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 F30: Quantum Computing Architectures IIFocus Live

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Sponsoring Units: DQI Chair: Matthias Steffen, IBM TJ Watson Research Center 
Tuesday, March 16, 2021 11:30AM  11:42AM Live 
F30.00001: Using chaotic quantum maps as a test of current quantum computing hardware fidelity* Max Porter, Ilon Joseph, Jeff B. Parker, Alessandro R Castelli, Vasily Geyko, Frank R Graziani, Stephen Bernard Libby, Yaniv J Rosen, Yuan Shi, Jonathan L DuBois In this work, the dynamics of chaotic quantum maps is explored via simulation as a means to test the fidelity of emerging quantum computing hardware. Quantum computers promise to deliver enormous gains in computational power that can potentially be used to benefit Fusion Energy Sciences (FES). Through the quantumclassical correspondence principle, quantum systems of sufficiently large quantum number (or number of qubits) can approximate classical dynamics. Here we study the simplest types of chaotic dynamical systems, defined by classical and quantum maps. It’s been shown that quantum maps of sufficient fidelity can recreate smallscale classical phase space structures in the limit of many qubits [G. Benenti, et al. Phys. Rev. Lett. 87, 2279011 (2001)]. They can also deviate from the classical dynamics and display dynamical Anderson localization. In this work phase space structures are sought on current hardware, using IBM’s 5qubit devices and the LLNL Quantum Design and Integration Testbed (QuDIT) platform, with verification from gate set tomography (GST). 
Tuesday, March 16, 2021 11:42AM  11:54AM Live 
F30.00002: A modular quantum computer based on a parametrically driven quantum state router Pinlei Lu, Chao Zhou, Mingkang Xia, TzuChiao Chien, Ryan Kaufman, Xi Cao, David Pekker, Roger Mong, Wolfgang Pfaff, Michael Jonathan Hatridge For superconducting quantum computers, most efforts seek to implement a “surface code” architecture, which only couples nearestneighbor qubits. In such a computer, operations between distant qubits require a large number of nearestneighbor gates to implement with concomitant increases in gate errors and run time. In contrast, a modular architecture allows for longrange couplings between distant qubits. We have realized a modular quantum state router based on threewave couplings with alltoall couplings between 4 modules. We have connected the router to four simple modules consisting of a high Q communication cavity which couples to the router, a single transmon qubit and a readout cavity to demonstrate feasibility of operating the router + module systems as a quantum machine. In this talk, we will demonstrate basic operations in our machine: transferring states and generating entanglement among the modules’ communication modes and qubits. Furthermore, we will discuss the potential for utilizing ancillary modes in the router as ancillary quantum storage, as well as expanding the router system to form a large scale quantum router for an arbitrary number of modules. 
Tuesday, March 16, 2021 11:54AM  12:06PM Live 
F30.00003: Characterization of quantum states based on creation complexity Zixuan Hu, Sabre Kais The creation complexity of a quantum state is the minimum number of elementary gates required to create it from a basic initial state. The creation complexity of quantum states is closely related to the complexity of quantum circuits, which is crucial in developing efficient quantum algorithms that can outperform classical algorithms. A major question unanswered so far is what quantum states can be created with a number of elementary gates that scales polynomially with the number of qubits. In this work we first show for an entirely general quantum state it is exponentially hard (requires a number of steps that scales exponentially with the number of qubits) to determine if the creation complexity is polynomial. We then show it is possible for a large class of quantum states with polynomial creation complexity to have common coefficient features such that given any candidate quantum state we can design an efficient coefficient sampling procedure to determine if it belongs to the class or not with arbitrarily high success probability. Consequently partial knowledge of a quantum state’s creation complexity is obtained, which can be useful for designing quantum circuits and algorithms involving such a state. 
Tuesday, March 16, 2021 12:06PM  12:18PM Live 
F30.00004: Adapting 5Gtelecom hardware for the control of quantum computers Riccardo Borgani, Mats Tholen, David Brant Haviland An important figure of merit for scalable quantum computing is the cost per channel per unit bandwidth of its control electronics. Fortunately, 5G telecom is a major market force pushing this cost down. These fields share in fact many technical requirements: multiple phasecoherent and wideband channels for output and input; low noise and low distortion with low crosstalk between channels; easy to reconfigure and fieldprogrammable, with highspeed logic for feedback and feedforward control. 
Tuesday, March 16, 2021 12:18PM  12:30PM Live 
F30.00005: Spontaneous patametric downconversion sources for boson sampling Reinier van der Meer, Jelmer Renema, Benjamin Brecht, Christine silberhorn, Pepijn Pinkse The next milestone in photonic quantum information processing is to 
Tuesday, March 16, 2021 12:30PM  12:42PM Live 
F30.00006: Generating nonclassical states for continuousvariable quantum computation Using PhotonNumber Selective Phase Gates and Displacements Marina Kudra, Daniel Perez Lozano, Marco Scigliuzzo, Ingrid Strandberg, Shahnawaz Ahmed, Per Delsing, Simone Gasparinetti Efficiently controlling the quantum state of 3D cavity modes is an important ingredient for exploiting their long lifetimes and restricted decoherence channels for quantum information processing. Here we experimentally explore the use of PhotonNumber Selective Phase (SNAP) gates and displacements to generate Wignernegative states useful for continuous variable quantum computing. Our statepreparation protocol consists of a sequence of interleaved SNAP gates and coherent displacements. We use gradient descent algorithm to optimize the parameters of the sequence, and characterize fidelities to the target state by Wigner tomography. It has been shown theoretically that just a few of these blocks can be used to generate highly nonclassical states with high fidelity. 
Tuesday, March 16, 2021 12:42PM  12:54PM Live 
F30.00007: Experimental demonstration of entangling gates across chips in a multicore QPU Alysson Gold, Anna Stockklauser, Matt Reagor, JeanPhilip Paquette, Andrew Bestwick, Cody James Winkleblack, Ben Scharmann, Feyza Oruc, Brandon Langley In addition to communication networks, routers and repeaters, actively under development in the context of quantum information processing, multicore architectures are a critical component of distributed computing. As quantum processors scale and quantum networks are realized, motherboards will be required to coherently route information between chips, enabling entanglement between cores in a quantum processor or possibly between quantum memory or processing modules deriving from different physical architectures. Furthermore, for superconducting qubit integrated circuits, a modular processor composed of several smaller individual chips mitigates the impact of exponentially decreasing chip yield as the number of qubits per die increases. Towards these aims, we present here experimental results from a multicore quantum processing unit: a 32qubit platform formed from shortrange interconnects across four 8qubit chips. We show that this interchip coupling does not significantly impact singlequbit or twoqubit performance, and conclude with examples of high fidelity multiqubit algorithms across these interchip edges. 
Tuesday, March 16, 2021 12:54PM  1:30PM Live 
F30.00008: The Role of Computer Architecture in Advancing QC (and the Role of QC in Advancing Computer Architecture!) Invited Speaker: Margaret Martonosi For decades, Quantum Computing has been seen as a compelling possibility as a novel form of computing offering the potential of speedups large enough to make useful intractable problems tractable. In recent years, nearterm quantum computers have been built demonstrating the basics of the approach. A huge gap exists, however, between the resource requirements of “useful” QC algorithms, and the resources available on current nearterm prototypes. 
Tuesday, March 16, 2021 1:30PM  1:42PM Live 
F30.00009: Quantum Optimal Control of Nuclear Spins in 87Sr for Quantum Logic with Qudits Sivaprasad Omanakuttan, Anupam Mitra, Michael J Martin, Ivan Deutsch Quantum optimal control is a powerful tool for the robust realization of quantum information processing tasks such as preparation of nonclassical quantum states and implementation of unitary maps. We studied quantum optimal control of the the spin I=9/2 nucleus of 87Sr, an alkaline earth atom that has attracted substantial recent attention for metrology, quantum simulation, and quantum computing. By employing nuclear spin magnetic resonance in the presence of a laserinduced nonlinear AC Stark shift, the system is controllable; we can design any SU(10) unitary matrix acting on the d=10 dimensional manifold of nuclear magnetic sublevels. We design control waveforms that generate the fundamental gates required for universal qudit logic gates. We also study experimental tradeoffs including the affects of decoherence and robustness to imperfections. 
Tuesday, March 16, 2021 1:42PM  1:54PM Live 
F30.00010: Dynamical mitigation of errors due to nonnegligible interactions in multiqubit system XiuHao Deng In current architecture of quantum computing, interaction between qubits needs to be turned on and off efficiently in a multiqubit system. Therefore, to perform singlequbit gates the interaction between the target qubit and the other qubits (spectators) must be turned off completely. While to perform twoqubit gates the interaction between the targeted two qubits must be turned on, and the interactions with the spectators must be remained off. A large on/off ratio of the tunable interaction is favorable. However, in fact, there is always residual interactions which introduces errors. We develop a framework including analytical protocol together with numerical method to dynamically mitigate such errors. Our method could be used to find robust control pulses to perform both single qubit gates and two qubit gates at the presence of unwanted interactions with other qubits. Further more, using our method one could perform perfect singlequbit rotations to a target qubit with strong alwayson interaction to spectator qubits. This gives an alternative solution for quantum computing with fixed frequency qubits. 
Tuesday, March 16, 2021 1:54PM  2:06PM Live 
F30.00011: Characterization of Parametric Entangling Gates on a MultiQubit Quantum Processor Larry Chen, Ravi K. Naik, John Mark Kreikebaum, David Ivan Santiago, Irfan Siddiqi Twoqubit gates activated via a parametrically driven, fluxtunable coupler are a promising approach to engineering fast, highfidelity entanglement without the potential sacrifice in qubit coherence that comes with a fully tunable architecture. Fluxmodulated couplers can also enable a universal hardwarenative gateset — an invaluable resource for reducing circuit depth and improving algorithm performance on noisy intermediate scale quantum (NISQ) devices. However, realizing a coherencelimited gate fidelity remains a challenge. In this work, we experimentally investigate the sources of coherent error that limit the parametric gate fidelity in a multiqubit device. Through identifying these error mechanisms, we explore a physically motivated, systematic approach to calibrating a continuous hardwarenative gateset for efficient nearterm algorithms. 
Tuesday, March 16, 2021 2:06PM  2:18PM Live 
F30.00012: Towards stabilizing manybody interacting flatstates in circuit QED Basil Smitham, Christie Chiu, Maria Mucci, Xi Cao, Michael Jonathan Hatridge, Andrew Houck In circuit quantum electrodynamics, systems with many types of energy spectra can be engineered, by coupling superconducting qubits and resonators. Arranging the components in certain geometries can lead to the presence of degenerate eigenstates, which  in a large chain  forms a dispersionless (or “flat”) band. In a flat band with nonlinearity (provided by transmon qubits), an external drive can populate strongly correlated quantum states. In this talk, we discuss experimental and theoretical progress towards realizing a circuit QED Lieb chain, a system with a gapped flat band that can be driven into a density wave state. We also discuss drivendissipative schemes that may allow us to stabilize strongly correlated quantum states, in particular one that makes use of a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement). 
Tuesday, March 16, 2021 2:18PM  2:30PM On Demand 
F30.00013: Largescale entanglement in superconducting quantum devices Gary Mooney, Gregory White, Charles Hill, Lloyd C. L. Hollenberg The ability to prepare sizeable multiqubit entangled states with full qubit control is a benchmark for the development of nearterm quantum computers. We demonstrate two forms of entanglement within IBM Quantum devices: entanglement in the sense of inseparability with fixed qubit bipartitions, and the stricter genuine multipartite entanglement (GME) which is the inability to express the state as a mixture of only product pure states. A graph state was prepared along a line on all 20 qubits of the ibmq_poughkeepsie device. Each neighbouring pair of qubits was found to be inseparable, indicating entanglement across the device. On the 65qubit ibmq_manhattan device, a native graph state was prepared and entanglement was detected for all neighbouring pairs of qubits. A parity verification method was implemented for GreenbergerHorneZeilinger states on the 27qubit ibmq_montreal device and an average fidelity increase of up to 0.056+0.023 was observed. Additionally, a fidelity of 0.61+0.05 was measured on a 27qubit state demonstrating GME across the entire device. 
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