Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J53: Invited Session: Progress in Electrically-Gated Quantum Dot Qubits |
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Sponsoring Units: DCMP Chair: HongWen Jiang, University of California, Los Angeles Room: Grand Ballroom C3 |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J53.00001: Control and Measurement of an Exchange-Only Spin Qubit Invited Speaker: James Medford Gate-defined semiconductor quantum dots have proven to be a versatile testbed for exploring quantum systems and quantum information. We demonstrate the fast all-electrical control of a spin qubit using the two coherent exchange interactions in a triple quantum dot. Our measurements identify the role of nuclear spins from the host GaAs in this system as a mechanism for both dephasing and leakage out of the qubit subspace. We also show that by increasing both exchange interactions in a balanced fashion, we enter a second regime of operation. In this regime, leakage from the subspace has been suppressed, resulting in a spin qubit with a tunable electric dipole moment, which we refer to as the resonant exchange qubit. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J53.00002: Silicon based quantum dot hybrid qubits Invited Speaker: Dohun Kim The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantum dots. The quantum dot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron's charge. On the other hand, quantum dot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications. This talk will present experimental progress of a `hybrid' qubit, formed by three electrons in a Si/SiGe double quantum dot, which combines desirable characteristics (speed and coherence) in the past found separately in qubits based on either charge or spin degrees of freedom. Using resonant microwaves, we first discuss qubit operations near the `sweet spot' for charge qubit operation. Along with fast ( \textgreater GHz) manipulation rates for any rotation axis on the Bloch sphere, we implement two independent tomographic characterization schemes in the charge qubit regime: traditional quantum process tomography (QPT) and gate set tomography (GST). We also present resonant qubit operations of the hybrid qubit performed on the same device, DC pulsed gate operations of which were recently demonstrated. We demonstrate three-axis control and the implementation of dynamic decoupling pulse sequences. Performing QPT on the hybrid qubit, we show that AC gating yields $\pi $ rotation process fidelities higher than 93{\%} for X-axis and 96{\%} for Z-axis rotations, which demonstrates efficient quantum control of semiconductor qubits using resonant microwaves. We discuss a path forward for achieving fidelities better than the threshold for quantum error correction using surface codes. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 4:18PM |
J53.00003: Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot Invited Speaker: Erika Kawakami Electron spins in Si/SiGe quantum dots are one of the most promising candidates for a quantum bit for their potential to scale up and their long dephasing time. We realized coherent control of single electron spin in a single quantum dot (QD) defined in a Si/SiGe 2D electron gas. Spin rotations are achieved by applying microwave excitation to one of the gates, which oscillates the electron wave function back and forth in the gradient field produced by cobalt micromagnets fabricated near the dot. The electron spin is read out in single-shot mode via spin-to-charge conversion and a QD charge sensor. In earlier work [1], both the fidelity of single-spin rotations and the spin echo decay time were limited by a small splitting of the lowest two valleys. By changing the direction and magnitude of the external magnetic field as well as the gate voltages that define the dot potential, we were able to increase the valley splitting and also the difference in Zeeman splittings associated with these two valleys. This has resulted in considerable improvements in the gate fidelity and spin echo decay times. Thanks to the long intrinsic dephasing time T2* $=$ 900 ns and Rabi frequency of 1.4 MHz, we now obtain an average single qubit gate fidelity of an electron spin in a Si/SiGe quantum dot of 99 percent, measured via randomized benchmarking. The dephasing time is extended to 70 us for the Hahn echo and up to 400 us with CPMG80. From the dynamical decoupling data, we extract the noise spectral density in the range of 30 kHz-3 MHz. We will discuss the mechanism that induces this noise and is responsible for decoherence. In parallel, we also realized electron spin resonance and coherent single-spin control by second harmonic generation, which means we can drive an electron spin at half the Larmor frequency. Finally, we observe not only single-spin transitions but also transitions whereby both the spin and the valley state are flipped. Altogether, these measurements have significantly increased our understanding and raised the prospects of spin qubits in Si/SiGe quantum dots.\\[4pt] This work has been done in collaboration with T.M. J. Jullien, P. Scarlino, V.V. Dobrovitski, D.R. Ward, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson, and L. M. K. Vandersypen. This work was supported in part by the Army Research Office (ARO) (W911NF-12-0607), the Foundation for Fundamental Research on Matter (FOM) and the European Research Council (ERC). Development and maintenance of the growth facilities used for fabricating samples was supported by the Department of Energy (DOE) (DE-FG02-03ER46028). E.K. was supported by a fellowship from the Nakajima Foundation. This research utilized NSF-supported shared facilities at the University of Wisconsin-Madison. \\[4pt] [1] E. Kawakami, P. Scarlino et al. Nature Nanotechnology 9, 666-670 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:54PM |
J53.00004: Control of Spin States in Triple Quantum Dots Invited Speaker: Andrew Sachrajda A brief review will be given on coherent behaviour in serial triple quantum dots in AlGaAs/GaAs heterostructure related to multi-spin states. One series of experiments involves the application of coherent superpositions of multi-electron states to the transfer of single spins and two-spin states non-locally between edge quantum dots while maintaining the center quantum dot occupation fixed at one or zero electrons. A second series of experiments involves the identification of coherent leakage mechanisms away from targeted encoded three-spin states qubits. Finally, results will be shown which reveal an unexpected control of the gap at the S-T$+$ anticrossing by taking advantage of different nuclear dynamic polarization pumping rates. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:30PM |
J53.00005: One- and two-qubit logic using silicon-MOS quantum dots Invited Speaker: Andrew Dzurak Spin qubits in silicon are excellent candidates for scalable quantum information processing [1] due to their long coherence times and the enormous investment in silicon MOS technology. While our Australian effort in Si QC has largely focused on spin qubits based upon phosphorus dopant atoms implanted in Si [2,3], we are also exploring spin qubits based on single electrons confined in SiMOS quantum dots [4]. Such qubits can have long spin lifetimes T1 $=$ 2 s, while electric field tuning of the conduction-band valley splitting removes problems due to spin-valley mixing [5]. In isotopically enriched Si-28 these SiMOS qubits have a control fidelity of 99.6{\%} [6], consistent with that required for fault-tolerant QC. By gate-voltage tuning the electron g*-factor, the ESR operation frequency can be Stark shifted by \textgreater 10 MHz [6], allowing individual addressability of many qubits. Most recently we have coupled two SiMOS qubits to realize CNOT gates [7] via either controlled rotation (CROT) or controlled phase (CZ) operations. The speed of the two-qubit CZ-operations is controlled electrically via the detuning energy and over 100 two-qubit gates can be performed within a two-qubit coherence time of 8 $\mu $s. \\[4pt] [1] D.D. Awschalom et al., ``Quantum Spintronics,'' Science 339, 1174 (2013).\\[0pt] [2] J.J. Pla et al., ``A single-atom electron spin qubit in silicon,'' Nature 489, 541 (2012).\\[0pt] [3] J.T. Muhonen et al., ``Storing quantum information for 30 seconds in a nanoelectronic device,'' Nature Nanotechnology, DOI: 10.1038/NNANO.2014.211 (2014).\\[0pt] [4] S.J. Angus et al., ``Gate-defined quantum dots in intrinsic silicon,'' Nano Lett. 7, 2051 (2007).\\[0pt] [5] C.H. Yang et al., ``Spin-valley lifetimes in a silicon quantum dot with tunable valley splitting,'' Nature Comm. 4, 2069 (2013).\\[0pt] [6] M. Veldhorst et al., ``An addressable quantum dot qubit with fault-tolerant control fidelity,'' Nature Nanotechnology, DOI: 10.1038/NNANO.2014.216 (2014).\\[0pt] [7] M. Veldhorst et al., ``A two-qubit logic gate in silicon,'' unpublished. [Preview Abstract] |
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