Observation of twin beam correlations and quadrature entanglement by frequency doubling in a two-port resonator

Abstract

We demonstrate production of quantum correlated and entangled beams by second harmonic generation in a nonlinear resonator with two output ports. The output beams at λ = 428.5 nm exhibit 0.9 dB of nonclassical intensity correlations and 0.3 dB of entanglement.PACS numbers: 03.67. Mn,42.50.Dv,42.65.Ky Continuous light beams that exhibit nonclassical statistics are of interest as a tool for studying quantum fields [1] and for a number of applications that include precision measurements [2], writing subwavelength spatial structures [3], and as resources for quantum information and communication protocols [4]. The most successful and widely used approach to generating nonclassical light employs parametric down conversion in crystals with a quadratic nonlinearity. At the microscopic level non-classical correlations and entanglement arise due to the possibility of converting a single high frequency photon at 2ω into a pair of entangled lower frequency photons at ω.In order to experimentally generate nonclassical beams with carrier frequency ω one typically starts with a coherent source at ω which is frequency doubled to 2ω. The light at 2ω is then used to drive a downconversion process to generate nonclassical light at frequency ω. These multiple steps add to the complexity of the experimental arrangement and limit the possibility of generating nonclassical light at high frequencies. In this letter we demonstrate for the first time that nonclassical intensity correlations, as well as quadrature entangled beams, can be generated directly by frequency upconversion. In this way a coherent source at frequency ω produces quantum correlated beams at frequency 2ω.Consider the interaction geometry shown in Fig. 1 where a beam of frequency ω pumps a resonator that has two exit ports for the second harmonic beams at frequency 2ω. The cavity mirrors are assumed perfectly transmitting for the harmonic beams which are generated in a single pass of the intracavity pump field through the nonlinear crystal. It is well known that second harmonic generation (SHG) results in squeezing of the fundamental and harmonic beams [5]. The generation of multibeam correlations in second harmonic generation is less well studied than in the case of parametric down conversion. Calculations have demonstrated the existence of correlations between the fundamental and harmonic fields [6] including entanglement between the fundamental and harmonic fields [7] and entanglement in type II SHG in the fundamental fields alone [8]. The possibility of nonclassical spatial correlations in either the funda- mental or harmonic fields alone [9] and of entanglement in the fundamental field [10] has also been shown in models that include diffraction. Here we consider a situation where there are two harmonic output beams that share the same intracavity pump field. The common pump field couples the output beams, and our recent analysis of this interaction geometry [11] reveals that the two outputs exhibit nonclassical intensity correlations, as well as quadrature en...

Generation of tripartite quantum correlation among amplitude-squeezed beams by frequency doubling in a singly resonant cavity

“…Using a singly resonant dual-port cavity SHG, generation of tripartite entanglement among the two SH fields and the fundamental field has also been proposed [24]. Moreover, it has been demonstrated that the two output SH fields of the dual-port SHG system exhibit nonclassical intensity correlation in experiment [25,26]. However, experi mental confirmation of tripartite quantum correlation is more interesting for the future application of quantum information and communications.…”