Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C26: Semiconductor Qubits - Gates and Robust Control |
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Sponsoring Units: GQI Chair: Hendrik Blhum, RWTH Aachen Room: 328 |
Monday, March 18, 2013 2:30PM - 3:06PM |
C26.00001: Interplay of charge and spin coherence in Landau-Zener interferometry in double quantum dots Invited Speaker: Hugo Ribeiro Landau-Zener-St\"{u}ckelberg-Majorana (LZSM) physics has been exploited to coherently manipulate two-electron spin states in a GaAs double quantum dot (DQD) at a singlet (S)-triplet ($\textrm{T}_+$) anti-crossing. The anti-crossing results from the hyperfine interaction with the nuclear spins of the host material [1,2]. However, the fluctuations of the nuclear spin bath result in spin dephasing within $T_2^* \sim 10-20$ ns. As a consequence, the sweep through the anti-crossing would have to be performed on a timescale comparable to $T_2^*$ to achieve LZSM oscillations with 100\% visibility. Moreover, the S-$\textrm{T}_+$ anti-crossing is located near the $(1,1)-(2,0)$ interdot charge transition, where $(n_{l}, n_{\mathrm{r}})$ denotes the number of electrons in the left and right quantum dot. As a result the singlet state involved in the dynamics is a superpostion of $(1,1)$ and $(2, 0)$ singlet states. Here we show that it is possible to increase the oscillation visibility while keeping sweep times less than $T_2^*$ using a tailored pulse with a detuning dependent level velocity. The pulse includes a slow level velocity portion that is chosen to coincide with the passage through the S-$\textrm{T}_+$ anti-crossing and two fast level velocity portions. The latter minimize the time spent in regions where spin and charge degrees of freedom are entangled, which renders the qubit susceptible to charge noise. The slow level velocity portion of the pulse results in a stronger effective coupling between the spins states, which increases the oscillations visibility [3,4]. In particular, we were able to obtain a visibility of $\sim 0.5$ for LZSM oscillations. This constitutes an important step towards the implementation of a Hadamard gate.\\[4pt] [1] J. R. Petta, H. Lu, and A. C. Gossard, Science 327, \textbf{669} (2010).\\[0pt] [2] H. Ribeiro, J. R. Petta, and G. Burkard, Phys. Rev. B \textbf{82}, 115445 (2010).\\[0pt] [3] H. Ribeiro, G. Burkard, J. R. Petta, H. Lu, and A. C. Gossard, arXiv:1207.2972 (2012). \\[0pt] [4] H. Ribeiro, J. R. Petta, G. Burkard, arXiv:1210.1957 (2012). [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C26.00002: Decoherence-protected nuclear spin quantum register in diamond Viatcheslav Dobrovitski, Wan Jung Kuo, Ronald Hanson, Tim H. Taminiau We analyze the decoherence-protected operation of a quantum register based on the nuclear spins surrounding a nitrogen-vacancy (NV) center in diamond. Combination of the decoherence protection with the quantum gates is achieved by applying the decoupling pulses to the NV center's electronic spin in resonance with the motion of one of the nuclear spins [1,2]. In this way, many weakly coupled (tens of kHz) nuclei located far from the NV center can be combined in a quantum register. We study the limits, set by realistic experimental parameters, on the size of such a register and on the duration of the quantum gates needed for its operation. We also consider the ways of accelerating the quantum gate operation, and integration of the decoherence-protected gates with the decoupling of the nuclear spins themselves. We conclude that creation of such registers is feasible with current experimental capabilities. Work at the Ames Laboratory was supported by the Department of Energy - Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [1] T. van der Sar et al., Nature 484, 82 (2012). [2] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012). [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C26.00003: Enhancement of Inter-qubit Coupling in Singlet-Triplet Qubits by Floating Metal Gate Shannon Harvey, Michael Shulman, Oliver Dial, Hendrik Bluhm, Vladimir Umansky, Amir Yacoby Spin qubits in semiconductors are promising systems for quantum computing, because they have long coherence times and are potentially scalable. However, their weak interaction with the environment, which gives their long coherence times, also makes inter-qubit interactions weak. Numerous proposals use electrostatic coupling between qubits for entangling operations, but these interactions require the qubits to be near one another. These proposals also suggest that adding a metallic gate between two qubits could increase coupling and allow the qubits to be spatially separated. We present results on two singlet-triplet (S-T$_{0})$ qubits connected by a floating metallic gate. Previous work on two-qubit operations, which use a capacitive coupling, showed that the inter-qubit coupling is weak and requires the qubits to be in close proximity. We find that the inter-qubit coupling is increased with the inclusion of a floating metal gate, which improves entangling operation fidelities and allows for these qubits to be spatially separated. Together, these improvements open the door to a scalable architecture for quantum information processing for all semiconductor spin qubit platforms. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C26.00004: Probing quantum phase transitions on a spin chain with a double quantum dot Yun-Pil Shim, Sangchul Oh, Jianjia Fei, Xuedong Hu, Mark Friesen We propose a local, projective scheme for detecting quantum phase transitions (QPTs) in a quantum dot spin chain [1]. QPTs in qubit systems are known to produce singularities in the entanglement, which could in turn be used to probe the QPT. Current proposals to measure the entanglement are challenging however, because of their nonlocal nature. We present numerical and analytical evidence that entanglement in a double quantum dot (DQD) coupled locally to a spin chain exhibits singularities at the critical points of the spin chain, and that these singularities are reflected in the singlet probabilities of the DQD. This result suggests that a DQD can be used as an efficient probe of QPTs through projective singlet measurements. We propose a simple experiment to test this concept in a linear triple quantum dot. [1]Y.-P. Shim {\it et al.}, arXiv:1209.5445 [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C26.00005: Coherent electron transfer between distant quantum dots in a linear array Floris Braakman, Pierre Barthelemy, Lieven Vandersypen Tunnel coupled quantum dots form the basis for electronic charge and spin qubits in semiconductors. The tunnel coupling gives rise to quantum coherent phenomena such as exchange oscillations of neighboring spins. However, tunnel coupling strength between non-neighbouring sites is negligible and it is therefore desirable to develop a form of long range coupling. In a linear array of three quantum dots, we demonstrate an effective tunnel coupling between the outer dots through virtual occupation of discrete levels in the center dot. The coupling strength depends strongly on the detuning between center and outer dot levels. The observation of Landau-Zener-Stueckelberg oscillations demonstrates the coherent nature of the coupling. In principle the effective long-range tunnel coupling should also allow coherent exchange of remote spins. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C26.00006: Dynamically Corrected Pulse Sequences for the Exchange Only Qubit Garrett Hickman, Jason Kestner In the exchange-only qubit, hyperfine interactions of qubit electrons with neighboring atoms introduce decoherence into the basis states and mix them with a third leaked state. We theoretically derive a scheme for performing arbitrary single-qubit rotations on the exchange-only qubit while canceling all hyperfine-induced errors to first order. We compare numerically the performance of the resulting pulse sequences with that of the simplest na\"ive implementations for a range of hyperfine interaction strengths. While for typical operations these sequences are roughly 50 times longer than a simple uncorrected pulse, error is significantly reduced. We show that for hyperfine field inhomogeneities less than one thirtieth of the maximum exchange strength, typical hyperfine-induced errors are reduced by at least an order of magnitude. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C26.00007: Composite pulses robust against charge noise and magnetic field noise for universal control of a singlet-triplet qubit Xin Wang, Edwin Barnes, Jason P. Kestner, Lev S. Bishop, Sankar Das Sarma We generalize our SUPCODE pulse sequences [1] for singlet-triplet qubits to correct errors from imperfect control. This yields gates that are simultaneously corrected for both charge noise and magnetic field gradient fluctuations, addressing the two dominant $T_2^*$ processes. By using this more efficient version of SUPCODE, we are able to introduce this capability while also substantially reducing the overall pulse time compared to the previous sequence. We show that our sequence remains realistic under experimental constraints such as finite bandwidth. [1] Wang et al., ``Composite pulses for robust universal control of singlet-triplet qubits'', Nat. Commun. 3, 997 (2012) [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C26.00008: Composite multi-qubit gates dynamically corrected against charge noise and magnetic field noise for singlet-triplet qubits Jason Kestner, Edwin Barnes, Xin Wang, Lev Bishop, Sankar Das Sarma We use previously described single-qubit SUPCODE pulses on both intra-qubit and inter-qubit exchange couplings, integrated with existing strategies such as BB1, to theoretically construct a CNOT gate that is robust against both charge noise and magnetic field gradient fluctuations. We show how this allows scalable, high-fidelity implementation of arbitrary multi-qubit operations using singlet-triplet spin qubits in the presence of experimentally realistic noise. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C26.00009: Dynamically corrected gates for singlet-triplet spin qubits with control-dependent errors N. Tobias Jacobson, Wayne M. Witzel, Erik Nielsen, Malcolm S. Carroll Magnetic field inhomogeneity due to random polarization of quasi-static local magnetic impurities is a major source of environmentally induced error for singlet-triplet double quantum dot (DQD) spin qubits. Moreover, for singlet-triplet qubits this error may depend on the applied controls. This effect is significant when a static magnetic field gradient is applied to enable full qubit control. Through a configuration interaction analysis, we observe that the dependence of the field inhomogeneity-induced error on the DQD bias voltage can vary systematically as a function of the controls for certain experimentally relevant operating regimes. To account for this effect, we have developed a straightforward prescription for adapting dynamically corrected gate sequences that assume control-independent errors into sequences that compensate for systematic control-dependent errors. We show that accounting for such errors may lead to a substantial increase in gate fidelities. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C26.00010: High fidelity gates in quantum dot spin qubits Mark Friesen, Teck Seng Koh, S. N. Coppersmith A variety of logical qubits and quantum gates have been proposed for quantum computer architectures using top-gated quantum dots. Despite their differences, we show that many combinations of qubits and gates can be evaluated on an equal footing by optimizing the gating protocols for maximum fidelity. Here, we evaluate single-qubit gate operations for two types of logical-qubits: singlet-triplet qubits and quantum dot hybrid qubits. In both cases, transitions between the qubit states are controlled by the exchange interaction between the dots, which in turn depends on the tunnel coupling and the detuning. We compute the fidelities for three exchange gate protocols: a dc pulsed gate, an ac resonant gate, and stimulated Raman adiabatic passage (STIRAP). Remarkably, we find that the optimized fidelities for all three gates follow a simple scaling law; the maximum fidelity depends only on the range of parameters that can be achieved experimentally. We show that a singlet-triplet qubit can be pulse-gated with significantly higher fidelity than a hybrid qubit, and that the highest overall fidelity should be achieved in a hybrid qubit using a STIRAP gating protocol. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C26.00011: Theoretical hyperfine decay functions in triple quantum dots Thaddeus Ladd Coherent oscillations in multiple quantum dots decay due to hyperfine interactions with nuclear spins. The decay functions observed in several double-dot experiments [1] agree well with simple formulae derived using the group SU(2), which is defined by exchange and hyperfine interactions in the singlet-triplet system [2]. We show that in triple dots, this theory generalizes to SU(3), with convenient representation in the basis of states of the exchange-only qubit in a decoherence-free subsystem~[3]. Using some intuition from SU(3), we derive analytic formulae for the hyperfine decay functions expected in coherent oscillations in triple dots~[4]. \\{} \newcommand\mybibformat[5]{#1, \textit{#2}~\textbf{#3}, #4 (#5)} [1]~\mybibformat{B.~M.~Maune et al.}{Nature}{481}{344}{2012}; \mybibformat{E.~A.~Laird et al.}{Phys. Rev. B}{82}{075403}{2012} \\{} [2]~\mybibformat{W. A. Coish and D. Loss}{Phys. Rev.~B}{72}{125337}{2005} \\{} [3]~\mybibformat{D.~P.~DiVincenzo et al.}{Nature}{408}{339}{2000}; \mybibformat{B.~H.~Fong and S.~M.~Wandzura}{Quantum Inf. Comput.}{11}{1003}{2011} \\{} [4]~\mybibformat{T. D. Ladd}{Phys. Rev. B}{86}{125408}{2012}. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C26.00012: High fidelity gates for exchange-only qubits in triple-quantum-dots Jianjia Fei, Jo-Tzu Hung, Teck Seng Koh, Yun-Pil Shim, Sangchul Oh, Susan Coppersmith, Xuedong Hu, Mark Friesen One of the main attractions of implementing exchange-only qubits in quantum dots is their ease of control. Gate operations are performed by changing the voltages on the top-gates, to vary the tunnel coupling and/or the detuning between the dots. One of the main challenges is that when exchange interactions are turned on, charge noise will cause dephasing. Here, we explore optimal strategies for implementing logical qubit rotations in exchange-only qubits. We take into account charge noise, and challenges due to hyperfine interactions, including leakage outside the logical qubit space, and dephasing caused by fluctuations of the local nuclear fields. Our method is based on optimizing the experimentally tunable parameters to maximize the fidelity of the gate operation. /newline /newline The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the U.S. Government. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C26.00013: Constructing Two-Qubit Gates for Exchange-Based Quantum Computing Daniel Zeuch, Robert Cipri, N.E. Bonesteel Exchange pulses are local unitary operations obtained by turning on and off the isotropic exchange interaction between pairs of spin-1/2 particles, for example electron spins in quantum dots. We present a procedure for analytically constructing sequences of exchange pulses for carrying out leakage free two-qubit gates on logical three-spin qubits. At each stage of our construction we reduce the problem to that of finding a sequence of rotations for an effective two-level system. The resulting pulse sequences are 39 pulses long, longer than the original 19-pulse sequence of DiVincenzo et al. [1] and the more recent 18-pulse sequence of Fong and Wandzura [2], both of which were obtained numerically. Like the latter sequence, our sequences work regardless of the total spin of the six spins used to encode two qubits. After introducing our method, we prove that any leakage-free sequence of exchange pulses must act on at least five of the six spins to produce an entangling two-qubit gate.\\[4pt] [1] D.P. DiVincenzo et al., Nature \textbf{408}, 339 (2000). \newline [2] B.H. Fong \& S.M. Wandzura, Quantum Info. Comput., \textbf{11}, 1003 (2011). [Preview Abstract] |
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