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 R32: Qubit MeasurementFocus Live
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Sponsoring Units: DQI Chair: Alexander Opremcak, University of Wisconsin - Madison |
Thursday, March 18, 2021 8:00AM - 8:12AM Live |
R32.00001: Realizing large, tunable dispersive shifts with parametric couplings – Part I Zhihao Xiao, Taewan Noh, Emery Doucet, Leonardo Ranzani, Luke Govia, Raymond W Simmonds, Jose Aumentado, Archana Kamal Parametric couplings are being deployed in a variety of applications, ranging from parametric gates or state preparation to quantum annealing, that require much stronger parametric coupling strengths, unlike their traditional applications such as quantum-limited amplification. The theoretical framework to capture the effect of such strong time-dependent couplings, however, remains rudimentary. In this talk, we will present a method that allows system diagonalization in the presence of such strong parametric interactions. Applying it to a single qubit parametrically coupled to a single bosonic mode, we show that even this simple system can support a rich structure in dispersive shifts that can be rendered large and tunable by choosing suitable parametric coupling drive amplitudes and frequencies. Further our analysis shows how significant corrections beyond the usual Jaynes-Cummings model can be crucial to describe new qualitative effects in dispersive regime, such as an exact cancellation of parametrically-induced shifts for optimal choice of pump frequencies. We will also introduce a circuit-QED realization of such a system based on a transmon coupled to a microwave cavity to investigate parametrically induced dispersive shifts, discussed further in the next talk. |
Thursday, March 18, 2021 8:12AM - 8:24AM Live |
R32.00002: Realizing large, tunable dispersive shifts with parametric couplings – Part II Taewan Noh, Zhihao Xiao, Emery Doucet, Leonardo Ranzani, Luke Govia, Archana Kamal, Jose Aumentado, Raymond W Simmonds Engineered dispersive shifts in cavity-QED systems are important for enabling high fidelity qubit measurement, fast logic gates, state preparation and error correction protocols. Standard superconducting circuit-QED systems typically rely on static coupling between qubits and cavities. Here, tunable dispersive shifts are only possible by in-situ frequency tuning of either the qubit or cavity. Manipulation of dispersive shifts in this way can be cumbersome and restricted. With the theoretical backbone for understanding strong parametric dispersive interactions covered in the previous talk, we will focus on describing our experiments with a transmon qubit coupled to a cavity via a dc-SQUID. We show that our system can avoid qubit decoherence by minimizing coupling to the readout cavity during qubit operations. With both the qubit and cavity at fixed frequencies, we can dynamically produce large positive or negative dispersive shifts, achieving high fidelity qubit measurements. In addition to realizing many features of standard cavity-QED, this system also exhibits a unique tunability along with qualitatively new features not supported by static circuit-QED setups, thus opening up a new paradigm for controlling light-matter interactions. |
Thursday, March 18, 2021 8:24AM - 8:36AM Live |
R32.00003: Continuous phase preserving measurement of a quantum N-level system Debmalya Das, John Steinmetz, Andrew N Jordan, Irfan Siddiqi We present an analysis of the continuous monitoring of a quantum N-level system using phase-preserving measurements. In recent times, many experiments, like those with superconducting qubits, involve interacting such a system to a resonator and then collecting signals via phase-preserving amplifiers. Using the measured quadratures at every time step, we construct a stochastic master equation, describing the quantum trajectories of the system. We use the dynamical equations that follow from the stochastic master equation, to discuss the realizations of the formalism for a few special quantum systems. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R32.00004: Single-shot number-resolved detection of microwave photons with error mitigation Jacob Curtis, Connor Hann, Salvatore Elder, Christopher Wang, Luigi Frunzio, Liang Jiang, Robert J Schoelkopf Single-photon detectors are ubiquitous and integral components of photonic quantum cryptography, communication, and computation. Many applications, such as Gaussian boson sampling, require not only detecting the presence of photons, but distinguishing the number present with a single shot. Here, we present a single-shot, high-fidelity photon number-resolving detector of up to 15 microwave photons in a cavity-qubit circuit QED platform. This detector functions by measuring a series of generalized parity operators which make up the bits in the binary decomposition of the photon number. Photon loss and ancilla readout errors can flip one or more bits, causing nontrivial errors in the outcome, but these errors have a traceable form which can be captured in a simple hidden Markov model. Relying on the independence of each bit measurement, we mitigate biases in the measurement result, showing good agreement with the predictions of the model. The mitigation improves the average total variation distance error of Fock states from 13.5% to 1.3%. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R32.00005: Complete quantum tomography for QND detectors and generalization of QNDness. Luciano Pereira, Tomas Ramos, Juan Jose Garcia-Ripoll The development of quantum non-demolition (QND) detectors for superconducting qubits, atoms or photons has been a topic of great interest in recent years. These detectors allow consecutive measurements of an observable without changing it, which has several applications in quantum computing and quantum metrology. In this work, we propose an unbiased tomographic protocol to fully characterize a QND detector, which provides us the POVM elements associated with the measurement, as well as the processes that define the state after the measurement. We test the protocol by simulating a dispersive measurement of transmon qubits, identifying the effects of several noise sources on the readout performance. Our proposal is a generalization to current characterization methods, which provides a complete quantification of the destructiveness of the detector, and hence, it can be used to improve the calibration and efficiency of state-of-the-art QND measurements. The generality of our theory allows us to improve the characterization of QND detectors for any quantum platform and under realistic experimental conditions, including photonic, atomic, and solid-state systems. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R32.00006: Circuit optimization to improve transmon qubit readout via cross-Kerr coupling Vladimir Milchakov, Timothee Guerra, Remy Dassonneville, Luca Planat, Tomas Ramos, Thibault Charpentier, Farshad Foroughi, Olivier Buisson, Nicolas Roch, Wiebke Guichard, Cécile Naud, Juan Jose Garcia-Ripoll As it has been demonstrated recently, transmon based on non-perturbative cross-Kerr coupling has shown a promising fast high-fidelity quantum non-demolition readout [1]. First generation of V-shape qubits had presented 4us relaxation time which limited fidelity and QND-ness of the readout. Moreover, unwanted qubit transitions were induced by readout photons in the cavity. In order to improve the qubit and readout performance, we have optimized different parameters: the circuit geometry has been adjusted to reduce the amplitude of the microwave electrical field at the interfaces; the electric circuit parameters were optimized to reach large detuning between the readout and the transmon frequencies; unwanted dipolar coupling between the transmon qubit and the cavity was reduced to avoid the Purcell effect; design and fabrication process were modified in order to optimize electron beam doses and angle of evaporation. We will present the readout performance based on the cross-Kerr coupling of the improved transmon circuit. |
Thursday, March 18, 2021 9:12AM - 9:48AM Live |
R32.00007: Fast high fidelity quantum non-demolition superconducting qubit readout Invited Speaker: Olivier Buisson
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Thursday, March 18, 2021 9:48AM - 10:00AM Live |
R32.00008: Unconditional reset of a qubit readout resonator using multichannel driving András Gunyhó, Suman Kundu, Joni Ikonen, Tianyi Li, Mikko Möttönen Rapid measurement is important for many quantum computing applications, such as quantum error-correction codes. We demonstrate a method for reducing the waiting time between successive measurements in a circuit QED system by actively depleting photons from a readout resonator after a measurement. The resonator ring-down time is reduced without feedback, independent of the measurement outcome. We achieve this by applying a readout tone near the resonator resonance frequency simultaneously to both the resonator and the qubit, which realizes an effectively longitudinal interaction between them [1, 2]. We present results from numerical simulations and from experiments on a typical circuit QED system. |
Thursday, March 18, 2021 10:00AM - 10:12AM Live |
R32.00009: Quantum reservoir computing using weakly-nonlinear Josephson junction networks under homodyne measurement Saeed Khan, Gerasimos Angelatos, Hakan E Tureci We analyze a model for quantum reservoir computing for quantum state classification based on single-shot measurements of a small-scale weakly-nonlinear quantum reservoir computer that is directly coupled to the quantum system of interest. This description is in contrast to more standard approaches where information about the states to be classified is described by ensemble-averaged quantities measured separately and subsequently fed to the reservoir computer. Our coupled model requires a complete quantum-mechanical description of the entire measurement chain undergoing conditional evolution. This description is relevant for experimental realizations of classification tasks based on single-shot readout schemes, and emerges naturally for weakly-nonlinear Josephson-junction networks in the circuit QED architecture under homodyne |
Thursday, March 18, 2021 10:12AM - 10:24AM Live |
R32.00010: A generalized figure of merit for qubit readout Benjamin D'Anjou Many approaches to fault-tolerant quantum computation require repeated quantum nondemolition (QND) readout of qubits. A commonly used figure of merit for readout performance is the error rate for binary assignment in a single repetition. However, binary assignment discards important information on the level of confidence in the analog outcomes observed in real experiments. Here, a generalized figure of merit that fully captures the information contained in the analog readout outcomes, the Chernoff information, is proposed. Unlike the single-repetition error rate, the Chernoff information uniquely determines the cumulative error rate for arbitrary readout noise. Importantly, this universal description persists for the small number of repetitions and non-QND imperfections relevant to real experiments. In addition, the Chernoff information rigorously quantifies the amount of information discarded by analog-to-binary conversion. These results provide a unified framework for qubit readout and should facilitate optimization and engineering of near-term quantum devices across all platforms. |
Thursday, March 18, 2021 10:24AM - 10:36AM Live |
R32.00011: Machine Learning Assisted Superconducting Qubit Readout – Part I: From Pulse Shaping to State Discrimination Benjamin Lienhard, Cole Hoffer, Antti Vepsäläinen, Luke Govia, Yanjie Qiu, David K Kim, Roni Winik, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Thomas A Ohki, Hari Krovi, Terry Philip Orlando, Simon Gustavsson, William Oliver Qubit-state readout is a significant error source in contemporary superconducting quantum processors. The fidelity of dispersive qubit-state readout depends on the readout pulse shape and resulting phase-shifted readout signal discriminator. For a single qubit, fast and high-fidelity readout is achieved with minor changes to the rising and falling edge of a rectangular pulse, and a linear matched filter discriminator. However, in resource-efficient, frequency-multiplexed readout of multiple qubits, optimizing the readout pulse shape and discriminator becomes a computationally intensive task. Here, we experimentally demonstrate deep machine learning techniques to improve superconducting qubit readout pulse shapes and discrimination compared to conventional methods. |
Thursday, March 18, 2021 10:36AM - 10:48AM Live |
R32.00012: Machine Learning Assisted Superconducting Qubit Readout – Part II: Deep Reinforcement Learning Cole Hoffer, Benjamin Lienhard, Antti Vepsäläinen, Luke Govia, Vilhelm L Andersen Woltz, David K Kim, Alexander Melville, Bethany Niedzielski, Jonilyn Yoder, Thomas A Ohki, Hari Krovi, Terry Philip Orlando, William Oliver Qubit-state readout of contemporary superconducting quantum processors is a significant error source. For a qubit dispersively coupled to a resonator, quick resonator ring-up and ring-down ensure fast readout and limited qubit dephasing in future operations. In an efficient, frequency-multiplexed readout of multiple qubits, effects such as drive crosstalk increase the complexity of optimal readout pulse shapes, requiring computationally intensive methods to discover high-fidelity pulse shapes. We present a pulse shaping optimization module using deep reinforcement learning (DRL). We compare DRL to conventional methods in readout pulse shaping experiments of multi-qubit devices and evaluate future generalized use of DRL methods in quantum computing. |
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