifically, we have used coherent manipulation of the NV spin to sense the resonator's Brownian motion under ambient conditions. Potential applications of th is technique include the detection of the zero-point uctuations of a mechanical resonator, the realization of strong spin-phonon coupling at a single quantum level, and the implementation of quantum spin transducers.   

Preparation, single-shot readout and long-distance coupling of solid-state quantum registers

A key challenge in quantum science is to robustly control and to couple long-lived quantum states in solids. In this talk, we report on our latest advances towards realizing long-distance quantum networks with spins in diamond. First, we demonstrate preparation and single-shot measurement of a quantum register containing up to four quantum bits [1]. Projective readout of the electron spin of a single NV center in diamond is achieved by resonant optical excitation. In combination with hyperfine-mediated quantum gates, this readout enables us to prepare and measure the state of multiple nuclear spin qubits with high fidelity. We show compatibility with qubit control by demonstrating initialization, coherent manipulation, and single-shot readout in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. Second, we observe quantum interference of photons emitted by two spatially separated NV centers [2]. By using electrical tuning of the optical transition frequencies, we are able to observe such interference even for initially dissimilar centers, indicating a viable path for scaling towards a multi-node diamond-based quantum network. We will present these results, along with our most recent data, and discuss the prospects of realizing quantum networks with NV centers in diamond in the near future. \\[4pt] [1] Nature 477, 574 (2011) \\[0pt] [2] arXiv 1110.3329   

Electrical tuning of single-photon emission in diamond devices

For quantum information applications, nitrogen-vacancy (NV) centers in diamond combine many of the advantages of atomic systems (optical access, millisecond spin-coherence times) with the engineering flexibility of solid-state devices. Recent demonstrations of coherent coupling between photons and individual NV-center spins [1,2] provide a route to integrating NV-center qubits within photonic networks and for on-demand generation of entangled single-spin/single-photon pairs. Such applications require the ability to tune the NV-center zero-phonon optical transitions, to compensate for natural sample inhomogeneities which perturb the electronic orbital states. Here we demonstrate the ability to electrically control the orbitals of individual NV centers by applying voltages to micron-scale surface gates [3]. Surprisingly, the local electric field experienced by an NV center is significantly enhanced by a photoinduced space charge resulting from photoionization of deep donor impurities within the diamond, even in high-purity single-crystal material ($<5$~ppb nitrogen content). Since the photoinduced electric fields are reproducible as a function of gate voltage and are predominantly directed perpendicular to the diamond surface, we can harness them to obtain three-dimensional control of the local electric field vector with surface gates alone. To demonstrate this technique, we tune the excited-state orbital doublet of a strained NV center to degeneracy, as required for some spin-photon entanglement protocols [2], and then adjust the optical transition frequency, showing that we can tune multiple NV centers to have the same degenerate transition energy. This method should enable the coherent coupling of multiple NV center spins to indistinguishable photons within a scalable photonic network. \\ \\ {[1]} B.~B.~Buckley, G.~D.~Fuchs, L.~C.~Bassett, and D.~D.~Awschalom, \emph{Science} \textbf{330}, 1212 (2010).\\ {[2]} E.~Togan \emph{et al.}, \emph{Nature} \textbf{466}, 730 (2010).\\ {[3]} L.~C.~Bassett, F.~J.~Heremans, C.~G.~Yale, B.~B.~Buckley, and D.~D.~Awschalom, \emph{Phys.~Rev.~Lett.} (in press).