Abstract
When a two-level system -- a qubit -- is used as a probe of a larger system, it naturally leads to answering a single yes-no question about the system state. Here we propose a method where a single qubit is able to extract, not a single, but many bits of information about the photon number of a microwave resonator using continuous measurement. We realize a proof-of-principle experiment by recording the fluorescence emitted by a superconducting qubit reflecting a frequency comb, thus implementing multiplexed photon counting where the information about each Fock state -- from 0 to 8 -- is simultaneously encoded in independent measurement channels. Direct Wigner tomography of the quantum state of the resonator evidences the back-action of the measurement as well as the optimal information extraction parameters. Our experiment unleashes the full potential of quantum meters by replacing a sequential quantum measurements with simultaneous and continuous measurements separated in the frequency domain.
Highlights
The most general measurement of a quantum system consists of using a quantum apparatus as a probe
We show that photon counting can be implemented in a time independent of the number of photons
X 11, 031045 (2021) (b) questions used time division multiplexing, while our experiment demonstrates the analog of frequency division multiplexing, where the qubit alone acts as the frequency multiplexing transducer [Fig. 1(a)]
Summary
The most general measurement of a quantum system consists of using a quantum apparatus as a probe. The system interacts with the probe before the probe gets measured projectively. The probe is a qubit whose readout answers a yes-no question about the system state. Identifying what is the state of a system comes down to playing a game of “guess who?” A series of binary questions are asked iteratively to refine our knowledge about the state. Each answer disturbs the state of the system. To give a concrete example, in order to determine how many photons are stored in a cavity, one may ask “is there an even number of photons?” or a series of binary questions such as “are there n photons?” for each integer n [Fig. 1(a)]
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