Bulletin of the American Physical Society
APS March Meeting 2023
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session Y64: Experiments on Current Noisy Quantum Hardware IFocus

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Sponsoring Units: DQI Chair: A. Baris Ozguler, Fermilab Room: Room 415 
Friday, March 10, 2023 8:00AM  8:12AM 
Y64.00001: Performance of Robust, HighOrder Dynamical Decoupling Sequences on Superconducting Quantum Hardware Amy F Brown, Vinay Tripathi, Bram Evert, Alexander D Hill, Xian Wu, Yuan Shi, Yujin Cho, Max D Porter, Vasily I Geyko, Ilon Joseph, Jonathan L DuBois, Eyob A Sete, Matthew J Reagor, Daniel A Lidar

Friday, March 10, 2023 8:12AM  8:24AM 
Y64.00002: Characterizing pulse distortions and crosstalks for superconducting qubits Zhang Jiang, ZePei Cian, Jonathan A Gross, Dripto M Debroy, Andre Petukhov Characterizing pulse distortions and crosstalks is essential to building scalable quantum computers. However, quantum chips are often operated at ultra low temperatures and insulated from the environment. This prevents one from direct characterizing the control electronics at the qubit level. We use superconducting qubits as a probe to estimate pulse distortions and crosstalks. Compared to former methods, ours does not require knowing a priori the qubit response as a function of the control pulse strengths. Moreover, its precision is not limited by the finite widths of the control pulses. 
Friday, March 10, 2023 8:24AM  8:36AM 
Y64.00003: Characterization and benchmarking of a phasesensitive twoqubit gate using direct digital synthesis Mats Tholen, Riccardo Borgani, David B Haviland, Jonas Bylander, Christian Križan We implemented an iSWAP gate with two transmon qubits using a fluxtunable coupler. Precise control of the relative phase of the qubitcontrol pulses and the parametric coupler drive was achieved with a multichannel instrument called Presto [1], using direct digital synthesis (DDS). We describe the process of tuning and benchmarking the iSWAP gate, where the relative phase of the pulses is controlled via software. Traditional iSWAP gates require multiple local oscillators and mixers with advanced synchronization methods based on external triggering. Presto synchronizes and generates all outputs using twostep digital mixing, where pulse envelopes are defined as arbitrary waveforms at 1 GS/s. In the first step the envelopes are mixed with a digital carrier where phase (and frequency) can be set arbitrarily each time a pulse is generated. In the second step the signal is upconverted to the sampling rate of the converter and digitally mixed to the target frequency. With Presto we perform the iSWAP gate in 254 ns, validate it with quantumstate tomography, and evaluate its fidelity with interleaved randomized benchmarking. 
Friday, March 10, 2023 8:36AM  9:12AM 
Y64.00004: Quantum Computing Systems and Results for Hybrid Quantum Classical Algorithms Invited Speaker: Matthew J Reagor In this talk, I will summarize recent advances for hybrid quantumclassical computer design and application benchmarking at Rigetti, including for our 80 qubit scale machines. I will highlight the role of noise tailoring, error mitigation, and speed to achieve performance at scale. 
Friday, March 10, 2023 9:12AM  9:24AM 
Y64.00005: Robust hardwareefficient implementation of a continuous twoqubit gate set for transmon qubits Colin Scarato, Christoph Hellings, Kilian Hanke, Ants Remm, Stefania Lazar, Dante Colao Zanuz, Michael Kerschbaum, Nathan Lacroix, Johannes Herrmann, Francois Swiadek, Graham J Norris, Mohsen Bahrami Panah, Alexander Flasby, Christopher Eichler, Andreas Wallraff Hardwareefficient implementations of continuous gate sets, such as the set of twoqubit controlledphase gates parameterized by the conditional phase, can improve the performance of noisy intermediatescale quantum computing by reducing the circuit depth, for example in variational quantum algorithms and quantum machine learning. In a previous experimental demonstration [1], such a continuous gate set was implemented on superconducting transmon qubits, but relied on pulse shapes that are sensitive to distortions and required adjusting multiple control pulse parameters simultaneously. Here, we present an alternative implementation by extending the controlledphase gate from [2], which is based on the resonant interaction between two fluxtunable transmons. In this implementation, an arbitrary conditional phase can be achieved by tuning a single pulse parameter, and the vanishing time integral of the employed netzero control pulses [2] provides robustness against memory effects. Furthermore, by activating the gate via flux control of both qubits, we demonstrate that the gate can be performed between fardetuned qubits, strongly suppressing residual interactions. We characterize the continuous gate set with crossentropy benchmarking for fixed values of the conditional phase and for phases randomly drawn from a uniform distribution, confirming a consistently high gate fidelity over the full range of phases. 
Friday, March 10, 2023 9:24AM  9:36AM 
Y64.00006: Experimental implementation and characterization of a virtual twoqubit gate Akhil P Singh, Kosuke Mitarai, Yasunari Suzuki, Kentaro Heya, Yutaka Tabuchi, Keisuke Fujii, Yasunobu Nakamura Quantum computers seem to be on a promising path to realize the capabilities which could prove to be more fast, secure, and efficient than their classical counterparts. However, the size of current quantum devices, termed as Noisy intermediatescale quantum (NISQ) devices, is strictly limited. The other major challenges faced by NISQ devices are finite noise and limited coherence times. In relation to the limited size issue, several techniques have been proposed to increase the effective size of the devices with additional classical processing costs. One such technique was proposed by K. Mitarai and K. Fujii [New J. Phys. 23 023021 (2021)], which constructs a general twoqubit gate from only singlequbit operations or referred here as a "virtual twoqubit" gate. This virtual twoqubit gate allows us to, for example, simulate the quantum circuit of 2N qubits by using only N physical qubits with sampling overhead when the goal of the quantum circuit is expectation value estimation. Hence, it enables us to expand the computing capabilities of NISQ devices in certain algorithms. In this work, we present the experimental demonstration and characterization of the "virtual CZ" gate. While local operations on each qubit involved in a virtual gate consist of singlequbit gates and measurements, measurement errors are usually much larger than singlequbit gate errors in experiments. Thus, we have also implemented measurement error mitigation to improve virtual gate fidelity. As a result, we experimentally achieved virtual CZ gate with average gate fidelity of 0.9975±0.0028. This technique helps us to obtain expectation values in "scaledup" quantum circuits, which are used in many quantum algorithms such as variational quantum eigensolver and others. 
Friday, March 10, 2023 9:36AM  9:48AM 
Y64.00007: Quasiparticle cooling via engineered dissipation in Floquet quantum circuits Xiao Mi, Alexios Michailidis, Sara Shabani, Jerome Lloyd, Vadim Smelyanskiy, Dmitry Abanin We introduce an approach to realise nontrivial steady states in systems of many superconducting qubits out of equilibrium. The approach combines periodic application of unitary gates, which implements Floquet evolution of the system, and ancilla qubits that are periodically reset. The coupling of ancillas to the system realises a dissipative channel engineered to extract quasiparticle excitations from the system. The quasiparticles are defined in a general case by the Floquet circuit, with Hamiltonian evolution being a particular example. This drives the system to a steady state where quasiparticle occupation numbers are low, despite the uncontrollable weak noise inevitably present in the device. We demonstrate applications ranging from manybody states with low energy density to quantum transport in correlated systems. Effect of external weak noise on the steady state properties, including quasiparticle occupation numbers and entanglement, is analyzed. This work provides a scalable experimental path to realising longlived entangled states in noisy intermediatescale quantum devices. 
Friday, March 10, 2023 9:48AM  10:00AM 
Y64.00008: Quantum Zeno effect in presence of engineered dissipation. Vidul R Joshi, Akshay Koottandavida, Alessandro Miano, Rodrigo G Cortiñas, Christopher Wang, Benjamin J Chapman, Michel H Devoret The quantum Zeno effect manifests itself as the freezing of the quantum state of a system subject to repeated measurements. Decay processes, like the spontaneous emission of a photon from a cavity into the environment, are generally modelled using a Markovian environment, and , in that framework, they should not be submitted to the quantum Zeno effect. A decay process, however, can in principle be frozen if the system is measured fast enough such that the Markov approximation breaks. In this talk, we present experiments to demonstrate how the decay of a photon from a cavity into an engineered environment can be reduced by exploiting the nonMarkovian nature of the bath. The experiment is implemented in a circuit QED architecture, where a photon is initially placed in a 3D microwave cavity, and the bath is engineered such that the photon decays through another cavity into which it is swapped using a transmon qubit. Experimental investigation of the "noclick" evolution of the system when a photodetector is placed in this engineered loss channel will be presented. 
Friday, March 10, 2023 10:00AM  10:12AM 
Y64.00009: Characterization of nonMarkovian noise effects in superconducting qubits using pseudoidentity gates Ivan Rungger, Deep Lall, Abhishek Agarwal In superconducting quantum computers available today, interactions between qubits and twolevel system (TLS) defects in the device are known to be a significant source of noise. Qubitdefect interaction can manifest itself as nonMarkovian noise in the dynamics of the qubit subsystem. Existing methods to identify such effects involve lowlevel noise spectroscopy experiments. We develop a method based on mirrored pseudoidentity gates to characterise qubitTLS interactions, and include them in a noise model to describe the effect of the TLS defects on the quantum circuits. We run experiments on superconducting quantum computers and find that our method is well suited to characterize the interactions between TLS defects and a qubit, and that their presence is an important source of noise. We also use the method to characterize residual crosstalk interactions between neighbouring qubits. Including the nonMarkovian components within our noise model allows us to significantly improve the accuracy of the predictions of the noise model when compared to experiment. 
Friday, March 10, 2023 10:12AM  10:24AM 
Y64.00010: Towards Testing Quantum Mechanics on NISQ Devices and Characterizing nonMarkovian noise on IBM Quantum Computers Tsz Chun Wu, Kevin Slagle We work towards testing the prediction of the emergent quantum mechanics (EmQM) by benchmarking a recently proposed protocol [1] on IBM's quantum computers. EmQM hypothesizes that quantum mechanics is not fundamental but instead emerges from theory with less computational power. The protocol involves simulating a nonClifford quantum circuit with random 1qubit and 2qubit gates, followed by its inverse, and then measuring the fidelity. We also characterize the nonMarkovian noise in the quantum devices by examining the fidelity decay rate. The presence of nonMarkovian noise is indicated by a significant deviation from exponential decay at small circuit depth. 
Friday, March 10, 2023 10:24AM  10:36AM 
Y64.00011: Waveguide Variational Quantum Algorithms Cristian Tabares, Alberto Muñoz de las Heras, Luca Tagliacozzo, Diego Porras, Alejandro GonzalezTudela Variational Quantum Algorithms (VQAs) [1], which make use of a classical optimizer to train a parametrized quantum circuit (PQC), have emerged as the leading proposal to leverage stateoftheart quantum computers. However, they are still subjected to important constraints, such as scalability problems, undesired interactions between qubits and a limited connectivity, which motivate the search of new resources when designing PQCs. In this work we capitalize on the longrange interactions naturally appearing between quantum emitters coupled to waveguideQED setups [24] as a resource for designing a novel class of PQCs, which we benchmark against other hardwareagnostic PQCs. Remarkably, these longrange interactions introduce entanglement between qubits in a controlled way, which allows an efficient classical training while reducing the gate count and noise impact. Our results suggest that such waveguidemediated interactions can be a useful resource when designing VQAs. 
Friday, March 10, 2023 10:36AM  10:48AM 
Y64.00012: A codesign superconducting quantum circuit for simulating nanoscaleNMR systems HsiangSheng Ku, Daria Gusenkova, Jayshankar Nath, Nicola Wurz, Julia Lamprich, Stefan Pogorzalek, Florian Vigneau, Ping Yang, Frank Deppe, Antti Vepsäläinen, Alessandro Landra, Vladimir Milchakov, Caspar OckeloenKorppi, Wei Liu, Liuqi Yu, LanHsuan Lee, SeungGoo Kim, Hermanni Heimonen, Pedro Figueroa Romero, Manish Thapa, Inés de Vega The codesign strategy, where applicationspecific algorithms and hardware are developed in parallel, is a viable strategy for reaching quantum advantage with NISQ computation devices. For efficiently simulating problems requiring alltoall interactions, startypearchitectures require a minimal number of SWAP operations to map the algorithm connectivity onto the physical qubit topology. At IQM, we have designed and fabricated a startype superconducting quantum circuit, where a distributedelement resonator is used as centralcomponent. To operate the central resonator as an effective qubit, we have developed two types of qubitresonator operations: a SWAP operation for shuffling around excitations and a CZgate for algorithmic execution. In this talk, we present the experimental progress towards digitally simulating a nanoscale NMR system [1] – such as an NV center coupled to multiple nuclear spins – on such a circuit. 
Friday, March 10, 2023 10:48AM  11:00AM 
Y64.00013: Quantum reservoir computing with superconducting nonlinear oscillators Guilhem J Ribeill, Leonardo Ranzani, Graham Rowlands Quantum reservoir computing (QRC) is an emerging approach to quantum machine learning that harnesses the dynamics of a quantum system in order to perform computation. It is especially attractive in the era of small and noisy quantum devices as it is inherently hardwareefficient and relaxes the requirements on precise control of the system state. While many approaches to building a practical QRC have been proposed, the strong nonlinearities acheivable with superconducting quantum devices make them an attractive platform for implementing a quantum reservoir. Here, we present our development of a small quantum reservoir based on superconducting Kerr oscillators. We focus on theoertical and experimental investigations of its computational capacity, and discuss challenges in scaling up this technology towards applicationrelevant performance. 
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