Quantum Rifling: Protecting a Qubit from Measurement Back Action

Abstract

Quantum mechanics postulates that measuring the qubit's wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate the qubit collapsed into. We show this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the time-averaged expectation value of the measurement operator, erasing information about the initial state of the qubit, while completely suppressing the measurement back-action. We call this regime "quantum rifling", as the fast spinning of the Bloch vector protects it from deflection into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to the same probe field and demonstrate that quantum rifling allows us to measure either one of the two qubits on demand while protecting the state of the other from measurement back-action. Our results allow for the implementation of selective read out multiplexing of several qubits, contributing to efficient scaling up of quantum processors for future quantum technologies.

Probabilistic motional averaging

“…Varying the driving amplitude, we observe two different regimes, analogously to Ref. 17 : (i) weakdriving regime and (ii) strong-driving regime. In the first one, the weak-driving regime (i), both the ground and excited qubit states are monitored, displaying their probabilistic energy-level occupations.…”

“…Our sample consists of a transmon qubit (named Qubit 2 in Ref. 17 ) with a transition frequency between its ground and first excited states ω q /(2π) = 6.44 GHz coupled to a superconducting co-planar waveguide resonator with resonance frequency ω r /(2π) = 7.643 GHz. The qubit state can be controlled by applying a coherent drive tone with frequency ω d ω q through a separate charge line.…”

“…This dispersive shift of χ/(2π) = 4.1 MHz allows us to perform the read out of the qubit state by applying a probe tone to the resonator at frequency ω p ω r and by subsequent detection of the transmitted signal with a standard heterodyne detection scheme (see Ref. 17 ). Numerical and analytical results of the previous two sections together with the experimental observations are presented in Figs.…”

“…Similar transitions from a "quantum" to "classical" regime was observed in Refs. 12,17,45,46,55 . Figure 3(d) shows the dependence of the characteristic driving amplitude Ω 2 , which separates regimes (i) and (ii), as a function of photon numbers obtained from our numerical solution as well as the experimental points.…”

“…1,3,5,[31][32][33][34][35][36][37][38][39] , when a chain of equations is restrained by keeping only the first-order correlators. To describe time-evolution experiments, a semiquantum approach is more correct 17,40,41 ; comparison of the two approaches can be found e.g. in Ref.…”