Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G41: Superconducting Qubit GatesFocus Recordings Available

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Sponsoring Units: DQI DCMP Chair: Raymond Simmonds, National Institute of Standards and Technology, Boulder Room: McCormick Place W196C 
Tuesday, March 15, 2022 11:30AM  11:42AM 
G41.00001: Bidirectional Generation of Itinerant Microwave Photons with Waveguide Quantum Electrodynamics (Part 1) Bharath Kannan, Aziza Almanakly, Youngkyu Sung, David A Rower, Roni Winik, David K Kim, Alexander Melville, Bethany M Niedzielski, Jonilyn L Yoder, Mollie E Schwartz, Terry P Orlando, Jeffrey A Grover, Joel I Wang, Simon Gustavsson, William D Oliver The ability to distribute and communicate quantum information between distinct processing nodes is a key requirement towards realizing a fully connected network of quantum processors. Typically, this communication is either mediated by photons that propagate between the nodes, or via a bus coupler that coherently couples adjacent nodes. However, the communication fidelity of protocols involving propagating photons are often limited due to the need for lossy components such as microwave circulators that fix the directionality. Furthermore, protocols that use bus couplers between nodes can have restricted connectivity. In this work, we present our progress towards realizing a device that is capable of bidirectionally generating and capturing propagating, or itinerant, microwave photons. We do so by taking advantage of the interference between the emission of quantum emitters in a waveguide quantum electrodynamics architecture. Such a device can then be used to form the basis of a network of quantum processors with alltoall connectivity without the need of lossy components between nodes. 
Tuesday, March 15, 2022 11:42AM  11:54AM 
G41.00002: Bidirectional Generation of Itinerant Microwave Photons with Waveguide Quantum Electrodynamics (Part 2) Aziza Almanakly, Bharath Kannan, Youngkyu Sung, David A Rower, Roni Winik, David K Kim, Alexander Melville, Bethany M Niedzielski, Mollie E Schwartz, Jonilyn L Yoder, Terry P Orlando, Jeffrey A Grover, Joel I Wang, Simon Gustavsson, William D Oliver A node that can communicate quantum information between processors is a necessary component of a general architecture for a largescale, fullyconnected quantum network. Quantum information is generally communicated between nodes via propagating (itinerant) photons, or via a bus coupler that coherently couples adjacent nodes. Protocols involving itinerant photons require lossy components such as microwave circulators, which limit the communication fidelity and fix the direction of communication between nodes. The hardware requirements for protocols involving bus couplers limit node connectivity. In this work, we present progress towards realizing a device that can bidirectionally emit and absorb itinerant microwave photons. The directionality is enabled by the interference between the emission of superconducting qubits in a waveguide quantum electrodynamics architecture. We also present progress towards implementing communication between two nodes by emitting and capturing a photon. This device can be used as the building block for an alltoall quantum network. 
Tuesday, March 15, 2022 11:54AM  12:06PM 
G41.00003: A versatile parametric coupler between two transmon qubits X. Y Jin, Shlomi Kotler, Florent Q Lecocq, Katarina Cicak, John Teufel, Jose Aumentado, Raymond W Simmonds Parametric coupling is a powerful tool that can be used to generate tunable coupling between superconducting qubits via a microwave pump. In this talk, we demonstrate a versatile parametric coupler between two transmon qubits, which can be used to eliminate the residual ZZ coupling between the qubits, to realize a cZ gate by onresonant parametric coupling, as well as a new kind of cZ gate by offresonant parametric ZZ manipulation. In the residual ZZ coupling elimination experiment, we show that the upper limit of the effective ZZ coupling after elimination is nominally zero, with an experimental upper limit of 110 kHz. Randomized benchmarking experiments show that the onresonant cZ gate has a fidelity of 99.4% with a gate time of 60 ns, whereas the offresonant cZ gate has a fidelity of 99.5% with a gate time of 30 ns. We show the gate time dependence of the fidelities of both types of cZ gates and discuss the source of error for those gates. 
Tuesday, March 15, 2022 12:06PM  12:42PM 
G41.00004: A Modular Superconducting Qubit Processor via Interchip Entanglement Invited Speaker: Alysson Gold Scaling superconducting qubit based processors to the hundreds and eventually thousands of physical qubits necessary for faulttolerant computing presents some formidable science and engineering challenges. One of the most fundamental of these challenges is fabrication yield, which decreases exponentially with the number of qubits per chip. A natural solution to this challenge is the assembly of processors out of small, specialized modules with high fidelity, lowlatency quantum interconnects between them. In this talk, we will show first demonstrations of this obtained with two different modular chip architectures. We will start with experimental results from a test platform with deterministic intermodule coupling between four physically separate, interchangeable superconducting qubit integrated circuits, achieving twoqubit gate fidelities as high as 99.1 ± 0.5% and 98.3 ± 0.3% for iSWAP and CZ entangling gates, respectively. We will then move on to results from Rigetti's first modular, multichip quantum processor consisting of two 40 qubit chips with interchip coupling between the chips. This modular approach, and the intermodule coupling technology which enables it, can accelerate nearterm experimental efforts and open up new paths to the faulttolerant era for solid state qubit architectures. 
Tuesday, March 15, 2022 12:42PM  12:54PM 
G41.00005: Highfidelity iToffoli gate for fixedfrequency superconducting qubits Yosep Kim, Alexis Morvan, Long B Nguyen, Ravi K Naik, Christian Jünger, Larry Chen, John Mark Kreikebaum, David I Santiago, Irfan Siddiqi Several threequbit gates have been implemented for superconducting qubits, but their use in gate synthesis has been limited due to their low fidelity. Here, using fixedfrequency superconducting qubits, we demonstrate a highfidelity iToffoli gate based on twoqubit interactions, the socalled crossresonance effect. As with the Toffoli gate, this threequbit gate can be used to perform universal quantum computation. The iToffoli gate is implemented by simultaneously applying microwave pulses to a linear chain of three qubits, revealing a process fidelity as high as 98.26(2)%. Moreover, we numerically show that our gate scheme can produce additional threequbit gates which provide more efficient gate synthesis than the Toffoli and iToffoli gates. 
Tuesday, March 15, 2022 12:54PM  1:06PM 
G41.00006: Analysis and Optimization of TunableCouplerBased ControlledPhase Gates Niklas J Glaser, Federico Roy, Stefan Filipp The use of tunable coupling elements in superconducting qubit architectures enables fast twoqubit gates, while minimizing crosstalk during idling time and singlequbit pulses. In particular, controlledphase (CPHASE) gates based on fluxtunable couplers have been realized with fidelities above 99%. To further improve the fidelity of CPHASE gates we employ optimal control principles and efficient numerical simulations, including decoherence effects that limit fidelities for long pulses. We analyze the system dynamics for different pulse parametrizations and find pulse shapes that harness destructive interference of the leakage amplitudes between the first and second half of the pulse. This recovers intermediate population losses during the pulse and minimizes final leakage. In simulations with experimentally realistic decay times, gate fidelities above 99.9% are obtained for a wide range of device parameters using 20ns long pulses with not more than six parameters. 
Tuesday, March 15, 2022 1:06PM  1:18PM 
G41.00007: Implementation of controlledcontrolledphase gates by refocusing threebody interactions Federico Roy, Niklas J Glaser, Stefan Filipp Many applications for noisy intermediate scale quantum (NISQ) computing devices require entanglement over a large number of qubits, which is typically generated using twoqubit gates. However, being able to access and control strong multiqubit interactions could significantly improve the capabilities of a device by allowing for more efficient generation of entanglement. For this purpose, we investigate a system of three superconducting qubits connected to a single tunable coupling element, in which conditional frequency shifts can be controlled by adiabatic flux pulses applied to the coupler. Using appropriate refocusing pulses and flux pulse timings, all accumulated phases of the computational states can be controlled leading to the implementation of the full family of pairwise controlledphase (CPHASE) and controlledcontrolledphase (CCPHASE) gates. Numerical simulations, including decoherence effects, result in gate fidelities above 99% for realistic experimental parameter settings and gate times between 200ns and 300ns. 
Tuesday, March 15, 2022 1:18PM  1:30PM 
G41.00008: Selectively Activated PhotonHopping, CrossKerr, and TwoMode Squeezing via Flux Modulation of a Tunable Coupler Jacob Koenig, Fatemeh Fani Sani, Giulio Barbieri, Marios Kounalakis, Gary Steele Access to a wide variety of qubitqubit interactions on a single device is desirable for the analog simulation of disparate quantum systems. At the most basic level, a plaquette containing two superconducting transmon qubits connected both capacitively and inductively by a fluxtunable coupler has shown promise for accessing distinct coupling regimes, such as those in which the single excitation transfer coupling dominates over the crossKerr (ZZ) coupling, and vice versa. Access to these regimes and others on a larger device is expected to allow for the analog simulation of several physical phenomena including fractional Bloch oscillations, various spinspin interactions, and lattice gauge theories. In this work, we show theoretically and demonstrate experimentally the ability to selectively enter into regimes in which the system dynamics are dominated by either single photonhopping, twomode squeezing, or crossKerr interactions. The primary interaction is solely determined by the DC flux bias point and choice of modulation frequency for the AC flux threading the tunable coupler. The ability to tune into and out of these coupling regimes demonstrates the ability of superconducting devices containing tunable couplers to perform as versatile analog quantum simulators. 
Tuesday, March 15, 2022 1:30PM  1:42PM 
G41.00009: Simultaneous paritydependent XY entangling gates in superconducting qubit chains Christian Schweizer, Maximilian Nägele, Federico Roy, Stefan Filipp Multiqubit operations have the potential to efficiently generate manybody entanglement. In this work, we investigate the simultaneous coupling of a chain of superconducting qubits. Starting from the wellknown full transfer of a single excitation along a chain of XY coupled qubits, we review parameter settings that allow for partial excitation transfer and extend it to multiple excitations. We find that every pair of qubits on opposite sides with respect to the center undergoes the same XY rotation with the angle determined by a set of coupling and detuning parameters. Moreover, by applying Zgates, we find that the sign of the rotation angle depends on the parity of the qubits in between. This multiqubit operation directly implements a parametrizable Trotter step for JordanWigner Fermions with potential application in Fermi simulations, VQE, parity measurement of long qubit chains, and the efficient generation of multiqubit gates. 
Tuesday, March 15, 2022 1:42PM  1:54PM 
G41.00010: LoopholeFree Bell Inequality Violation with Superconducting Circuits: Concepts Josua Schär, Simon Storz, Anatoly Kulikov, Paul Magnard, Philipp Kurpiers, Janis Luetolf, Adrian Copetudo Espinosa, Kevin Reuer, Abdulkadir Akin, JeanClaude Besse, Mihai Gabureac, Graham J Norris, Andrés Rosario, Baptiste Royer, Alexandre Blais, Andreas Wallraff Nonlocality is an essential resource for protocols in deviceindependent quantum processing [1,2]. For the first time, we bring this capability to the platform of superconducting circuits, one of the main contenders for realizing quantum computer systems. 
Tuesday, March 15, 2022 1:54PM  2:06PM 
G41.00011: LoopholeFree Bell Inequality Violation with Superconducting Circuits: Experiment Simon Storz, Josua Schär, Anatoly Kulikov, Paul Magnard, Philipp Kurpiers, Janis Lütolf, Adrian Copetudo, Kevin Reuer, Abdulkadir Akin, JeanClaude Besse, Mihai Gabureac, Graham J Norris, Andres Rosario, Baptiste Royer, Alexandre Blais, Andreas Wallraff To our knowledge, no loopholefree Bell tests have been performed with macroscopic degrees of freedom to date. Here we present our progress towards performing such a test with superconducting qubits separated by 30 meters of physical distance. In that setting, we attempt to close the locality, detection and freedomofchoice loopholes and present first data. Realizing loopholefree Bell tests is a key ingredient for future applications such as deviceindependent quantum information processing [1, 2]. 
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