Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B36: Supercondcuting Qubits: Circuit AnalysisRecordings Available

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Sponsoring Units: DQI Chair: David Roberts, University of Chicago Room: McCormick Place W194A 
Monday, March 14, 2022 11:30AM  11:42AM 
B36.00001: Controlling Qubit Relaxation in 13 Dimensions Param J Patel In the circuit QED architecture, the coupling of a qubit through its associated cavity to the cavity drive port sets the ultimate limit on coherence. For lumped element realizations this socalled singlemode Purcell limit stringently limits qubit coherence, unless an external filter (often referred to as a Purcell filter) is used to suppress photon emission at the qubit's frequency. However, in 2 and 3D circuits, we can often greatly exceed the single mode Purcell limit [Houck, et.al, PRL (2008)]. In this talk, we present a method for visualizing transmon fields to find port placements which can achieve both large cavity bandwidths and long qubit lifetimes. We find that when the qubit frequency is below the lowest cavity mode, this effect cannot be explained by interference between multiple cavity modes, but rather the variation of the qubit mode's fields in space. We will present data from transmons in both 3D post cavity and chipintube geometries. We have achieved coherences as high as 100 microseconds in systems with strong qubitcavity couplings, short cavity lifetimes, and no external Purcell filtering. This technique can also readily be extended to planar qubit/resonator designs and is thus broadly useful across many implementations of superconducting circuits. 
Monday, March 14, 2022 11:42AM  11:54AM 
B36.00002: A Tunable MultiQubit Coupling Element for Superconducting Circuits Brian Marinelli, Jie Luo, David I Santiago, Irfan Siddiqi We propose a SQUID tunable microwave coplanar waveguide (CPW) resonator to 
Monday, March 14, 2022 11:54AM  12:06PM 
B36.00003: A nonreciprocal dispersive interaction in circuit QED: Part I, formulation and setup Yingying Wang, Sean van Geldern, Yuxin Wang, Thomas Connolly, Aashish Clerk, Chen Wang Nonreciprocity is a valuable property for building interesting and complex quantum systems. Previous experimental studies have primarily focused on nonreciprocal excitation transfer between linear modes, which can be fully captured by a nonHermitian effective Hamiltonian matrix. To go beyond these semiclassical descriptions, we implement a dispersive type of nonreciprocal interaction between a transmon qubit and a superconducting cavity. We achieve this by utilizing a custom designed ferrite circulator to mediate Bfield tunable nonreciprocity between two cavities[1], with one of them housing the qubit. We were able to describe the inputoutput spectrum of this device with a 2 mode nonHermitian linear model coupled to a qubit. By adjusting the coupling parameters we were able to put the system in a regime where the intermediary mode could be eliminated and the model could be adiabatically reduced to a master equation involving only the cavity and the qubit, utilizing a nonlinear jump operator. This model can then be used to predict dispersive shifts and dephasing, showing notable asymmetry across magnetic fields due to the nonreciprocity. 
Monday, March 14, 2022 12:06PM  12:18PM 
B36.00004: A nonreciprocal dispersive interaction in circuit QED: Part II, measurements and analysis Sean van Geldern, Yingying Wang, Yuxin Wang, Thomas Connolly, Aashish Clerk, Chen Wang Nonreciprocity is a valuable property for building interesting and complex quantum systems. Previous experimental studies have primarily focused on nonreciprocal excitation transfer between linear modes, which can be fully captured by a nonHermitian effective Hamiltonian matrix. To go beyond these semiclassical descriptions, we implement a dispersive type of nonreciprocal interaction between a transmon qubit and a superconducting cavity, where the system dynamics can be captured by a cavityqubit nonlinear jump operator in a new effective theory of nonreciprocity. This theory is a result of adiabatic elimination and its predictions must be compared to timeaveraged quantities. Thus, we experimentally measured the timeaveraged qubit dispersive shifts and dephasing rates from a decaying photon population, which agree excellently with model predictions, showing a notable asymmetry between positive and negative applied magnetic fields due to the nonreciprocity. We further verify the usefulness of the model by studying the dispersive shifts and dephasing rates in the steadystate regime under a continuous cavity drive, which show good agreement with theory. Furthermore, we explore potential applications of this model, such as qubit phase gate operations. 
Monday, March 14, 2022 12:18PM  12:30PM 
B36.00005: Direct Calculation of the ZZinteraction Rates in the MultiMode circuitQED Firat Solgun, Srikanth Srinivasan Superconducting quantum processors might suffer from unwanted ZZinteractions which naturally arise as a result of the finite and small anharmonicities of the Transmon qubits. Nonzero ZZterms in the qubit Hamiltonians can lead to the accumulation of the spurious phases during the execution of certain twoqubit gates. Recently novel qubitqubit couplers have been proposed to suppress the ZZinteractions while keeping a finite exchange coupling strength necessary to run the twoqubit gates. These new couplers typically involve multiple modes and their engineering requires diagonalization of the multimode Hamiltonians which can very quickly become computationally demanding as the number of modes of the coupler increases. Here we describe a new method for the accurate calculation of the ZZrates in the multimode circuitQED by capturing their frequency dependence with simple relations of the entries of the multiport impedance matrix defined between the qubit ports. We observe very good agreement between the predictions of our method and the measurement data collected from the multiqubit devices. Our method being an extension of the framework developed in (**) makes the quantum microwave engineering of the superconducting qubits more streamlined. 
Monday, March 14, 2022 12:30PM  12:42PM 
B36.00006: Majorana fermions revealing the true nature of quantum phase slip junctions Christina Koliofoti, RomanPascal Riwar Quantum phase slips (QPS) are famously considered to be the exact dual to the Josephson effect. However, if taken at face value, this duality stipulates that charge transported across a QPS junction is continuous, as opposed to regular Josephson junctions, which perserve charge quantization. We here analyse a circuit consisting of a QPS junction shunted by a topological Josephson junction hosting Majoranas, the latter being sensitive to charge transport in units of half a Cooper pair charge. Contrary to common phenomenological approaches, we consider a microscopic derivation of the phase slip processes, using a large Josephson junction array to model the QPS junction. We find that there is a crossover when increasing the number of junctions in the array, from a regime where the standard, dual description of phase slip junctions is valid, to a regime where charge has to be quantized in integer units of the Cooper pair charge. This crossover isstraightforwardly visible in the eigenspectrum of the circuit as a function of an externally applied flux. Importantly, we expect that the latter (quantized) regime renders QPS junctions useless for circuit realizations of the GottesmanKitaevPreskill code. 
Monday, March 14, 2022 12:42PM  12:54PM 
B36.00007: Directional photon transport in a superconducting circuit lattice Joachim Lauwens, Arnau Sala, Bart Soree One of the current bottlenecks in quantum computing devices based on circuit quantum electrodynamics (cQED) is in the circulators used for routing microwave photons. The circulators currently used in novel devices are bulky and require large, localized magnetic fields in order to achieve directional transport of the electromagnetic waves, making them incompatible for onchip implementation with superconducting circuits. We address these limitations by studying a simple superconducting circuit design that achieves directional photon transport using components currently present in standard cQED devices. In a lattice of coupled microwave resonators, with an effective complex hopping term between lattice sites, we achieve directionality by engineering an effective magnetic field for photons that results in a Peierls phase. Implementations that make use of such a phase have already been proposed, using frequency mixers or magnetic fields in Josephson junction rings, but such devices are currently not considered due to their complexity. We instead investigate the possibility of using voltagedriven planar transmission lines to generate the effective Peierls phase. This will result in a simpler implementation of a superconducting circuit that can also be used for onchip signal routing. 
Monday, March 14, 2022 12:54PM  1:06PM 
B36.00008: Nonperturbative analytical diagonalization of Hamiltonians with application to ZZcoupling suppression and enhancement in circuitQED Boxi Li, Tommaso Calarco, Felix Motzoi Deriving effective Hamiltonian models plays an essential role in many quantum control and engineering problems. In this talk, we present two symbolic methods for effectively eliminating auxiliary space: the Nonperturbative Analytical Diagonalization (NPAD) and the Recursive SchriefferWolff Transformation (RSWT). While NPAD makes use of the Jacobi iteration and works without the assumption of perturbation, RSWT takes advantage of the recursive structure and avoids the exponentially increasing number of terms in highorder perturbation. Both methods consist of elementary expressions and can be easily automated to obtain closedform expressions. This opens the possibility for fast operations using adiabatic techniques such as DRAG and STA beyond the perturbative regime. 
Monday, March 14, 2022 1:06PM  1:18PM 
B36.00009: Tunable Directional Photon Scattering from a Pair of Superconducting Qubits Elena Redchenko, Alexander V Poshakinskiy, Martin Zemlicka, Alexander Poddubny, Johannes M Fink Nonreciprocal integrated devices provide flexibility and scalability for a wide range of onchip applications, such as integrated photonics, quantum information processing, and nonlinear optics. We demonstrate tunable directional scattering of microwave radiation with just two transmon qubits coupled to a transmission line. When the qubits are tuned in resonance with each other such that the effective distance between them is equal to a quarter of the qubit excitation wavelength, we periodically modulate their transition frequency. The resulting interference between the two qubits enables directional forward or backward scattering of Stokes and antiStokes components depending on the relative phase between the local modulation tones in analogy to the optomechanical Kerker effect. Such a nonreciprocal device is compatible with modern superconducting qubit technology and can be used to route microwave radiation for the realization of chiral networks. 
Monday, March 14, 2022 1:18PM  1:30PM 
B36.00010: Tunable Multiqubit couplers with low residual ZZ interactions Ivan Tsitsilin, Gerhard Huber, Leon Koch, Niklas Bruckmoser, Leonhard Hoelscher, Federico Roy, Niklas Glaser, Max Werninghaus, Malay Singh, Stefan Filipp Maintaining high gate fidelities when scaling quantum processors to a large number of qubits is key to practically useful quantum computing. Twoqubit gates based on tunable couplers now reach fidelities above 99.5% in superconducting qubit quantum processors, in which the qubits are typically arranged on a square grid with pairwise qubitqubit couplers. However, involving multiqubit couplers that provide higher connectivity may lead to more efficient algorithms. In this work, we specifically discuss the challenges imposed by residual ZZ interactions in such multiqubit tunablecoupler architectures. We analyze possibilities to suppress these offresonant interactions by connecting qubits to the coupler with different polarities of the coupling. Moreover, we investigate the use of additional twoqubit couplers as a way to mitigate ZZ interactions. 
Monday, March 14, 2022 1:30PM  1:42PM 
B36.00011: Exact parameterization of qubit dynamics with coupling to a resistive element in the drive line Antti P Vaaranta, Marco Cattaneo, Russell E Lake Understanding the coupling between a quantum circuit and its environment is crucial for qubit development. In this work we derive a complete description of the qubit dynamics, where the solution is given in terms of the experimental circuit parameters. We start from the circuit model for the qubit plus environment, the latter realized by a variable temperature attenuator [1] acting as a thermal bath with JohnsonNyquist noise. We apply methods of circuit quantum electrodynamics for the qubitenvironment combination, following Ref. 2, and obtain a Hamiltonian from which we derive a master equation in the open quantum systems formalism. This approach gives insight into how each circuit element and the drive line temperature affect the qubit dynamics. As proof of concept, we solve the master equation for the specific case of a fixedfrequency, dispersive transmon, recovering the expected result of decohering qubit without dissipation. 
Monday, March 14, 2022 1:42PM  1:54PM 
B36.00012: Parasitic free gate Xuexin Xu, Mohammad H Ansari Implementation of highperformance twoqubit gates is a key factor for scalable quantum computation. However, the stateoftheart superconducting twoqubit gates are yet far from being perfect due to the parasitic ZZ coupling. In this paper, we propose a parasitic free (PF) gate to suppress such unwanted interaction in use of the tunable coupler. The gate is operated in two modes: in idle mode the coupler frequency is tuned such that the two qubits are effectively decoupled; in driven mode the coupler frequency is tuned so as to result in a nonzero static ZZ interaction, which later can be cancelled by the dynamical part. Our theory shows that using this method the fidelity of a CR gate is able to achieve the coherence limit. 
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