Selective Amplification of a Quantum State

TL;DR

The paper predicts a new quantum effect where a two-level system selectively amplifies a specific photon number in a cavity based on its initial state, enabling quantum state readout with robustness to noise.

Contribution

It introduces a novel selective amplification effect in a quantum two-level system coupled to a cavity, useful for quantum state measurement.

Findings

Achieves coherent excitation of an N-photon state conditioned on initial TLS state

Demonstrates robustness of the effect against noise and decoherence

Provides a potential method for quantum state readout in qubit-resonator systems

Abstract

We predict a novel effect in a quantum two-level system (TLS) coupled to a resonant cavity. By bringing the TLS in and out of resonance with the cavity by a series of $N$ rectangular bias pulses (the length of the $m$th pulse scaling as $1/\sqrt{m}$), we will coherently excite the $N$-photon state, $|N,$ of the cavity only if the TLS was initially in an appropriate quantum state ("go" state). Otherwise the number of photons in the cavity will remain small compared to $N$ ({\em selective amplification}). If the TLS was in a coherent superposition of the "go" and "no go" states, the cavity will be in a superposition of states, in which the state $|N$ will enter with the same weight as the initial "go" component. The effect is due to $\sqrt{N+1}$-dependence of the Rabi oscillation frequency on the number $N$ of photons in the cavity. It is stable with respect to noise, pulse shape, finite temperature, TLS decoherence and TLS detuning from resonance with the cavity. The effect can be used as a means to read out a quantum state of a qubit coupled to a resonator.