Session H26: Spin Qubits - Diamond, III-Vs, and Coupling to Cavities

A tunable few electron triple quantum dot

We report on a new design to realize a fully tunable lateral triple quantum dot. The electrostatically defined quantum dots are arranged in series. The number of electrons in the quantum dots can be controlled and fundamental electronic configurations such as the (0,0,0) and (1,1,1) are obtained. Control of the number of electrons is important to perform quantum information processes using electron spins as qubits. Individual control of the tunnel barriers between the dots as well as resonant conditions called quadruple points where the chemical potential of the three quantum dots are aligned are achieved. A neighboring quantum point contact is used as a charge sensor to map out the charge stability diagram. We explore the back-action effects of the charge sensor on the triple quantum dot system and observe a series of additional resonances directly linked to the bias applied across the charge sensor.   

Coherent manipulation of a single charge in an isolated double quantum dot

The key requirement that a quantum computer needs to be scalable has motivated recent work demonstrating coherent control of two-level systems in semiconductor devices.\footnote{J. R. Petta et al., Science 309, 2180 (2005).} One very simple semiconductor two-level system is the `charge qubit' in which a single excess electron occupies either the left or right dot of a tunnel-coupled double quantum dot.\footnote{T. Hayashi et al., Phys Rev. Lett. 91, 226804 (2003).} In this work we establish coherent control of an isolated and tunable double dot containing a single electron. The qubit is manipulated by applying non-adiabatic voltage pulses to the surface gate electrodes. State readout is then performed non-invasively using a proximal quantum point contact charge detector.\footnote{M. Field et al., Phys Rev. Lett. 70, 1311 (1993).} We also perform microwave spectroscopy to verify that the frequency of the observed oscillations is consistent with the tunnel coupling strength. This isolated qubit provides a very fundamental system with which to study quantum coherence in semiconductor devices.   

Dynamic Nuclear Polarization in InAs Nanowire Few Electron Double Quantum Dots

Electrical transport measurements are performed in few electron double quantum dots defined by local gates in InAs nanowires. In the Pauli spin blockade regime the current through the double dot exhibits hysteresis in detuning and magnetic field. The abrupt switching between the low and the high current states is interpreted as polarization and depolarization of nuclear spins in the nanowire due to spin flips of electrons. The effective nuclear magnetic field is in the direction opposite to the applied magnetic field. The hysteresis can be extended to magnetic fields of several Tesla, suggesting a large degree of nuclear polarization.   

Mapping out spin blockade in the presence of strong spin-orbit and hyperfine interactions

Mixing of singlet and triplet states is investigated in the few electron double quantum dot defined in an InAs nanowire. Regular spin filling of the first four electrons in both quantum dots is confirmed by the Pauli spin blockade. For given magnetic field and detuning spin blockade can be suppressed or restored by changing the tunnel coupling. Contributions of spin orbit and hyperfine interactions are revealed by comparing the magnetic field and detuning dependences of the leakage current with the rate equation model.   

Electron transfer between distant quantum dots by surface acoustic waves

Quantum dots (QDs) provide a useful system for manipulating and storing quantum information. Tunneling of electrons between double dots has been demonstrated but over larger distances a tunnel barrier is inadequate. We show such long range transfer of single electrons between QDs through a depleted 1D channel using a surface acoustic wave (SAW) pulse. Surface gates define two QDs (RQD \& LQD) connected by a 4$\mu$m channel. Electrons trapped in RQD are raised above the Fermi energy by a gate sequence. Having set RQD to be ``full'' and LQD ``empty'' a SAW pulse is sent through the RQD towards the LQD. The SAW potential lifts and carries any electrons from RQD to LQD where they are trapped by a large exit gate voltage (transfer reliability 70\%.) Changes in QD occupation are monitored by 1D charge detectors. We have demonstrated the transfer of electrons between static and dynamic QDs. This technique may allow the movement of quantum information (spin) between processor and storage, or to a region of holes for conversion to photon qubits.   

Towards Quantum Computing with Electron Spin Ensembles

We describe a recent proposal [1] where a register of quantum bits is encoded into different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. Accessing different spin waves can be achieved by applying gradient magnetic fields across the sample [2], while a Cooper pair box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations. We also present experimental progress towards coupling spins to on-chip superconducting cavities. \\[4pt] [1] Wesenberg, et. al. Phys. Rev. Lett. 103, 070502 (2009) \\[0pt] [2] Anderson, et. al. J. Ap. Phys. 26, 11-1324 (1955)   

Coupling of Electron Spin Ensembles to Superconducting Transmission Line Resonators

Recent proposals have suggested using a mesoscopic ensemble of electron spins to create a quantum memory for superconducting qubits in solid state systems[1]. Such ensembles can have large cavity couplings ($\sim $MHz) and should have long coherence times. Here we show the measurement and coupling of electron spins in ruby and diamond to multiplexed superconducting coplanar waveguide (CPW) cavities, as well as broadband spectroscopy of ruby using a CPW transmission line. We discuss the application of these techniques to electron spin resonance at low power, millikelvin temperatures, and over many gigahertz and evaluate the suitability of our materials for quantum computing. \\[4pt] [1] Wesenberg J et al 2009 Phys. Rev. Lett. 103 070502   

Laser spectroscopy of a doubly charged quantum dot molecule

The spectra of a self-assembled quantum dot molecule (QDM) precharged with two carriers is presented. The QDM is deterministically charged with two electrons, where one resides in each dot. The finite tunnel coupling between these two electrons results in triplet states and a well separated singlet state. These four ground states can be optically excited to four molecular trion states. The fan diagram of this exciton system is investigated in Faraday and Voigt fields. At certain fields an anticrossing between the singlet and triplet ground states is observable, the magnitude of which gives a direct measure of the asymmetric exchange interaction in this system. The complimentary two-hole molecule is also studied (with a hole on each dot), where optical spin pumping is observed. These experiments are important in realizing an optically operated singlet-triplet qubit and realizing two-qubit entanglement in self-assembled quantum dots.   

Optical Control of an Electron Spin in a Single Self-Assembled Quantum Dot

Optically driven self-assembled quantum dots are a leading candidate for next generation quantum computers because of their high speed and potential for relatively compact design. In this approach, each dot is charged with a single electron (or hole) whose spin serves as the quantum bit (``qubit''). We present our recent experimental results demonstrating qubit initialization, rotation through a stimulated Raman excitation, and optical readout, as well as a geometric phase gate. Optically induced coupling between the dots through the negatively charged exciton should allow for the critical entangling operations and for conditional two-qubit gates. Conditional gates combined with single qubit gates form the building blocks from which any quantum computing algorithm can be constructed.   

Ultrafast optical spin echo of a single electron spin in a quantum dot

We report on the ultrafast optical implementation of a Hahn Echo sequence on a single electron spin in an InGaAs quantum dot. With this technique, we were able to overcome the shot-to-shot variations of the electron spin's magnetic environment in our multi-shot, time-averaged read-out scheme. We measured the electron spin coherence time T$_{2}$, both as a function of applied magnetic field, and for different types of sample surface treatment. Measured T$_{2}$-times of 3 $\mu $s, together with our experiment all-optical single spin rotation times of 30 ps, would allow 10$^{5}$ single qubit gate operations. Furthermore, we observe pronounced non-linear, hysteretic effects in a 2-pulse Ramsey interference experiment, which we attribute to an electron-spin dependent polarization of the nuclear spins. .   

Resilient neutral exciton qubits in self-assembled quantum dot molecules

We investigate the optical coherent manipulation of neutral indirect excitonic qubits in self-assembled quantum dot molecules (QDMs) [1]. Our studies include initialization with near unity fidelity, state shelving and rotations. A numerical Lindblad master equation approach, as well as analytical Feshbach self-consistent projection, allows us to extract the effective qubit dynamics and decoherence times. Near resonant excitation and electric field pulses drive vacuum-indirect transitions mediated by virtual levels outside the qubit subspace. Subsequent adiabatic pulses drive the initial state into a level anticrossing, where the qubit is controllably rotated by the internal dynamics of the system. The qubit subspace is surprisingly well isolated from external driving fields and is resilient to spontaneous exciton recombination. With low excitation power the dynamics is nearly coherent over hundreds of nanoseconds, while high excitation power produces virtual transitions mediated by biexciton states reducing somewhat the coherent time scales, but well within appropriate limits for quantum information processing. This opens the possibility of using indirect neutral exciton states as operating qubits, or auxiliary qubits within more complex quantum computation schemes in QDMs. [1] J.E. Rolon and S.E. Ulloa, Phys. Rev. B 79, 245309 (2009).   

Spatial placement limits of nitrogen vacancy centers in diamond via ion implantation

Nitrogen-vacancy (NV) centers in diamond are an interesting candidate for spin qubits in quantum information processing due to their long coherence times at room temperature. A common technique to place NV centers in diamond is via ion implantation [1] and thermal annealing. We discuss the contributions of ion beam spot size, range straggling and ion channeling as well as diffusion during annealing on the placement accuracy of nitrogen ions and NV-centers in diamond for ion implantation energies below 40 keV. Placement accuracy limits are compared to requirements for NV-center placement in proposed multi-qubit coupling schemes. [1] C. D. Weis et al., J.Vac.Sci.Tech. B 26, 2596 (2008)   

Decoherence of nitrogen-vacancy center spins in diamond and its control

We theoretically investigated the pure dephasing of NV spins in diamond coupled to carbon-13 spin baths with dipolar intra-bath interaction, using a many-body quantum theory [1]. Moreover, we study the control of decoherence with the Uhrig dynamical decoupling [2]. Under zero external magnetic field, the calculated free-induction decay time is about 1$\mu$s, and a 5- pulse Uhrig sequence brings the coherence time to above 250 $\mu$s. Under a strong magnetic field (5300 G), the free- induction decay time is about 2$\mu$s, and the coherence time under the 5-pulse Uhrig control is above 2.5 ms. This work was supported by Hong Kong GRF Project CUHK401906 and CUHK402209. References: 1. W. Yang and R. B. Liu, Phys. Rev. B 78, 085315 (2008); ibid 79, 115320 (2009). 2. G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007).   

Coherent spin manipulation of a single NV center in its orbital excited state

Nitrogen vacancy centers in diamond have emerged as a promising candidate for quantum information processing in the solid state at room temperature. Recent spectroscopic investigations of the spin levels in the orbital excited state have revealed sensitivity to strain and strong hyperfine interactions with the nitrogen nuclear spin [1]. By fabricating coplanar waveguides directly on diamond substrates [2] and using pulsed laser excitation combined with sub-nanosecond timing, we investigate the coherent dynamics of a single NV center spin in its excited state. We find spin lifetimes from the decay of Rabi oscillations that are much shorter than the orbital lifetime. Results are discussed in the context of orbital and spin dynamics of the NV center excited state manifold. \\[4pt] [1] G. D. Fuchs \textit{et al}., \textit{Phys. Rev. Lett.} \textbf{101}, 117601 (2008). \\[0pt] [2] G. D. Fuchs, \textit{et al}., \textit{Science} (2009) (10.1126/science.1181193).   

Spin decoherence of a single NV center in its orbital excited state

Nitrogen vacancy (NV) centers in diamond are promising candidates for quantum information processing at room temperature. Due to the peculiar structure of their excited state [1,2], the spin of a single NV center can be initialized and read out by optical means. Theoretical descriptions are provided for the spin decoherence in the excited state of NV centers caused by orbital coupling to phonons. We explain the shortened decay times seen in recent experiments on the excited-state Rabi oscillations and Ramsey fringes. Spin decoherence is found to be governed by the Raman processes, which also determine orbital decoherence [3]. This theory explains the appearance and positions of the excited-state ESR lines. [1] G. D. Fuchs et al., Phys. Rev. Lett. 101, 117601 (2008). [2] L. J. Rogers et al., New J. Phys. 11, 063007 (2009); A. Batalov et al., Phys. Rev. Lett. 102, 195506 (2009). [3] K.-M. C. Fu et al., arXiv:0910.0494.