Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P33: Superconducting Parametric/Tunable InteractionsFocus

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Sponsoring Units: DQI Chair: David McKay, IBM T J Watson Res Ctr Room: LACC 408B 
Wednesday, March 7, 2018 2:30PM  3:06PM 
P33.00001: Enhancing cavity QED via antisqueezing: synthetic ultrastrong coupling Invited Speaker: Luke Govia We present and analyze a method where parametric (twophoton) driving of a cavity is used to exponentially enhance the lightmatter coupling in a generic cavity QED setup, with timedependent control. Our method allows one to enhance weakcoupling systems, such that they enter the strong coupling regime (where the coupling exceeds dissipative rates) and even the ultrastrong coupling regime (where the coupling is comparable to the cavity frequency). As an example, we show how the scheme allows one to use a weakcoupling system to adiabatically prepare the highly entangled ground state of the ultrastrong coupling system. The resulting state could be used for remote entanglement applications. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P33.00002: Parametric entangling gates in a superconducting quantum processor, Part I: Theory Nicolas Didier, Eyob Sete, Marcus da Silva, Chad Rigetti Realizing highfidelity twoqubit gates is one of the main challenges in building quantum processors. Controllability and selectivity are particularly advantageous to reduce crosstalks in experimental implementations. We present twoqubit gates based on parametricallyactivated entangling interactions and fully controlled by a RF flux pulse on the SQUID of a tunable transmon that is capacitively coupled to other transmon qubits. The modulation frequency acts as a pump to enable onresonance Rabi oscillations between two desired twoqubit states, the gate is switched on and off by the modulation amplitude. We show that several kinds of gates can be realized, selected by the modulation frequency. We discuss the rate and fidelity of iSWAP and controlledphase gates for experimentally realized superconducting circuits. arXiv:1706.06566 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P33.00003: Parametric entangling gates in a superconducting quantum processor, Part II: Dephasing due to modulation Eyob Sete, Nicolas Didier, Chad Rigetti

Wednesday, March 7, 2018 3:30PM  3:42PM 
P33.00004: Parametric Entangling Gates in a Superconducting Quantum Processor, Part III: Experiment Shane Caldwell, Nicolas Didier, Colm Ryan, Eyob Sete, Alexander Hudson, Peter Karalekas, Riccardo Manenti, Marcus da Silva, Rodney Sinclair, Chad Rigetti A central challenge in building a scalable quantum computer is the execution of highfidelity entangling gates within an architecture containing many resonant elements. As elements are added, or as the multiplicity of couplings between elements is increased, the frequency space of the design becomes crowded and device performance suffers. By applying flux modulation to tunable transmons, one can drive the resonant exchange of photons directly between energy levels of a statically coupled multitransmon system. This obviates the need for mediating qubits or resonator modes and allows the full utilization of all qubits in a scalable architecture. The resonance condition is selective in both the frequency and amplitude of modulation and thus alleviates frequency crowding. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P33.00005: Quantum information processing with multimode cavity systems using parametrically modulated components Peter Groszkowski, Srivatsan Chakram, Ravi Naik, Nelson Leung, Yao Lu, David Schuster, Jens Koch In recent years superconducting circuits have come to the forefront as systems realizing early quantum information processing devices. Some of the key reasons for this success are their flexibility as well as capacity for easy manipulation and measurement. In this talk, we theoretically explore a setup consisting of multiple harmonic modes coupled to a qudit, where the qudit is parametrically modulated in order to carry out logical operations and readout. Recently, a superconducting circuit implementing such a system has been experimentally demonstrated in [1]. We explore various properties of this architecture related to its random access nature and long state lifetimes, as well as provide a thorough discussion of unwanted crosstalk and frequency crowding, which helps to understand its detailed physics. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P33.00006: Parametric coupling with superconducting circuits X. Y. Jin, Florent Lecocq, K. Cicak, Shlomi Kotler, Gabriel Peterson, John Teufel, Jose Aumentado, Raymond Simmonds We discuss parametric coupling with superconducting circuits sharing a common dcSQUID coupler. The dcSQUID coupling element provides a variable inductance that can be modulated to provide a tunable parametric coupling. We present the latest experimental progress with these systems. 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P33.00007: Implementing logical oscillators by parametric two photon blockade Andrei Vrajitoarea, Ziwen Huang, Peter Groszkowski, Jens Koch, Andrew Houck Present superconducting quantum architectures make use of transmon qubits as logical elements and harmonic oscillators as quantum memories due to their improved coherence times. In this talk, we report a design for performing logical operations directly on the oscillator single photon manifold by strong hybridisation of the two photon fock state without compromising coherence. We implement a two cavity architecture with fluxtunable coupling using a superconducting quantum interference device. The cavity resonances can be statically tuned to be first order insensitive to flux noise while the coupling can be parametrically modulated to selectively activate a three wave mixing process for generating anharmonicity. Experimental results of coupler parametric modulation will be presented. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P33.00008: Suppression of Qubit Crosstalk in a Tunable Coupling Superconducting Circuit Device Pranav Mundada, Gengyan Zhang, Andrew Houck Ondemand qubitqubit interactions without unwanted crosstalk are crucial for realization of a scalable quantum computer. We demonstrate a tunable coupler between two transmon qubits which is able to achieve zero static ZZ coupling strength due to interference effects, in agreement with our theoretical model. We are able to tune the ZZ coupling from +2MHz to 100kHz. We characterize the effect of the ZZ crosstalk on single qubit gate fidelity by simultaneous randomized benchmarking. Furthermore, by statically biasing at the low crosstalk point and selectively modulating the flux through the coupler, we implement an iSWAP gate between the two qubits. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P33.00009: Universal stabilization of singlequbit states using a tunable coupler Ziwen Huang, Yao Lu, Eliot Kapit, David Schuster, Jens Koch We theoretically analyze a scheme for fast stabilization of arbitrary qubit states with high fidelities, extending a protocol recently demonstrated experimentally [1]. Our scheme utilizes red and blue sideband transitions in a system composed of a fixedfrequency transmon qubit, a lowQ LCoscillator, and a coupler enabling us to tune the interaction between them. Under parametric modulations of the coupling strength, the qubit can be steered into any desired pure or mixed singlequbit state. For realistic circuit parameters, we predict that stabilization can be achieved within 100 ns. By varying the ratio between the oscillator's damping rate and the effective qubitoscillator coupling strength, we can switch between underdamped, criticallydamped, and overdamped stabilization and find optimal working points. We further analyze the effect of thermal fluctuations and show that the stabilization scheme remains robust for realistic temperatures. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P33.00010: Tunable Dissipator for Highfidelity Cavity and Qubit Initialization FNU Naveen, Alexander Opremcak, Bradley Christensen, Chris Wilen, Ivan Pechenezhskiy, JJ Nelson, Clement Wong, Maxim Vavilov, Britton Plourde, Robert McDermott Superconducting qubit dephasing times are limited by photon shot noise in the proximal readout resonator, due either to stray thermal photons or to coherent photons left over from measurement. At the same time, a small excess population of the qubit 1 state leads to error, and approaches to suppress this error based either on postselection or premeasurement and feedback are unsatisfactory. Here we describe the design and operation of tunable dissipative modes for the fast, deterministic initialization of linear cavities and qubits. We show that the dissipator can be used to provide a fast cavity reset following strong measurement and to suppress photon shot noise dephasing. Sideband gates applied to the qubitcavity system followed by fast cavity reset are used to achieve high qubit initialization fidelity. We discuss prospects for the integration of tunable dissipators into a scalable multiqubit system. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P33.00011: ExponentialOff Qubit Couplers David Clarke, Ryan Epstein, Alex Marakov, Joel Strand Creating qubitqubit interactions that can be turned on or off is key to making multiqubit circuits, whether for gatebased or quantum annealing applications. It is critical that the “off” state be robust, both in terms of residual coupling and sensitivity to flux noise. This ensures good isolation of the circuit elements and prevents unwanted terms from being introduced into the Hamiltonian. Here we present an extensible framework for a coupler consisting of a chain of elements connecting two qubits. While the “on” state coupling decreases polynomially in the length of the chain, the unwanted “off” coupling decreases exponentially, leading to a broad range of control flux with negligible “off” coupling for chains of reasonable length. Additionally, the individual elements may be engineered to control aspects of the “on” point, allowing one to mitigate noise or errors in fabrication. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P33.00012: A new paradigm in open quantum systems: a transmon qubit coupled to a mesoscopic and tunable environment Javier Puertas, Sebastien Leger, Nicolas Gheereart, Remy Dassonneville, Luca Planat, farshad foroughi, yuriy krupko, Olivier Buisson, cecile naud, Wiebke Guichard, serge florens, izak snyman, Nicolas Roch At ultrastrong coupling, an open quantum system forms a nonperturbative hybridized state with its environment. In the past few years, this regime has been reached for superconducting qubits coupled to single mode resonators or to the continuum of modes in a superconducting transmission. Here we go a step further down this route by realizing an insitu tunable multimode environment, the degrees of which can be individually monitored. To do so we interface for the first time a transmon qubit to a superconducting metamaterial consisting of a onedimensional array of 4700 SQUIDs. We demonstrate ultrastrong coupling with a qubit decay rate of up to 10% of its natural frequency, and hybridization with many (up to 10) environmental modes at a time. Owing to the simplicity of the transmon qubit design, we are able to obtain quantitative agreement between experiment and a theoretical model with no unknown parameters, over a broad parameter range. 
Wednesday, March 7, 2018 5:18PM  5:30PM 
P33.00013: Parametrically activated twoqubit interactions in a superconducting qubit system for quantum simulations Marc Ganzhorn, Marco Roth, Gian Salis, Nikolaj Moll, Daniel Egger, Andreas Fuhrer, Peter Müller, Sebastian Schmidt, Stefan Filipp A current bottleneck for quantum computation and simulation is the realization of flexible highfidelity twoqubit operations. Gates based on parametrically driven tunable couplers offer a convenient method to entangle qubits by selectively activating different interaction terms in the effective Hamiltonian. Here, we present experimental results on a system comprising fixedfrequency superconducting transmons capacitively coupled to a tunable coupler realized in our setup by a fluxtunable qubit. The twoqubit interactions are activated via a parametric frequency modulation. We realize different types of interactions with fidelities up to 97% by adjusting the frequency and phase of this modulation. Our experimental findings are backed by analytical calculations and numerical simulations, revealing that the fidelities of these two qubit operations are currently limited by the coherence of the tunable coupler. Eventually, using such a set of single and two qubit interactions, analog simulations of quantum systems such as molecules or spin systems could be realized. 
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