Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W39: Spin Qubit Arrays IFocus Recordings Available

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Sponsoring Units: DQI DCMP Chair: Jake Taylor, Riverlane Room: McCormick Place W196A 
Thursday, March 17, 2022 3:00PM  3:36PM 
W39.00001: Away from voltages: Generating and using abstractions to operate arrays of quantum dots Invited Speaker: Reed Andrews As quantum dot qubits mature as a technology, operation and characterization of larger and larger devices must become robust and routine. This talk describes automated tuning and characterization methods that focus on extracting and storing relevant and physicallymeaningful information contained in measurements of dot devices. These methods, which use a combination of machine learning techniques and simple physical models, are shown to work with a variety of data types of varying quality acquired from Si/SiGe SLEDGE arrays of quantum dots. The resulting information forms a highlevel 'device API' that allows interaction with, e.g., quantities of charge and tunnel coupling rather than applied gate voltages. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W39.00002: Simulation Aided Design of a Six Dot Si/SiGe Spin Qubit Device Weiheng Fu, Adam R Mills, Fabio Ansaloni, Mark F Gyure, Chris R Anderson, Jason R Petta High fidelity single and twoqubit gates have been achieved in small Si/SiGe quantum processors [1,2]. In larger quantum dot arrays, the quantum dot charge detectors can have an appreciable impact on the confinement potential of the adjacent spin qubit array [3], complicating device tuneup. Optimization of the charge detector placement to maximize charge sensing fidelity and minimize undesired impacts on the spin qubit array is an outstanding challenge. We simulate the electron density of a six dot linear array in Si/SiGe to characterize the impact of the adjacent charge sensors using a SchrödingerPoisson based device model. 
Thursday, March 17, 2022 3:48PM  4:00PM 
W39.00003: Approaching Unit Readout Visibility in a LossDiVincenzo Spin Qubit with >99.9% Control Fidelity Adam R Mills, Charles Guinn, Mayer Feldman, Anthony Sigillito, Michael J Gullans, Erik Nielsen, Jason R Petta High fidelity gate operations and high visibility readout are required for faulttolerant implementations of spinbased quantum computing. Typical measurement fidelities for quantum dot and donor spin qubits using coupling to a reservoir for readout range from 68% to 99.5%, with the quantum dot based systems largely limited to <99% [1]. Here we control a single spin qubit in a sixdot linear array using electric dipole spin resonance and achieve single qubit gate fidelities exceeding 99.9%, as verified by randomized benchmarking and gate set tomography [2]. Optimization of the spintocharge conversion and electrical charge detection process yields a measurement fidelity exceeding 99% [3,4]. Our results bring the total operation fidelity of a silicon spin qubit above 99%. 
Thursday, March 17, 2022 4:00PM  4:12PM 
W39.00004: Large coupling in a silicon quantum dot array John Rooney, Xuedong Hu, HongWen Jiang Gatedefined silicon quantum dots have been successfully used to encode twoqubit devices [1]. Central to attaining this multiqubit control is the method used for interqubit coupling, where qubits may be coupled through capacitive or tunneling (exchange) interactions. Previous works have studied the capacitive coupling between two adjacent double quantum dots (DQDs) in accumulationmode, overlapping gate architectures in SiGe [2,3] and have achieved capacitive couplings on the order of 2050 GHz. In this talk, we examine the charge dynamics between two DQDs in a similar depletionmode SiGe device we have fabricated. This device contains a linear array of four coupled QDs, and we study the interdot coupling’s transition from 45 GHz to measurements exceeding 200 GHz. We explore the mechanism responsible for the observed trend in interdot coupling as the barrier separating the two DQDs is decreased and remark on the device’s ability to reach such large couplings. 
Thursday, March 17, 2022 4:12PM  4:24PM 
W39.00005: NearlyHeisenberg precision scaling in spatiotemporally correlated noise environments by optimized sensor geometry Francisco U Riberi, Lorenza Viola We study the influence of noncollective couplings on frequency 
Thursday, March 17, 2022 4:24PM  4:36PM 
W39.00006: A sparse quantum dot crossbar with sublinear scaling of interconnects at cryogenic temperature Peter L Bavdaz, Harmen G Eenink, Job van Staveren, Mario Lodari, Fabio Sebastiano, Menno Veldhorst, Giordano Scappucci, Carmina G Almudever A practical spinbased quantum computer will require millions of qubits that operate at cryogenic temperature and interface to room temperature with only a few electrical wires. Sublinear scaling of interconnects is also required for a highthroughput fabricationmeasurement cycle of quantum devices. We demonstrate a 36x36 gate electrode crossbar that supports 648 narrowchannel field effect transistors (FET) for gatedefined quantum dots and enables a quadratic increase in quantum dot device count with a linear increase in control lines. The multigate FET are fabricated on industrial ^{28}SiMOS stacks and integrate two tunable tunnel barriers per device, with interleaved ohmic contacts and cryoCMOS control circuitry to measure each device independently from all others. Electrical characterisation at 1.7 K of a grid with geometry variations demonstrates 100% device yield and shows a decreasing threshold voltage for narrow channel devices. Statistical data obtained in this way provide means for device optimisation by design as a stepping stone towards large quantum dot spinqubit arrays. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W39.00007: A 16 quantum dot crossbar array in germanium Francesco Borsoi, Nico W Hendrickx, Sayr Motz, Floor Van Riggelen, Sander L de Snoo, Amir Sammak, Giordano Scappucci, Menno Veldhorst Quantum dots in semiconductors are a promising platform for quantum information. The steady advancement in the material growth and in the electrical control of gatedefined quantum dots in germanium quantum wells enabled the demonstration of quantum logic in a one, then two and recently in a fourspin qubit processor. Scaling up to larger quantum systems presents outstanding challenges such as solving the interconnect bottleneck caused by the high number of degrees of freedom of the circuits. Here, we take this step and demonstrate the operation of the first 4x4 quantum dot crossbar array. Despite the reduced input voltages, the shared control of plunger and barrier gates enables the definition of 16 quantum dots with similar occupancy and addressable interdot coupling. This result is a crucial milestone for scaling and controlling spin qubits in two dimensions. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W39.00008: Benchmarking singleelectron transistor charge sensitivity using machine learning Jacob F ChittockWood, Dominic T Lennon, Bogdan Govoreanu, Stefan Kubicek, Sofia M Patomaki, John Morton, Fernando GonzalezZalba As silicon quantum device manufacturing shifts from academic laboratories to industrial settings, metrics to assess fabrication processes and optimal geometries will need to be developed. Preferentially, these metrics should be extracted using automated protocols to facilitate the largescale characterisation required to obtain statistical evidence of improvement. In this talk, we propose a device characterisation strategy that aims to compare device performance across different quantum dot technologies. We present this approach applied to charge sensing, where the objective is to sample from the volume in gate voltage space in which the device can be operated with a satisfactory charge sensitivity, this allows us to efficiently quantify this volume we denote the charge sensitivity volume. This volume is determined by a uniform sampling algorithm that searches gate voltage space to identify regions where the charge sensor’s Coulomb blockade oscillations are measured to have a charge sensitivity exceeding a user defined satisfactory threshold. With the increasing demand for fast automatic testing of entire wafers of devices, this characterisation strategy could provide a universal benchmark through which to compare singleelectron transistor technologies. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W39.00009: Synthetic spin1 chain in an array of InAsP quantum dots embedded in an InP nanowire Jacob Manalo Here we describe the potential realization of a synthetic spin1 Haldane chain in an array of InAsP quantum dots embedded in an InP nanowire for the possible construction of a topologically protected singlettriplet qubit. Using an abinitio derived tightbinding Hamiltonian for a single and a double InAsP quantum dot containing millions of atoms, the single particle states were obtained. Though the distribution of As atoms in each quantum dot is random, the conduction band states of a quantum dot array can still be understood in terms of s, p, and d quantum dot orbitals with interdot tunneling. We then constructed the manybody Hamiltonian in the basis of Nelectron configurations with the single particle states from the tightbinding model. Using exact diagonalization, we determined that the ground state of a single quantum dot with a half filled pshell has a total electronic spin of S = 1 and that the ground state for a double dot is a singlet with total spin S = 0, which is consistent with Heisenberg model^{1}. The lowenergy spectrum of the double quantum dot array was then successfully fitted to both the HubbardKanamori and Heisenberg model Hamiltonians to confirm that the quantum dot array with 4 electrons in each dot hosts the Haldane quasipartcles of a spin1 chain. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W39.00010: Flipflop Qubits for Scalable Quantum Computing Architectures Marco De Michielis, Elena Ferraro, Davide Rei Flipflop qubits (FFQ) are a promising qubit type based on a mixture of nuclear and bonded electron spin states of a ^{31}P atom in a spinfree ^{28}Si substrate [1]. Highfidelity onequbit operations can be obtained by exploiting electric dipole spin resonance and twoqubit ones are created by using electric dipole interaction [1,2]. The longrange dipoledipole interaction between FFQs could relax the usually stringent fabrication requirements for spinbased qubits, particularly for the lateral positioning of gates/donors thus moving the specs from some tens of nm to few hundreds of nm range. The simultaneous application of gates, i.e. parallel gating, is a central ingredient for quantum computation, but gate parallelism is inevitably limited by unwanted interqubit interactions. The effects on gate fidelities of those unwanted mutual interactions, due to multi onequbit and multi twoqubit gates executed in parallel, is simulated in different arrays embedded in a realistic noisy environment [3]. Such information helps to infer those for logical qubits defined by a quantum error correction code, also providing insights for system scalingup in view of longterm siliconbased quantum computers. [1] Tosi et al., Nat. Comm. 2017, 8450. [2] Ferraro et al., arXiv:2104.14341v12021. [3] Rei et al., arXiv:2110.12982. 
Thursday, March 17, 2022 5:24PM  5:36PM 
W39.00011: Twobody Wigner molecularization in asymmetric quantum dot spin qubits Jose Carlos AbadilloUriel, Biel Martinez Diaz, Michele Filippone, YannMichel Niquet We model the effects of Coulomb repulsion in doublyoccupied anisotropic quantum dots. Indeed, Coulomb interactions strongly influence the spectrum and the wave functions of a few electrons/holes confined in a quantum dot. When the confinement potential is not too strong, the Coulomb repulsion triggers the formation of a correlated state, the Wigner molecule, where the particles tend to split apart. We show that the anisotropy of the confinement potential strongly enhances the molecularization process. We support this conclusion with a simple harmonic potential model as well as full configurationinteraction calculations in realistic qubit devices. We highlight the exponential suppression of the singlettriplet gap with increasing anisotropy and discuss how molecularization effects specifically hamper Pauli spin blockade readout and reduce the exchange interactions in twoqubit gates. We compare trends in different semiconductor materials and show that the molecularization effects are much stronger in silicon than in germanium, due to the heavier effective masses. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W39.00012: SteadyState Tunable Entanglement Switch with Quantum Dots Sai Vinjanampathy, Parvinder Solanki, Bhaskaran Muralidharan, Bitan De, Adil A Khan, Anuranan Das SV acknowledges support from a DSTSERB Early Career Research Award (ECR/2018/000957) and DSTQUEST grant number DST/ICPS/QuST/Theme4/2019. BM acknowledges Science and Engineering Research Board (SERB), Government of India, Grant No. STR/2019/000030. 
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