Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H28: Charge Noise Mitigation in Quantum Dot QubitsFocus

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Sponsoring Units: DQI Chair: John Nichol, Univ of Rochester Room: LACC 405 
Tuesday, March 6, 2018 2:30PM  3:06PM 
H28.00001: Charge Noise Characterization in SiGe TripleDot Qubits Invited Speaker: Matthew Borselli Gated semiconductor quantum dots are attractive candidates for quantum information processing because of the large existing base of fabrication processes providing nearly defectfree host materials and gate patterning techniques. Isotopicallyenhanced silicon precursors are now routinely leveraged to create spinbased qubits with long coherence times [1]. Electrical control of these qubits is sensitive to unwanted charge fluctuations originating from the device itself. This noise can be mitigated  but not eliminated  through clever device designs and modes of operation [2,3]. Complementing these design and operation advances are the efforts to understand the origins of charge noise and methods to reduce the noise [4]. In this talk, I will review one of the promising spin qubits, Si/SiGe triplequantum dots, and general techniques that have been developed to gain insight into what limits the performance of these devices. An emphasis will be placed on correlating qubit performance to simpler and more universal charge noise characterization methods. In particular, power spectral densities will be presented as a function of different device designs and temperatures. In addition, crosscorrelation measurements will show the noise to be a local phenomenon. Simple characterization structures and methods will also be discussed that have general applicability to all semiconductor quantumdot qubits in an effort to drive to a common understanding of the microscopic origins of charge noise in these systems. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H28.00002: Quantum Decoherence of Dressed Central Spins in Nuclear Spin Baths Gengli Zhang, Wenlong Ma, Vincent Jacques, Renbao Liu We present a physically clear, reliable and easy to implement solution to the decoherence problem of central spin induced by entanglement with the nuclear spin bath for a general class of systems. The critical feature of this class of decoherence problems is that the renormalized energy for the central spin has a dressed form in the dressed basis, which is also fluctuating together with the fluctuation of the Overhauser field when nuclear spins flip or flop in the bath. By proving that fluctuation has negligible effects on the dressed basis compared with dressed energy, we approximate dressed basis as static and treat dressed energy exactly in our model. In the renormalized basis for the bath, hyperfine mediated long range interactions are absorbed as fluctuation of the Overhauser field and contribute as single spin correlations in the total decoherence, so the central spin decoherence caused by the nuclear spin bath can be systemically solved by the cluster correlation expansion method. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H28.00003: Theory of Barrier vs Tilt Exchange Gate Operations in Spinbased Quantum Computing YunPil Shim, Charles Tahan We present a theory for understanding the exchange interaction between electron spins in neighboring quantum dots, either by changing the detuning of the two quantum dots or independently tuning the tunneling barrier between quantum dots [1]. The Hubbard model and a more realistic confiningpotential model are used to investigate how the tilting and barrier control affect the effective exchange coupling and thus the gate fidelity in both the detuning and symmetric regimes. We show that the exchange coupling is less sensitive to the charge noise through tunnel barrier control (while allowing for exchange coupling operations on a sweet spot where the exchange interaction has zero derivative with respect to the detuning). Both GaAs and Si quantum dots are considered and we compare our results with experimental data showing qualitative agreements. Our results answer the open question of why barrier gates are preferable to tilt gates for exchangebase gate operations. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H28.00004: Critical role of tunneling noise on spin decoherence in a twoqubit gate Peihao Huang, Neil Zimmerman, Garnett Bryant Rapid progress in semiconductor spin qubits enables experimental demonstrations of a twoqubit logic gate, which is necessary for universal quantum computing. Here, we study the decoherence of two electronspin qubits due to 1/f charge noise in a silicon double quantum dot used for a twoqubit logic gate. We find that, even though the amplitude of tunneling fluctuation is small, its effect on the spin qubit is first order in the charge admixture in comparison with an effect, second order in the admixture, due to detuning fluctuation. As a consequence, the tunneling noise can dominate over detuning noise under conditions typical for accumulation mode quantum dots. The different orders of contributions also result in the different detuning dependencies of the decoherence for detuning and tunneling noise, which enables the identification of noise sources. By comparing with a recent twoqubit experiment, we find that the decoherence was dominated by tunneling fluctuation from charge noise instead of detuning fluctuation. When tunneling noise dominates, more, rather than less, asymmetry in the device detuning can increase the number of twoqubit operations, pointing to the importance of considering tunneling noise to design optimal operation of spin qubits. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H28.00005: Simulating HighFidelity TwoQubit Gates with SingletTriplet Qubits Generated by Capacitive Coupling and Interqubit Exchange Interaction Michael Wolfe, Pascal Cerfontaine, Fernando CalderonVargas, Jason Kestner, Hendrik Bluhm Twoqubit gates in singlettriplet qubits can be generated via capacitive coupling or interqubit exchange interaction. Both methods suffer considerably from charge noise and nearly all approaches to mitigate this effect rely on the fact that the noise is slow compared to the gate time. We show that in the strictly capacitive case where gate times are much slower, maximally entangling gates with fidelities above 99% are achievable by operating the qubit in a sweet spot regime that is predicted by a HundMulliken model [1]. The advantage of this control method is that it naturally suppresses twoqubit errors regardless of the noisefrequency profile. In addition, we find comparable fidelities when both interqubit exchange and capacitive interactions are simultaneously used to generate entanglement. We compare these theoretical results with gates that are found using an optimization technique that numerically searches for highfidelity twoqubit gates using a fullnoise and control error model [2]. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H28.00006: Charge Sensing in a HighMobility MetalOxideSilicon Dual Quantum Dot Device JinSung Kim, Stephen Lyon Spins confined in metaloxidesilicon (MOS) heterostructures are promising qubits, demonstrating long coherence times and a large valley splitting. One of the key challenges in fabricating MOS quantum devices is to maintain a highquality Si/SiO_{2} interface without introducing shallow electron traps during highenergy processes like electronbeam lithography. In previous work we have developed a fabrication process yielding recordhigh mobility thinoxide (30 nm) MOS transistors (23,000 cm^{2}/Vs) with very low shallow defect densities and percolation thresholds (8×10^{10} cm^{2}). In addition, we have shown that we can maintain a highquality Si/SiO_{2} interface even after irradiating these samples with an electronbeam lithography dose. Leveraging this fabrication process, we have fabricated a MOS dual quantum dot device similar to the “dualrail” structures pioneered in Si/SiGe, featuring two parallel conduction channels defined by three overlapping layers of polysilicon and aluminum gates. We present preliminary characterization of this device with DC transport measurements at 300 mK. Biasing one of the quantum dots into a charge sensor, we demonstrate the tunability of our device down to the few electron regime. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H28.00007: Lifting of Spin Blockade by Charged Impurities in SiMOS Double Quantum Dot Devices Cameron King, Joshua Schoenfield, Maria Calderon, Belita Koiller, Andre Saraiva, Xuedong Hu, HongWen Jiang, Mark Friesen, Susan Coppersmith One obstacle that has impeded the development of electrically gated MOS singlettriplet qubits is the lack of observed spin blockade, where the tunneling of a second electron into a dot is fast when the twoelectron state is a singlet and slow when the state is a triplet, even in samples with large singlettriplet energy splittings. We present theoretical and experimental evidence that the cause of this commonly exhibited problem in MOS double quantum dots is the presence of stray positive charges in the oxide layer that induce accidental dots near the device’s active region that allow spin blockade lifting. We also present evidence that these effects can be mitigated with device design modifications. 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H28.00008: Impact of gateinduced strain on silicon MOS quantum dot tunnel barriers Ryan Stein, Joshua Pomeroy, Neil Zimmerman, M. Stewart Jr. Gate defined quantum dots in silicon are extremely sensitive to disorder in the local environment of the quantum dot. In the Si MOS system, disorder can originate from oxide charge defects, substrate impurities, and strain. One significant source of inhomogeneous strain in silicon quantum dots is induced by the gate materials. This strain originates in the intrinsic stress in the gate material as deposited and differences in the thermal properties between the gate and substrate. At low temperatures, this strain leads to local modifications of the conduction band strong enough to form unintentional quantum dots and to affect the tunnel coupling between dots. In this work, we investigate the role of gateinduced strain by comparing measurements of the 4terminal IV characteristics of tunnel barrier devices at 2K. The devices are fabricated on bulk silicon wafers with Al and polysilicon gate electrodes separated by tunnel gap lengths ranging from 2040nm and gate widths ranging from 50 to 500 nm. Using a WKB tunnel barrier model, we find that Al gates devices have on order of 1 meV larger barrier heights as compared to polysilicon devices when the extracted barrier width is about 30 nm. We will discuss these results in terms of the strained induced modulation of the barrier. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H28.00009: Coherence Time of a Semiconductor Hybrid Qubit in Presence of Environmental Noises Marco De Michielis, Elena Ferraro, Marco Fanciulli The effects of magnetic and charge noises on the dynamics of the hybrid qubit [1] (HQ) is theoretically investigated. The HQ consisting of three electrons arranged in a double quantum dot deserves special interest in quantum computation applications due to its advantages in terms of fabrication, control and manipulation in view of implementation of fast operations through only electrical tuning. The presence of the environmental noise due to nuclear spins and charge traps, in addition to fluctuations in the applied magnetic field and charge fluctuations on the electrostatic gates, is taken into account including random magnetic field and random coupling terms in the Hamiltonian [2]. The dymanics of the return probability for two initial conditions of interest is presented when HQs are hosted in ^{28}Si, natural Si and GaAs [3]. Coherence time of HQs is extracted when model parameters take values achievable experimentally, providing interesting predictions on its maximization. [1] Z. Shi et al., Phys. Rev. Lett. 108, 140503 (2012). [2] R.E. Throckmorton et al.,, Phys. Rev. B 95, 085405 (2017). [3] E. Ferraro et al., arXiv:1710.10032 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H28.00010: Quantum dot qubits with quadrupolar geometry Joydip Ghosh, Mark Friesen, Mark Eriksson, Susan Coppersmith Robust implementations of semiconducting quantum dot qubits must overcome the effects of charge noise that currently limit coherence during gate operations. Here, I describe a scheme for protecting solidstate qubits from uniform electric field fluctuations with a specific physical implementation: a quadrupole qubit in a triple quantum dot. The unique design of the quadrupole qubit enables particularly simple pulse sequences for suppressing the effects of noise during gate operations. Finally, I will discuss an advanced implementation of the quadrupole qubit where the spin and the charge degrees of freedom are hybridized in order to generate a very wide sweet spot that can provide even stronger protection against charge noise. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H28.00011: Effects of phonons on the coherence of a charge quadrupole qubit Viktoriia Kornich, Maxim Vavilov, Mark Friesen, Susan Coppersmith Many types of qubits are operated in a regime where the energy splittings between qubit states are large. In this regime, phonons can be the dominant source of decoherence. The charge quadrupole qubit, based on one electron in a triple quantum dot, uses a symmetric charge distribution to suppress the influence of charge noise. However, phonons couple the qubit subspace to a leakage state, potentially limiting the qubit’s coherence. We study the effect of phonons considering Larmor and Ramsey pulse sequences and identify favorable operating parameters and temperatures for them. We show that both pulse sequences can be implemented with >99.9% fidelity in the presence of phonons, but care must be taken to operate the qubit in an appropriate parameter range. This work was supported in part by ARO (W911NF1510248, W911NF1710274) and the Vannevar Bush Faculty Fellowship program sponsored by the Basic Research Office of the Assistant Secretary of Defense for Research and Engineering and funded by the Office of Naval Research through Grant No. N000141510029. The views and conclusions contained here are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office (ARO), or the U.S. Government. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H28.00012: Structural Sources of Electronic Disorder in GaAs Quantum Devices Anastasios Pateras, Joonkyu Park, Youngjun Ahn, Jack Tilka, Martin Holt, Honghyuk Kim, Luke Mawst, Werner Wegscheider, Christian Reichl, Timothy Baart, J.P Dehollain, Uditendu Mukhopadhyay, Lieven Vandersypen, Paul Evans The fabrication of quantum dots in GaAs/AlGaAs heterostructures involves the formation of metallic gate electrodes lithographically patterned on their surfaces. The gates are used for the definition and tuning of the electrostatic potential of the dot via applied voltages. We show using synchrotron xray nanobeam diffraction that the gates generate significant stresses that influence the operation of quantum devices. The stress is transferred through the metal/semiconductor interface to the depth where a twodimensional electron gas (2DEG) forms, inducing unintentional lattice distortions to the active region of the device. The xray diffraction studies reveal lattice tilts on the order of 0.04° in the quantum dot region and strain up to 4×10^{5} at the depth of the 2DEG. The piezoelectric effect in zincblende structures is the main source of electronic disorder with respect to the deformation potential which is an order of magnitude smaller. We estimate the piezoelectric potential induced by the lattice distortions to be approximately 10 mV. The strain leads to an energy shift of the minimum of the conduction band near the Γ point of GaAs due to the deformation potential by only 0.4 meV. The results indicate that such effects should be considered in the design of quantum devices. 
Tuesday, March 6, 2018 5:18PM  5:30PM 
H28.00013: Hyperfinephonon spin relaxation in a singleelectron GaAs quantum dot Leon Camenzind, Liuqi Yu, Peter Stano, Jeramy Zimmerman, Arthur Gossard, Daniel Loss, Dominik Zumbuhl Understanding and control of the spin relaxation time T_{1} is among the key challenges for spin based qubits. We present measurements of the spin relaxation rate W in a gate defined singleelectron GaAs quantum dot as a function of direction and strength of magnetic field, spanning an unprecedented range from 0.6 T to 14 T applied in the plane of the 2DEG. At high fields, the spin relaxation relies on phonon emission and spinorbit (SO) leading to a characteristic dependence W~B^{5} and a pronounced Bfield anisotropy, due to the interplay of the Rashba and Dresselhaus SO contributions. Along the axis with weak SO, a T_{1} of 57 ± 15 sec is obtained at 0.6 T  setting a new record for the spin lifetime in a nanostructure. Surprisingly, this is more than one order of magnitude shorter than the expected value based on SO mediated spin relaxation. Also, W shows a B^{3} dependence and becomes isotropic at low magnetic fields. These observations indicate hyperfine interaction mediated spin relaxation via phonons in the low field regime as predicted already 15 years ago. In this process the HF replaces the SO interaction as the source of admixture of the spin and orbital degree of freedom. 
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