Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V28: Spin-Based Quantum ComputingFocus
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Sponsoring Units: DQI Chair: Jason Petta, Princeton University Room: LACC 405 |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V28.00001: A programmable two-qubit quantum processor in silicon Invited Speaker: Thomas Watson Building small-scale quantum computers where initialisation, readout, single and two-qubit gates are combined to perform computation result in new challenges such as qubit cross talk, state leakage, and calibration. Here, we overcome these challenges to demonstrate a programmable two-qubit quantum processor using single electron spins in silicon [1]. In the natural Si/SiGe double quantum dot device, single qubit gates (2 MHz) with fidelities > 98% are achieved using electric dipole spin resonance [2] while a two-qubit gate (5-20 MHz) is realised using the exchange coupling between the two electron spins [3]. We characterise entanglement in our processor by performing quantum state tomography on Bell states where we achieve state fidelities between 85-90% and concurrences between 73-80%. Finally, we demonstrate the programmability of the processor by successfully running both the Deutsch-Jozsa and the Grover search algorithms. |
Thursday, March 8, 2018 3:06PM - 3:42PM |
V28.00002: Quantum CNOT Gate for Spins in Silicon [1] Invited Speaker: David Zajac Realizing robust two qubit gates has been one of the major hurdles for semiconductor spin qubits. Extremely long coherence times and high fidelity single qubit gates have been realized in spin qubits, but conventional exchange based two qubit couplings have suffered from a high sensitivity to charge noise. We demonstrate a resonantly driven single-step CNOT gate in a regime where exchange is a small perturbation to a large magnetic field gradient [2]. By placing a double quantum dot (DQD) in the fringing field of a Co micromagnet, we are able to electrically drive single spin resonance with Rabi frequencies greater than 10 MHz. We achieve single qubit fidelities of 99.3 ± 0.2 % and 99.7 ± 0.1 % for each qubit, as determined by Clifford randomized benchmarking. By turning on an exchange coupling between the two spins we split the single qubit frequencies by 19.7 MHz and use a frequency selective drive to realize a CNOT gate in ~200 ns. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V28.00003: Two-qubit gates in silicon quantum dots Anthony Sigillito, David Zajac, Maximilian Russ, Jacob Taylor, Guido Burkard, Jason Petta Silicon spin qubits offer long coherence times and exquisite single-qubit control in a platform that has proven to be scalable by the semiconductor electronics industry. Despite these advantages, there are very few demonstrations of two-qubit gates [1-3]. By incorporating a micromagnet with a double quantum dot device, we have recently demonstrated a fast (<200 ns), resonantly-driven CNOT gate [2]. In the same device, we have also implemented a CPHASE gate by applying dc voltage pulses to the barrier gate separating the two qubits. In this talk, we will discuss the operation of the CPHASE and address complications due to the field gradient generated by the micromagnet. We compare the CPHASE operation to the resonantly-driven CNOT and estimate the corresponding two-qubit gate fidelities. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V28.00004: Progress in exchange-based 2-qubit logic gates with donors in silicon Mateusz Madzik, Arne Laucht, Vincent Mourik, Fay Hudson, Kohei Itoh, David Jamieson, Andrew Dzurak, Andrea Morello Donor spin qubits have been extensively studied for future applications in quantum computing. In top-down ion-implanted devices, coherence times as long as 30 seconds and gate fidelities beyond 99.9% have been demonstrated. Building on those achievements, we report on progress towards two-qubit operations mediated by exchange interactions. Newly fabricated devices with an increased implantation dose revealed a high number of donors in the vicinity of a single-electron transistor. Electron spin resonance (ESR) spectra show spectroscopic evidence of exchange-coupled donor pairs. Experiments are underway to demonstrate 2-qubit logic gates using state-conditional ESR pulses. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V28.00005: Experimental Measurement of Leakage-Error in Exchange-Only SiGe Quantum Dot Qubits by Extending Randomized Benchmarking Aaron Weinstein Randomized benchmarking is a common method for quantifying qubit gate error, but has questionable validity or reliability for some physically relevant error sources. The focus of this talk will be leakage out of the computational subspace of a decoherence-free subsystem, which can introduce additional benchmarking decay and unreliable error estimates. Though techniques have been developed to characterize leakage, it is not clear how best to use that information to inform computational error rates. Here we develop an extension to the randomized benchmarking protocol that estimates both computational and leakage errors by means of preparing and tracking different final states of the benchmarking sequence. Using this protocol, we experimentally measure the leakage error per gate in an exchange-only SiGe triple quantum dot at rates well below that of state-preparation-and-measurement (SPAM) error and pulse error from electrical noise. These leakage rates are in close agreement with a noise model with electrical and hyperfine-induced magnetic noise terms. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V28.00006: Triple-Quantum-Dots with Overlapping Gates for Si/SiGe Qubits John Dodson, Trevor Knapp, Nathan Holman, Ryan Foote, Tom McJunkin, Brandur Thorgrimsson, Evan MacQuarrie, Mark Gyure, Lisa Edge, Peter Deelman, Robert McDermott, Mark Friesen, Susan Coppersmith, Mark Eriksson Silicon is an attractive host material for quantum bits due to its spinless isotope 28Si, and weak spin-orbit coupling, allowing for long spin decoherence and relaxation times. A state-dependent dipole moment is inherent in many silicon-based qubits, aiding in fast gate times and strong two qubit interactions. However, these types of qubits are susceptible to the decohering effects of charge noise. We present data from devices with potential to reduce the effects of charge noise by making use of symmetry in the quantum dot layout. Of particular interest is the charge quadrupole qubit. A linear triple-dot with a locally integrated charge sensor is used to explore the charge quadrupole qubit. The linear triple dot device utilizes a three-layer overlapping aluminum gate architecture, which has the potential to reduce charge noise due to a terraced field oxide with less bulk oxide in the active region. This architecture allows for high tunability of tunneling rates, and for closely spaced dots, which is in line with the requirements for the charge quadrupole qubit. It is also scalable in a one-dimensional array, allowing for multi-qubit devices in the near future. Additionally, we present charge sensing measurements of quantum dots using fast cryogenic amplification. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V28.00007: Exploring quantum chaos within a single 123-Sb donor in silicon Vincent Mourik, Serwan Asaad, Hannes Firgau, Mark Johnson, Mateusz Madzik, Arne Laucht, Fay Hudson, Catherine Holmes, Gerard Milburn, Jarryd Pla, Andrew Dzurak, Jeffrey McCallumn, Andrea Morello The 123-Sb atom is a group-V element with a nuclear spin quantum number of 7/2, resulting in an 8-dimensional Hilbert space. This atom can be implanted in a silicon Metal-Oxide-Semiconductor structure, and its quantum state can be controlled using the same infrastructure that has been proven to yield high-fidelity control [1] and single-shot readout [2] on the 31-P donor. The key difference in 123-Sb is that the nucleus possesses a quadrupole moment, which can introduce a quadratic term in the spin Hamiltonian when the atom is placed in a strained silicon crystal. By further adding a strong periodic drive, this results in a single-atom quantum system that accurately maps a classically chaotic one: the driven nonlinear top [3]. Therefore, we can engineer a highly controllable, individual quantum system that enables an experimental study of the emergence of chaos and the quantum-to-classical crossover [3]. In this presentation, we will provide a detailed theoretical description of the system, as well as initial results on the spectrum and coherence times of a 123-Sb donor. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V28.00008: Characterization of a scalable donor based singlet-triplet qubit architecture Prasanna Pakkiam, Matthew House, Matthias Koch, Michelle Simmons We present a donor based quadruple quantum dot device, capable of hosting two singlet-triplet qubits fabricated by scanning tunnelling microscope lithography, with just two leads per qubit. The design is geometrically compact with each pair of dots independently controlled via one gate and one reservoir that both supplies electrons for the dots and measures the singlet-triplet state of the given qubit via dispersive sensing. We verify the locations of the four phosphorus donor dots via an electrostatic model of the device. We study one of the observed singlet-triplet states with a tunnel coupling of 39GHz and a S0 to T– decay of 2ms at zero detuning. We measure a 5GHz electrostatic interaction between two pairs of dots separated by 70nm. The results outline a low gate density pathway to a scalable 1D building block of atomic precision donors with dispersive readout. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V28.00009: Single and two hole spin qubits formed in a lateral GaAs/AlGaAs double quantum dot Sergei Studenikin, Alex Bogan, Louis Gaudreau, Marek Korkusinski, Geof Aers, Piotr Zawadzki, Andrew Sachrajda, Lisa Tracy, John Reno, Terry Hargett Single holes localised in electrostatically tuneable quantum dot devices are explored as candidates for spin qubits. Here we study single hole and two-hole hybrid spin-charge qubits in the presence of a strong spin-orbit interaction. For the single hole regime we present results on the |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V28.00010: Time-Resolved Dynamics of a Double Quantum Dot Charge Qubit Probed by Dispersive Readout David Van Woerkom, Pasquale Scarlino, Jonne Koski, Anna Stockklauser, Michele Collodo, Simone Gasparinetti, Christian Reichl, Werner Wegscheider, Thomas Ihn, Klaus Ensslin, Andreas Wallraff We study the dynamics of a GaAs based double quantum dot (DQD) charge qubit with a dephasing rate of γ2 ~ 4 MHz which is strongly coupled to a high impedance SQUID array resonator. In time-resolved measurements, realized in the dispersive regime, we discriminate between contributions of relaxation and dephasing to the decoherence rate. We realize measurements of Rabi-oscillations, Ramsey-fringes, charge relaxation and Hahn-echo for a DQD charge qubit, with timescales T2* ~ 25 ns, T1 ~ 100 ns and T2,echo ~ 50 ns extracted for interdot detuning δ = 0 and tunnel rate 2t ~ 4 GHz. Furthermore, we investigate the dependence of these time scales on the interdot tunnel rate 2t at δ = 0 and on the detuning δ ≠ 0 for 2t ~ 4 GHz. We conclude that T1 and T2* are dominated by the interaction with phonons and by charge noise, respectively. We extract a standard deviation of the detuning noise more than an order of magnitude lower than reported previously in literature for semiconductor DQDs. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V28.00011: Measuring Back-action of a Single Qubit on its Nuclear Spin Environment Through Spin-echo Correlations Patrick Bethke, Tim Botzem, Thomas Fink, Simon Humpohl, Robert McNeil, Arne Ludwig, Andreas D. Wieck, Hendrik Bluhm Decoherence of a qubit arising from its interaction with an environment is of great practical and fundamental interest. Commonly the effects of the environment are described as a classical, fluctuating field whose dynamics is unaffected by the qubit. A fully quantum description, on the other hand, implies some back-action from the qubit on the environment. Here we show direct experimental evidence for such a back-action for an electron-spin-qubit in a GaAs quantum dot coupled to an environment of order 106 nuclear spins. We are able to detect the back-action of a single qubit-environment interaction whose duration is comparable to the qubit’s coherence time by a correlation measurement technique. We perform two Hahn spin-echo measurement (SE1/2), between which the qubit is reinitialized and manipulated but not measured. The correlations between SE1 and SE2 are strongly affected by the nuclear-spin dynamics during the intermediate manipulation. The complete suppression or revival of the correlation depending on the choice of intermediate qubit sequence shows the action of a single electron spin on the nuclear spins. Quantifying such back-action effects is important for understanding the limitations of semi-classical descriptions of decoherence processes. |
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