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

We demonstrate production of short wavelength beams with nonclassical intensity correlations and quadrature entanglement using second harmonic generation in a nonlinear resonator with two output ports. The two output beams at the harmonic frequency have no direct interaction but are both nonlinearly coupled to a common pump beam. The nonlinear coupling produces amplitude and phase correlations between the beams which results in quadrature entanglement. The output beams at λ= 428.5 nm are shown to exhibit 0.6 dB of amplitude quadrature squeezing, 0.9 dB of nonclassical intensity correlations, and 0.3 dB of entanglement using about 30mW of pump power. Copyright c © EPLA, 2007 Continuous light beams that exhibit nonclassical statistics and entanglement are of interest in studies of 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 continuous variable entangled light beams employs parametric downconversion in crystals with a quadratic nonlinearity [5]. 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 [6]. In order to experimentally generate entangled 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 entangled light at short wavelengths. 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 which may be important for applications such as quantum lithography [3] which will benefit from short wavelength entanglement. 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 [7]. The generation of multibeam correlations in second harmonic generation is less well studied than in the case of parametric downconversion. Calculations have demonstrated the existence of correlations between the fundamental and harmonic fields [8] including entanglement between the fundamental and harmonic fields [9] and entanglement in type-II SHG in the fundamental fields alone [10]. The possibility of nonclassical spatial correlations in either the fundamental or harmonic fields alone [11] and of entanglement in the fundamental field [12] has also been shown in models that include diffraction. Here we consider a situation where there are two harmonic output beams which share the same intracavity pump field. The common pump field couples the output beams, and our recent analysis of this interaction geometry [13] reveals that the two outputs exhibit nonclassical