Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F17: Focus Semiconductor Spin Qubit ReadoutFocus

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Sponsoring Units: DQI Chair: Jason Petta, Princeton University Room: 203 
Tuesday, March 3, 2020 8:00AM  8:36AM 
F17.00001: Readout of smallscale semiconductor spinqubit arrays Invited Speaker: Anasua Chatterjee The engineering of readout methods becomes crucial as single qubits are scaled up in linear and twodimensional arrays to form NISQ processors. In particular, fast, highfidelity and simultaneous measurements across these small arrays are essential. I will present our work on readout techniques and scaling efforts, in particular involving RFreflectometry, in small twodimensional arrays of quantum dots in GaAs and silicon. In these systems we utilize techniques such as crosstalk mitigation, multiqubit DC and pulse calibration, sensor compensation, virtual gates, and adaptive searching in highdimensional spaces. First, I will discuss simultaneous, singleshot, and interlaced measurements for singlettriplet spin qubits, performed via four independent sensors spaced across a GaAs chip. I will also show singleshot reflectometry measurements via compact gatebased dispersive sensing, carried out in a twodimensional CMOS silicon array. These methods, combined with pulsedgate techniques, enable deterministic singleelectron shuttling within the array, and may be beneficial for many other quantumdot devices or spin and chargebased hybrid systems. 
Tuesday, March 3, 2020 8:36AM  8:48AM 
F17.00002: CascadeBased Fast, HighFidelity and Scalable Spin Readout Cornelis van Diepen, TzuKan Hsiao, Uditendu Mukhopadhyay, Christian Reichl, werner wegscheider, Lieven M Vandersypen Spinqubits based on gatedefined semiconductor quantum dots are a promising platform for quantum computation and simulation. An important advantage of quantum dots is their small footprint. The dot pitch is about 100 nm, hence 100 million dots fit on 1 mm^{2}. A problem is that qubit readout with charge sensing based on capacitive coupling only enables to sense nearby quantum dots and placing charge sensors within the quantum dot array hosting the qubits is detrimental for connectivity. Here, we report on cascadebased fast, highfidelity and scalable spin readout. The cascade consists of an initial charge transition, far away from the sensor, and subsequent charge transitions induced by Coulomb repulsion, with the final transition nearby the sensor. Combined with spintocharge conversion a cascade enables the readout of charge and spin occupation of quantum dots remote from the charge sensor. We demonstrate fast and highfidelity spin readout by performing Pauli spin blockade with a cascade implemented in a quadruple dot with a sensing dot. The cascadebased readout is a promising alternative for readout of large quantum dot arrays compared to gatedispersive readout or state transfer via either shuttling or logical operations. 
Tuesday, March 3, 2020 8:48AM  9:00AM 
F17.00003: Fast charge sensing in undoped silicon quantum dots with radiofrequency reflectometry Akito Noiri, Kenta Takeda, Jun Yoneda, Takashi Nakajima, Tetsuo Kodera, Seigo Tarucha Fast and sensitive charge sensing is an essential ingredient in quantum dot (QD) based spin qubit experiments. A performance of the charge sensing can be drastically improved by embedding the sensor in a radiofrequency (rf) tank circuit [1]. While the technique is commonly used in depletionmode devices, applying it to accumulationmode devices is a challenge due to a large parasitic capacitance by accumulation gates [2]. In this presentation, we report how to reduce this capacitance and realize charge sensing by the rf reflectometry in undoped Si/SiGe QDs. To this end, we introduce a specially designed device geometry comprising a small accumulation gate area [3]. We observe that the reflected rf power changes more than 30 dB by modulating the sensor conductance, which allows sensitive charge sensing. We achieve singleshot singlettriplet readout with a SNR of 6 in an integration time of 0.8 μs. 
Tuesday, March 3, 2020 9:00AM  9:12AM 
F17.00004: Rapid highfidelity spin state readout in Si/SiGe quantum dots via radiofrequency reflectometry JJ Nelson, Elliot Connors, John Nichol Radiofrequency (rf) reflectometry, while enabling rapid highfidelity readout of GaAs spin qubits, is challenging to implement in accumulationmode Si quantum dot devices. The difficulty arises when a large parasitic gate capacitance is distributed along a relatively large resistance twodimensional electron gas (2DEG). Here we will present minor design changes that when implemented in a Si/SiGe quantum dot device enable charge readout fidelity above 99.9% in 300ns using rf reflectometry. The rf compatible Si design can perform singleshot readout of spin states via spinselective tunneling in microsecond integration times. We will also show high fidelity singlet and triplet readout of a double quantum dot via Pauli spin blockade. With the use of a charge latching mechanism we achieve a maximum fidelity of 99.0%. 
Tuesday, March 3, 2020 9:12AM  9:24AM 
F17.00005: Repetitive Quantum Nondemolition Measurement of a Silicon Spin Qubit Using Different Decodings Xiao Xue, Benjamin D'Anjou, Thomas F Watson, Dan R. Ward, Donald E Savage, Max G Lagally, Mark G Friesen, Susan Nan Coppersmith, Mark Alan Eriksson, William Coish, Lieven M Vandersypen Silicon spin qubits show great promise for faulttolerant quantum computing. As an essential step towards practical quantum error correction, quantum nondemolition (QND) measurements are needed to efficiently detect the state of a logical qubit without totally losing track of its state. Here we implement QND measurements in a Si/SiGe twoqubit system [1], with one qubit as the logical qubit and the other as the ancilla. Making use of a twoqubit controlledrotation gate, the state of the logical qubit is mapped onto the ancilla, followed by a destructive readout of the ancilla. In contrast, QND measurement does not destroy the logical qubit but keeps it at the same state after the collapse caused by the first measurement, which allows us to enhance the readout fidelity by multiple QND measurements. Moreover, we also make use of a new analysis method called soft decoding to extract additional information on the state of the logical qubit [2]. We compare the two methods and discuss the conditions for which soft decoding provides an advantage. 
Tuesday, March 3, 2020 9:24AM  9:36AM 
F17.00006: Singleshot singleelectron spin readout with a singlelead quantum dot charge sensor Mark R Hogg, Andrey V Timofeev, Prasanna Pakkiam, Samuel Keith Gorman, Matthew House, Michelle Y Simmons “Gatebased”, or dispersive, charge sensors for the readout of semiconductor spin qubits have been growing in popularity because these compact sensors integrate readout capability into electrical control structures. However, dispersive readout can only directly measure a spin state in a double quantum dot, either by Pauli blockade or with a large magnetic field gradient. Here we demonstrate a dispersivelyprobed singlelead quantum dot (SLQD) charge sensor which senses the charges on four quantum dots, each defined with P atoms in Si using scanning tunnelling microscope lithography. With this charge sensor we demonstrate singleshot, singlespin readout via energyselective tunnelling. We achieve a voltage signaltonoise ratio of 8 for charge detection with a measurement bandwidth of 15 kHz and demonstrate spin readout with high fidelities of up to 97.4% at 2.5 Tesla. The capacitive coupling between all four quantum dots and the charge sensor is strong enough that there is no loss of sensitivity for detecting the more distant quantum dots, up to about 100 nm away from the sensor. Good sensitivity, compact geometry, and longrange sensing makes the SLQD a promising choice of sensor for scaling up future atomicprecision singlespin qubit devices to larger numbers of qubits. 
Tuesday, March 3, 2020 9:36AM  9:48AM 
F17.00007: Theory of Pulsed Spectroscopy in Quantum Dots: Interdot Dynamics Andrew Pan Understanding the energy structure and dynamics of excited states in quantumdot qubits is critical for device design and operation. Incoherent methods for probing these quantities typically call for loading from neighboring baths or applying large external magnetic or RF fields. These methods may be problematic if they require operating conditions far from the intended qubit environment. We have developed a technique to characterize these quantities using typical tuneup conditions, relying on the incoherent dynamics of interdot charge transitions. In the first half of this twopart talk, we examine theoretically how tunneling, dephasing, and detuningdependent relaxation affect the timedependent populations near charge transition points and allow excited states to be resolved. We compare these expectations to experimental results in Si/SiGe quantum dots. Potential connections to the dynamics relevant for spin qubit readout are considered. 
Tuesday, March 3, 2020 9:48AM  10:00AM 
F17.00008: Pulsed Spectroscopy of Si/SiGe Quantum Dots: One and TwoElectron ValleyOrbit Excited States Kate Raach The one and twoelectron energy spectra of silicon quantum dots can be complicated due to the interplay of valley and orbital degrees of freedom, affecting all aspects of spin qubit operation. For instance, in exchangeonly qubits, lowlying single electron excitations are relevant during coherent evolution, while the twoelectron singlettriplet splitting limits the initialization and readout fidelity. In the second half of this twopart talk, we present experimental data and analysis of this energy structure using pulsed techniques to simultaneously probe multiple valley and orbital states of double quantum dots. We discuss the variations in one and twoelectron structure across dots and as a function of bias and how they compare with theoretical expectations. 
Tuesday, March 3, 2020 10:00AM  10:12AM 
F17.00009: Developing Monolithic Superconducting Resonators for GateBased Quantum Dot Readout Zac Barcikowski, Michael David Stewart, Mustafa Bal, Christopher Richardson, David Pappas, Joshua Pomeroy Gatebased sensing using resonant circuitry has been identified as a promising path towards high fidelity, fast, and compact readout for semiconductor quantum computing architectures. By coupling a quantum dot (QD) system to an LC resonator, the state is inferred via the reflected phase response. The resistive loss in the discrete inductors and loss associated with wirebonds often limit measurement sensitivity. In contrast to oft used surface mount components, we have fabricated Nb superconducting resonators to reduce resistive loss and will explore additional high kinetic inductance materials. The resonators are intended for monolithic integration with QD gates to reduce resonatorgate loss. We present transmission and reflection data used to assess our ability to deterministically fabricate resonators with designed inductance and capacitance values. Efforts to optimize resonance quality factor and integrate resonators with QDs are discussed. 
Tuesday, March 3, 2020 10:12AM  10:24AM 
F17.00010: NonMarkovian qubit spectroscopy in cavity QED Zoé McIntyre, William Coish The strongcoupling regime of cavity quantum electrodynamics (QED) can be identified spectroscopically from a peak splitting (~qubitcavity coupling) that is large compared to the qubit decoherence and cavity decay rates. This interpretation normally relies on a Markovian model of qubit dynamics together with inputoutput theory. A Markovian description may however break down for charge qubits (coupling to ~1/f noise) or spin qubits (coupling to a slow nuclearspin bath). Motivated by very recent work showing strong coupling of spin/charge qubits coupled to superconducting microwave resonators, we use a generalized inputoutput theory to account for a generic nonMarkovian environment. This allows us to calculate a spectroscopic lineshape that fully accounts for nonMarkovian features and which may be relevant to recent experiments. 
Tuesday, March 3, 2020 10:24AM  10:36AM 
F17.00011: Lowmagnetic field singlespin qubit operation in isotopically enriched silicon RUICHEN ZHAO, Tuomo Tanttu, Kuan Yen Tan, Bas Hensen, Kok Wai Chan, Jason Hwang, Ross Leon, ChihHwan Yang, Will Gilbert, Fay E. Hudson, Kohei M Itoh, Andrey Kiselev, Thaddeus D Ladd, Andrea Morello, Arne Laucht, Andrew Steven Dzurak Singlespin qubit readout traditionally relies on selective tunneling to a neighboring reservoir. It requires sophisticated microwave engineering to deliver highfrequency qubit drives to the quantum chip from roomtemperature electronics, which will be a challenge for scaling of a siliconbased quantum processor. Here we present an alternative scheme where we use highfidelity Pauli spin blockade readout to enable singlespin qubit operation in a magnetic field as low as 150mT. We discover the qubits decohere faster in low magnetic fields due to the background (800 ppm) ^{29}Si nuclear spins in the isotopically enriched substrate. A simulation modeling the nuclear spin induced qubit frequency fluctuation produced results consistent with our experimental data. This work indicates that further isotopic enrichment may be needed to achieve the high fidelities required for a scalable quantum processor. 
Tuesday, March 3, 2020 10:36AM  10:48AM 
F17.00012: Adiabatic conversion for qubit readout: Optimal pulse shapes and dephasing Felix Fehse, Michel PioroLadriere, William Coish Adiabatic conversion schemes are commonly used to measure quantum states. These schemes may include, e.g., spintocharge conversion for spins in quantum dots or paritytocharge conversion for qubits based on Majorana zero modes. In all of these schemes, a common element is that dephasing is minimized at an "operating point" (e.g., for the 'spin' or 'parity' quantum number), but measurementinduced dephasing is maximized at the "measurement point". The balance between 'adiabaticity' and dephasing can be optimized to improve performance of these readout schemes. We give an explicit construction that allows for optimal state conversion in qubit readouts. Applying this scheme to the specific case of spin qubits in quantum dots, we show that a highfidelity (better than 99.9%) singleshot allelectrical readout is possible. 
Tuesday, March 3, 2020 10:48AM  11:00AM 
F17.00013: Machine LearningBased control of 2D Arrays of Quantum Dots Ali Izadi Rad, Sandesh Kalantre, Jacob Taylor, Justyna Zwolak Recent advances towards employing quantum dots (QD) as a platform for quantum computation 
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