Abstract
A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier. Here, we realize a device circumventing this problem by directly embedding a two-level artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier. Surprisingly, this configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction. This is accomplished by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise. By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80 percent. Our theoretical model accurately describes the observed interplay of gain and measurement backaction, and delineates the parameter space for future improvement. The device is compatible with standard fabrication and measurement techniques, and thus provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.
Highlights
The sum of interactions between a quantum system and all environmental channels produces a continuous flow of quantum information into the environment, causing dephasing at a rate Γφ
For a two-level qubit described by σz and measured along that axis, one may define the fraction of this information flux experimentally captured per unit time to be the measurement efficiency ηmeas 1⁄4 Γmeas=2Γφ, a critical parameter for continuous quantum measurements, where Γmeas represents the rate at which the experimentalist learns about σz and is defined such that ηmeas ranges from
The use of off-chip superconducting parametric amplifiers has enabled a variety of experiments investigating quantum measurement dynamics [1,2,3,4,5,6], with improvements in efficiency reported using multijunction circuits [7]; the remaining approximately 30% of the information is typically lost in dissipative elements such as circulators
Summary
The sum of interactions between a quantum system and all environmental channels produces a continuous flow of quantum information into the environment, causing dephasing at a rate Γφ. The QPA implements on chip the parametric mode of operation that has been widely applied in continuous measurements of qubits This scheme presents a novel challenge, as the in situ microwave amplification and squeezing opens a parasitic measurement channel inducing excess dephasing. A microwave readout tone at frequency ωQPA reflects off the QPA, acquiring qubit-state information. A pump tone of the form cos1⁄22ðωQPAt þ ΦÞ applied to the pump port of the QPA concurrent with the readout modulates the QPA resonance frequency, producing on-chip phase-sensitive amplification of the measurement field. The QPA consists of a transmon qubit dispersively coupled to a JPA acting as the readout resonator. A superconducting coil housed below the chip enables static tuning of ωQPA, while a pump applied via the flux line (cyan) modulates ωQPA to produce parametric gain. A succinct theoretical analysis of the system is given in Appendix B, with further details available in Ref. [26]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
