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 R30: MultiMode and 3D Cavity Circuit QED Systems IIFocus Live

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Sponsoring Units: DQI Chair: Srivatsan Chakram, Rutgers 
Thursday, March 18, 2021 8:00AM  8:12AM Live 
R30.00001: Bathengineering qubit systems with 3wave mixing Maria Mucci, Xi Cao, Basil Smitham, Christie S. Chiu, Andrew Houck, Michael Jonathan Hatridge Threewave parametric couplings have traditionally been used to create parametric gain between two lowQ modes by driving at the sum of their frequencies, and photon conversion by driving at their difference. In this talk we extend this idea to qubits by coupling a lowQ mode containing a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement) to a highQ transmon. Driving the SNAIL at the sum of the SNAIL and transmon 0 to 1 photon transition frequencies creates a pair of photons. The SNAIL is designed to promptly lose its photon, and this, together with the transmon anharmonicity, creates a parametrically controlled heating rate from the ground to first excited state. Driving the SNAIL and qubit 0 to 1 transition difference frequency creates cooling via a similar process. Heating and cooling between higher qubit states are also accessible via distinct drive frequencies. By combining these drives we can create quite unusual transmon steady states, with full control over both the transmon temperature and relaxation time scale, which we demonstrate experimentally. We will also discuss plans to use this parametrically driven bath to control more complicated multimode systems of qubits and linear modes. 
Thursday, March 18, 2021 8:12AM  8:24AM Live 
R30.00002: Integrating a nonreciprocal amplifier with a 3D transmon qubit Benton Miller, Florent Lecocq, Bradley Hauer, Katarina Cicak, Raymond W Simmonds, John Teufel, Jose Aumentado In conventional superconducting qubit dispersive readout schemes, the measurement signal is routed from the readout cavity to the amplification chain using magnetic circulators. These circulators allow for a modular approach in which the amplifiers and qubitcavity system can be developed separately, but their intrinsic loss and the required wiring significantly reduce the measurement efficiency. 
Thursday, March 18, 2021 8:24AM  8:36AM Live 
R30.00003: Faulttolerant error syndrome detection in the GKP code Christian Siegele, Mazyar Mirrahimi, Phillipe CampagneIbarcq The bosonic encoding proposed by Gottesman, Kitaev, and Preskill (GKP) is one of the 
Thursday, March 18, 2021 8:36AM  9:12AM Live 
R30.00004: Symmetries of bosonic codes Invited Speaker: Arne Grimsmo Most popular bosonic codes obey either rotation symmetry or translation symmetry in phase space. In this talk I will discuss how the symmetries dictate which logical operations are fault tolerant, and some of the pros and cons of rotation symmetric vs. translation symmetric codes for scalable, faulttolerant quantum computing. I will also introduce a new rotation symmetric code family called the Minimum Uncertainty Codes, which performs particularly well under loss and dephasing. Finally, I will show how the translation symmetry of GottesmanKitaevPreskill codes allows for a particularly efficient gate implementation where all singlequbit Clifford gates are done in software and twoqubit gates are implemented with a threewave mixing element. 
Thursday, March 18, 2021 9:12AM  9:24AM Live 
R30.00005: High fidelity interactions between cavities, part 1 Stijn de Graaf, Benjamin Chapman, Yaxing Zhang, Shantanu O Mundhada, Luigi Frunzio, Steven Girvin, Robert J Schoelkopf The long lifetime and large Hilbert space offered by bosonic modes in microwave cavities makes them excellent candidates for quantum memories. By engineering a beamsplitter interaction between these cavities, one can entangle and manipulate these memories. Implementations using 4wave mixing provided by a driven transmon coupled to two cavities have been limited by other, undesired 4th order processes such as the AC Stark Shift. We mitigate this by replacing the transmon with a custommade conversion element, diluting the 4th order nonlinearity of the conversion element while correspondingly driving it harder, and pumping the element further offresonance. Achieving this requires careful engineering to deliver strong offresonant pump tones without inducing unwanted radiative losses. Here, I will talk about the expected performance of such a device, and how we solve these engineering challenges. 
Thursday, March 18, 2021 9:24AM  9:36AM Live 
R30.00006: Mediating highfidelity interactions between superconducting microwave cavities, part II. Benjamin Chapman, Stijn de Graaf, Yaxing Zhang, Shantanu O Mundhada, Luigi Frunzio, Steven Girvin, Robert J Schoelkopf The coherence and large Hilbert spaces of threedimensional superconducting microwave cavities make them an attractive platform for building quantum memories. Such memories may be coupled together by the fourthorder nonlinearity of a transmon, allowing for bilinear (beamsplitter, singlemode, and twomode squeezing) interactions. These in turn may form the building blocks for cavitycavity gates. We examine how these bilinear interactions can be improved by designing custom nonlinear coupling elements. These ‘converters’ can be engineered to suppress spurious fourthorder processes, such as self and crossKerr effects, and improve the desired bilinear coupling. In this talk, we discuss preliminary measurements on pairs of threedimensional microwave cavities coupled by such a custom conversion element. 
Thursday, March 18, 2021 9:36AM  9:48AM Live 
R30.00007: Superstrong coupling between a transmon and a metamaterial resonator Sagar Indrajeet, Tianna A. McBroom, Bradley G. Cole, Britton Plourde A high density of modes can be produced using metamaterial resonant structures made from arrays of lumped circuit elements, to which a fluxtunable transmon qubit can be coupled. For such a system, we have measured the coupling strength of the qubit to multiple modes by tuning the flux and observing the splitting in the transmission of each mode. To enhance the coupling strength in our next generation devices, we can increase the resonator impedance and the number of unit cells of the metamaterial resonator. In addition, by increasing the coupling capacitance between the qubit and metamaterial resonator, our simulations indicate that it will be possible to reach the superstrong coupling regime, where the coupling strengths exceed the intermode spacing and the qubit can thus be coupled to multiple modes simultaneously. The inclusion of onchip fluxbias lines can enable the nonadiabatic modulation of the qubit transition or parametric modulation, which can be used to swap excitations between the qubit and the metamaterial. Superstrong coupling with fast qubit modulation will have a variety of applications in analog quantum simulations and multimode cQED. We report on our progress with implementing metamaterial devices with enhanced qubit coupling. 
Thursday, March 18, 2021 9:48AM  10:00AM Live 
R30.00008: Manybody states of radiation and quantum thermalization in multimode circuit QED (Part 1) Nitish Mehta, Roman Kuzmin, Cristiano Ciuti, Vladimir Manucharyan We explore a quantum impurity problem arising from the galvanic coupling of a superconducting fluxonium qubit to a long section of a 1D waveguide. The resulting multimode circuit QED has presented theoretical challenges in the context of gauge invariance and convergence of perturbative expansions. The fluxonium’s strong anharmonicity prevents the use of semiclassical blackbox circuit quantization procedure, standard to transmonbased circuits. We have uncovered a qualitatively new effect, the dressing of photons by photons: a onephoton state hybridizes with nearly resonant nphoton states. In the spectral range 57 GHz, we resolve twophoton and threephoton splittings and reproduce hundreds of energy levels without adjustable parameters via an Hamiltonian with asymptotic freedom and truncationfriendly gauge. We observe a fine structure of discrete manybody resonances that turns into a broad continuum as we go to higher energies. This occurs because the density of multiphoton states rapidly grows with increasing energy. Such transition shows that our finitesize 1D quantum system can act as a bath to itself. We believe this is the first instance of quantum thermalization of radiation in a closed system. 
Thursday, March 18, 2021 10:00AM  10:12AM Live 
R30.00009: Manybody states of radiation and quantum thermalization in multimode circuit QED (Part 2) Nitish Mehta, Roman Kuzmin, Cristiano Ciuti, Vladimir Manucharyan We explore a quantum impurity problem arising from the galvanic coupling of a superconducting fluxonium qubit to a long section of a 1D waveguide. The resulting multimode circuit QED has presented theoretical challenges in the context of gauge invariance and convergence of perturbative expansions. The fluxonium’s strong anharmonicity prevents the use of semiclassical blackbox circuit quantization procedure, standard to transmonbased circuits. We have uncovered a qualitatively new effect, the dressing of photons by photons: a onephoton state hybridizes with nearly resonant nphoton states. In the spectral range 57 GHz, we resolve twophoton and threephoton splittings and reproduce hundreds of energy levels without adjustable parameters via an Hamiltonian with asymptotic freedom and truncationfriendly gauge. We observe a fine structure of discrete manybody resonances that turns into a broad continuum as we go to higher energies. This occurs because the density of multiphoton states rapidly grows with increasing energy. Such transition shows that our finitesize 1D quantum system can act as a bath to itself. We believe this is the first instance of quantum thermalization of radiation in a closed system. 
Thursday, March 18, 2021 10:12AM  10:24AM Live 
R30.00010: Long range connectivity in a scalable superconducting qubit network Sumeru Hazra, Anirban Bhattacharjee, Madhavi Chand, Kishor V Salunkhe, Sriram Gopalakrishnan, Meghan P Patankar, R Vijay Qubit connectivity plays a pivotal role in the performance of a quantum processor by minimizing the gate count for an arbitrary multiqubit operation. However, most superconducting quantum processors exhibit only nearest neighbor coupling. Hence the quantum information may require to be transferred across several qubits to perform an arbitrary operation resulting in reduced fidelity due to finite coherence of the qubits. 
Thursday, March 18, 2021 10:24AM  10:36AM Live 
R30.00011: Chiral cavity quantum electrodynamics in a 3D microwave lattice coupled to a transmon qubit (Part 1) Margaret Panetta, Clai Owens, Srivatsan Chakram, Brendan Saxberg, Gabrielle Roberts, Ruichao Ma, Andrei Vrajitoarea, Jon Simon, David I Schuster Recent advancements in the ability to create and manipulate superconducting quantum systems have created an exciting opportunity to construct from the ground up quantum materials tailored to host rich interactions. We have designed a twodimensional metamaterial in which microwave photons inhabiting a lattice of superconducting 3D microwave cavities interact strongly with ferrimagnets, realizing a quarterflux Hofstadter model for light. This is the first photonic topological lattice platform compatible with strong interactions. We perform state tomography on the lattice and demonstrate chiral, timereversal symmetry broken edge transport with lifetimes ~1000 times larger than the site to site tunneling rate. We have coupled a transmon qubit to this lattice, achieving, for the first time, a platform for chiral cavity quantum electrodynamics. Here we discuss the design and testing of this system and describe prospects for its application. 
Thursday, March 18, 2021 10:36AM  10:48AM Live 
R30.00012: Chiral cavity quantum electrodynamics in a 3D microwave lattice coupled to a transmon qubit (Part 2) Clai Owens, Margaret Panetta, Srivatsan Chakram, Brendan Saxberg, Gabrielle Roberts, Ruichao Ma, Andrei Vrajitoarea, Jon Simon, David I Schuster We describe work combining a single transmon with this topological lattice, achieving, for the first time, a platform for chiral cavity quantum electrodynamics. We show strong coupling between the qubit and the topological lattice. Using the qubit we prepare arbitrary quantum states of the lattice edge modes and we can nondestructively measure their photon occupation. Finally, we describe a path towards coupling multiple transmons to edge sites of the lattice, enabling exploration of photonphoton interactions in a topological band structure. 
Thursday, March 18, 2021 10:48AM  11:00AM On Demand 
R30.00013: Manipulating complex hybrid entanglement and testing multipartite Bell inequalities in a superconducting circuit Xiaoxuan Pan, Yuwei Ma, Weizhou Cai, Xianghao Mu, Yuan Xu, Ling Hu, Weiting Wang, Haiyan Wang, Yipu Song, zhenbiao yang, ShiBiao Zheng, Luyan Sun Multipartite entangled states are crucial not only for fundamental interest, but also for important applications of quantumbased technologies, such as quantum communication, quantum metrology and quantum simulation. Quantum correlations in multipartite observables are direct consequences of entanglement and can be used to characterize the nonclassical nature of the entangled states. As a signature of quantum correlation, the violation of Bell inequalities has not been demonstrated in multipartite hybrid quantum systems involving both discrete and continuous variables. Here we generate a fivepartite entangled state with three superconducting transmon qubits and two photonic qubits, each encoded in the mesoscopic field of a microwave cavity. We reveal the quantum correlations among these distinct elements by joint Wigner tomography of the two cavity fields conditional on the detection of the qubits and by test of a fivepartite Bell inequality. The measured Bell signal is 8.381±0.038, not only surpassing the classical bound of 4 allowed by local realism, but also above the quantum bound of 8 for fourpartite entanglement, demonstrating the genuine fivepartite entanglement in a hybrid quantum system. 
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