Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B29: Semiconductor Qubits  Spin Qubit Readout IFocus Live

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Sponsoring Units: DQI Chair: Matthew Rakher, HRL Laboratories, LLC 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B29.00001: Reflectometry of charge transitions in a silicon quadruple dot Heorhii Bohuslavskyi, Fabio Ansaloni, Anasua Chatterjee, Federico Fedele, Torbjørn Rasmussen, Bertram Brovang, Jing LI, Louis HUTIN, Benjamin Venitucci, Benoit Bertrand, Maud Vinet, YannMichel Niquet, Ferdinand Kuemmeth Silicon is considered a promising candidate for realizing large qubit processors [1], making quantum dots in silicon nanowire transistors, fabricated in industrial cleanrooms on 300mm wafers, particularly attractive [2]. We perform gatebased reflectometry measurements of various charge states in a foundryfabricated twodimensional quadruple quantum dot. From a wiring perspective, the low number of control channels (one gateelectrode per dot and global top and back gates) is desirable, provided that tunnel couplings are adjustable to allow controlled singleelectron movements [3] and singleshot reflectometry readout. 
Monday, March 15, 2021 11:42AM  11:54AM Live 
B29.00002: Spin Readout of a CMOS Quantum Dot by Gate Reflectometry and SpinDependent Tunneling Virginia CirianoTejel, Michael A. Fogarty, Simon Schaal, Louis HUTIN, Benoit Bertrand, Lisa A. Ibberson, M Fernando GonzalezZalba, Jing LI, YannMichel Niquet, Maud Vinet, John J. L. Morton We report the measurement of the electron spin orientation in a singlyoccupied gatedefined quantum dot, fabricated using CMOS compatible processes at the 300 mm waferscale [1]. For readout, we employ spindependent tunnelling [2] combined with a lowfootprint singlelead quantum dot charge sensor, measured using radiofrequency gate reflectometry [3]. We demonstrate spin readout, obtaining valley splittings in the range 0.50.7 meV and a maximum electron spin relaxation time (T_{1}) of 9 ± 3 s at 1 Tesla. These long lifetimes indicate that the silicon nanowire geometry and fabrication processes possess considerable promise for qubit devices, while this spinreadout method is wellsuited to scalable architectures. We will discuss progress towards integrating such spinreadout with quantumlimited amplifiers [4]. 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B29.00003: Permutation of two electrons within a twodimensional array of quantum dot Fabio Ansaloni, Anasua Chatterjee, Heorhii Bohuslavskyi, Benoit Bertrand, Louis HUTIN, Maud Vinet, Ferdinand Kuemmeth Silicon spin qubits have achieved highfidelity one and twoqubit gates [1,2] and promise an industrial route to faulttolerant quantum computation. A significant next step for the development of scalable multiqubit processors is the operation of foundryfabricated, extendable twodimensional (2D) quantumdot arrays. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B29.00004: Bellstate tomography in a silicon manyelectron artificial molecule Ross C C Leon, Chih Hwan Yang, Jason Hwang, Julien Camirand Lemyre, Tuomo Tanttu, Wei Huang, Jonathan Y Huang, Kohei M Itoh, Arne Laucht, Michel PioroLadriere, Andre Saraiva, Andrew Steven Dzurak An errorcorrected quantum processor will require millions of qubits, accentuating the advantage of nanoscale devices with small footprints, such as silicon quantum dots. However, as for every device with nanoscale dimensions, disorder at the atomic level is detrimental to qubit uniformity. Here we investigate two spin qubits confined in a silicon double quantumdot artificial molecule. Each quantum dot has a robust shell structure and, when operated at an occupancy of 5 or 13 electrons, has single spin½ valence electron in its p or dorbital, respectively. These higher electron occupancies screen atomiclevel disorder. The larger multielectron wavefunctions also enable significant overlap between neighbouring qubit electrons, while making space for an interstitial exchangegate electrode. We implement a universal gate set using the magnetic field gradient of a micromagnet for electricallydriven single qubit gates, and a gatevoltagecontrolled interdot barrier to perform twoqubit gates by pulsed exchange coupling. We use this gate set to demonstrate a Bell state preparation between multielectron qubits with fidelity 90.3%, confirmed by twoqubit state tomography using spin parity measurements. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B29.00005: Large Dispersive Interaction between a CMOS Double Quantum Dot and Microwave Photons David J. Ibberson, Theodor Lundberg, James A. Haigh, Louis HUTIN, Benoit Bertrand, Sylvain Barraud, ChangMin Lee, Nadia A. Stelmashenko, Jason Robinson, Maud Vinet, M Fernando GonzalezZalba, Lisa A. Ibberson To implement quantum errorcorrection, readout must be performed much faster than the coherence time, up to ~100 μs for spins in silicon [1]. With that goal in mind, here we demonstrate the readout of an interdot charge transition, the basis of parity readout, in 50 ns with a SNR of 3.3. We do so using dispersive gate sensing with a CMOS splitgate nanowire transistor. This fast readout is achieved firstly by maximising the coherent coupling rate between the microwave photons and the charge dipole. We measure a coupling of 183 MHz owing to the large interdot lever arm (0.72) of our asymmetric splitgate device, and the high impedance (560 Ω) of our readout cavity, which comprises of an offchip superconducting spiral [2] that is inductively coupled to the microwave feedline. Secondly, this inductive coupling is tuned so that the cavity linewidth is similar to the statedependent shift in the resonant frequency, the regime of optimal state visibility [3]. 
Monday, March 15, 2021 12:30PM  12:42PM Live 
B29.00006: Accurate spin and valley state identification in silicon double quantum dots Theodor Lundberg, David J. Ibberson, Jing LI, Louis HUTIN, Benoit Bertrand, ChangMin Lee, David J. Niegemann, Matias Urdampilleta, Nadia A. Stelmashenko, Tristan Meunier, Jason Robinson, Maud Vinet, Lisa A. Ibberson, YannMichel Niquet, M Fernando GonzalezZalba To read the state of silicon spin qubits, the mechanism that has provided highest fidelity is spintocharge conversion via Pauli spin blockade [1]. However, given the valley degree of freedom in silicon quantum dots, which can lead to complex energy spectra, accurate identification of the spin states involved in Pauli spin blockade is a key requirement for reliable readout and operation of silicon spin qubits. 
Monday, March 15, 2021 12:42PM  1:18PM Live 
B29.00007: Multiqubit and multidot reflectometry measurements in arrays of semiconductor quantum dots Invited Speaker: Ferdinand Kuemmeth My talk on highfrequency reflectometry measurements has two parts, explaining its use to tune gate voltages into desired charge configurations of quantumdot arrays, and to perform simultaneous singleshot readout of multiple qubits. 
Monday, March 15, 2021 1:18PM  1:30PM Live 
B29.00008: Low Temperature Radiofrequency Reflectometry setup for Charge sensing in CMOS devices Joffrey Rivard, Clément Godfrin, Alexei Orlov, Eva DupontFerrier Spin qubits in silicon are great candidates for scalable quantum information processors due to their long coherence time combined with compatibility with industrial CMOS fabrication lines [1]. The spinreadout is obtained by spintocharge conversion using a nearby SET. This require multiple additional leads and limits the scalability of the system. RFreflectometry measurement provides a compact alternative [2,3] as only one lead is necessary to control and read the qubit [4]. The critical part for this measurement is to obtain an impedance matching, at low temperature, between the resonant circuit and the RFline. This is due to the temperature dependence of each of the tank circuit components and the sampletosample capacitance variability. In this talk, we report a chargesensing measurement of a CMOS device with optimized reflectometry setup and discuss the use of tunable capacitors to target high sensitivity RFmeasurement for spin qubit readout. 
Monday, March 15, 2021 1:30PM  1:42PM Live 
B29.00009: Lowfrequency electron spinqubit detuning noise in highly purified ^{28}Si/SiGe* Tom Struck, Arne Hollmann, Floyd Schauer, Andreas Schmidbauer, Veit Langrock, Olexiy Fedorets, Kentarou Sawano, Helge Riemann, Nikolay V Abrosimov, Lukasz Cywinski, Dominique Bougeard, Lars Schreiber

Monday, March 15, 2021 1:42PM  1:54PM Live 
B29.00010: A resetifleaked procedure for encoded spin qubits Veit Langrock, David Peter DiVincenzo The spin of electrostatically confined electrons in semiconductor heterostructures has proven to be a promising candidate for hosting longlived quantum information. Single and two spin qubit realizations rely on manipulation using (engineered) magnetic fields, but universal control using exclusively pairwise Heisenberg exchanges becomes possible when encoding the qubit in the spin1/2 subspace of three electrons. While such qubits have proven to be operable with high fidelity [1], an issue is the presence of the spin3/2 leakage subspace which tends to be occupied under sustained operation and has to be actively depopulated via so called Leakage Reduction Units. 
Monday, March 15, 2021 1:54PM  2:06PM Live 
B29.00011: NonMarkovian qubit spectroscopy in cavity QED Zoe McIntyre, Bill Coish Markovian models of qubit dynamics break down for charge qubits coupled to 1/f noise and for spin qubits coupled to slow nuclearspin baths. For spin/charge qubits also coupled to a cavity, it can be difficult to directly extract timedomain coherence dynamics because the AC control fields used to prepare and measure these qubits have the potential to excite the cavity mode. In this talk, we present a way of extracting the coherence dynamics of a qubit coupled to a cavity purely from frequencydependent measurements of the cavity response in cavity quantum electrodynamics (QED). In contrast to a more standard equationofmotion approach, we make neither a Markov approximation nor a weakcoupling approximation for the qubitbath dynamics. Using this approach, we calculate the spectroscopic response of a spin qubit coupled to nuclear spins. This response shows pronounced nonLorentzian features, indicative of nonMarkovian dynamics, arising from a manyspin collective mode. We also consider the case of a qubit coupled to a single bosonic mode corresponding to, e.g., a mechanical degree of freedom, phonon, or cavity mode. In this instance, strong coupling gives rise to higher harmonics in the qubit coherence spectrum. 
Monday, March 15, 2021 2:06PM  2:18PM On Demand 
B29.00012: Asymmetric Sensing Dot for Scaleable Baseband Readout of Spin Qubits Eugen Kammerloher, Matthias Kuenne, Inga Seidler, Arne Ludwig, Andreas D. Wieck, Lars Schreiber, Hendrik Bluhm High fidelity scalable readout is one of the key requirements for quantum computers with more than just a few qubits. Charge sensing dots are in this regard the most sensitive sensors for spin qubit readout. The most widespread readout technique is based on RF reflectometry, satisfying the requirement of high fidelity, but requires bulky, powerhungry components and is not well scalable. A more scalable alternative is to use transistors in close proximity to the qubit [1,2]. 
Monday, March 15, 2021 2:18PM  2:30PM On Demand 
B29.00013: State preparation fidelity by QND readout of a silicon electron spin qubit Jun Yoneda, Takashi Kobayashi, Kenta Takeda, Akito Noiri, Takashi Nakajima, Sen Li, Jun Kamioka, Tetsuo Kodera, Seigo Tarucha A quantum nondemolition (QND) singleshot qubit readout is crucial for largescale quantum computing, and unlike conventional counterparts, can serve as a state preparation process with its error extinguishable by repetition. Here we discuss the state preparation fidelity of a cumulative QND readout of an electron spin qubit in a silicon quantum dot with its neighboring electron spin used as an ancilla [1]. By repeating the single QND readout process, a qubit spin state is prepared in the state correlated to the readout outcomes with a state dependent fidelity. Furthermore, a series of readout outcomes can be converted to the likelihood ratio of successful state preparation, which allows one to select events with a higher preparation fidelity. Combined with realtime signal processing, such protocols offer alternative routes for highfidelity state preparation. 
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