Quantum jumps of light recording the birth and death of a photon in a cavity

Abstract

A microscopic quantum system under continuous observation exhibits at random times sudden jumps between its states. The detection of this quantum feature requires a quantum non-demolition (QND) measurement repeated many times during the system's evolution. Whereas quantum jumps of trapped massive particles (electrons, ions or molecules) have been observed, this has proved more challenging for light quanta. Standard photodetectors absorb light and are thus unable to detect the same photon twice. It is therefore necessary to use a transparent counter that can 'see' photons without destroying them. Moreover, the light needs to be stored for durations much longer than the QND detection time. Here we report an experiment in which we fulfil these challenging conditions and observe quantum jumps in the photon number. Microwave photons are stored in a superconducting cavity for times up to half a second, and are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms, highly correlated in the same state, are interrupted by sudden state switchings. These telegraphic signals record the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons should open new perspectives for the exploration of the quantum-to-classical boundary.

Understanding quantum measurement from the solution of dynamical models

“…of particle detectors or of avalanche processes, deserve to be studied through models. Measurements of a more elaborate type, in which some quantum property of S is continuously followed in time, are now being performed owing to experimental progress [307,308,361,362,319]. 152 In the Schrödinger picture, the expectation value of any (time-independent) observable for the subensemble E i was found from the stateD i of Eq.…”

“…Here, it is obtained from the evolution (13. For instance, non-destructive (thus non-ideal) repeated observations of photons allow the study of quantum jumps [362], and quantum-limited measurements, in which a mesoscopic detector accumulates information progressively [363], are of interest to optimize the efficiency of the processing of q-bits. Quantum measurements are by now employed for designing feedback control processes [319,364], a task that in the classical domain is routinely done via classical measurements.…”

Nanophotonic quantum phase switch with a single atom

“…Cavity quantum electrodynamics (cavity QED), where a photon is confined to a small spatial region and made to interact strongly with an atom, is a promising approach to overcome this challenge [4]. Over the last two decades, cavity QED has enabled advances in the control of microwave [17][18][19] and optical fields [13,[20][21][22][23]. While integrated circuits with strong coupling of microwave photons to superconducting qubits are currently being developed [24], a scalable path to integrated quantum circuits involving coherent qubits coupled via optical photons has yet to emerge.…”

Corrections to the expected signal in quantum metrology using highly anisotropic Bose-Einstein condensates

“…Since γ 1 and a 22 are linearly related through g 22 with the scaling of the measurement uncertainty in both being identical, in the following, we will consider γ 1 as the measured parameter but we will keep in mind that a 11 (and g 11 ) are known quantities. Comparing with conventional Ramsey interferometry [15] in which the parameter dependent evolution of the probe is generated by a Hamiltonian proportional to J z we can see how the relative phase evolves N times faster (assuming N 1). As detailed in [6] the quantum Cramer-Rao bound on the measurement uncertainty in γ 1 will scale with N as…”