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Bulletin of the American Physical Society

Continuously monitoring the parity of superconducting qubits in a 2D cQED architecture

Machiel Blok, Emmanuel Flurin, William P. Livingston, James Colless, Allison Dove, Irfan A. Siddiqi

Published Mar 16, 2017• 0 citations
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Real-time monitoring of a quantum state provides powerful tools for studying the backaction of quantum measurement and performing quantum feedback. Historically, this monitoring capability has been the exclusive province of atomic and optical physics. This thesis describes the implementation of the first such high-fidelity readout scheme in a solid state circuit, a superconducting quantum bit (qubit) coupled to a microwave cavity in the circuit quantum electrodynamics (circuit QED) architecture. The qubit-state-dependent resonance frequency of the cavity is probed with a microwave drive tone, and the resulting signal amplified using a fast, ultralow-noise superconducting parametric amplifier. This arrangement enables the observation of quantum jumps between the qubit states in real time. The ability to monitor the qubit continuously with high fidelity and resolve quantum jumps can be used to investigate the backaction of the measurement process on the qubit. This thesis examines the quantum Zeno effect--where strong measurement inhibits the evolution of a quantum system-- as well as the transition to non-ideal measurement with increasing measurement strength in the circuit QED architecture, a phenomenon shown to be due to the upconversion of low-frequency dephasing noise. These data allow probes of ``universal'' flux noise in previously inaccessible frequency ranges. The work presented here opens the door for quantum feedback and error correction in solid-state quantum systems using continuous weak measurement.

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Superconducting qubit in a resonator : test of the Legget-Garg inequality and single-shot readout

Agustin Palacios-Laloy

This thesis presents a series of circuit QED (cQED) experiments, in which artificial atoms consisting of superconducting circuits are strongly coupled to the electromagnetic field stored in a microwave resonator. This resonator can be used as a measurement apparatus for the atom, allowing both to address fundamental issues in quantum mechanics, and to develop building blocks for a quantum processor. In a first experiment we continuously monitored the evolution of a quantum system while varying the measurement strength. We observed the crossover from weak to strong measurement regimes, and, ultimately, the freezing of the dynamics by the Quantum Zeno Effect. In the weak measurement regime, we probed if the artificial atom complies with the macroscopic realism hypotheses, from which Leggett and Garg have derived a Bell's inequality in time. The violation of this inequality confirms that although macroscopic, the artificial atom is a truly quantum object. On the quantum information side, we have enriched the proposed cQED quantum processor architecture by demonstrating a high fidelity single-shot qubit readout, a crucial element for quantum processors. This new circuit relies on a dynamical transition of a non-linear resonator. The coupled system formed by the qubit and the non-linear resonator allows in addition to investigate the interplay between strong coupling and non-linear effects --parametric amplification, squeezing...-- opening the new field of non-linear cQED. Finally we have operated a circuit which would potentially be able to mediate operations between two arbitrary qubits: a superconducting resonator which is tunable in frequency.

•Sep 23, 2010•2 citations
Real-time monitoring of a quantum state provides powerful tools for studying the backaction of quantum measurement and performing quantum feedback. Historically, this monitoring capability has been the exclusive province of atomic and optical physics. This thesis describes the implementation of the first such high-fidelity readout scheme in a solid state circuit, a superconducting quantum bit (qubit) coupled to a microwave cavity in the circuit quantum electrodynamics (circuit QED) architecture. The qubit-state-dependent resonance frequency of the cavity is probed with a microwave drive tone, and the resulting signal amplified using a fast, ultralow-noise superconducting parametric amplifier. This arrangement enables the observation of quantum jumps between the qubit states in real time. The ability to monitor the qubit continuously with high fidelity and resolve quantum jumps can be used to investigate the backaction of the measurement process on the qubit. This thesis examines the quantum Zeno effect--where strong measurement inhibits the evolution of a quantum system-- as well as the transition to non-ideal measurement with increasing measurement strength in the circuit QED architecture, a phenomenon shown to be due to the upconversion of low-frequency dephasing noise. These data allow probes of ``universal'' flux noise in previously inaccessible frequency ranges. The work presented here opens the door for quantum feedback and error correction in solid-state quantum systems using continuous weak measurement.
This thesis presents a series of circuit QED (cQED) experiments, in which artificial atoms consisting of superconducting circuits are strongly coupled to the electromagnetic field stored in a microwave resonator. This resonator can be used as a measurement apparatus for the atom, allowing both to address fundamental issues in quantum mechanics, and to develop building blocks for a quantum processor. In a first experiment we continuously monitored the evolution of a quantum system while varying the measurement strength. We observed the crossover from weak to strong measurement regimes, and, ultimately, the freezing of the dynamics by the Quantum Zeno Effect. In the weak measurement regime, we probed if the artificial atom complies with the macroscopic realism hypotheses, from which Leggett and Garg have derived a Bell's inequality in time. The violation of this inequality confirms that although macroscopic, the artificial atom is a truly quantum object. On the quantum information side, we have enriched the proposed cQED quantum processor architecture by demonstrating a high fidelity single-shot qubit readout, a crucial element for quantum processors. This new circuit relies on a dynamical transition of a non-linear resonator. The coupled system formed by the qubit and the non-linear resonator allows in addition to investigate the interplay between strong coupling and non-linear effects --parametric amplification, squeezing...-- opening the new field of non-linear cQED. Finally we have operated a circuit which would potentially be able to mediate operations between two arbitrary qubits: a superconducting resonator which is tunable in frequency.