Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S39: Superconducting Circuits: Modeling |
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Sponsoring Units: DQI Chair: Antonio Corcoles, IBM T J Watson Res Ctr Room: LACC 501B |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S39.00001: A Simple Impedance Formula for the Exchange Coupling Rates between Low Anharmonicity Qubit Modes in Superconducting Quantum Processors Firat Solgun, David DiVincenzo, Jay Gambetta For superconducting quantum processors consisting of low anharmonicity qubits such as transmons we give a complete microwave description of the system in the qubit subspace. We assume that the qubits are dispersively coupled to a distributed microwave structure such that the detunings of the qubits from the internal modes of the microwave structure are stronger than their couplings. We define “qubit ports” at the terminals of the Josephson junctions and “drive ports” where transmission lines carrying drive signals launch the chip and we obtain a multiport impedance response of the linear passive part of the system between the ports. We then relate interaction parameters in between qubits and between the qubits and the environment to the entries of this multiport impedance function: in particular we show that the exchange coupling rate J between qubits is a simple function of the off-diagonal entry connecting the qubit ports evaluated at the qubit frequencies. Similarly we relate couplings of the qubits to voltage drives and lossy environment to the entries connecting the qubits and the drive ports. Our treatment takes into account all the modes (possibly infinite) that is present in the distributed electromagnetic structure. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S39.00002: Design and Analysis of Superconducting Qubits for Extensible Surface Coding Nadia Haider, Stefano Poletto, Alessandro Bruno, David Michalak, Roman Caudillo, Nandini Muthusubramanian, Ramiro Sagastizabal, Brian Tarasinski, Cornelis Christiaan Bultink, Michiel Adriaan Rol, James Clarke, Leonardo DiCarlo We present the design and the finite-element electromagnetic analysis of superconducting qubits intended for an extensible surface-code architecture. Surface code, a promising architecture for fault-tolerant quantum computing, requires qubits with connectivity to all nearest neighbors. This interconnectivity, when combined with requirements for microwave and flux controllability, fast gating and readout, makes the design of superconducting qubits for surface coding challenging. We present the design, simulation, and experimental verification of several variants of seven-port high-coherence transmons. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S39.00003: Surface Current and DC Cross Talk in Superconducting Quantum Devices Alireza Najafi-Yazdi Unwanted electromagnetic interference between qubits, transmission lines, flux lines and other elements of a superconducting quantum processor poses a challenge in engineering such devices. This problem is exacerbated with scaling up the number of qubits. High fidelity, massively parallel computational toolkits, which can simulate the 3D electromagnetic environment and all features of the device, are instrumental in addressing this challenge. While RF crosstalk has to some extent been studied, DC crosstalk remains an intriguing topic to address. In this work, we investigated the DC surface current on a superconducting chip and crosstalk between various elements of a multi-qubit superconducting quantum processor. The simulations are carried out with a high fidelity, massively parallel EM solver developed at Anyon Systems. We will present our findings regarding the sources of crosstalk in the device, design guidlines, as well as numerical model setup. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S39.00004: A Machine Learning Approach to Superconducting Circuit Design Tim Menke, Florian Häse, Simon Gustavsson, Andrew Kerman, William Oliver, Alan Aspuru-Guzik Superconducting qubits are electrical circuits comprising inductors, capacitors, and Josephson junctions. Although circuit parameters may in principle assume a wide range of values, existing architectures generally span only a small subset of the available design space, and design variations are often based largely on experience and engineering intuition. However, future circuit architectures – such as radically new qubit designs or many-body couplers for adiabatic quantum computing – will likely extend beyond an intuitive regime. Towards this end, we have developed a numerical search engine that screens the parameter design space to achieve circuits with desired properties. The engine performs Bayesian optimization on a graphical model of arbitrary circuits with several nodes. We present novel circuit designs with targeted spectral properties, robustness to parameter variation, and noise sensitivity. We expect our work to facilitate the design of more complex and robust superconducting circuit architectures. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S39.00005: Effective Hamiltonian in superconducting qubits Mohammad Ansari Qubits with more than two energy levels, such as superconducting transmons, usually are externally driven in order to engineer one and two qubit gates. However due to the presence of higher excited levels the fidelity of the gates requires improvement. Such a system carries a large Hilbert space and recognizing effective qubits requires to use perturbation theory. This puts a lage limitation on the system parameters and interactions. We discuss a method that allows to go beyond regular perturbative limitations and separates classical effects from quantum fluctuations in the Hamiltonian of a weakly-anharmonic qubits. We compare results taken from applying Schrieffer-Wolff transformation, Least action principle, and our method. Our results will become practical tools for experimental efforts in circuit QED. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S39.00006: Shape Design Optimization for 3D Integrated Superconducting Quantum Processors Alireza Najafi-Yazdi, Guillaume Duclos-Cianci, Kevin Lalumiere As the number of qubits increase in superconducting quantum processors, so does the complexity and the number of design variables. Optimizing the design parameters, including shape and layout, is of crucial importance to the successful engineering and operation of complex quantum processors such as 3D integrated devices. Such optimization problems may involve a large number of geometric design parameters with several optimization goals such as maximizing signal integrity, maximize qubit and interconnect density while minimizing crosstalk. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S39.00007: Characterization of Transmon Qubit Gate Operations based on Time-dependent Simulations with Realistic Noise Michael O'Keeffe, Andrew Kerman, Kevin Obenland Fault tolerant quantum computation requires improving scalable control protocols for specific qubit implementations to mitigate noise-induced error. Simulations that accurately capture observed physical qubit behavior while scaling to multi-qubit circuit models provide a useful tool to address this challenge. We present results on time-dependent simulations of coupled transmon qubits. The model captures driven multi-level dynamics in and out of the computational subspace. Noise incorporated into the model includes fast and slow dephasing, excited-state decay, timing jitter, and phase noise. Process tomography of gates including these noise sources goes beyond standard models that typically make simplifying assumptions about the noise. This will enable analysis of small circuit encodings of quantum error correction and algorithms being proposed as demonstrations on near-term qubit devices. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S39.00008: Impact of Non-Local Electrodynamics on Flux Noise and Inductance of Superconducting Qubits Pramodh Senarath Yapa Arachchige, Tyler Makaro, Rogério de Sousa We present explicit numerical calculations of current density, inductance, and impurity-induced flux noise of simple superconducting devices in the non-local Pippard regime, which occurs when the Pippard coherence length is larger than the London penetration depth. In this regime the current density displays a peak away from the surface of the superconductor, signalling a breakdown of the usual approximation of local London electrodynamics with a renormalized penetration depth. Our calculations show that the internal inductance and the bulk flux noise power increases with increasing non-locality. In contrast, the kinetic inductance is reduced and the surface flux noise remains the same. As a result, spins in the bulk may dominate the flux noise in superconducting qubits in the Pippard regime, such as the ones using aluminum superconductors with large electron mean free path. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S39.00009: Quantum Langevin Equations for the Brune Multiport Hamiltonian Nicholas Materise, Frank Graziani, Keith Ray, Heather Whitley, Vincenzo Lordi Current models of superconducting qubit dynamics rely on Lindblad master equations expressed in the Schrödinger picture. Traditionally the Lindblad terms are lumped into relaxation and dephasing contributions, T1 and Tφ, respectively. The system is described as the qubit and electromagnetic environment bilinearly coupled to a thermal bath, most often as a collection of harmonic oscillators. We present a methodology for including multiple baths with linear and non-linear coupling between the system and bath operators in the Heisenberg picture. Quantum Langevin equations describe the evolution of the canonically conjugate variables of flux and charge in a superconducting circuit of qubits and linear passive circuit elements. We derive Langevin equations for the special case of a Brune multiport Hamiltonian bilinearly coupled to a harmonic bath and discuss our approach to couple other baths to this system, including the two-bath model where ensembles of two level systems are coupled to the system and lose their energy to a phonon bath. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S39.00010: Longitudinal Coupling for Fast QND Measurement: Numerical Study Mathieu Lachapelle, Jerome Bourassa, Alexandre Blais Dispersive qubit readout in circuit QED relies on transverse coupling to a microwave resonator. This type of coupling, however, leads to mixing between qubit and resonator eigenstates. In turn, this leads to Purcell decay and to a breakdown of the quantum non-demolition (QND) aspect of the measurement associated with the critical number photons. An alternative approach avoiding these two issues is longitudinal qubit-resonator coupling [1,2,3]. In this talk, we consider circuit realizations of this idea and perform realistic numerical simulations of the longitudinal measurement. We show that the measurement remains QND at higher power than can be expected under transverse coupling. Moreover, comparing transverse and longitudinal qubit readouts, we show that the latter leads to fast and high-fidelity QND measurements. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S39.00011: Systematic Perturbation Theory for Frequency and Lifetime Renormalization of Superconducting Qubits Moein Malekakhlagh, Alexandru Petrescu, Hakan Tureci We introduce two quantitatively consistent methods for perturbatively calculating the renormalization of both frequency and relaxation time for transmon-type superconducting qubits, relevant to recent experiments [1]. We consider a general situation where the qubit is embedded in an open linear electromagnetic environment, whose properties are determined by its Maxwell Green's function [2]. The two calculation strategies provide systematic corrections to the system poles to arbitrary order and are not limited by Hilbert space truncation. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S39.00012: T1 renormalization of transmon qubits versus drive power Hakan Tureci, Moein Malekakhlagh, Alexandru Petrescu Recent experiments in circuit QED with a transmon qubit have found a strong dependence of the qubit relaxation rate on the read-out drive power [1]. We discuss a plausible mechanism that is consistent with available experimental data up to intermediate drive powers (cavity occupation n < 7). |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S39.00013: Critical Slowing Down in Circuit QED Giovanna Tancredi, Paul Brookes, Themis Mavrogordatos, Andrew Patterson, Joseph Rahamim, Eran Ginossar, Marzena Szymanska, Peter Leek We explore an intermediate non-linear driving regime in circuit QED for a 3D microwave cavity coupled to a transmon qubit, both experimentally and theoretically. In this regime, we measure the response of the cavity to a step function drive pulse, with the qubit initialized in either its ground or excited state, and observe critical slowing down, i.e. a very slow approach to the steady state, due to quantum bistability. We find that the system reaches the steady state in a time much longer than both the individual qubit and cavity lifetimes. We characterise the critical slowing down as a function of driving frequency and power and find good agreement with simulations. This regime may be exploited to improve circuit-QED based qubit readout. |
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