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 X31: Novel QubitsFocus Live
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Sponsoring Units: DQI Chair: Vivekananda Adiga; David Zajac, IBM TJ Watson Research Center |
Friday, March 19, 2021 8:00AM - 8:12AM Live |
X31.00001: Towards High-Fidelity Gates in the Soft Zero-Pi Qubit Anjali Premkumar, Andras Gyenis, Xanthe Croot, Pranav Mundada, Agustin Di Paolo, Jens Koch, Alexandre Blais, Andrew Houck The zero-pi qubit [1, 2] is intrinsically protected against relaxation and dephasing, making it a promising candidate for high-fidelity quantum processing. A recent implementation of the “soft” zero-pi circuit showed experimental evidence of protection and achieved logical operations via a Raman transition [3]. Here, we address some outstanding challenges facing the soft zero-pi qubit. For example, we achieve faster gates via a fast-flux drive through an inductively-coupled bias line. The improvements we made allow for longer coherence times and higher fidelity gates. |
Friday, March 19, 2021 8:12AM - 8:24AM Live |
X31.00002: Design Progress for Tunable Current Mirror Qubits David Ferguson, Moe S Khalil Superconducting qubits with intrinsic noise protection have made considerable advances in the last few years. Here we report on new design innovations for tunable current mirror qubits including circuits that allow novel forms of readout and coupling between qubits. |
Friday, March 19, 2021 8:24AM - 8:36AM Live |
X31.00003: Protected C-Parity Qubits Part 1: Characterization and Protection Kenneth Dodge, Yebin Liu, Brad Cole, Jaseung Ku, Michael Senatore, Abigail Shearrow, Shaojiang Zhu, Sohair Abdullah, Andrey Klots, Lara Faoro, Lev B Ioffe, Robert McDermott, Britton Plourde Superconducting qubits with topological protection against local noise hold the promise of significantly enhanced coherence times and higher gate fidelities than is possible with conventional qubits. We are developing one such protected design — the hybrid charge-parity qubit — that combines arrays of compact, high inductances and conventional Josephson junctions with individual flux control for tuning each plaquette to a regime with a π-periodic Josephson energy. With this scheme, concatenating multiple π-periodic elements further enhances the degree of protection. Here, we will describe our experimental characterization of devices with concatenated π-periodic elements through spectroscopy and time-domain measurements. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X31.00004: Protected C-Parity Qubits Part 2: Gate Operations Abigail Shearrow, Shaojiang Zhu, Sohair Abdullah, Kenneth Dodge, Yebin Liu, Bradley G. Cole, Jaseung Ku, Michael Senatore, Andrey Klots, Lara Faoro, Lev B Ioffe, Britton Plourde, Robert McDermott Topologically protected qubits are an area of growing interest and active research, with the potential for orders-of-magnitude improvements in coherence. One such protected qubit is the charge-parity (C-parity) qubit, which consists of a pi-periodic Josephson element shunted by a large capacitance. In this talk we describe the implementation of protected gate operations in the C-parity qubit. Operations involve the modulation of capacitive and inductive shunt elements using SQUID switches that are integrated into the qubit device. We describe the fabrication and characterization of C-parity qubits involving integrated SQUID switches, and we discuss the path to experimental implementation of protected operations. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X31.00005: Modular and compact designs for multi-qubit devices in 2D and 3D cQED architecture Anirban Bhattacharjee, Madhavi Chand, Sumeru Hazra, Kishor V Salunkhe, Gaurav Bothara, Meghan P Patankar, R Vijay Although not highly scalable, the 3D cQED architecture provides a more modular design for few qubit devices. On the other hand, the 2D architecture lacks modularity since both qubit and resonators are defined on a single chip. This often puts stringent requirements on the fabrication of 2D chips as variability in Josephson junction parameters can render the entire chip unusable. |
Friday, March 19, 2021 9:00AM - 9:36AM Live |
X31.00006: Implementation of Protected Qubits with π-periodic Josephson Elements Invited Speaker: Britton Plourde The coherence times of superconducting qubits have undergone tremendous growth over the past two decades through a combination of materials science efforts and improvements in device design and measurement. Moving significantly beyond the current state-of-the-art coherence achievable with transmon qubits will require new device designs. Circuits that have a natural topological protection against decoherence due to local noise are particularly attractive. The 0-π qubit was an early example of such a protected design, with a combination of Josephson junctions and superinductors resulting in an effective π-periodic Josephson element. However, the challenging requirements for extremely large inductances and tight tolerances on parameter uniformity have made the original 0-π design difficult to implement. More recently, the charge-parity qubit design, composed of chains of concatenated imperfect π-periodic Josephson elements, offers a more feasible approach to achieving protection. In this case, the level of protection against local noise increases exponentially with the number of concatenated π-periodic elements. I will present progress with the implementation and characterization of these devices and the pathway to high-fidelity protected gate operations. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X31.00007: Improved resonator measurement using 1-port cryogenic calibration Haozhi Wang, Suren Singh, Corey Rae H McRae, Joseph Bardin, Josh Y Mutus, David Pappas Loss measurements are critical for the superconducting qubit community as the coherence of quantum systems are limited by loss through energy relaxation. Due to geometry limitations, many 3D cavities as well as quantum limited amplifiers, are measured in reflection. However, there’s no known method to correct errors caused by impedance mismatch in Qi extraction for reflection mode. Thus, a calibration down to the device under test is necessary to improve measurement accuracy. In this talk, we will briefly discuss why we need calibration for reflection measurement and then show the difference of before and after applying the 1-port databased SOL calibration introduced in the earlier talk in experiment of measuring both 3D and 2D over-coupled resonators. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X31.00008: Magnifying quantum phase fluctuations with Cooper-pair pairing Clarke Smith, Marius Villiers, Antoine Marquet, José Palomo, Matthieu Delbecq, Takis Kontos, Phillipe Campagne-Ibarcq, Benoit Douçot, Zaki Leghtas Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. The inductive wires contribute a parabolic confinement, while the tunnel junctions add a cosinusoidal corrugation. Usually, the ground state wavefunction is localized within a single potential well—that is, quantum phase fluctuations are small—although entering the regime of delocalization holds promise for metrology and qubit protection. A direct route is to loosen the inductive confinement and let the ground state phase spread over multiple Josephson periods, but this requires a circuit impedance vastly exceeding the resistance quantum and constitutes an ongoing experimental challenge. Here we take a complementary approach and fabricate a generalized Josephson element that can be tuned in situ between one- and two-Cooper-pair tunneling, doubling the frequency of the corrugation and thereby magnifying the number of wells probed by the ground state. We measure a tenfold suppression of flux sensitivity of the first transition energy, implying a twofold increase in the vacuum phase fluctuations. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X31.00009: Ultrahigh-impedance suspended Josephson circuits for quantum computing, simulations, and metrology Ray Mencia, Roman Kuzmin, Ivan V Pechenezhskiy, Vladimir Manucharyan Chains of Josephson junctions probably have the largest kinetic inductance per-unit-length, which can exceed the geometric one by about 104, primarily limited by quantum phase-slip fluctuations. However, the total inductance is also limited by the stray capacitance, which grows linearly with the chain length. This stray capacitance is unnecessarily large in most circuits because of the high dielectric constant of silicon or sapphire substrates. By releasing Josephson chains off the substrate, we can combine the maximal per-unit-length inductance with the minimal stray capacitance, thereby obtaining the highest impedance electromagnetic structures available today. As a first demonstration, we created a superconducting quasicharge qubit [1] (blochnium), a dual of transmon, made of a weak junction shunted by such a large inductance (hyperinductance) whose impedance reaches over 30 × RQ (200 kΩ). In the second demonstration, we fabricated suspended "telegraph" transmission lines, composed of 5,000+ junctions, whose wave impedance exceeds 5 × RQ (32.5 kΩ). These lines are a unique resource in exploring DC current metrology via Bloch oscillations, as well as in analog quantum simulations of many strongly-correlated 1D systems. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X31.00010: Inductively shunted transmon qubit for ZZ interaction cancellation Kun Zuo, Yoshiro Urade, Zhiguang Yan, Shuhei Tamate, Yutaka Tabuchi, Hirotaka Terai, Yasunobu Nakamura To improve the cross-resonance (CR) gate fidelity between superconducting qubits, unwanted ZZ interaction has to be eliminated. Previously, it has been demonstrated that combining a capacitively shunted flux qubit (CSFQ) and a transmon can suppress the ZZ interaction when the anharmonicities of the two qubits are exactly opposite. However, this implementation has several challenges such as the CSFQ requires an external magnetic flux bias and it is difficult to achieve controlled anharmonicity at the flux-noise insensitive point. In light of this, here we present the so-called inductively shunted transmon that has a positive anharmonicity for the purpose of ZZ cancellation. The proposed qubit is easy to fabricate since it contains a single Josephson junction with junction parameters similar to that of the coupled transmon; Both frequency and anharmonicity of such qubit can be easily modelled and well controlled, enabling seamless integration with current CR-gate systems. Furthermore, by including a pi-junction or by trapping a flux in the qubit loop with a gradiometric design, one can fully eliminate the external magnetic flux for scaling up. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X31.00011: Investigating unwanted transitions in dispersive qubit measurement at high readout power Kishor V Salunkhe, Suman Kundu, Nicolas Gheeraert, Meghan P Patankar, R Vijay In the cQED architecture, the Josephson parametric amplifier (JPA) improves the signal to noise ratio (SNR) and enables high fidelity measurement. Improvement in the SNR is also possible by increasing the readout power and integration time, but unwanted transitions within and outside the computational subspace limit the fidelity. We investigate a multimodal circuit [1], nicknamed the “quantromon”, with cross-Kerr coupling between the modes: a split transmon qubit and a linear cavity. We measured a fidelity of 97.63% for 1.2 us integration time without using a JPA while the best fidelity obtained with a JPA was 99.37% for a 170 ns integration time. The fidelity in both cases was limited by unwanted transitions. Numerical simulations suggest that at high readout power, these transitions are suppressed for cross-Kerr coupling when compared with a transmon transversely (σxσx) coupled to a readout cavity. In the quantromon, the asymmetry between the two Josephson junctions introduces a small transverse coupling between the qubit mode and cavity mode. We theoretically and experimentally investigate the effect of this residual transverse coupling on readout fidelity with the help of a quantromon device with tunable asymmetry. |
Friday, March 19, 2021 10:36AM - 10:48AM On Demand |
X31.00012: Investigation of databased calibration for 1-Port cryogenic measurements Suren Singh, Haozhi Wang, Josh Y Mutus, David Pappas, Corey Rae H McRae, Joseph Bardin This talk will cover the application of the databased standards for the calibration of a vector network analyzer when used to measure devices at millikelvin temperatures, specifically 3D cavities measured in reflection. In this talk we will start with a basic understanding on the systematic errors that are inherent in a VNA measurement and classical methodology for correction of these errors. The next part of the talk will discuss the application of these error correction methods as applied to a distributed network analysis configuration used with a dilution refrigerator. As part of this discussion we will present results for a databased calibration versus using traditional polynomial calibration method. The focus of these results will be evaluation of a 1-Port SOL calibration and measurement of a 3D cavity and standard short as verification devices. |
Friday, March 19, 2021 10:48AM - 11:00AM On Demand |
X31.00013: Electronically-tunable quantum phase slips in voltage-biased superconducting rings Ahmed Kenawy, Wim Magnus, Bart Soree Superconducting qubits hinge on the coherent tunneling of Cooper pairs across the insulating barrier of a Josephson junction, two superconducting electrodes separated by an insulating barrier. For example, in a superconducting loop interrupted by a thin insulator, the Cooper-pair tunneling coherently couples the macroscopic flux states of the loop, giving rise to the simplest implementation of a flux qubit, the rf SQUID. Likewise, the superconducting loop can be interrupted by a nanowire where, due to its small cross-sectional area, quantum-phase-slip rates in the gigahertz regime can be achieved, giving rise to a phase-slip flux qubit. Here, we present the use of a bias voltage across a superconducting loop to electrostatically induce a weak link, thereby enhancing the rate of quantum phase slips without physically interrupting the loop. Our Ginzburg-Landau simulations show that the bias voltage modulates the energy barrier separating the adjacent flux states of the loop, suggesting a route towards a phase-slip flux qubit whose transition frequency is electronically tunable. |
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