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Title: Correlators Exceeding One in Continuous Measurements of Superconducting Qubits

Journal Article · · Physical Review Letters
 [1];  [2];  [3];  [4]
  1. Univ. of California, Riverside, CA (United States); Univ. of California, Berkeley, CA (United States)
  2. Univ. of California, Berkeley, CA (United States); Technion-Israel Inst. of Technology, Haifa (Israel)
  3. Univ. of California, Berkeley, CA (United States)
  4. Univ. of California, Riverside, CA (United States); Google Inc., Venice, CA (United States)
+ Show Author Affiliations

Here, e ponder the effect of phase backaction on the correlator ⟨I(t)I(t+τ)⟩ for the output signal I(t) from continuous measurement of a qubit. We demonstrate that the interplay between informational and phase backactions in the presence of Rabi oscillations can lead to the correlator becoming larger than 1, even though |⟨I⟩|≤1. The correlators can be calculated using the generalized "collapse recipe," which we validate using the quantum Bayesian formalism. Moreover, he recipe can be generalized to the case of multitime correlators and arbitrary number of detectors, measuring non-commuting qubit observables. The theory agrees well with experimental results for continuous measurement of a transmon qubit. The experimental correlator exceeds the bound of 1 for a sufficiently large angle between the amplified and informational quadratures, causing the phase backaction. The demonstrated effect can be used to calibrate the quadrature misalignment.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC)
Grant/Contract Number:
AC02-05CH11231
OSTI ID:
1572818
Journal Information:
Physical Review Letters, Journal Name: Physical Review Letters Journal Issue: 22 Vol. 122; ISSN 0031-9007; ISSN PRLTAO
Publisher:
American Physical Society (APS)Copyright Statement
Country of Publication:
United States
Language:
English

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