IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 44 (2011) 135102 (8pp) doi:10.1088/0022-3727/44/13/135102
Improvement of the signal-to-noise ratio
of laser-induced-fluorescence
photon-counting signals of single-atoms
magneto-optical trap
Jun He, Baodong Yang, Tiancai Zhang and Junmin Wang
1
State Key Laboratory of Quantum Optics and Quantum Optics Devices, and Institute of Opto-Electronics,
Shanxi University, No. 92 Wucheng Road, Taiyuan 030006, Shanxi Province, People’s Republic of China
E-mail: wwjjmm@sxu.edu.cn
Received 16 October 2010, in final form 21 January 2011
Published 10 March 2011
Online at
stacks.iop.org/JPhysD/44/135102
Abstract
Employing grating extended-cavity diode lasers as the cooling/trapping and repumping lasers
for preparing and manipulating single atoms, we have implemented a large-magnetic-gradient
caesium magneto-optical trap (MOT). To detect and evaluate single caesium atoms trapped in
MOT, laser-induced-fluorescence (LIF) photons of trapped atoms driven by MOT lasers are
collected and counted by an avalanched photodiode worked in photon-counting mode. The
dependences of LIF photon-counting signals of single atoms on a cooling laser’s intensity,
frequency detuning and frequency fluctuation are analysed and investigated. Remarkable
improvement of the signal-to-noise ratio of LIF photon-counting signals is achieved by
optimizing the cooling laser’s intensity and frequency detuning and using the modulation-free
polarization spectroscopic technique with feedback to both the slow channel (piezoelectric
transducer channel with typical bandwidth of ∼2 kHz in the grating extended cavity) and the
fast channel (current modulation channel with typical bandwidth of ∼200 kHz in the current
driver).
(Some figures in this article are in colour only in the electronic version)
1. Introduction
The interaction of atoms with light has been an important
theme in investigations of behaviour of the microscopic world.
Typically, a variety of fundamental investigations require
a sample consisting of an individual atom that is spatially
localized with small kinetic energy. Single atoms in a
magneto-optical trap (MOT) [1–9] or in an optical dipole trap
[5, 10–14] are very significant to demonstrate and perform
quantum information processing. Several applications, such
as quantum register [10], triggered single-photon source [11],
atom-photon entanglement [12] and coherent manipulation of
atomic qubit [13, 14] have been demonstrated. Quite often,
the detection schemes of these experiments employ a near-
resonance laser beam that drives a cycling atomic transition
1
Author to whom any correspondence should be addressed.
and a high numerical aperture lens assembly to collect the
light-induced-fluorescence (LIF) photons of trapped atoms to
a photomultiplier tube (PMT) or an avalanche photodiode
(APD). For single atoms, the fluorescence signal is so weak
that it is strongly restrained by the intensity and fluctuation of
scattering rate and the accuracy of manipulation. Many of the
experiments rely on the ability not only to trap and manipulate
atoms but also to collect and distinguish weak fluorescence
signals.
The signal-to-noise ratio (SNR) of LIF photon-counting
signals of single atoms is crucial. In the process of atoms
interacting with laser beams, the magnitude and fluctuation
of signals depend on the fluctuation of the intensity and
frequency of exciting laser. When one makes the improvement
in LIF photon-counting signals, in addition to enhancing the
signal magnitude, better frequency stabilization techniques
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