Measurement of the squeezed vacuum state by a
bichromatic local oscillator
WEI LI,
1,2
XUDONG YU,
1,2
AND JING ZHANG
1,3,
*
1
The State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University,
Taiyuan 030006, China
2
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
3
Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China,
Hefei, Anhui 230026, China
*Corresponding author: jzhang74@sxu.edu.cn
Received 4 September 2015; revised 19 October 2015; accepted 20 October 2015; posted 21 October 2015 (Doc. ID 249434);
published 9 November 2015
We present the experimental measurement of a squeezed
vacuum state by means of a bichromatic local oscillator
(BLO). A pair of local oscillators at 5 MHz around the
central frequency ω
0
of the fundamental field with equal
power are generated by three acousto-optic modulators
and phase-locked technology and used as a BLO. The
squeezed vacuum light is detected by a phase-sensitive
balanced-homodyne detection with a BLO. The baseband
signal around ω
0
combined with a broad squeezed field
can be detected with the sensitivity below the shot-noise
limit, in which the baseband signal is shifted to the vicinity
of 5 MHz (the half of the BLO separation). This work has
important applications in quantum state measurement and
quantum information.
© 2015 Optical Society of America
OCIS codes: (270.5585) Quantum information and processing;
(270.6570) Squeezed states.
http://dx.doi.org/10.1364/OL.40.005299
The squeezed state of the light is an important resource of
quantum information [1–9] and quantum metrology [10 –14].
Especially, in the modern research focus, the squeezed state be-
comes crucial for gravitation wave detection. In recent years,
some significant improvements have been made in this field,
such as 12.7 dB squeezing being obtained [15], the very lower
frequency squeezing measurement being realized, and the fre-
quency-dependence squeezing being investigated [16]. A single
broadband squeezed light can be split into N pairs of upper and
lower single sideband fields with spatial separation, which cor-
respond to N independent Einstein–Podolsky–Rosen (EPR)
entangled fields [17]. This scheme was demonstrated experi-
mentally by using a pair of frequency-shifted local oscillators
to measure this EPR entanglement [18,19]. The theoretical
scheme based on a bichromatic local oscillator (BLO) to detect
the squeezed state was proposed [20] in which several advan-
tages and applications were given. The phase-sensitive detec-
tion with a BLO or a double-sideband signal field were
studied [21–23]. In this Letter, we utilize a BLO to detect a
broadband squeezed light with a phase-sensitive balanced-ho-
modyne detection. This work demonstrates quantum correla-
tion between the upper and lower sideban d modes [17]ofa
single broadband squeezed light from another perspective.
Generating and measuring the low-frequency squeezing for
the terrestrial gravitational wave detectors are very difficult be-
cause of the extreme challenges in the technique. The BLO
technique can circumvent the challenge of detecting low-fre-
quency squeezing, which is usually obscured by technical noise.
We present the result that the baseband signal is shifted into the
vicinity of 5 MHz (half of the BLO separation), and subshot-
noise detection is implemented. Thus, this work with the BLO
and broadband squeezing can be used to enhance the signal-to-
noise ratio (SNR) of an interferometer for lower-frequency
phase measurement [24,25].
The schematic diagram of the detection is shown in Fig. 1.A
strong BLO (at Ω
0
around the central frequency ω
0
of the
fundamental field with equal power) is mixed with the signal
light field at a 50/50 beam splitter. The relative phase θ of
the local oscillator and the signal field can be controlled by
the reflective mirror mounted on a PZT (piezoelectric trans-
ducer). The annihilation operators of the BLO and the signal
field can be written as ˆatˆa
t exp−iω
0
Ω
0
t
ˆa
−
texp−iω
0
− Ω
0
t and
ˆ
bt
ˆ
b
0
t exp−iω
0
t, where
Fig. 1. Schematic diagram of measuring a single broadband
squeezed light with the central frequency ω
0
using a phase-sensitive
balanced-homodyne detection with a BLO.
Letter
Vol. 40, No. 22 / November 15 2015 / Optics Letters 5299
0146-9592/15/225299-04$15/0$15.00 © 2015 Optical Society of America