Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H3: Charge Noise Mitigation in Multiple Quantum Dot QubitsInvited
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Sponsoring Units: DCMP GQI Chair: Thaddeus Ladd, Hrl Laboratories Room: Ballroom III |
Tuesday, March 15, 2016 2:30PM - 3:06PM |
H3.00001: Reduced sensitivity to charge noise in semiconductor spin qubits via symmetric operation Invited Speaker: Matthew Reed Gated semiconductor quantum dots controlled with the exchange interaction are attractive candidates for quantum information processing because of their long coherence time and electrical controllability. Exchange is conventionally modulated by detuning the chemical potentials of neighboring dots over a fixed tunnel barrier, an approach whose precision is limited by charge noise. In this talk we demonstrate a "symmetric" mode of operation which substantially reduces the sensitivity of exchange operations to gate fluctuations. The method involves biasing a double-dot symmetrically between the charge-state anti-crossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We propose a metric, insensitivity, to quantify the technique’s improvement and find that it increases by at least a factor of five between operating regimes. We also demonstrate a substantial increase in the number of Rabi fringes observed. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:42PM |
H3.00002: Symmetric operation and nuclear notch filtering in GaAs double quantum dots Invited Speaker: Ferdinand Kuemmeth Spin qubits based on few-electron semiconducting quantum dots are promising candidates for quantum computation, due to their potential for miniaturization, scalability and fault tolerance. In this talk I will present recent results on how to mitigate electrical and nuclear noise in GaAs singlet-triplet qubits. \\ The traditional way of implementing exchange rotations in singlet-triplet qubits involves detuning the qubit away from the symmetric (1,1) charge configuration, thereby temporarily hybridizing with the (0,2) charge state. Due to the large dipole coupling the resulting qubit oscillation suffers from detuning noise, motivating operation at sweet spots [1] or in the multi-electron regime [2]. Alternatively, exchange rotations can be implemented by symmetrically lowering the middle barrier. This method yields less relative exchange noise, significantly enhanced free induction decay times, and quality factors comparable to those reported in silicon quantum dot devices using similar techniques [3]. \\ In order to decouple the singlet-triplet qubit from nuclear spin fluctuations, we investigate Carr-Purcell-Meiboom-Gill (CPMG) sequences in more detail. At high magnetic fields we find that qubit dephasing is limited by narrow-band high-frequency noise arising from Larmor precession of $^{69}$Ga, $^{71}$Ga, $^{75}$As nuclear spins, similar to what has been observed at intermediate magnetic field [4]. By aligning the notches of the CPMG filter function with differences of the discrete nuclear Larmor frequencies we demonstrate a qubit coherence time of 0.87 ms, i.e. more than five orders of magnitude longer than the duration of a $\pi$ exchange gate in the same device. \\ [1][1] O. E. Dial et al. Physical Review Letters 110, 146804 (2013). [2] A. P. Higginbotham et al, Phys Rev Lett 112, 026801 (2014). [3] M. D. Reed et al, arXiv:1508.01223 (2015). [4] H. Bluhm et al. Nature Physics 7, 109 (2011). [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 4:18PM |
H3.00003: Optimizing fidelities of quantum dot hybrid qubits Invited Speaker: Susan Coppersmith The quantum dot hybrid qubit, which can be viewed as a hybrid between a spin and charge qubit, has an attractive combination of speed and fabrication simplicity. The initial experiments implementing this qubit yielded process fidelities of $\sim $88{\%} [1] and $\sim $93{\%} [2] for pulsed-gating and ac-gating, respectively. We present experimental evidence that these fidelities were limited by charge noise, and we present theoretical and experimental evidence that the sensitivity of qubit operations to charge noise can be reduced substantially by appropriate adjustment of the tunnel couplings. Our work indicates that, with suitable optimization, this qubit can achieve gate fidelities of well over 99{\%}. [1] D. Kim et al., \textit{Nature} \textbf{511}, 70 (2014). [2] D. Kim, et al., \textit{npj Quant. Inf.} \textbf{1}, 15004 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:54PM |
H3.00004: Double sweet-spot operation of the resonant exchange qubit in three-electron quantum dots Invited Speaker: Guido Burkard The resonant exchange (RX) qubit is a promising variant of the exchange-only spin qubit in a triple quantum dot which responds to a narrow-band resonant frequency. But the advantage of a permanently applied exchange splitting for spin control generally entails an increased susceptibility to charge noise. We have investigated the influence of electrical charge noise on a resonant exchange (RX) qubit by taking into account uncorrelated noise in each quantum dot, giving rise to two independent noisy bias parameters $\varepsilon$ and $\Delta$ [1]. Calculating the energy splitting of the two qubit states as a function of these two bias detuning parameters, we have identified “sweet spots,” where the qubit is least susceptible to noise. Our investigation shows that the sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters between the dots. By calculating and comparing the charge dephasing rates at the various operating points of the RX qubit, we identify a new favorable operating regime for the RX qubit in the case of weak noise, based on these double sweet spots. In contrast, spin noise can be mitigated using exchange-based dynamical decoupling sequences that have been optimized using two different strategies, Uhrig dynamical decoupling (UDD) and optimized filter function dynamical decoupling (OFDD) [2]. Finally, we give a brief outlook towards the possibility of long-distance coupling between resonant exchange qubits mediated by a microwave cavity [3].\newline [1] M. Russ and G. Burkard, Phys. Rev. B 91, 235411 (2015).\newline [2] N. Rohling and G. Burkard, arXiv:1510.04098.\newline [3] M. Russ and G. Burkard, Phys. Rev. B (accepted) [arXiv:1508.07122]. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:30PM |
H3.00005: Feedback-tuned, noise resilient gates for encoded spin qubits Invited Speaker: Hendrik Bluhm Spin 1/2 particles form native two level systems and thus lend themselves as a natural qubit implementation. However, encoding a single qubit in several spins entails benefits, such as reducing the resources necessary for qubit control and protection from certain decoherence channels. While several varieties of such encoded spin qubits have been implemented, accurate control remains challenging, and leakage out of the subspace of valid qubit states is a potential issue. Optimal performance typically requires large pulse amplitudes for fast control, which is prone to systematic errors and prohibits standard control approaches based on Rabi flopping. Furthermore, the exchange interaction typically used to electrically manipulate encoded spin qubits is inherently sensitive to charge noise. I will discuss all-electrical, high-fidelity single qubit operations for a spin qubit encoded in two electrons in a GaAs double quantum dot. Starting from a set of numerically optimized control pulses \footnote{Pascal Cerfontaine, Tim Botzem, David P. DiVincenzo, and Hendrik Bluhm, Phys. Rev. Lett. \textbf{113}, 150501 (2014)}, we employ an iterative tuning procedure based on measured error syndromes to remove systematic errors.Randomized benchmarking yields an average gate fidelity exceeding 98 \% and a leakage rate into invalid states of 0.2 \%. These gates exhibit a certain degree of resilience to both slow charge and nuclear spin fluctuations due to dynamical correction analogous to a spin echo. Furthermore, the numerical optimization minimizes the impact of fast charge noise. Both types of noise make relevant contributions to gate errors. The general approach is also adaptable to other qubit encodings and exchange based two-qubit gates \footnote{Sebastian Mehl, Hendrik Bluhm, and David P. DiVincenzo, Phys. Rev. B \textbf{90}, 045404 (2014)}. [Preview Abstract] |
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