April 2008
EPL, 82 (2008) 24003 www.epljournal.org
doi: 10.1209/0295-5075/82/24003
Experimental investigation about the influence of pump phase
noise on phase-correlation of output optical fields
from a non-degenerate parametric oscillator
Dong Wang, Yana Shang, Zhihui Yan, Wenzhe Wang, Xiaojun Jia, Changde Xie
(a)
and Kunchi Peng
The State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electro nics,
Shanxi University - Taiyuan, 030006, PRC
received 14 September 2007; accepted in final form 28 February 2008
published online 28 March 2008
PACS 42.50.-p – Quantum optics
PACS 42.65.Yj – Optical parametric oscillators and amplifiers
PACS 42.65.Lm – Parametric down conversion and production of entangled photons
Abstract – The influence of the phase fluctuation of the pump laser on the phase correlation
between the signal and idler modes of the output fields from a non-degenerate optical parametric
oscillator operating above oscillation threshold was experimentally investigated. The noise spectra
of the intensity-difference and the phase-sum of the entangled optical beams were measured with
a pair of unbalanced fiber Mach-Zehnder interferometers specifically designed. The experimental
results prove the previous theoretical prediction and are in reasonable agreement with the
calculation based on semiclassical theory involving the phase fluctuation of pump laser.
Copyright
c
EPLA, 2008
Optical parametric oscillators both degenerate (DOPO)
and nondegenerate (NOPO) have been one of the most
important entanglement sources in the quantum informa-
tion of continuous variables (CV) [1]. As a useful tool in
quantum optics for the generation of nonclassical states
of light, the quantum fluctuation characteristics of the
output fields from OPO have been extensively studied
since the late 1980s. Firstly Reid and Drummond theo-
retically predicted that the signal and idler optical modes
produced from NOPO operating above or below the oscil-
lation threshold are in an entangled state with the quan-
tum correlation of amplitude and phase quadratures [2–5].
In 1992, the CV entanglement of optical modes was
experimentally demonstrated by Ou et al. with a NOPO
below threshold [6]. Although the quantum correlation of
amplitude quadratures between intense signal and idler
from above-threshold NOPO (named twin beams) was
measured experimentally and effectively applied by several
groups [7–12], the phase correlation of the twin beams with
nondegenerate frequencies was not observed for a long
time. For measuring the phase correlation of twin beams
using general homodyning detector, Laurat et al. forced
the NOPO to oscillate in a strict frequency-degenerate
situation by inserting a λ/4 plate inside the NOPO and
(a)
E-mail: changde@sxu.edu.cn
observed a 3 dB phase-sum variance above the shot-noise-
limit (SNL) [13]. Then, in 2005 CV entanglement of
frequency-nondegenerate twin beams was experimentally
measured by two groups with scanning a pair of tunable
ring analysis cavities [14] or with two sets of unbalanced
Mach-Zehnder (M-Z) interferometers [15], respectively. In
2006 the entangled twin beams with stable frequency
difference was obtained by Pfister’s group [16]. However,
those experimentally measured phase-correlations were
always smaller than that of the theoretical prediction. At
the end of 1980s, Fabre et al. presented the expressions for
calculating quantum correlations of amplitude and phase
quadratures between twin beams [17], in which the pump
laser was regarded as an ideal coherent state thus the
influence of the pump excess phase noise on entanglement
of twin beams was not considered. In fig. 3 of ref. [15]
achieved by our group before, we compared the normal-
ized noise power spectra of the intensity difference and
the phase sum calculated from the expressions given in
ref. [17] and the experimentally measured values, which
showed that the measured value of the intensity difference
perfectly agreed with that calculated theoretically but the
measured phase correlation noise was higher by 0.08 than
the calculated values. Earlier, Reid and Drummond theo-
retically proved that a correlation between signal and idler
phase diffusion existed and hence that the sum-phase
24003-p1