Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L17: Focus Exchange Based Spin QubitsFocus

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Sponsoring Units: DQI Chair: Thaddeus Ladd, HRL Laboratories Room: 203 
Wednesday, March 4, 2020 8:00AM  8:36AM 
L17.00001: Strong photon coupling to the quadrupole moment of an electron in a triple quantum dot Invited Speaker: Thomas Ihn We experimentally couple the photonic excitations of a superconducting microwave resonator to a single electron hosted in a quantum dot charge qubit. The two qubit states with a 4.3 GHz separation arise in a fourelectron triple quantum dot formed by locally gating a twodimensional electron gas in GaAs. We demonstrate strong electronphoton coupling via the quadrupole moment in a parameter regime with negligible dipole coupling. The quadrupolar qubitphoton coupling strength is estimated from the vacuum Rabi mode splitting to be g_{0}/2π = 150 MHz. The qubit can also be tuned into a regime where it operates as a conventional double quantum dot charge qubit diplole coupled to the resonator. The experiment is motivated by a recent proposal [1,2] which aims at avoiding decoherence by distant charge fluctuations. Using spectroscopy measurements we determine the decoherence rate of the quadrupolar qubit to be γ_{2}/2π = 32 MHz. Comparing the coupling of charge noise to the conventional dipolar qubit and the quadrupolar qubit, we find that the coherence of the system is limited by shortrange charge noise originating from noise sources residing near the triple quantum dot. 
Wednesday, March 4, 2020 8:36AM  8:48AM 
L17.00002: Efficient orthogonal control of tunnel couplings in a quantum dot array TzuKan Hsiao, Cornelis van Diepen, Uditendu Mukhopadhyay, Christian Reichl, werner wegscheider, Lieven M Vandersypen Electrostatically defined semiconductor quantum dot arrays offer a promising platform for quantum computation and quantum simulation. However, crosstalk of gate voltages to dot potentials and interdot tunnel couplings complicates the tuning of the device parameters. To date, crosstalk to the dot potentials is routinely and efficiently compensated using virtual gates, but crosstalk to the tunnel barriers is currently compensated through a slow iterative process, due to exponential dependence of tunnel couplings on gate voltages. Here we show that the crosstalk on tunnel barriers can be compensated using a linear combination of gate voltages, since the exponential dependence applies to all gates. We demonstrate efficient calibration of crosstalk in a quadruple dot and define a set of virtual barrier gates as linear combinations of physical gate voltages. We then demonstrate orthogonal control of tunnel couplings in the quadruple dot using these virtual barrier gates. Our method marks a key step forward in the scalability of the tuning process of largescale quantum dot arrays. 
Wednesday, March 4, 2020 8:48AM  9:00AM 
L17.00003: BarrierControlled MultiQubit Exchange Haifeng Qiao, Yadav Kandel, Saeed Fallahi, Geoff C Gardner, Michael Manfra, John Nichol Heisenberg exchange coupling between neighboring electron spins in semiconductor quantum dots provides a powerful tool for coherent qubit manipulation in spinbased quantum computing and quantum information processing. Various other phenomena such as many body localization and time crystals in semiconductor quantum dots also require tunable exchange couplings. However, controlling multiple exchange couplings in large quantum dot arrays proves to be challenging, due to nonlinear and nonlocal dependence of the exchange couplings on the confinement gate voltages. In this work we demonstrate simultaneous control of multiple barrierinduced exchange couplings in a fourqubit processor. We model the dependence of the exchange couplings on all barrier gate voltages, which provides a means of precisely and simultaneously controlling multiple exchange couplings. We demonstrate two, three, and fourqubit exchange oscillations, and compare the experimental data to the simulated predictions. 
Wednesday, March 4, 2020 9:00AM  9:12AM 
L17.00004: Coherent SpinState Transfer via Heisenberg Exchange Yadav Kandel, Haifeng Qiao, Saeed Fallahi, Geoffrey C. Gardner, Michael Manfra, John Nichol Spin qubits in semiconductors are a promising scalable architecture for quantum computing. Long chains of spin qubits have already been realized in different semiconductor platforms but transferring quantum information throughout such chains is still a challenge. Here we present the first experimental demonstration of the coherent transfer of electron spin states back and forth across a chain of four quantumdot spin qubits using Heisenberg exchange coupling between neighboring electrons, which arises from the overlap of their wavefunctions. We swap the spin states of a pair of electrons via preciselycontrolled exchange pulses. Successive SWAP operations between different pairs can transfer a quantum spin state to a distant qubit, without moving any electrons. Because this method is scalable to long chains of spin qubits, coherent spin state transfer using Heisenberg exchange will be useful in multi qubit operations, measurementbased entanglement swapping and gate teleportation, and error correction in spinbased quantum computers. 
Wednesday, March 4, 2020 9:12AM  9:24AM 
L17.00005: Noiseresilient driven exchange gate for quantum dot spin qubits Stephan Philips, Maximilian Russ, Lieven M Vandersypen Spin qubits in silicon quantum dots are a promising candidate for highfidelity quantum computation due to long decoherence times and fast operations. 
Wednesday, March 4, 2020 9:24AM  9:36AM 
L17.00006: Interplay of exchange and superexchange in triple quantum dots Kuangyin Deng, Edwin Barnes Recent experiments on semiconductor quantum dots have demonstrated the ability to utilize a large quantum dot to mediate superexchange interactions and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. We will describe our theoretical efforts to understand the controllability of superexchange interactions in these systems. We focus on a tripledot system arranged in a triangular configuration and use configuration interaction calculations to investigate the interplay of superexchange and nearestneighbor exchange interactions as the geometry and detuning of the mediating dot are varied. We also study how the strength and sign of the superexchange coupling depends on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, twodimensional quantum dot arrays. 
Wednesday, March 4, 2020 9:36AM  9:48AM 
L17.00007: Coherent manipulation of threespin states in a Si/SiGe triple quantum dot Kenta Takeda, Akito Noiri, Takashi Nakajima, Jun Yoneda, Takashi Kobayashi, Seigo Tarucha Quantum dot arrays provide a promising platform for quantum information processing. In Sibased devices, recent technological advances make it possible to implement single and twoqubit operations with high fidelities. A threequbit system is a next step toward scaling up and it is particularly important for realizing a quantum errorcorrecting code. In this work, we demonstrate coherent manipulation of three individual spins in a Si/SiGe triple quantum dot by siteselective electric dipole spin resonance. Spins in the left and right dots are read out by a spinselective tunneling technique while the center spin is measured by reading out the left spin subsequently to a controlledrotation operation. We also report our recent progress on controlledcontrolledrotation where the resonance frequency of the center spin depends on both left and right spin states. 
Wednesday, March 4, 2020 9:48AM  10:00AM 
L17.00008: Full Permutation Dynamical Decoupling in an Encoded TripleDot Qubit Bo Sun Dynamical decoupling (DD) sequences can mitigate decoherence induced by slowly varying interactions between a qubit and a bath by using appropriately timed qubit rotations. For an exchange only qubit encoded in the spins of three electrons, dynamical decoupling can be achieved by applying a series of fullSWAP operations on the constituent spins which fully permute their locations across the three dots [1]. Using gatedefined quantum dots in an enriched Si/SiGe device, we demonstrate that repeated applications of the full permutation sequence can echo low frequency charge and magnetic noise, resulting a dynamically decoupled coherence time of 100s of microseconds. We find that the first order DD sequence is susceptible to rotations about the Yaxis of the Bloch sphere, driven by either real magnetic field gradients or spinorbit pseudogradients. Initializing the qubit along the Y axis renders the experiment insensitive to these errors, as implied by measurements of qubit leakage in this configuration. The resulting experiment is only sensitive to highfrequency noise sources and is similar to noise spectroscopy. We compare our results to simulations which include various types of highfrequency charge noise. 
Wednesday, March 4, 2020 10:00AM  10:12AM 
L17.00009: Highly tunable exchangeonly singletonly qubit in a GaAs triple quantum dot Arnau Sala, Jørgen Holme Qvist, Jeroen Danon Quantumdot based exchangeonly spin qubits offer fast manipulation and full electric control but decohere rapidly in the presence of Zeeman field gradients, such as those resulting from the hyperfine interaction with randomly polarized nuclearspin ensembles in the dots. A solution is to encode the qubit in the decoherencefree subspace spanned by two fourparticle singlet states in a quadrupledot setup [1]. 
Wednesday, March 4, 2020 10:12AM  10:24AM 
L17.00010: Correcting Distortion of Baseband Exchange Pulses in Quantum Dot Qubits David Barnes Repeatedly identical control waveforms are necessary for high fidelity qubit control. In tripledot exchangeonly spin qubits, full Blochsphere control is achieved via baseband voltage pulses alone. This makes them sensitive to the effects of pulse distortion extending from kHz to GHz frequencies. Here we present a technique to correct the known distortion of our arbitrary waveform generators. The application of this correction lead to a reduction of single qubit infidelity from 0.5% to 0.2% as measured by the “blind randomized benchmarking” protocol [1]. Additionally, we will cover an extension to this routine where we are able to estimate and potentially correct residual pulse distortion using coherent qubit control experiments. 
Wednesday, March 4, 2020 10:24AM  10:36AM 
L17.00011: LongDistance Charge Transport in a Single Electron Conveyor Device in (Al,Ga)As Matthias Kuenne, Stefan Trellenkamp, Julian Ritzmann, Arne Ludwig, Andreas Wieck, Hendrik Bluhm For the realization of scalable quantum computing architectures enabling topological error correction, a transfer of the qubit information over distances of at least a few microns is required for making space for signal vias and allowing for tiling of qubit registers with classical controlcircuits. Instead of coupling qubits by employing surface acoustic waves, we chose a onedimensional, gatedefined conveyor beltlike device layout where electrons are moved between qubit sites by translating the potential minima in which the electrons are trapped. Here, the direction and the velocity of the electron transport are not limited by the crystal’s properties. 
Wednesday, March 4, 2020 10:36AM  10:48AM 
L17.00012: Computerautomated tuning procedures for semiconductor quantum dot arrays Adam Mills, Mayer M Feldman, Cara Monical, Phillip J Lewis, Kurt W Larson, Andrew M Mounce, Jason Petta As with any quantum computing platform, semiconductor quantum dot devices require sophisticated hardware and controls for operation. The increasing complexity of quantum dot devices necessitates the advancement of automated data collection and control software. By automating the analysis of charge stability diagrams, we can easily create tools to tune charge occupancy and interdot tunnel couplings in our quantum dot arrays. We use an image analysis toolbox developed in Python to automate the calibration of virtual gates, a process that previously involved a large amount of user intervention. Moreover, we show that straightforward feedback protocols can be used to simultaneously tune multiple tunnel couplings in a triple quantum dot^{1}. 
Wednesday, March 4, 2020 10:48AM  11:00AM 
L17.00013: Designing CPHASE Gates with Arbitrary Phase by Structural Modification of the FongWandzura Sequence Daniel Zeuch, Nicholas Evans Bonesteel We design efficient arbitrary CPHASE gates for exchangeonly spinbased quantum computation in which quantum gates are carried out by sequences of exchange pulses acting on qubits encoded using three or more spins. The construction we present is motivated by our analytic derivation [1] of the FongWandzura sequence [2], the shortest known pulse sequence for an exact CNOT gate (for a linear array of spins). This earlier derivation is based on a type of elevation of a simple fivepulse sequence consisting only of SWAP or trivial pulses, which captures the essential structure of the FongWandzura sequence. In the present construction, we introduce and evaluate a modified simple fivepulse sequence consisting of four SWAP pulses and one pulse of arbitrary duration. We then show how this sequence can be elevated in a fashion similar to that of our FongWandzura derivation, to yield a leakagefree entangling twoqubit sequence that carries out a gate operation locally equivalent to arbitrary CPHASE. 
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