Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B45: Semiconductor Qubits: Multidot Qubits and Dynamical ControlFocus

Hide Abstracts 
Sponsoring Units: GQI Chair: Vanita Srinivasa, Laboratory for Physical Sciences/University of Maryland, College Park, MD Room: 348 
Monday, March 14, 2016 11:15AM  11:51AM 
B45.00001: Multiple quantum dot spin qubits Invited Speaker: Seigo Tarucha To date various techniques of implementing spin qubits and entangling gates have been developed with quantum dos (QDs). The necessary step for further scaling up the qubit system is to increase the number of QDs with a wellcontrolled charge state to prepare multiple qubits and improve the fidelity of the qubit gates as well. I will first review spin1/2 qubit gates with triple QDs for operating three qubits, local and nonlocal entangling gates, and SWAP gates. I show that the fidelity of these spin manipulations is significantly increased by decreasing the data acquisition time. Secondly I will refer to quadruple and quintuple QDs to implement multiple spin qubits. For the triple QD we use two sets of two coupled dots in the spin blockade regime to demonstrate operation of three individual spin qubits. We use an exchange coupling between the neighboring dots to make two sets of SWAPs and an inhomogeneous Zeeman field difference between the neighboring dots (between the remote dots) to make local (nonlocal) control of ST$_{0}$ oscillations. We apply the same technique for the quadruple QD to coherently manipulate individual four spins. We finally discuss a way to further scale up the qubit system using multiple QDs. [Preview Abstract] 
Monday, March 14, 2016 11:51AM  12:03PM 
B45.00002: A new look at encodedqubit quantum dot quantum computing in silicon Charles Tahan, YunPil Shim, Rusko Ruskov Although the properties of spinbased qubits are specified by the material system they reside in, it's possible to modify those properties by encoding a qubit into multiple physical spins. Here we consider new operating regimes for encoded spin qubits and discuss their relevance to spinbased quantum computing and qubitqubit coupling, especially in silicon quantum dot systems. We will also briefly discuss recent developments in gfactor theory in silicon quantum dots and their possible implications. [Preview Abstract] 
Monday, March 14, 2016 12:03PM  12:15PM 
B45.00003: High quality exchange rotations in spin qubits using symmetric gating F. Martins, F. K. Malinowski, P. D. Nissen, C. M. Marcus, F. Kuemmeth, E. Barnes, G. C. Gardner, S. Fallahi, M. J. Manfra We present results on a singlettriplet qubit implemented in a GaAs/AlGaAs heterostructure and we show that exchange oscillations can be realized either by tilting the double well potential, the conventional method, or by symmetrically lowering the barrier, as originally suggested by Loss and DiVincenzo. The two methods are compared here. We find that lowering the barrier between dots has much less relative exchange noise compared to tilting the potential. Since exchange rotations are sensitive to electrical noise and relatively insensitive to nuclear noise, this yields significantly enhanced free induction decay times and quality factors. Our results are comparable to those reported recently in silicon quantum dot devices, obtained using similar techniques. [Preview Abstract] 
Monday, March 14, 2016 12:15PM  12:27PM 
B45.00004: Charge noise mitigation in tripledot encoded spin qubits Emily Pritchett The immediate scalability of electrons confined to semiconductor quantum dots makes them one of the most attractive platforms for quantum information processing; however, 1/f charge noise associated with electrical confinement has been a leading source of noise in quantum dot systems. Recently, there has been a surge of experimental and theoretical work aimed at charge noise mitigation in quantum dot systems implementing AC or DC control of triple dots at "sweet spots''. In this talk, we compare the symmetric operation point (SOP) DC control technique implemented in Reed, et al. [arXiv:1508.01223] to the resonant exchange (RX) AC control technique [Medford, et al., PRL 111, 050501 (2013), Taylor, et al., PRL 111, 050502 (2013), Russ, et al., Phys. Rev. B 91, 235411 (2015)] . Numerical results suggest that both DC and AC tripledot control can offer a comparably substantial reduction in charge noise; however, the validity of the rotating wave approximation forces a tradeoff between speed and accuracy for RX qubits, while the performance of SOP qubits actually improves at shorter gate times. [Preview Abstract] 
Monday, March 14, 2016 12:27PM  12:39PM 
B45.00005: The validity of the RWA and gate operation speedup by violating RWA in resonantdriven qubit systems Yang Song The rotating wave approximation (RWA) is ubiquitously used in understanding (quasi)resonant driven systems and designing pulses for state evolution. Following the practice in atomic and NMR physics, a wide range of semiconducting qubit systems are driven resonantly to manipulate the qubit, including singlespin/resonant exchange (RX)/various singlettriplet(ST)/spincharge hybrid qubits. The purpose of this talk is twofold: (I) Examine the validity of RWA in different qubit systems and analyze the error in terms of quantum computation; (II) Present faster gate operations by going into RWAinvalid regime for resonantdriven qubits (esp. for ST and RX types). We measure the RWAinduced infidelity and discuss it in view of the faulttolerant error correction threshold and operation speeds. Applying the analytical extension (two orders higher than RWA) greatly reduces the infidelity, in the regime where the RWA is attempted to be used. Moreover, we show that the resonantdriven system is not limited by the Rabilike weak coupling limit and the associated slow gate speed, much smaller than the level splitting (e.g., the small Zeeman energy gradient in ST qubits). We demonstrate the universal one qubit gates for driving strength up to a few level splitting, achieving fast control with only simple sinusoidal pulses. We also solve for the `shifted sinusoidal' pulses needed for ST qubits where the exchange coupling cannot change signs. [Preview Abstract] 
Monday, March 14, 2016 12:39PM  12:51PM 
B45.00006: Leakage of The Quantum Dot Hybrid Qubit in The Strong Driving Regime YuanChi Yang, Mark Friesen, S. N. Coppersmith Recent experimental demonstrations of highfidelity singlequbit gates suggest that the quantum dot hybrid qubit is a promising candidate for largescale quantum computing. The qubit is comprised of three electrons in a double quantum dot, and can be protected from charge noise by operating in an extended sweetspot regime. Gate operations are based on exchange interactions mediated by an excited state. However, strong resonant driving causes unwanted leakage into the excited state. Here, we theoretically analyze leakage caused by strong driving, and explore methods for increasing gate fidelities. [Preview Abstract] 
Monday, March 14, 2016 12:51PM  1:03PM 
B45.00007: Tenfold increase in the Rabi decay time of the quantum dot hybrid qubit Brandur Thorgrimsson, Dohun Kim, C.B. Simmons, Daniel R. Ward, Ryan H. Foote, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson The quantum dot hybrid qubit is formed from three electrons in a double quantum dot. In previous work, we showed that the hybrid qubit has the speed of a charge qubit and the stability of a spin qubit. Here, we show that the hybrid qubit is also highly tunable, and can be tuned into regimes with desirable coherence properties. By changing the interdot tunnel rate by only 25\%, from 5 GHz to 6.25 GHz, we are able to increase the Rabi decay time by a factor of ten, from 18 ns to 177 ns. We attribute this improvement to the refinement of an extended “sweet spot” in the energy dispersion of the hybrid qubit, where the qubit is less susceptible to charge noise, which is a dominant source of decoherence. This work was supported in part by ARO (W911NF120607) and NSF (DMR1206915 and PHY1104660). Development and maintenance of the growth facilities used for fabricating samples is sup ported by DOE (DEFG0203ER46028). This research utilized NSFsupported shared facilities at the University of WisconsinMadison. [Preview Abstract] 
Monday, March 14, 2016 1:03PM  1:15PM 
B45.00008: Effect of Charge Noise on LandauZener Interferometry in double quantum dots Zhenyi Qi, Mark Friesen, Susan Coppersmith, Maxim Vavilov We study the effect of charge noise on the dynamics of semiconductor quantum dot qubits. Recent experiments have demonstrated relatively long coherence times in these systems; however at the same time, the visibility of the LandauZener interference pattern is relatively low. We argue that the electromagnetic noise of the environment affects the coherence of the qubit near the charge degeneracy point, including the singlettriplet avoided level crossing, and results in the reduced visibility of the LandauZener interferometry when the singlettriplet avoided level crossing happens in the vicinity of the charge degeneracy point. Using a master equation, we describe the evolution of the density matrix for the qubit assuming weak coupling of the quantum dot to its electromagnetic environment and compare our results to experimental data. [Preview Abstract] 
Monday, March 14, 2016 1:15PM  1:27PM 
B45.00009: Noiseinduced collective quantum state preservation in spin qubit arrays Edwin Barnes, DongLing Deng, Robert Throckmorton, YangLe Wu The hyperfine interaction with nuclear spins (or, Overhauser noise) has long been viewed as a leading source of decoherence in individual quantum dot spin qubits. We show that in a coupled multiqubit system consisting of as few as four spins, interactions with nuclear spins can have the opposite effect where they instead preserve the collective quantum state of the system. This noiseinduced state preservation can be realized in a linear spin qubit array using current technological capabilities. Our proposal requires no control over the Overhauser fields in the array; only experimental control over the average interqubit coupling between nearest neighbors is needed, and this is readily achieved by tuning gate voltages. Our results illustrate how the role of the environment can transform from harmful to helpful in the progression from singlequbit to multiqubit quantum systems. [Preview Abstract] 
Monday, March 14, 2016 1:27PM  1:39PM 
B45.00010: Decoupling a spin qubit from highfrequency Larmor dynamics of a GaAs nuclear spin bath Filip K. Malinowski, Frederico Martins, Peter D. Nissen, Mark S. Rudner, Charles M. Marcus, Ferdinand Kuemmeth, Edwin Barnes, Saeed Fallahi, Geoffrey C. Gardner, Michael J. Manfra We present a technique of decoupling a spin qubit in a GaAs/AlGaAs heterostructure from low and highfrequency noise arising from hyperfine interaction of electrons with nuclear spins. We use CarrPurcellMeiboomGill sequences in which we synchronize the repetition rate of $\pi$ pulses to difference Larmor frequencies of $^{69}$Ga, $^{71}$Ga and $^{75}$As nuclei. This decouples the qubit both from lowfrequency noise due to diffusion of nuclear spins and from noise at selected high frequencies, allowing us to apply more than a thousand $\pi$ pulses in a sequence. We demonstrate a coherence time of a singlettriplet qubit of 0.87 ms, i.e. five orders of magnitude longer than the inhomogeneous dephasing time intrinsic to GaAs. [Preview Abstract] 
Monday, March 14, 2016 1:39PM  1:51PM 
B45.00011: Noise filtering of composite pulses for singlettriplet qubits Xin Wang, XuChen Yang Dynamically corrected gates are useful measures to combat decoherence in spin qubit systems. They are, however, mostly designed assuming the staticnoise model and may thus be considered lowfrequency noise filters. In this talk we carefully examine the applicability of a particular type of dynamically corrected gates, namely the \textsc{supcode} designed for singlettriplet qubits, under realistic $1/f^\alpha$ noises. Through randomized benchmarking, we have found that \textsc{supcode} offers improvement of the gate fidelity for $\alpha>1$ and the improvement becomes exponentially more pronounced with the increase of the noise exponent $\alpha$ up to 3. On the other hand, for small $\alpha$ \textsc{supcode} will not offer any improvement. We also present the computed filter transfer functions for the \textsc{supcode} gates for nuclear and charge noise respectively and have found that they are consistent with the finding from the benchmarking. [Preview Abstract] 
Monday, March 14, 2016 1:51PM  2:03PM 
B45.00012: Dynamical Decoupling with pulse errors for ensembles of interacting spins E. S. Petersen, A. M. Tyryshkin, S. A. Lyon Dynamical decoupling (DD) is a wellknown approach for decoupling quantum (spin) systems from their environments. Theoretically, the performance of DD pulse sequences is often analyzed using a single spin approximation in which environmental noise is included through single spin operators. This approach has successfully analyzed the effectiveness of many popular DD pulse sequences (like CPMG and XY4) to cancel environmental noise even in the presence of unavoidable pulse errors. However, this methodology does not describe the effect of DD on the spinspin interactions present in experiments involving large numbers of spins. Here, we go beyond the usual singlespin model, extending the analysis of DD sequences to include such spinspin interactions. We find that when using certain popular DD sequences (like CPMG), coherence times of ensembles with dipolar interactions between spins can be drastically influenced by pulse errors. While sequences with ideal pulses do not decouple the spinspin interactions, the presence of even small pulse errors can partially (or even greatly) decouple the spinspin interactions thus leading to longer coherence times. Furthermore, the extent that these interactions are decoupled is highly dependent on the type of DD sequence used, and not necessarily the number of pulses involved. These calculations explain results of past experiments (Tyryshkin et al, arxiv: 1011.1903). [Preview Abstract] 
Monday, March 14, 2016 2:03PM  2:15PM 
B45.00013: Dynamic fieldfrequency lock for tracking magnetic field fluctuations in electron spin resonance experiments Abraham Asfaw, Alexei Tyryshkin, Stephen Lyon Global magnetic field fluctuations present significant challenges to pulsed electron spin resonance experiments on systems with long spin coherence times. We will discuss results from experiments in which we follow instantaneous changes in magnetic field by locking to the free induction decay of a proton NMR signal using a phaselocked loop. We extend conventional fieldfrequency locking techniques used in NMR to follow slow magnetic field drifts by using a modified CarrPurcellMeiboomGill (CPMG) pulse sequence in which the phase of the pipulses follows the phase of the proton spins at all times. Hence, we retain the ability of the CPMG pulse sequence to refocus local magnetic field inhomogeneities without refocusing global magnetic field fluctuations. In contrast with conventional fieldfrequency locking techniques, our experiments demonstrate the potential of this method to dynamically track global magnetic field fluctuations on timescales of about 2 seconds and with rates faster than a kHz. This frequency range covers the dominant noise frequencies in our electron spin resonance experiments as previously reported. [Preview Abstract] 
Follow Us 
Engage
Become an APS Member 
My APS
Renew Membership 
Information for 
About APSThe American Physical Society (APS) is a nonprofit membership organization working to advance the knowledge of physics. 
© 2023 American Physical Society
 All rights reserved  Terms of Use
 Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 207403844
(301) 2093200
Editorial Office
1 Research Road, Ridge, NY 119612701
(631) 5914000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 200452001
(202) 6628700