Session H48: Single-Spin Systems in Semiconductors

Increasing Spin Coherence in Nanodiamond via Dynamic Nuclear Polarization

Nanodiamonds are of interest for quantum information technology, as metrological sensors, and more recently as a probe of biological environments. Here we present results examining how intrinsic defects can be used for dynamic nuclear polarization that leads to a dramatic increase in both T$_{1}$ and T$_{2}$ for $^{13}$C spins in nanodiamond. Mechanisms to explain this enhancement are discussed.   

Dynamically Decoupled $^{13}$C Spins in Hyperpolarized Nanodiamond

The spin-spin relaxation time, T$_2$, which determines how long a quantum state remains coherent, is an important factor for many applications ranging from MRI to quantum computing. A common technique used in quantum information technology to extend the T$_2$, involves averaging out certain noise spectra via dynamical decoupling sequences. Depending on the nature of the noise in the system, specific sequences, such as CPMG, UDD or KDD, can be tailored to optimize T$_2$. Here we combine hyperpolarization techniques and dynamical decoupling sequences to extend the T$_2$ of $^{13}$C nuclear spins in nanodiamond by three orders of magnitude.   

Sensitive Detection of Weak Dynamic Nuclear Polarization

Capable of detecting nanotesla magnetic fields with nanometer resolution, shallow NV centers in diamond have recently been investigated as NMR sensors that can distinguish different biological molecules at the single molecule level. While NV centers closer to the diamond surface are generally more sensitive to a smaller number of molecular nuclear spins, NV centers shallower than about 2 nm are unstable and eventually become insensitive to the nuclear magnetic fields of nearby molecules. This places a significant limit on the strength of dipolar coupling between molecular spins and NV centers. The challenge in distinguishing between different molecules is in obtaining a strong structure dependent signal despite this weak coupling. One such signature could potentially be obtained by using the NV center as both a detector of nuclear magnetic fields and a source of dynamic nuclear polarization. However, the weak dipolar coupling significantly limits the amount of dynamic nuclear polarization the NV can impart to external molecules. Here we introduce a measurement scheme to sensitively detect weak levels of dynamic nuclear polarization even when greatly exceeded by the statistical polarization of nearby nuclei.   

Accelerating quantum control of spins in semiconductors with superadiabatic dynamics

Adiabatic processes are widely used tools in quantum science, providing robust control of spin or motional states and enabling adiabatic quantum simulation. However, the long evolution time required to maintain adiabaticity exacerbates the effect of quantum decoherence. A general strategy termed ‘shortcuts to adiabaticity’ (STA) aims to remedy this vulnerability by designing judicious fast dynamics to reproduce the results of slow, adiabatic evolutions. Recently, an STA technique known as superadiabatic transitionless driving (SATD) was proposed to provide a flexible set of dressed-state shortcuts [1]. In this talk, I discuss the demonstration of SATD to speed up stimulated Raman adiabatic passage in a solid-state lambda system based on the spin of a single nitrogen-vacancy center in diamond [2]. Driving optical transitions to a dissipative excited state with resonant laser pulses shaped on nanosecond timescales, I characterize the accelerated performance of different SATD shortcuts for the initialization and transfer of quantum states, including coherent superpositions. SATD protocols exhibit robustness to dissipation and experimental uncertainty, and can be optimized when these effects are present. This realization of superadiabatic dynamics in a three-level system suggests useful applications of STA to a variety of dissipative settings, including hybrid quantum systems of spins linked by intermediary couplings. \\[4pt] [1] A. Baksic, H. Ribeiro, and A. A. Clerk, Phys. Rev. Lett. 116, 230503 (2016). \\[0pt] [2] B. B. Zhou, et al., arXiv:1607.06503 (2016). Accepted in Nature Phys. \\[4pt] In collaboration with A. Baksic, H. Ribeiro, C. G. Yale, F. J. Heremans, P. C. Jerger, A. Auer, G. Burkard, A. A. Clerk, and D. D. Awschalom.   

Shortcut toAdiabatic Passage in a NV Center

Adiabatic population transfer in a three-level lambda system can enable quantum control of the electron spin states of a nitrogen vacancy (NV) center in diamond through optical transitions, facilitating the control of spin as well as mechanical degrees of freedom. However, in practical experiments with desired duration, the adiabaticity is not attainable due to the residual excitation and population of the excited state, which inevitably leads to optically-induced decoherence. Here we apply the technique of shortcut-to-adiabatic-passage to a three-level lambda system in a NV center. Our method uses a microwave pulse to drive the transition between the two lower states of the lambda system. This microwave coupling serves as a counterdiabatic driving, effectively suppressing the excitation of the excited states. For a coupled spin-phonon system with excited-state mediated coupling, this technique can be used to increase the spin-phonon coupling rate, while avoiding optically induced decoherence.   

Coherent coupling between microwave and optical fields in NV centers in diamond

The spin and electronic structure of Nitrogen Vacancy centers in diamond has been shown to have promising properties for use in quantum information, including a ground state spin coherence lifetime on the order of milliseconds. Transitions in the ground state triplet occur at radio frequencies while transitions into the first excited state occur in the optical spectrum (637nm). The prevalent use of both frequency regimes as well as the benefits of using hybrid systems in quantum information encourages us to explore the coherent coupling between microwave and optical fields within the energy level structure of the NV center.   

Development of nanoscale ESR techniques using nitrogen-vacancy centers in diamond

A nitrogen-vacancy (NV) center in diamond is a promising sensor for nanoscale magnetic sensing. Spin sensitivity of electron spin resonance (ESR) spectroscopy and the spatial resolution of the ESR active volume are drastically improved using NV centers. Here we discuss development of nanoscale ESR techniques using a single NV center in diamond. We employ the free-induction decay, T$_{\mathrm{2}}$ and T$_{\mathrm{1}}$ measurements of the single NV center to study static and dynamic properties of nanoscale bath spins surrounding the NV center. The detected bath spins can be identified by analyzing ESR spectrum of the bath spins using double electron-electron resonance spectroscopy [1]. We also plan to discuss the investigation of NV centers in various diamond crystals [1] C. Abeywardana et al, J. Appl. Phys. 120, 123907 (2016).   

Spin cat state generation for quadrupolar nuclei in semiconductor quantum dots or defect centers

Implementing spin-based quantum information encoding schemes in semiconductors has a high priority. The so-called cat codes offer a paradigm that enables hardware-efficient error correction. Their inauguration to semiconductor-based nuclear magnetic resonance framework hinges upon the realization of coherent spin states (CSS). In this work, we show how the crucial superpositions of CSS can be generated for the nuclear spins. This is through the intrinsic electric quadrupole interaction involving a critical role by the biaxiality term that is readily available, as in strained heterostructures of semiconductors, or defect centers having nearby quadrupolar spins. The persistence of the cat states is achieved using a rotation pulse so as to harness the underlying fixed points of the classical Hamiltonian. We classify the two distinct types as polar- and equator-bound over the Bloch sphere with respect to principal axes. Their optimal performance as well as sensitivity under numerous parameter deviations are analyzed. Finally, we present how these modulo-2 cat states can be extended to modulo-4 by a three-pulse scheme.   

Probing the coherent oscillations of the electron-nuclear spin system of Ga paramagnetic centers in GaAsN by band-to-band photoluminescence.

Optically or electrically detected magnetic resonance techniques are consistently employed for manipulating and probing the defect spins through the hyperfine interaction, or to identify the defect chemical nature. Here, we demonstrate a novel detection scheme of the hyperfine interaction features of gallium paramagnetic centers in GaAsN based on the measurement in the time domain of the coherent electron-nuclear spin oscillations through the band-to-band photoluminescence. A pump and probe photoluminescence experiment leads to the measurement of the hyperfine constant of the coupled electron-gallium center by directly tracing the hyperfine interaction dynamical behavior. The hyperfine constants, defect configuration and the relative abundance of the nuclei involved can be determined without the need of electron spin resonance techniques and in the absence of any magnetic field. Information on the nuclear and electron spin relaxation damping parameters can also be estimated from the oscillations damping and the long delay decay of nuclear spin memory effect.   

Optimized polarization dynamics through dissipation in a central-spin system

We study the zero-temperature phase diagram and the dissipative dynamics of the central-spin system, where one ``central" spin is homogeneously coupled with many ``ancilla" spins. An archetypical example of this model is given by an electron spin coupled to nuclear spins in a quantum dot via hyperfine interactions. This same central-spin model has been shown to improve the efficiency of quantum-annealing protocols. We establish the zero-temperature phase diagram with phases characterized by the polarization of the ancilla spins relative to the central spin. By rapidly tuning a parameter in the Hamiltonian, the ancilla-spins polarization can be rapidly modified through a dissipative equilibration process mediated by the central spin. Remarkably, we find that the dissipation rate can be optimized to minimize the time scale for polarization dynamics. These results may be important for protocols to quickly polarize nuclear spins in semiconductor quantum dots or to rapidly and efficiently equilibrate a quantum annealer.   

Electron Spin Coherence of Silicon Vacancies in Proton-Irradiated 4H-SiC

We report $T_{\mathrm{2}}$ spin coherence times for electronic states of Si vacancies in 4H-SiC. Our spin coherence study included two SiC samples that were irradiated with 2 MeV protons at different fluences (10$^{\mathrm{13}}$ and 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}})$. Using optically detected magnetic resonance and spin echo, the coherence times for each sample were measured across a range of temperatures from 8 K to 295 K. All echo experiments were done at a magnetic field strength of 0.371 T and a microwave frequency of 10.49 GHz. The longest coherence times were obtained at 8 K, being 270 $\mu $s for the 10$^{\mathrm{13}}$~cm$^{\mathrm{-2}}$ proton-irradiated sample and 104 $\mu $s for the 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}}$ sample. The coherence times for both samples displayed unusual temperature dependences; in particular, they decreased with temperature until 60 K, then increased until 160 K, then decreased again. This increase between 60 and 160 K is tentatively attributed to a motional Jahn-Teller effect. The consistently longer lifetimes for the 10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ sample suggest that a significant source of the spin dephasing can be attributed to dipole-dipole interactions between Si vacancies or with other defects produced by the proton irradiation. The lack of a simple exponential decay for our 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}}$ sample indicates an inhomogeneous distribution of defect spins.   

Nitrogen-vacancy centers orientation identification by Rabi frequency for vector magnetometry

Nitrogen-vacancy (NV) centers are atomic defects in diamond which can be initialized and read-out by laser pulses and manipulated by microwaves, in an optically detected magnetic resonance experiment (ODMR). The microsecond coherence time of NV ensembles at room temperature and the four possible NV orientations can be used for vectorial magnetometry. However, the NV orientation identification in an ODMR spectra requires the application of a known external magnetic field, which can affect the magnetization of the object under study. In this work, the magnetic bias field is replaced by a microwave field. Instead of measuring ODMR lines displacement, the Rabi frequency of three optically detected magnetic resonance features is measured. The four possible NV orientations are linked to the Rabi frequencies by an optimization technique. We show that taking the average of the Rabi frequency for the $m_s$=+1 and $m_s$=-1 states is a valid approximation in the case of a quasi perpendicular external magnetic field. The technique allows quick, non-invasive, vectorial magnetometry, applicable to other atomic defects and the measurement of dynamical magnetization phenomena.   

First-principles simulations of transition metal ions in silicon as potential quantum bits

Optically active spin defects in semiconductors have gained increasing attention in recent years for use as potential solid-state quantum bits (or qubits). Examples include the nitrogen-vacancy center in diamond, transition metal impurities, and rare earth ions. In this talk, we present first-principles theoretical results on group 6 transition metal ion (Chromium, Molybdenum and Tungsten) impurities in silicon, and we investigate their potential use as qubits. We used density functional theory (DFT) to calculate defect formation energies and we found that transition metal ions have lower formation energies at interstitial than substitutional sites. We also computed the electronic structure of the defects with particular attention to the position of the defect energy levels with respect to the silicon band edges. Based on our results, we will discuss the possibility of implementing qubits in silicon using group 6 transition metal ions.