Session D29: Focus Session: Qubits in Diamond II

Control of hybrid quantum registers in diamond

Nitrogen Vacancy (NV) centers in diamond provide a robust quantum register consisting of the NV electron spin, the nuclear spin of the associated nitrogen atom and nearby nuclear spins of carbon impurities ($^{13}$C). The constituents of this hybrid system each have different properties that make them ideal for different roles. For example, the electron spin is readily polarized and read-out optically using spin-dependent fluorescence, whereas long coherence times make the nuclear spins ideal quantum memories. However, because all these systems evolve and decohere on very different time scales, it is challenging to control the register and to protect it from decoherence due to the surrounding environment at the same time. Here we discuss our recent progress in initializing, controlling and reading out all the spins in such few-qubit NV quantum registers. To this end, we combine dynamical decoupling of the electron spin with control of all the nuclear spins, and explore different initialization schemes. These few qubit registers might then be used to implement quantum search algorithms and error correction protocols.   

Quantum interference of single photons from two remote Nitrogen-Vacancy centers in diamond

The interference of two identical photons impinging on a beam splitter leads to perfect photon coalescence where both photons leave through the same output port. This fundamental effect, known as Hong-Ou-Mandel (HOM) interference [1], can be used to characterize the properties of quantum emitters with high accuracy. This is a particularly useful tool for quantum emitters embedded in a solid state matrix because their internal properties, unlike those of atoms in free space, can vary substantially from emitter to emitter due to interactions with the environment. Here, we demonstrate HOM interference of photons emitted from two single Nitrogen-Vacancy (NV) centers in diamond that are spatially separated by 2 meters. The frequencies of the photons are controlled by tuning individual optical transitions of associated NVs via a DC electric field. The indistinguishability of the photons paves the way for entanglement generation between remote solid state qubits. [1] C. K. Hong, Z.Y. Ou, and L. Mandel, Phys. Rev. Lett. 59, 2044 (1987).   

Coherent excitation dynamics of single spins in diamond

The spin dynamics of nitrogen vacancy (NV) centers in diamond during non-resonant optical excitation are fundamentally important for both optical initialization and fluorescence-based spin read-out. While such processes rely on the preservation of the longitudinal spin projection, an NV center's lack of orbital coherence at room temperature might suggest that its quantum phase would be destroyed during excitation. We address this question using Ramsey experiments, quantum process tomography and theoretical modeling and establish limits on the coherence loss of an NV center during optical excitation [1]. By treating the excitation and spin precession as a quantum process, we measure a process fidelity of $F=0.87\pm0.03$, which includes excited state dephasing during measurement. Extrapolation to the moment of optical excitation yields $F\approx0.95$. These results provide a new understanding of NV centers' spin coherence during optical excitation and are crucial for efforts to use coherent evolution in the excited state for spin control. \\[4pt] [1] G. D. Fuchs, A. L. Falk, V. V. Dobrovitski, and D. D. Awschalom, \emph{submitted} (2011)   

Optimizing the resolution and the sensitivity of a scanning NV magnetometer

A nitrogen-vacancy (NV) center in diamond has been recently considered as an outstanding atomic-scale magnetic field sensor. At the heart of NV based magnetometry is the ability to control the position of an NV within few nanometers to a sample, while preserving its spin coherence and readout fidelity. To this end, we previously developed a fabrication procedure for creating a monolithic scanning diamond nanopillar containing a single NV center. Here we present further optimization of our devices by locating NV centers with nanoscale accuracy as well as improving their magnetic field sensitivty. For locating the NV center, we developed a nanoscale imaging of a NV center in the device via near-field fluorescence quenching, which facilitates post selection of devices with NV centers being closer to the surface. In addition to enhancing photon collection via wave guiding through the nanopillar, we also significantly improved the spin coherence times of our devices via dynamic decoupling. This results in a magnetic field sensitivity of 30 nT/sqrt(Hz), unprecedented for scanning NV centers.   

High Dynamic Range Magnetometry with a Single Spin in Diamond

Detection of the weak magnetic fields associated with nanometer sized volumes of spins could allow for non-invasive, element-specific probing of a variety of important physical and biological systems. Averaging out random noise which is the commonly used standard measurement strategy (SM) in most nano-sensors, will at best lead to a field variance that is inversely proportional to the total averaging time. Further, there exists a trade-off between the field sensitivity and the dynamic range in the SM. In this work, we demonstrate an alternative approach for accurate magnetic sensing, using novel phase estimation algorithms (PEA), implemented on a single electronic spin associated with the nitrogen-vacancy (NV) defect center in diamond. The field variance in our approach scales down faster than the SM. The trade-off between the field sensitivity and the dynamic range no longer exists in this approach. Our results show an improvement of $\sim 6.25 dB$ in the field sensitivity compared to the SM, over a large field sensing range ($ \sim \pm 0.3 mT$). Besides their direct impact on applications in demonstrated nanoscale magnetic sensing and imaging, this may also open the way for application of other quantum feedback and control techniques to magnetometry.   

Room temperature solid-state quantum bit with second-long memory

Realization of stable quantum bits (qubits) that can be prepared and measured with high fidelity and that are capable of storing quantum information for long times exceeding seconds is an outstanding challenge in quantum science and engineering. Here we report on the realization of such a stable quantum bit using an individual $^{13}$C nuclear spin within an isotopically purified diamond crystal at room temperature. Using an electronic spin associated with a nearby Nitrogen Vacancy color center, we demonstrate high fidelity initialization and readout of a single $^{13}$C qubit. Quantum memory lifetime exceeding one second is obtained by using dissipative optical decoupling from the electronic degree of freedom and applying a sequence of radio-frequency pulses to suppress effects from the dipole-dipole interactions of the $^{13}$C spin-bath. Techniques to further extend the quantum memory lifetime as well as the potential applications are also discussed.   

Mechanism for optical initialization of spin in NV$^{-}$ center in diamond

Optical initialization of the negatively charged nitrogen-vacancy (NV$^{-})$ center in diamond, the experimental manipulation of its degenerate mixed ground state into an un-entangled spin state through optical means, makes it one of the best candidates for realization of individually addressable spins in the solid state for quantum computing and other studies under ambient conditions. However, its exact mechanism is still not clear. Based on exact diagonalization of a many-electron Hamiltonian with parameters derived from ab initio GW calculations, the present study elucidates the electronic structure of the NV$^{- }$center and puts forward a concrete optical initialization mechanism. We calculated the ordering and energy surfaces of the low-energy many-body states and the relaxation processes of photo-excitation responsible for the optical initialization. Intersystem crossings are shown to be essential.   

Long-range quantum gates using dipolar crystals

We propose to use the magnetic dipole interaction in high density arrays of nitrogen-vacancy centers to enable long-range quantum logic between distant spin qubits. In our approach, an effective interaction between remote qubits is achieved by adiabatically following the ground state of the dipolar chain across a quantum phase transition [1]. We demonstrate that the proposed quantum gate is particularly robust against disorder and enables coherent coupling between qubits on distances that are compatible with sub-wavelength addressing techniques.\\[1em] [1] H. Weimer et al., arXiv:1009.1003 (2011).   

Magnetic imaging of a single electron spin using a scanning NV magnetometer under ambient conditions

It has long been an outstanding challenge to develop a magnetometer capable of detecting individual spins under ambient conditions. Nitrogen-vacancy (NV) centers are an attractive candidate for such a sensor, as even at room temperature their spins can be initialized and read out optically, have long coherence times, and are localized on atomic lengthscales. Here we present measurements using a scannable NV center to magnetically image the dipole field of a single electron spin. For a target spin, we chose to use an additional NV spin as it can be initialized and driven independently from other proximal spins, allowing us to perform dynamical decoupling schemes on both sensing and target NV spins. Magnetic images are taken under ambient conditions and are achieved through optimizing NV magnetic sensitivity ($<$60 nT/sqrt(Hz)) as well as minimizing the distance between the NV center and our target spin ($<$50nm).   

Fabrication of thin diamond membranes for quantum information processing

Coupling of nano-photonic devices to color centers in diamond offers exceptional opportunities to enhance our understanding of light-matter interactions. The formation of thin single crystal diamond membranes containing such centers, is an important prerequisite for the fabrication of diamond based devices. However, there are challenges in forming such membranes in ways that do not compromise the quality of the cavities or the optical properties of the emitters. Here we report the formation of optically active diamond membranes and the subsequent fabrication of optical cavities. In our approach, 1.7 $\mu $m thick diamond membranes were generated by forming a sacrificial layer using ion implantation, followed by thermal annealing. These membranes then served as templates for the epitaxial overgrowth of $\sim $ 300 nm of diamond using CVD. Remarkably, the regrown films reveal the presence of optically active defects which were \textit{not} present in the template, such as silicon-vacancy (SiV) or nitrogen vacancy centers. Microdisk cavities were then formed from the regrown single crystal diamond membranes. Whispering gallery modes (WGMs) with quality factors of $\sim $ 3000 were measured from the diamond cavities. Spectral overlap of WGMs with the zero phonon line of SiV centers was observed and lifetime reduction of the coupled emitter -- cavity system was measured. The demonstration of coupling between diamond emitters and a single crystal diamond cavity is a crucial step towards diamond integrated nano-photonic networks.   

Noises of spin baths for qubits in diamond

Nitrogen-vacancy (NV) center in diamond is a promising qubit candidate for quantum information processing and high precision magnetometry and is an excellent platform for studying quantum spin dynamics [1,2]. Overcoming spin decoherence of NV centers is critical to the applications. Coupling to spin baths of paramagnetic impurities and nuclei is a major decoherence source for NV centers. Therefore, recent theoretical and experimental efforts have aimed at suppressing the bath noises. In this presentation, we will discuss effects of the spin baths on the qubits at different regimes including high magnetic fields where the degree of the electron spin polarization is almost complete [3]. We will also discuss dynamical decoupling sequences to investigate spin bath noises. \\[4pt] [1] R. Hanson et al., \textit{Science.} \textbf{320}, 352 (2008). \\[0pt] [2] G. de Lange et al., \textit{Science}. \textbf{330}, 60 (2010) \\[0pt] [3] S. Takahashi et al., \textit{Phys. Rev. Lett.}\textbf{101}, 047601 (2008)   

Spectroscopy of composite solid-state spin environments for improved metrology with spin ensembles

For precision coherent measurements with ensembles of quantum spins the relevant Figure-of-Merit (FOM) is the product of spin density and coherence lifetime, which is generally limited by the dynamics of spin coupling to the environment. Significant effort has been invested in understanding the causes of decoherence in a diverse range of spin systems in order to increase the FOM and improve measurement sensitivity. Here, we apply a coherent spectroscopic technique to characterize the dynamics of a composite solid-state spin environment consisting of Nitrogen-Vacancy (NV) color centers in room temperature diamond coupled to baths of electronic spin (N) and nuclear spin (13C) impurities. For diamond samples with a wide range of NV densities and impurity spin concentrations we employ a dynamical decoupling technique to minimize coupling to the environment, and find similar values for the FOM, which is three orders of magnitude larger than previously achieved in any room-temperature solid-state spin system, and thus should enable greatly improved precision spin metrology. We also identify a suppression of electronic spin bath dynamics in the presence of a nuclear spin bath of sufficient nuclear spin concentration. This suppression could inform efforts to engineer samples with even larger FOM for solid-state spin ensemble metrology and collective quantum information processing.