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 S30: Superconducting Qubits and CouplersFocus Live
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Sponsoring Units: DQI Chair: Matthew Ware, BBN Technology - Massachusetts |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S30.00001: Fast Tunable Coupler Architecture for Fixed Frequency Transmons Jiri Stehlik, David Zajac, Devin Underwood, Timothy Phung, Muir Kumph, John Blair, Santino Carnevale, Dave Klaus, April Carniol, George Keefe, Matthias Steffen, Oliver E. Dial The limits of fixed coupling architectures for superconducting quantum computation have long been recognized. This has led to the development of various tunable coupling schemes [1,2]. In this talk we will explore a tunable capacitive coupler, which interferes a direct coupling capacitance with coupling through a tunable qubit. Unlike previous proposals [2], we place the direct capacitance between opposite islands of the transmons compared to the coupling qubit. This arrangement allows us to operate with the coupler frequency below the qubits and thus in a regime with reduced noise susceptibility and a better on/off ratio compared to previously published designs [2]. With this coupler we demonstrate operation of cPhase gates in low detuning regimes and further demonstrate a two qubit CZ gate with 99.8% fidelity. |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S30.00002: Spectators Errors in Multiqubit Tunable Coupling Architectures David Zajac, Jiri Stehlik, Devin Underwood, Timothy Phung, Muir Kumph, John Blair, Santino Carnevale, Dave Klaus, April Carniol, George Keefe, Matthias Steffen, Oliver E. Dial The addition of tunable couplers to superconducting quantum architectures offers significant advantages for scalability compared to fixed coupling approaches. Couplers such as the one proposed in Ref. [1] allow exact cancellation of qubit-qubit coupling by interfering the coupling through a capacitor with the coupling through a tunable qubit. However, the cancellation is necessarily narrowband, and in a multiqubit environment with stray couplings the cancellation condition can change depending on which gate is operating and even the states of the qubits. Here we investigate a modified version Ref. [1] coupler in multiqubit environment. With near-detuned qubits, we find that stray coupling can induce gate errors of order 1% when performing simultaneous gates if the microwave crosstalk is not carefully engineered. |
Thursday, March 18, 2021 11:54AM - 12:06PM Live |
S30.00003: A high-fidelity, two-qubit cross-resonance gate using interference couplers Abhinav Kandala, Xuan Wei, Srikanth Srinivasan, Easwar M Magesan, Santino Carnevale, George Keefe, Dave Klaus, Oliver E. Dial, David McKay Improving two-qubit gate performance and suppressing crosstalk are major challenges for building fault-tolerant quantum computers. In this talk, we will discuss a novel, fixed-frequency, multi-element coupling architecture for transmon qubits. This architecture features enhanced J exchange interaction strength and intrinsic idle ZZ suppression. We observe no degradation of qubit coherence (T1,T2 > 100μs) and measure a J/ZZ ratio greater than 100. Using the cross-resonance interaction we demonstrate a novel 200ns single-pulse CNOT gate and measure a gate fidelity of 99.77% (2.3e-3 gate error) from interleaved randomized benchmarking. |
Thursday, March 18, 2021 12:06PM - 12:42PM Live |
S30.00004: Advances in gates with tunable qubits and tunable couplers Invited Speaker: Brooks Foxen Quantum algorithms offer a dramatic speedup for computational problems in material science and chemistry. However, any near-term realizations of these algorithms will need to be optimized to fit within the finite resources offered by existing noisy hardware. Here, taking advantage of the adjustable coupling of gmon qubits, we demonstrate a continuous two-qubit gate set that can provide a threefold reduction in circuit depth as compared to a standard decomposition. We implement two gate families: an imaginary swap-like (iSWAP-like) gate to attain an arbitrary swap angle, θ, and a controlled-phase gate that generates an arbitrary conditional phase, φ. Using one of each of these gates, we can perform an arbitrary two-qubit gate within the excitation-preserving subspace allowing for a complete implementation of the so-called Fermionic simulation (fSim) gate set. We benchmark the fidelity of the iSWAP-like and controlled-phase gate families as well as 525 other fSim gates spread evenly across the entire fSim(θ,φ) parameter space, achieving a purity-limited average two-qubit Pauli error of 3.8×10−3 per fSim gate. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S30.00005: Scalable quantum computer with superconducting circuits in the ultrastrong coupling regime Roberto Stassi, Mauro Cirio, Franco Nori So far, superconducting quantum computers have certain constraints on qubit connectivity, such as nearest-neighbor couplings. To overcome this limitation, we propose a scalable architecture to simultaneously connect several pairs of distant qubits via a dispersively coupled quantum bus. The building block of the bus is composed of orthogonal coplanar waveguide resonators connected through ancillary flux qubits working in the ultrastrong coupling regime. This regime activates virtual processes that boost the effective qubit–qubit interaction, which results in quantum gates on the nanosecond timescale. The interaction is switchable and preserves the coherence of the qubits. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S30.00006: Entangling gates at dynamical sweet spots. Part 1: Theory. Joseph Valery, Shoumik Chowdhury, Nicolas Didier Scaling up superconducting quantum processors with optimized performance requires a sufficient flexibility in the choice of operating points for single and two qubit gates to maximize their fidelity and cope with imperfections. Flux control is an efficient technique to manipulate the parameters of tunable qubits, in particular to activate entangling gates. At flux sensitive points of operation, the ubiquitous presence of 1/f flux noise however gives rise to dephasing by inducing fluctuations of the qubit frequency. We show how two-tone modulation of the flux bias, a bichromatic modulation, gives rise to a continuum of dynamical sweet spots where dephasing due to slow flux noise is suppressed to first order for a wide range of time-averaged qubit frequencies. The qubits can be operated at these dynamical sweet spots to realize protected entangling gates and to avoid collisions with two-level-system defects. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S30.00007: Entangling gates at dynamical sweet spots. Part 2: Experiment. Joseph Valery, Shoumik Chowdhury, Nicolas Didier Current superconducting quantum processors require methods of performing two-qubit entangling operations that are robust to material defects and imperfect parameter targeting. Known techniques for achieving high fidelity gates include modulation of magnetic flux to control tunable transmon frequencies and activate sideband interactions between capacitively-coupled neighbors. However, viable operating points for these gates have historically been limited to specific modulation amplitudes at which there is first-order protection against dephasing due to 1 / f flux noise. As a result, there is little freedom to select time-averaged detunings that avoid unintended frequency collisions. This restriction can be relaxed, however, by introducing a second flux modulation frequency [1]. We show that by varying the mixing parameters and frequency relationship of the two tones, we can use these bichromatic pulses to maintain qubit insensitivity to flux noise across a wide continuum of effective frequencies, greatly improving the gate scheme’s robustness to defects and gate collisions. The presentation will review experimental results achieved via this novel control technique. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S30.00008: Demonstration of an All-Microwave Controlled-Phase Gate between Far-Detuned Qubits Sebastian Krinner, Philipp Kurpiers, Baptiste Royer, Paul Magnard, Ivan Tsitsilin, Jean-Claude Besse, Ants Remm, Alexandre Blais, Andreas Wallraff A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit-qubit coupling strength, and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and maintain qubit coherence during the gate, but suffer from frequency crowding and limited addressability in multi-qubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transmon qubits which are far detuned compared to the qubit anharmonicity [1]. The gate is activated by applying a single, strong microwave tone to one of the qubits, inducing a coupling between the two-qubit |f,g〉 and |g,e〉 states. We model the gate in presence of the strong drive field using Floquet theory. Our gate could have hardware scaling advantages in large-scale quantum processors as it neither requires additional drive lines nor tunable couplers. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S30.00009: Demonstration of Entangling Gate for All-to-All Connected Superconducting Qubits Marie Lu, Jean-Loup Ville, Joachim Cohen, Alexandru Petrescu, Sydney Schreppler, Alexei N Marchenkov, William Livingston, Archan Banerjee, John Mark Kreikebaum, Alexandre Blais, David Ivan Santiago, Irfan Siddiqi Exploring highly connected networks of qubits is invaluable for implementing various quantum codes and simulations. All-to-all connectivity allows for entangling qubits with reduced gate depth. In the ion community, the Mølmer-Sørensen gate is routinely used to entangle over a dozen qubits with high fidelity. We demonstrate a Mølmer-Sørensen-like interaction through the use of shared coplanar waveguide (CPW) resonators to couple multiple superconducting qubits and analyze the effect of qubit lifetimes in the Rabi dressed frame on our gate fidelity. |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S30.00010: Investigating the speed limit of two-qubit entangling gates with superconducting qubits Joel Howard, Junling Long, Mustafa Bal, RUICHEN ZHAO, Haozhi Wang, Tongyu Zhao, David Pappas, Zhexuan Gong, Meenakshi Singh Fast two-qubit entangling gates are essential for quantum computers with finite coherence times. Due to the limit of interaction strength among qubits, there exists a theoretical speed limit for a given two-qubit entangling gate. This speed limit has been explicitly found only for a two-qubit system and under the assumption of negligible single qubit gate time. We seek to demonstrate such a speed limit experimentally using two superconducting transmon qubits with a fixed capacitive coupling. Moreover, we investigate a modified speed limit when single qubit gate time is not negligible, as in any practical experimental setup. Finally, we present a three-qubit device design aimed at demonstrating that coupling to additional qubits can significantly increase the speed limit of a two-qubit entangling gate, thus requiring the co-design of the quantum computer from both theorists and experimentalists for optimal gate performance. |
Thursday, March 18, 2021 1:54PM - 2:06PM Live |
S30.00011: Novel Coupling for RIP Gate Based Devices Muir Kumph, James J Raftery, Will Shanks, Aaron Finck, John Blair, George Keefe, Santino Carnevale, Vincent Arena, Shawn Hall, Dave Klaus, Oliver E. Dial Resonator Induced Phase (RIP) gates are an intriguing way to command many-bodied interactions in a superconducting-qubit based device. Here we present a novel RIP-coupler based 6 qubit device with 2D square lattice connectivity to demonstrate two-qubit gates with a fidelity exceeding 98.8% running algorithmic benchmarks. Theoretical and experimental limits of the RIP gate are also discussed. |
Thursday, March 18, 2021 2:06PM - 2:18PM Live |
S30.00012: High-fidelity controlled-Z gate with maximal intermediate leakage operating at the speed limit in a superconducting quantum processor Hany Ali, Victor Negirneac, Nandini Muthusubramanian, Francesco Battistel, Ramiro Sagastizabal, Miguel S Moreira, Jorge Marques, Wouter Vlothuizen, Marc Beekman, Nadia Haider, Alessandro Bruno, Leonardo DiCarlo Simple tuneup of high-fidelity two-qubit gates is essential for the scaling of quantum processors. Here, we introduce the sudden variant (SNZ) of the Net Zero scheme realizing high-fidelity, repeatable controlled-Z (CZ) gates by baseband flux control of transmon frequency. SNZ achieves CZ gates at the speed limit of transverse coupling between computational and non-computational states by maximizing intermediate leakage. Beyond speed, the key advantage of SNZ over fast-adiabatic approaches is tuneup simplicity, owing to the regular structure of conditional phase and leakage as a function of two control parameters. We realize SNZ CZ gates in a multi-transmon processor, reaching 99.87±0.27% fidelity with 0.15 ± 0.02% leakage. We use numerical simulations with experimental input parameters to dissect the error budget and compare SNZ to conventional NZ , finding SNZ to outperform. SNZ is compatible with scalable schemes for quantum error correction and adaptable to arbitrary conditional-phase gates useful in NISQ applications. |
Thursday, March 18, 2021 2:18PM - 2:30PM Live |
S30.00013: Superconducting-qubit coupler design with exponentially large on-off ratio Catherine Leroux, Agustin Di Paolo, Alexandre Blais We present a design for a superconducting-qubit coupler featuring an exponentially large on-off ratio that is controlled by the amplitude of a microwave drive. In this architecture, the tunable bus interacts with a driven mode such as to, on demand, exponentially suppress the qubit-qubit virtual interactions with respect to the amplitude of the external control field. We demonstrate how this scheme can be used to reduce residual cross-Kerr (or~ZZ) interactions in transmon-qubit-based quantum processors. We also provide a superconducting-circuit implementation of the coupler, which can enable high-fidelity parametric two-qubit gates and help to reduce cross-talk. |
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