Fluctuations and Noise in Photonics and Quantum Optics II

Derek Abbott, Jeffrey H. Shapiro, Y. Yamamoto

Philip Hemmer, Julio R. Gea-Banacloche, P. Heszler +1 more

Leon W. Cohen

P. Heszler, Sociedad Española de Optica

Prem Kumar, Jianming Huang, Orhan Aytür

ABSTRACT We demonstrate the use of a commercial mode-locked and Q-switched Nd:YAG laser in photon-noisereduction experiments. There are many frequency bands wherein the photodetection of such a laser isshot-noise limited. By appropriately filtering the photocurrent produced in direct detection of light froman optical parametric amplifier that is pumped with this laser, we have successfully generated twin-beam states of light. We demonstrate that such twin beams can be used to make measurements below the shot-noise limit. 2. INTRODUCTION The fundamental noise in optical measurements using lasers results from the fact that photons of light are detected at random. In a lossless system, one can ascribe the random detection of photons to the random emission of photons by the laser itself. It can be shown from the detailed laser theory' that anideal laser emits photons in a Poissonian fashion2 as shown in Fig. 1. This fact implies that there willalways be fluctuation in the number of photons that are counted in a given time interval. If = ii photonsare counted by the detector in an observation time T, then the variance in the photon count will be givenby the Poisson formula Var(N) = ii. This fluctuation in the detected photon number leads to the followingmaximum signal-to-noise ratio (SNR) that can be obtained using laser light:

Carlton M. Caves

The interferometers now being developed to detect gravitational waves work by measuring the relative positions of widely separated masses. Two fundamental sources of quantum-mechanical noise determine the sensitivity of such an interferometer: (i) fluctuations in number of output photons (photon-counting error) and (ii) fluctuations in radiation pressure on the masses (radiation-pressure error). Because of the low power of available continuous-wave lasers, the sensitivity of currently planned interferometers will be limited by photon-counting error. This paper presents an analysis of the two types of quantum-mechanical noise, and it proposes a new technique---the "squeezed-state" technique---that allows one to decrease the photon-counting error while increasing the radiation-pressure error, or vice versa. The key requirement of the squeezed-state technique is that the state of the light entering the interferometer's normally unused input port must be not the vacuum, as in a standard interferometer, but rather a "squeezed state"---a state whose uncertainties in the two quadrature phases are unequal. Squeezed states can be generated by a variety of nonlinear optical processes, including degenerate parametric amplification.