Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J36: Focus Session: Semiconductor Qubits: Multi-Spin Gates & Sequence Optimization |
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Sponsoring Units: GQI Chair: Jake Taylor, National Institute of Standards and Technology Room: 703 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J36.00001: Composite Sequences for Triple-dot Qubits that Compensate for Miscalibration and Hyperfine Gradients Invited Speaker: Thaddeus Ladd Exchange-only qubits defined in triple quantum dots form a promising means for all-electrical semiconductor quantum control, but they suffer from both charge noise and random magnetic field gradients. Low-frequency noise sources can be compensated using composite sequences, but the development of such sequences is constrained by the fact that exchange energies are always positive and the control axes are non-orthogonal. Here, we present the results of both analytical approaches and computational searches for composite pulse sequences, which compensate for simultaneous low-frequency miscalibration (due to fixed random electric fields) and hyperfine effects (due to nuclear magnetic fields) in a single triple-dot qubit. We also present compensation sequences for multi-qubit gates. These results can substantially improve the working fidelity of quantum operations in semiconductor quantum dot devices. Sponsored by United States Department of Defense. 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 United States Department of Defense or the U.S. Government. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J36.00002: Robust Two-Qubit Gates for Exchange-Coupled Exchange-Only Qubits FNU Setiawan, Hoi-Yin Hui, Jason Kestner, Xin Wang We show how to perform dynamically corrected two-qubit gates, with the leading hyperfine error term cancelled, for various geometries of an exchange-only qubit network. These sequences are designed to obey the realistic experimental constraint of strictly non-negative couplings. Moreover, we show that these corrected sequences lead to substantial improvement in the gate fidelity. Together with single-qubit dynamically corrected gates, our results facilitate universal and robust multi-qubit quantum operations and pave the way towards scalable fault-tolerant quantum computation on the exchange-only qubit platform. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J36.00003: Optimizing efficiency of noise cancelling in a singlet-triplet spin-qubit array Muhed Rana, Jason Kestner, Fernando Calderon Singlet-triplet qubits are a very good candidate for use in quantum computing and quantum operations due to their long coherence time and rapid gate operations. The fluctuations of the background nuclear spin bath and fluctuations in electrostatic quantum dot confinement potential affects the precise manipulation of the qubit. Recently, a method was developed to create an identity operation that corrects both sources of errors to the first order [1]. We now consider how much the compensating identity pulse sequence can be shortened in cases where only one dominant source of error needs correction. [1] J.P. Kestner et al., Phys. Rev. Lett. 110, 140502 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J36.00004: High-Fidelity Single-Qubit Gates for Two-Electron Spin Qubits Tim Botzem, Pascal Cerfontaine, David P. DiVincenzo, Hendrik Bluhm High fidelity gate operations for manipulating individual and multiple qubits in the presence of decoherence are a prerequisite for fault-tolerant quantum information processing. However, the control methods used in earlier experiments on semiconductor two-electron spin qubits are based on unrealistic approximations which preclude reaching the required fidelities. An attractive remedy is to use control pulses found in numerical simulations that minimize the infidelity from decoherence and take the experimentally important imperfections and constraints into account. Using this approach and experimentally determined noise spectra, we find pulses for singlet-triplet qubits in GaAs double quantum dots with fidelities as high as 99.9{\%}. Fully eliminating systematic pulse errors will likely require a calibration of the pulses on the experiment using some form of self-consistent approach. Starting with inaccurate control pulses we show that elimination of individual systematic gate errors is possible by applying a modification of the bootstrap protocol proposed by Dobrovitski et al. (PRL 105, 2010) while still retaining the pulses' high fidelities. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J36.00005: Dynamically Corrected Quantum Gates for Two-Electron Spin Qubits Pascal Cerfontaine, Matthias Loebl, Hendrik Bluhm Two-electron spin qubits in double quantum dots offer the possibility of fast and fully electrical manipulation via the exchange interaction. Arbitrary single-qubit gates have been demonstrated while maintaining a magnetic field gradient. However, simple gate constructions are extremely sensitive to noise in the Hamiltonian and thus incur considerable decoherence. Dynamically corrected gates are first-order insensitive to disturbances and present an appealing solution if slow noise sources are dominant. Using a numerical model that reflects the experimentally important imperfections and hardware constraints, we find control pulses for singlet-triplet qubits in GaAs double quantum dots which decouple in both the electrical control and the hyperfine magnetic field gradient. Additionally, dephasing effects from fast noise sources are minimized by favoring operating points close to a sweet spot. For experimentally determined noise levels the resulting gates feature fidelities as high as 99.9\% and are mainly limited by high-frequency noise and nonlinearities. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J36.00006: Quantum optimal local control of coherent dynamics in custom-made nanostructures Mario Borunda, Thomas Blasi, Esa Rasanen, Eric Heller We apply quantum optimal control theory to establish a local voltage-control scheme that operates in conjunction with the numerically exact solution of the time-dependent Schr\"odinger equation. The scheme is demonstrated for high-fidelity coherent control of electronic charge in semiconductor double quantum dots. We find tailored gate voltages in the viable gigahertz regime that drive the system to a desired charge configuration with $>99\%$ yield. The results could be immediately verified in experiments and would play an important role in applications towards solid-state quantum computing. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J36.00007: Coherent Manipulation of a Silicon Spin-Charge Hybrid Qubit Dohun Kim, Zhan Shi, Christie B. Simmons, Daniel R. Ward, Jon R. Prance, Teck Seng Koh, John King Gamble, Donald E. Savage, Max G. Lagally, Mark Friesen, Susan N. Coppersmith, Mark A. Eriksson The recently proposed quantum dot hybrid qubit enables fast, coherent quantum operations [1, 2]. We demonstrate rotations of a hybrid qubit in a three-electron Si/SiGe double quantum dot about two axes of the Bloch sphere (X and Z). We perform Larmor oscillations (x-rotations on the Bloch sphere) between the 0 and 1 hybrid states, demonstrating a T2* time of 2.1 ns at the charge degeneracy point [3,4]. Using tailored pulse gating sequences, we perform fast (\textgreater 10GHz) phase (z-axis) rotations of the hybrid qubit states. We measure a lower bound of the coherence time T$_{2}$* of 10 ns and high figure of merit \textgreater 150. This work was supported in part by ARO (W911NF-12-0607) and the United States Department of Defense. 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 US Government. \\[4pt] [1] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012).\\[0pt] [2] Teck Seng Koh, et al, Phys. Rev. Lett. 109, 250503 (2012)\\[0pt] [3] Z. Shi, et al., Phys. Rev. B 88, 075416 (2013) \\[0pt] [4] Z. Shi, et al., e-print: http://arxiv.org/abs/1308.0588 [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J36.00008: Statistical benchmarking for orthogonal electrostatic quantum dot qubit devices John Gamble, Adam Frees, Mark Friesen, S.N. Coppersmith Quantum dots in semiconductor systems have emerged as attractive candidates for the implementation of quantum information processors because of the promise of scalability, manipulability, and integration with existing classical electronics. A limitation in current devices is that the electrostatic gates used for qubit manipulation exhibit strong cross-capacitance, presenting a barrier for practical scale-up. Here, we introduce a statistical framework for making precise the notion of orthogonality. We apply our method to analyze recently implemented designs at the University of Wisconsin-Madison that exhibit much increased orthogonal control than was previously possible. We then use our statistical modeling to future device designs, providing practical guidelines for devices to have robust control properties. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy Nuclear Security Administration under contract DE-AC04-94AL85000. 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 US Government. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J36.00009: Enhancing the performance of exchange-only qubits in triple-quantum-dots Jianjia Fei, Jo-Tzu Hung, Teck Seng Koh, Yun-Pil Shim, Susan Coppersmith, Xuedong Hu, Mark Friesen The exchange-only qubit has several potential advantages for quantum computation: all-electrical control, fast gate operations, and robustness against global magnetic noise. Such a device has recently been implemented in a GaAs triple-quantum-dot. In this talk, we discuss theoretical simulations of the fidelity of pulsed gate operations of the exchange-only qubit, based on a master equation approach. Our model accounts for several different dephasing mechanisms, including hyperfine interactions and charge noise arising from double-occupation errors and fluctuations of the detuning parameter. Our investigations indicate the optimal working regimes and maximum gate fidelities for these devices, in terms of experimentally tunable parameters. This work was supported by the Army Research Office, the National Science Foundation, and the United States Department of Defense. 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 US Government. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J36.00010: Universal Set of Quantum Gates for Double-Dot Exchange-Only Spin Qubits Under Realistic Conditions Marco De Michielis, Elena Ferraro, Davide Rotta, Giovanni Mazzeo, Marco Tagliaferri, Alessandro Crippa, Marco Fanciulli, Enrico Prati We report on a universal set of quantum logic gates for hybrid qubits. In a hybrid qubit the information is encoded in the spin state of three electrons elettrostatically confined in a silicon double quantum dot (QD), in (2,1) filling [1]. All electrical operations, reduced fabrication complexity and high scalability are the strengths of this technology. Schrieffer-Wolff effective models for both one [2] and two coupled hybrid qubit [3] are developed including the inescapable exchange interaction between electrons in the same QD. Optimal sequences of exchange interactions creating a complete set of quantum operations, namely Hadamard, $\pi$/8 and CNOT gates [4], are obtained by using a search algorithm, based on simplex and genetic ones. Silicon devices have been designed by SDFT-based program and efforts in its fabrication have produced in-plane inter-QDs distances down to 100 nm by means of electron beam lithography. Double QDs devices operating in few electron filling regime have been preliminary characterized at 4.2 K. Ref: [1] T.S. Koh et al., PRL 109, 250503 (2012) [2] E. Ferraro et al., quant-ph/arXiv, 1304.1800 (2013) [3] In preparation [4] M. De Michielis et al., submitted (2013) [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J36.00011: Quantum Interference between Three Spin Qubits Andrew Sachrajda, Gabriel Poulin-Lamarre, Joelle Thorgrimson, Sergei Studenikin, Geof Aers, Alicia Kam, Piotr Zawadzki, Zbigniew Wasilewski Recently both hyperfine and exchange based qubits based on three spin states in triple quantum dot circuits have been individually demonstrated. The effective targeting of a specific qubit species required a carefully designed pulse shape and measurement sequence. We discuss results where pulses are chosen to activate both three spin qubit species simultaneously. In our results two novel coherent behaviors have been identified which are related to quantum interference effects involving an interplay between the two qubits types. Such experiments are important to gain an understanding of critical leakage paths which drive the system away from the intended qubit states. Certain features of the data are analyzed in terms of a breakdown of the usual spin blockade spin to charge conversion technique for three spin experiments and the consequences of charge noise on the measurements. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J36.00012: Maximal Rabi frequency of an electrically driven spin in a disordered magnetic field Andras Palyi, Gabor Szechenyi We present a theoretical study of the spin dynamics of a single electron confined in a quantum dot. Spin dynamics is induced by the interplay of electrical driving and the presence of a spatially disordered magnetic field, the latter being transverse to a homogeneous magnetic field. We focus on the case of strong driving, i.e., when the oscillation amplitude $A$ of the electron's wave packet is comparable to the quantum dot length $L$. We show that electrically driven spin resonance can be induced in this system by subharmonic driving, i.e., if the excitation frequency is an integer fraction (1/2, 1/3, etc) of the Larmor frequency. At strong driving we find that (i) the Rabi frequencies at the subharmonic resonances are comparable to that at the fundamental resonance, and (ii) at each subharmonic resonance, the Rabi frequency can be maximized by setting the drive strength to an optimal, finite value. Our simple model is applied to describe electrical control of a spin-valley qubit in a weakly disordered carbon nanotube. Reference: http://arxiv.org/abs/1310.7350 [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J36.00013: Deterministic photonic cluster state generation from quantum dot molecules Sophia Economou, Mercedes Gimeno-Segovia, Terry Rudolph Currently, the most promising approach for photon-based quantum information processing is measurement-based, or one-way, quantum computing. In this scheme, a large entangled state of photons is prepared upfront and the computation is implemented with single-qubit measurements alone. Available approaches to generating the cluster state are probabilistic, which makes scalability challenging. We propose to generate the cluster state using a quantum dot molecule with one electron spin per quantum dot. The two spins are coupled by exchange interaction and are periodically pulsed to produce photons. We show that the entanglement created by free evolution between the spins is transferred to the emitted photons, and thus a 2D photonic ladder can be created. Our scheme only utilizes single-spin gates and measurement, and is thus fully consistent with available technology. [Preview Abstract] |
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