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arXiv:1009.2969 · Physical Review Letters

Observation of Quantum Jumps in a Superconducting Artificial Atom

Rajamani Vijay, D. H. Slichter, Irfan A. Siddiqi

Published Mar 14, 2011• 361 citations
arXivHTMLPDF

Abstract

We continuously measure the state of a superconducting quantum bit coupled to a microwave readout cavity by using a fast, ultralow-noise parametric amplifier. This arrangement allows us to observe quantum jumps between the qubit states in real time, and should enable quantum error correction and feedback--essential components of quantum information processing.

DOI
10.1103/physrevlett.106.110502

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We continuously measure the state of a superconducting quantum bit coupled to a microwave readout cavity by using a fast, ultralow-noise parametric amplifier. This arrangement allows us to observe quantum jumps between the qubit states in real time, and should enable quantum error correction and feedback--essential components of quantum information processing.
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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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Fast discrimination between quantum states of superconducting artificial atoms is an important ingredient for quantum information processing. In circuit quantum electrodynamics, increasing the signal-field amplitude in the readout resonator, dispersively coupled to the artificial atom, improves the signal-to-noise ratio and increases the measurement strength. Here, we employ this effect over 2 orders of magnitude in readout power, made possible by the unique combination of a dimer-Josephson-junction-array amplifier with a large dynamic range and the fact that the readout of our granular aluminum fluxonium artificial atom remains quantum nondemolition (QND) at relatively large photon numbers in the readout resonator, up to n‾=110\overline{n}=110n=110. Using Bayesian inference, this allows us to detect quantum jumps faster than the readout-resonator response time $2/\ensuremath{\kappa}$, where $\ensuremath{\kappa}$ is the bandwidth of the readout resonator.