arXiv:1001.2670v1  [quant-ph]  15 Jan 2010
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The Linewidth of Ramsey Laser with Bad Cavity
Y ang Li, Wei Zhuang, Jinbiao Chen, ∗ and Hong Guo †
CREAM Group, State Key Laboratory of Advanced Optical Commu nication
Systems and Networks (Peking University) and Institute of Q uantum Electronics,
School of Electronics Engineering and Computer Science,
and Center for Computational Science and Engineering (CCSE ), Peking University, Beijing 100871, P . R. China
(Dated: October 29, 2018)
We investigate a new laser scheme by using Ramsey separated- ﬁeld technique with bad cavity. By studying
the linewidth of the stimulated-emission spectrum of this k ind of laser inside the cavity, we ﬁnd its linewidth
is more than two orders of magnitude narrower than atomic nat ural linewidth, and it is far superior to that
of conventional optical Ramsey method and any other availab le subnatural linewidth spectroscopy at present.
Since any cavity related noise is reduced to cavity-pulling e ﬀect in bad cavity laser, this Ramsey laser provides
the possibility of precision subnatural linewidth spectro scopy, which is critical for the next generation of optical
clock and atom interferometers.
PACS numbers: 42.55.Ah, 42.50.Ar, 42.60.Da, 32.30.-r
Introduction: Since the invention of the separated-ﬁeld
technique [1], it has played an important role in the ﬁeld of
precision spectroscopy due to its linewidth narrowing eﬀect
via multiple coherent interaction. Atomic clocks based on
this technique have greatly extended our ability for frequency
measurement, further, almost all the atom interferometers are
based on this technique [2].
Though, the natural linewidth of quantum transition was
regarded as the ultimate limit to high-resolution laser spe c-
troscopy [4], several methods of subnatural linewidth spec -
troscopy have been proposed to gain subnatural linewidth [3 –
10]. However, in all these e ﬀorts, including optical Ramsey
spectroscopy, subnatural line is realized at the expense of a
quick reduction in signal-to-noise (SNR) ratio due to the ex -
ponential decaying of signal, thus all these schemes can onl y
get the linewidth several times narrower than the atomic nat -
ural linewidth. In the past three decades, this situation do es
not change in the ﬁeld of the precision laser spectroscopy.
On the other hand, the thermal noise of the cavity mirrors is
the main obstacle for further linewidth reduction of a laser
[11, 12], and it is a challenge to substantially reduce this noise
further[13]. Recently, a new scheme, called active optical
clock [14–18], was proposed to substantially reduce the las er
linewidth. With lattice trapped atoms, it is possible to rea ch
mHz linewidth laser based on the mechanism of active optical
clock [14, 15, 19]. The principal mechanism of active optica l
clock is to directly extract light emitted from the ultranar row
atomic transition with a cavity mode linewidth much wider
than that of lasing. This bad cavity ensures that any frequen cy
shift due to cavity noise reduces to cavity-pulling e ﬀect [15–
17], then the thermal noise is not the major obstacle again fo r
reducing the linewidth. This means the bad cavity can play an
indispensable role in new subnatural linewidth spectroscopy.
In this Letter, we propose a new scheme called Ramsey
laser with bad cavity. Distinct from any previous applicati ons
of conventional Ramsey separated oscillating ﬁelds method
[1], which focuses on the absorption spectrum, we here fo-
cus on the stimulated emission spectrum via multiple coher-
ent interactions inside the cavity. We ﬁnd this Ramsey laser
can provide a stimulated-emission spectrum with a linewidth
much narrower than that of any conventional optical Ramsey
seperated-ﬁeld spectroscopy, which is commonly applied in
optical atomic clock. Our results also show that a subnatural
linewidth spectroscopy, superior to any other available su bnat-
ural spectroscopy technique at present [3–10], can be reach ed