Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C42: Semiconducting QC: Gates and Architectures |
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Sponsoring Units: GQI Chair: Susan Coppersmith, University of Wisconsin-Madison Room: 389 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C42.00001: Donor spin qubits with robust long-distance coupling Vivien Schmitt, Guilherme Tosi, Fahd Mohiyaddin, Stefanie Tenberg, Arne Laucht, Rajib Rahman, Gerhard Klimeck, Andrea Morello Single-donor spin qubits in silicon have been shown to be among the most coherent in the solid state [1]. However, scaling up beyond one donor to build a scalable quantum computer architecture remains a great challenge. Here we propose to use the flip-flop states of electron and nucleus of an implanted phosphorus atom as a qubit. We induce an electric dipole by biasing the electron wavefunction between the donor and an interface state. This dipole couples the flip-flop states to a resonant oscillating electric field, which can drive fast transitions between the qubit states. The electric dipole-dipole interaction between two donors allows robust two-qubit logic gates at long-distance (~200 nm). We present simulated single- and two-qubit gate fidelities exceeding 99\% in the presence of realistic values of charge noise, and show that the ability to electrically drive and couple the qubits does not result in a deterioration of their coherence properties. Prototypical devices are currently being tested to demonstrate the predicted behavior. [1] J. T. Muhonen, et.al. Nature Nanotechnol. 9, 986 (2014). [2] G. Tosi, et.al. arXiv:1509.08538 (2015). [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C42.00002: Adiabatically-controlled two-qubit gates using quantum dot hybrid qubits Adam Frees, John King Gamble, Mark Friesen, S. N. Coppersmith With its recent success in experimentally performing single-qubit gates, the quantum dot hybrid qubit is an excellent candidate for two-qubit gating. Here, we propose an operational scheme which exploits the electrostatic properties of such qubits to yield a tunable effective coupling in a system with a static capacitive coupling between the dots. We then use numerically calculated fidelities to demonstrate the effect of charge noise on single- and two-qubit gates with this scheme. Finally, we show steps towards optimizing the gates fidelities, and discuss ways that the scheme could be further improved. This work was supported in part by ARO (W911NF-12-0607) (W911NF-12-R-0012), NSF (PHY-1104660), ONR (N00014-15-1-0029). The authors gratefully acknowledge support from the Sandia National Laboratories Truman Fellowship Program, which is funded by the Laboratory Directed Research and Development (LDRD) Program. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C42.00003: Dynamically correcting a CNOT gate for any systematic logical error Fernando Calderon-Vargas, Jason Kestner The reliability of quantum information processing depends on the ability to deal with noise and error in an efficient way, and a significant source of error in many settings is coherent, systematic gate error. We address this problem by deriving a set of composite pulse sequences that generate CNOT gates and correct all systematic errors within the logical subspace to arbitrary order [1]. These sequences are applicable for any two-qubit interaction Hamiltonian, and make no assumptions about the underlying noise mechanism except that it is constant on the timescale of the operation. We do assume access to error-free single-qubit gates, so single-qubit gate imperfections eventually limit the achievable fidelity. However, since single-qubit gates generally have much higher fidelities than two-qubit gates in practice, these pulse sequences offer useful dynamical correction for a wide range of coupled qubit systems. [1] F.A. Calderon-Vargas, J.P. Kestner, "Dynamically correcting a CNOT gate for any systematic logical error", arXiv:1607.04638 (2016). [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C42.00004: CNOT sequences for heterogeneous spin qubit architectures in a noisy environment Elena Ferraro, Marco Fanciulli, Marco De Michielis Explicit CNOT gate sequences for two-qubits mixed architectures are presented in view of applications for large-scale quantum computation. Different kinds of coded spin qubits are combined allowing indeed the favorable physical properties of each to be employed. The building blocks for such composite systems are qubit architectures based on the electronic spin in electrostatically defined semiconductor quantum dots. They are the single quantum dot spin qubit, the double quantum dot singlet-triplet qubit and the double quantum dot hybrid qubit [1]. The effective Hamiltonian models expressed by only exchange interactions between pair of electrons are exploited in different geometrical configurations [2,3,4]. A numerical genetic algorithm that takes into account the realistic physical parameters involved is adopted. Gate operations are addressed by modulating the tunneling barriers and the energy offsets between different couple of quantum dots. Gate infidelities are calculated considering limitations due to unideal control of gate sequence pulses, hyperfine interaction and unwanted charge coupling. [1] Z. Shi et al, PRL 108,140503 (2012) [2] E. Ferraro et al, QIP 13,1155 (2014) [3] E. Ferraro - M. De Michielis et al, QIP 14,47 (2015) [4] M. De Michielis et al, JPA 48,065304 (2015) [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C42.00005: Optimal local control of entangled states in semiconductor quantum wells Mario Borunda, Robert Radford, Esa Rasanen 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\"{o}dinger equation. The scheme is demonstrated for high-fidelity coherent control of electronic charge in many-particle states of 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 semiconducting quantum information. [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C42.00006: Coherent probing and manipulation of valley states in silicon double quantum dot devices with fast pulses Joshua Schoenfield, Blake Freeman, HongWen Jiang, Jason Petta We report the coherent manipulation of a qubit based on two valley states of an electron confined in a silicon quantum dot. Coherent evolution between the states, which have a relatively small energy splitting of 20 $\mu$eV, is excited by a fast electrical pulse and the phase information is projected to a charge state for read-out by a nearby charge sensing channel. Coherent control was demonstrated at multiple charge configurations of the same device. The energy dispersion as a function of detuning as well as the phase coherence time of the valley qubit is obtained by varying pulse height and duration. Such coherent manipulation also provides a method of measuring valley splittings which are too small to probe with conventional methods of magneto-spectroscopy. Using these same techniques, we have produced analogous results in a different device. Coherent time domain oscillations of roughly 350 MHz, corresponding to a valley-like splitting of 1.4 $\mu$eV, are observed. Coherence times of up to 15 ns, in excess of reported values for charge qubits, have been observed in this system when a Ramsey-like pulse shape is applied. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C42.00007: Suppressing leakage by composition of pulses for single-qubit operations in a three-level system Joydip Ghosh, Mark Friesen, Susan Coppersmith Many realizations of solid-state qubits are constructed from elements with more than two energy levels. The tunneling of quantum information to these additional energy levels, often called leakage errors, remains an impediment to devising high-fidelity quantum gate operations. Mitigating the leakage errors becomes more challenging if the couplings between the computational subspace and the leakage states are unknown, which is, in fact, the case for some semiconducting qubits. Here we propose an approach based on composition of pulses to suppress such leakage errors for a qubit encoded in a three-level system, and apply our theory specifically to the Charge Quadrupole (CQ) quantum dot qubit. The proposed scheme thus brings us one step closer to constructing a fault-tolerant quantum computer with solid-state elements. [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C42.00008: Faster pulse sequences for performing arbitrary rotations in singlet-triplet qubits Robert Throckmorton, Edwin Barnes, Xin Wang We present new composite pulses that perform universal single-qubit operations in singlet-triplet spin qubits faster than existing methods. We introduce two types of composite pulses: one that generalizes the standard Hadamard-$x$-Hadamard sequence used to perform rotations about the $z$ axis, and another that generalizes a sequence proposed by Guy Ramon (G. Ramon, Phys. Rev. B {\bf 84}, 155329 (2011)). We determine how much time it takes to perform each set of pulses and find that our ``generalized Hadamard-$x$-Hadamard'' sequence can be made faster than any of the other sequences. We also present composite pulses for performing $x$ rotations and show that a generalization of the Hadamard-$z$-Hadamard sequence is faster than other existing sequences, as well as faster and more precise than performing $x$ rotations with single pulses. We present versions of these gates that also dynamically correct for noise-induced errors along the lines of SUPCODE (X.\ Wang {\it et. al.}, Phys. Rev. A {\bf 89}, 022310 (2014)). [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C42.00009: Enhancing the gate fidelity of silicon-based singlet-triplet qubits under symmetric exchange control using optimized pulse sequences Chengxian Zhang, Robert Throckmorton, Xu-Chen Yang, Xin Wang, Edwin Barnes We perform Randomized Benchmarking of a family of recently introduced control scheme for singlet-triplet qubits in semiconductor double quantum dots, which is optimized to have substantially shorter gate times. We study their performances under the recently introduced symmetric control scheme of changing the exchange interaction by raising and lowering the barrier between the two dots (barrier control) and compare these results to those under the traditional tilt control method in which the exchange interaction is varied by detuning. It has been suggested that the barrier control method encounters a much smaller charge noise. We found that for the cases where the charge noise is dominant, corresponding to the device made on isotopically enriched silicon, the optimized sequences offer much longer coherence time under barrier control compared to the tilt control method of the strength of the exchange interaction. [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C42.00010: A molecular orbital study of the energy spectrum, exchange interaction and gate crosstalk of a four-quantum-dot system Xu-Chen Yang, Xin Wang The manipulation of coupled quantum dot devices is crucial to scalable, fault-tolerant quantum computation. We present a theoretical study of a four-electron four-quantum-dot system based on molecular orbital methods, which depicts a pair of singlet-triplet (S-T) qubits. We find that while the two S-T qubits are coupled by the capacitive interaction when they are sufficiently far away, the admixture of wave functions undergoes a substantial change as the two S-T qubits get closer. We find that in certain parameter regime the exchange interaction may only be defined in the sense of an effective one when the computational basis states no longer dominate the eigenstates. We further discuss the gate crosstalk as a consequence of this wave function mixing. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C42.00011: Coherence of an electron bound to a moving quantum dot Reinaldo de Melo e Souza, Andre Saraiva, Xuedong Hu, Belita Koiller Several problems have hindered the development of logic gates based on electrons bound to a quantum dot. Strong exchange interactions implicate short coherence time (as compared to processing time) and this constitutes one of the greatest obstacles. As an alternative, in recent years there has been much study involving flying qubits, in which we have the coherent transport of a particle. In the case of electrons, it has been experimentally shown that they can be successfully transported from a quantum dot to another one (separated by few micrometers) by a surface acoustic wave (SAW). Nevertheless, distinct factors may affect the electron coherence along the motion. In this work we analyze theoretically the transport of an electron from one quantum dot to another one by an applied potential. Although the time dependence for the entire process is usually very complicate, we may describe the problem in several steps so that each one can be modeled as a harmonic oscillation with time-dependent parameters, enabling us to obtain general analytic expressions (spin-orbit coupling neglected) in situations of experimental interest. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C42.00012: Single electron transport using surface acoustic waves in semiconductor devices Hugo Lepage, Crispin Barnes A numerical approach to single electron transport provides the means to interpret results obtained experimentally and guide further experimental designs. We use surface acoustic waves (SAWs) to generate a quantized electron current. In piezoelectric materials, an oscillating stress and strain wave is accompanied by an electric potential modulation of similar waveform. By tuning the amplitude of the SAW, flying quantum dots can be created, trapping single electrons in the potential minima. Numerical solutions to the time dependent Schrodinger equation offer an accurate description of an electron's wavefunction as it is being transported by a SAW. We first model a 2D channel using a harmonic potential in the y dimension and add a sinusoidal SAW confinement in the x dimension. After introducing a tunnelling barrier allowing the electron to escape the channel, the system becomes akin to an electron beam splitter, where the electron wavefunction oscillates between both possible channels (or states). Be replicating typical potential layouts used by experimental groups, we were able to find the dependence of an electron tunnelling out of a 2D channel on the device's surface gate voltages. A model quantum computer using SAW-driven single electron qubits was proposed by Barnes in 2000. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C42.00013: Development of SAW-driven single-photon source in an undoped AlGaAs/GaAs/AlGaAs quantum well structure Tzu-Kan Hsiao, Yousun Chung, Antonio Rubino, Ateeq Nasir, Hangtian Hou, Seok-Kyun Son, Jonathan Griffiths, Thomas Mitchell, Ian Farrer, David Ritchie, Chris Ford A lot of effort has been made to study single-photon sources due to their applications such as quantum key distribution and quantum repeater. In this research, a single-photon source driven by a surface acoustic wave (SAW) is in development. In this device, electrons and holes are induced in adjacent regions to form an n-i-p junction in an undoped AlGaAs/GaAs/AlGaAs quantum well by gates on the surface. A SAW launched by a transducer creates a moving electric potential and drags electrons from the induced region of electrons to the region of holes across a 1-D channel defined by a pair of side gates. A single-photon source can thereby be realised if the 1-D channel allows only one electron in each SAW-created potential minimum to reach the region of holes and recombine with holes. Recently, we have observed the SAW-pumped recombination current, which can be modulated by the side gates. This means that it is possible to transport a stream of single-electrons by SAW. In addition, the spectrum of the electroluminescence shows that the recombination happens in the quantum well when the n-i-p junction is under forward bias. We are now working on detecting the emission from the SAW-pumped recombination, and getting quantised current to realise the SAW-driven single-photon source. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C42.00014: Measurement-free implementations of small-scale surface codes for quantum dot qubits H. Ekmel Ercan, Joydip Ghosh, Daniel Crow, Vickram N. Premakumar, Robert Joynt, Mark Friesen, S. N. Coppersmith Quantum error correction schemes and their corresponding error thresholds depend sensitively on the physical implementation of the qubits. For example, in quantum dot spin qubits, readout can be much slower than gate operations; however qubit reset---without readout---can be fast, via tunneling to a reservoir. Conventional surface code implementations rely heavily on syndrome measurements, and could therefore be challenging for quantum dots. Here, we propose small-scale surface code implementations for which syndrome measurements are replaced by a combination of Toffoli gates and qubit reset. For quantum dot qubits, this enables much faster error correction than measurement-based schemes, but requires additional ancilla qubits and non-nearest-neighbor interactions. We have performed numerical simulations of two different coding schemes, obtaining error thresholds on the order of $10^{-3}$ for a 1D architecture that only corrects bit-flip errors, and $10^{-5}$ for a 2D architecture that corrects bit- and phase-flip errors. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C42.00015: Grid-bus surface code architecture Simon Nigg, Andreas Fuhrer, Daniel Loss We present a scalable hybrid architecture for the 2D surface code combining superconducting resonators and spin qubits in nanowires with tunable spin-orbit coupling. The back-bone of this architecture is a square lattice of capacitively coupled coplanar waveguide resonators each of which hosts a nanowire spin-orbit qubit. A simple circuit QED model is derived for the coupling between the spin degree of freedom and the quantized resonator modes on the lattice. The electrically tunable qubit frequency allows for fast single qubit phase gates. A two-qubit $\sqrt{i\rm SWAP}$ gate between neighboring qubits can be realized by a third order process, whereby a virtual photon in one cavity is created by a first qubit, coherently transferred to a neighboring cavity, and absorbed by a second qubit in that cavity. Numerical simulations with realistic parameters predict high gate fidelities. [Preview Abstract] |
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