Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L37: Focus Session: Semiconductor Qubits - Electrically Controlled Quantum Dots |
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Sponsoring Units: GQI Chair: Charles Marcus, University of Copenhagen Room: 212A |
Wednesday, March 4, 2015 8:00AM - 8:36AM |
L37.00001: Addressable single-spin control in multiple quantum dots coupled in series Invited Speaker: Takashi Nakajima Electron spin in semiconductor quantum dots (QDs) is promising building block of quantum computers for its controllability and potential scalability [1]. Recent experiments on GaAs QDs have demonstrated necessary ingredients of universal quantum gate operations: single-spin rotations by electron spin resonance (ESR) which is virtually free from the effect of nuclear spin fluctuation [2], and pulsed control of two-spin entanglement [3]. The scalability of this architecture, however, has remained to be demonstrated in the real world. In this talk, we will present our recent results on implementing single-spin-based qubits in triple, quadruple, and quintuple QDs based on a series coupled architecture defined by gate electrodes. Deterministic initialization of individual spin states and spin-state readout were performed by the pulse operation of detuning between two neighboring QDs. The spin state was coherently manipulated by ESR, where each spin in different QDs is addressed by the shift of the resonance frequency due to the inhomogeneous magnetic field induced by the micro magnet deposited on top of the QDs. Control of two-spin entanglement was also demonstrated. We will discuss key issues for implementing quantum algorithms based on three or more qubits, including the effect of a nuclear spin bath, single-shot readout fidelity, and tuning of multiple qubit devices. Our approaches to these issues will be also presented. This research is supported by Funding Program for World-Leading Innovative R\&D on Science and Technology (FIRST) from JSPS, IARPA project ``Multi-Qubit Coherent Operations'' through Copenhagen University, and Grant-in-Aid for Scientific Research from JSPS. \\[4pt] [1] D. Loss et al., Phys. Rev. A \textbf{57}, 120 (1998).\newline [2] J. Yoneda \textit{et al}., to appear in Phys. Rev. Lett.\newline [3] J. R. Petta \textit{et al}., Science \textbf{309}, 2180 (2005), R. Brunner \textit{et. al}, Phys. Rev. Lett. \textbf{107}, 1 (2011). [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L37.00002: Multi-qubit read-out of spin qubits in GaAs in a CCD like manner: the Spin qubit CCD Tim Baart, Mohammad Shafiei, Jules van Oven, Christian Reichl, Werner Wegscheider, Lieven Vandersypen Efficient implementation and characterization of quantum information protocols requires the ability to measure multiple qubits individually and in a single-shot manner. We reported a succesfull demonstration of two-qubit read-out in [1]. We now demonstrate the next step by reading out three individual spin qubits formed by a linear array of three quantum dots where each electron forms a single spin qubit. We introduce several strategies for multi-qubit measurements in dot arrays and demonstrate and implement the following protocol experimentally. We first read-out the right qubit using standard spin-to-charge conversion [2]. Next we shuttle the centre electron to the right dot and read out its spin state. Afterwards we shuttle the left qubit through the centre dot to the right, and complete the three-qubit read-out. Due to its resemblance with reading out a CCD, we coin this the Spin qubit CCD. This is the first demonstration of reading out multiple qubits through the same reservoir and allows scaling to larger arrays of qubits.\\[4pt] [1] K.C. Nowack \emph{et al.}, Science {\bf 333}, 1269 (2011).\\[0pt] [2] J.M. Elzerman \emph{et al.}, Nature {\bf 430}, 431-435 (2004). [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L37.00003: Experimental Implementation of High-Fidelity Single-Qubit Gates for Two-Electron Spin Qubits in GaAs Pascal Cerfontaine, Tim Botzem, Hendrik Bluhm High fidelity gate operations for manipulating individual and multiple qubits in the presence of decoherence are a prerequisite for fault-tolerant quantum information processing. However, the control methods used in earlier experiments on GaAs two-electron spin qubits are based on unrealistic approximations which preclude reaching the required fidelities. An attractive remedy is to use control pulses found in numerical simulations that minimize the infidelity from decoherence and take the experimentally important imperfections and constraints into account [1]. We show that the experimental implementation of these numerically optimized control pulses is possible by using a self-consistent calibration routine we proposed earlier [1]. In our experiment this calibration routine succeeds in removing systematic gate errors to a high degree without increasing the pulses' decoherence. We extract the Bloch sphere trajectories of the resulting gate sequences using self-consistent state tomography and find good agreement with the theoretically predicted trajectories. Furthermore, we prepare different states using these gates and determine their fidelities.\\[4pt] [1] P. Cerfontaine, T. Botzem, D. P. DiVincenzo, and H. Bluhm, Physical Review Letters \textbf{113}(15), 2014. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L37.00004: Directly accessible entangling gates for capacitively coupled singlet-triplet qubits Fernando Calderon-Vargas, Jason Kestner In view of recent experimental demonstration of entanglement in capacitively coupled singlet-triplet qubits, we address the open question of what type of entangling gates the system's Hamiltonian can produce directly via a single square pulse. In the analysis we consider the system's Hamiltonian from first principles, incorporating the three different ways in which the system can be biased, and use the representation of its nonlocal properties in terms of local invariants. We find that, in one of the possible biasing modes, the Hamiltonian has an especially simple form, which can directly generate a wide range of different entangling gates including the iSWAP gate. Moreover, using the complete form of the Hamiltonian we find that, for any biasing mode, a CNOT gate can be generated directly [1]. \\[4pt] [1] F.A. Calderon-Vargas, J.P. Kestner, ``Directly accessible entangling gates for capacitively coupled singlet-triplet qubits,'' arXiv:1409.6292 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L37.00005: Improving the gate fidelity of capacitively coupled spin qubits Xin Wang, Edwin Barnes Precise execution of quantum gates acting on two or multiple qubits is essential to quantum computation. For semiconductor spin qubits coupled via capacitive interaction, the best fidelity for a two-qubit gate demonstrated so far is around 70\%, insufficient for fault-tolerant quantum computation. In this talk we present control protocols that may substantially improve the robustness of two-qubit gates against both nuclear noise and charge noise. Our pulse sequences incorporate simultaneous dynamical decoupling protocols and are simple enough for immediate experimental realization. Together with existing control protocols for single-qubit gates, our results constitute an important step toward scalable quantum computation using spin qubits. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L37.00006: Multi-level interference resonances in strongly-driven three-level systems Jeroen Danon, Mark Rudner We study multi-photon resonances in a strongly-driven three-level quantum system, where one level is periodically swept through a pair of levels with constant energy separation $E$. Near the multi-photon resonance condition $n\hbar\omega = E$, where $n$ is an integer, we find qualitatively different behavior for $n$ even or odd. We explain this phenomenon in terms of families of interfering trajectories of the multi-level system. Remarkably, the behavior is insensitive to fluctuations of the energy of the driven level, and survives deep into the strong dephasing regime. The setup can be relevant for a variety of solid state and atomic or molecular systems. In particular, it provides a clear mechanism to explain recent puzzling experimental observations in strongly-driven double quantum dots. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L37.00007: Fast Long-Distance Control of Spin Qubits by Photon Assisted Cotunneling Peter Stano, Jelena Klinovaja, Floris Braakman, Lieven Vandersypen, Daniel Loss We investigate theoretically the long-distance coupling and spin exchange in an array of quantum dot spin qubits in the presence of microwaves. We find that photon assisted cotunneling is boosted at resonances between photon and energies of virtually occupied excited states and show how to make it spin selective. We identify configurations that enable fast switching and spin echo sequences for efficient and non-local manipulation of spin qubits. We devise configurations in which the near-resonantly boosted cotunneling provides non-local coupling which, up to certain limit, does not diminish with distance between the manipulated dots before it decays weakly with inverse distance. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L37.00008: Efficient suppression of Overhauser field fluctuations with DNP Robert McNeil, Tim Botzem, Stefanie Tenberg, Sebastian Rubbert, Hendrik Bluhm In certain spin-qubit schemes the Overhauser field is a tuned control parameter and in many spin qubits this fluctuating nuclear field is a significant factor limiting coherence. Nuclear spins can be driven via dynamic nuclear polarisation (DNP) to a chosen field and selective feedback applied narrowing the distribution of nuclear Overhauser field fluctuations[1]. The achievable narrowing of the Overhauser field is related to the maximum pump rate and previous experiments on gated GaAs quantum dots were limited by the pump rate of the pumping mechanism used. We present a method to reduce nuclear fluctuations by increasing the max achievable pump rate. Sequentially applying two ac electric fields with frequencies slightly detuned from the desired Larmor frequency results in a pump curve with a stable fixed point. In the absence of spin-orbit interaction, driving electron spin flips via electric dipole spin resonance (EDSR)[2] will also drive nuclear spin flips and this scheme is expected to result in stronger pumping and efficient suppression of the Overhauser field fluctuations. We will present experimental evidence of this driven nuclear polarization including tracking of EDSR resonances.\\ 1. Bluhm, et al. PRL 105, 216803 ('10)\\ 2. Laird, et al. PRL 99, 246601 ('07)\\ [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L37.00009: Anisotropy and quadrupolar effects on dephasing in two-electron spin qubits in GaAs Tim Botzem, Robert McNeil, Hendrik Bluhm Understanding the dynamics of nuclear spins causing decoherence of gate-defined two-electron spin qubits in GaAs is a crucial prerequisite for a potential use in quantum computation. We present B-field dependent Hahn echo measurements giving new insight on the mechanism causing dephasing due to the nuclear spin bath of the host material GaAs. By rotating the magnetic field inplane we discover two effects ultimately limiting coherence times. We find that quadrupolar interaction between nuclear spins and electrical fields contributes to broadening of the nuclear Lamor frequencies, which in turn degrades electron coherence. By rotation the magnetic field towards the [100] direction, we can minimize this effect, but an additional envelope modulation that can be attributed to a electron g-factor anisotropy occurs. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L37.00010: Pumping of Dynamic Nuclear Polarization in GaAs Double Quantum Dots Arijeet Pal, John Nichol, Michael Shulman, Shannon Harvey, Emmanuel Rashba, Amir Yacoby, Bertrand Halperin Control of nuclear spins in semiconductors is essential for spin qubits realized using gate-defined quantum dots (QDs) for quantum computing. One qubit realization uses the singlet ($S$) and triplet $S_z=0$ $(T_0)$ states of two electrons in a double QD. The difference in the Overhauser fields on the two dots provides an axis of rotation on the Bloch sphere orthogonal to the one produced by the exchange interaction. These fields, in turn, may be modified by a dynamic nuclear polarization protocol, in which the electronic system is swept repeatedly through the level crossing between the $S$ and $T_+$ states. In any given sweep, the hyperfine interaction may cause a transition from $S$ to $T_+$, thereby transferring electronic spin polarization to the nuclear spins. We find the dependence of the polarization process on the asymmetry of the electron wave function, which is induced by the Zeeman field even in geometrically symmetric dots and which leads to pumping of the difference in Overhauser fields. We further report on correlations between $S-T_+$ transitions, which capture the macroscopic nuclear spin dynamics and the various relaxation mechanisms in this system. A semi-classical theoretical model is formulated which is in good agreement with the experimental observations. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L37.00011: Microscopic models for charge-noise-induced dephasing of solid-state qubits F\'elix Beaudoin, William A. Coish Several experiments have shown qubit coherence decay of the form $\exp[-(t/T_2)^\alpha]$ due to environmental charge-noise fluctuations. We present a microscopic description for temperature dependences of the parameters $T_2$ and $\alpha$. Our description is appropriate to qubits in semiconductors coupled to spurious two-level charge fluctuators coupled to a thermal bath. We find distinct power-law dependences of $T_2$ and $\alpha$ on temperature depending on the nature of the interaction of the fluctuators with the associated bath. We consider fluctuator dynamics induced by first- and second-order tunneling with a continuum of delocalized electron states. We also study one- and two-phonon processes for fluctuators in either GaAs or Si. These results can be used to identify dominant charge-dephasing mechanisms and suppress them. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L37.00012: Scaling of Decoherence for a Decoupled Multi-spin System Xuedong Hu, Jun Jing We study the decoherence of $n$ confined but decoupled electron spin qubits by examining the state fidelity for various initial states under the influence of hyperfine interaction with local environmental nuclear spins. We find that $n$-qubit inhomogeneous broadening time $T_2^*(n)$ and the narrowed-state free induction decay time $T_2(n)$ have the same scaling behaviors. For a superposed state whose product basis states are all from the same Zeeman manifold, both $T_2^*(n)$ and $T_2(n)$ are scale-free with respect to $n$ and the number of basis states, $m$. For a superposed state whose product basis states are selected from different Zeeman manifolds, both $T_2(n)$ and $T_2^*(n)$ are roughly inversely proportional to $\sqrt{n}$. Our results can be extended to other decoherence mechanisms, and to decoherence mechanisms in the presence of dynamical decoupling, such as decay of spin echo. This analysis should allow a more meaningful discussion on the scalability of any spin-based solid state quantum technology. [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L37.00013: Coherence preservation of a qubit inflicted by classical non-Gaussian charge noise Guy Ramon The efficiency of decoupling pulse sequences in removing noise due to several charge fluctuators is studied. Both numerical simulations and analytics are used to explore the qubit's dephasing and dissipative dynamics. Special emphasis is placed on qubit dynamics at the optimal point, where it is found that fluctuators that are strongly coupled to the qubit induce a non-Gaussian noise. Exact analytical results for this limit reveal a nontrivial scaling of the noise with the number of fluctuators. Furthermore, a crossover between distinct qubit dynamics is demonstrated by increasing the number of control pulses and/or varying the qubit's working position. While we consider as a test case exchange-coupled spin qubits in gate-defined GaAs double dots, our results are relevant to other systems such as superconducting Josephson qubits, and Si/SiGe quantum dots. [Preview Abstract] |
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