High sensitivity spectroscopy of cesium Rydberg
atoms using electromagnetically induced
transparency
Jianming Zhao, 1*Xingbo Zhu,1 Linjie Zhang,1 Zhigang Feng,1 Changyong Li,1 Suotang
Jia1
1
State Key Laboratory of Quantum Optics and Quantum Optics Devices, and College of Physics and Electronics
Engineering, Shanxi University, Taiyuan 030006, P. R. China
* [email protected]
Abstract: A high sensitivity spectroscopy of Rydberg atoms is presented by
using electromagnetically induced transparency (EIT) in the 6S1/2-6P3/2-nD
ladder-type system of cesium vapor cell at room temperature. The EIT
spectra of 40D Rydberg state are measured and the dependences of the EIT
magnitude and linewidth on the coupling laser power are investigated in
detail. The Rydberg EIT linewidth is measured to be about 5.6MHz when
the powers of probe and coupling lasers are 50µW and 5.2mW, respectively,
and which is close to the natural linewidth of cesium atoms. The effect of
double resonance optical pumping on EIT is also investigated. The fine
structures of nD (n = 39-55) are measured and the experimental result is in
agreement with quantum defect theory.
©2009 Optical Society of America
OCIS codes: (270.1670) Coherent optical effects; (020.5780) Rydberg states; (190.4180)
Multiphoton processes; (300.6210) Spectroscopy atomic.
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Received 1 Jul 2009; revised 12 Aug 2009; accepted 13 Aug 2009; published 21 Aug 2009
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1. Introduction
Rydberg atoms are excited atoms with one or more electrons that have very high principal
quantum number and have been extensively studied over many decades [1]. Rydberg atoms
display exaggerated collision properties and rich many-body behavior and became a attractive
candidate for quantum logic gates [2,3] due to their enhanced interactions. An important
process is the dipole blockade in the Rydberg excitation of atoms which due to the long-range
dipole-dipole interaction shifting the Rydberg energy from its isolated atomic level. Rydberg
atoms are usually produced by two or three photons excitation and detected by using the
selective pulsed field ionization method [1] in room temperature atomic beam or laser cooled
cold atoms [4,5]. This detection method has high efficiency but is destructive and atoms
cannot be reused. For application of quantum information, a nondestructive detection of the
Rydberg atoms is necessary [6]. Electromagnetically induced transparency (EIT) will be a
good candidate for nondestructive detection, which is manifest as a decrease or absence of
absorption of probe laser due to the quantum coherence effect when the probe laser is
resonance with the atomic transition in three-level system. EIT has been widely studied in
atomic vapor [7] and in laser cooled atoms [8] in Lamdba, Vee and ladder-type three-level
systems. The quantum coherence is induced between two ground states for Lamdba-type
system or between the ground and excited states for ladder-type system. Clarke group [9]
observed the EIT signal of excited D states with a principal quantum number is up to n = 8 in
ladder-type rubidium atomic system. Mohapatra group [6] demonstrated the coherent optical
detection of highly excited Rydberg states using EIT in rubidium atomic cell. Raithel group
[10] obtained Autler-Townes spectroscopy of the 5S1/2-5P3/2-44D ladder-type system in cold
rubidium atoms. The ladder-type atomic system combines the attractive features of both
ground dark states and Rydberg states. We can use this system to map the long range
interaction of Rydberg state on to the ground state. Recently EIT involving Rydberg states has
been observed in thermal rubidium atomic vapor cell and a strontium atomic beam [11];
Adams group [12] demonstrated a giant electro-optic effect based on polarizable dark states
and obtained an electro-optic coefficient of 10−6 m/V2, which is 6 orders of magnitude larger
than the Kerr cell based on Nitrobenzene.
In this work, we obtain the high sensitivity spectroscopy of Rydberg atoms (n = 39-55) by
using EIT in cesium atomic vapor cell at room temperature. A nondestructive probe of nD
Rydberg level is observed and the EIT linewidth is measured to be about 5.6MHz which near
the natural linewidth. The dependences of the EIT linewidth and magnitude on the coupling
laser power are investigated in detail. The fine structures of Rydberg states (n = 39-55) are
measured and the result is in good agreement with the quantum defect theory.
2. Experiments
Relevant atomic levels and the experimental setup are shown in Fig. 1. We present a ladder
three-level system with a ground state |1> (6S1/2 F = 4), an excited state |2> (6P3/2 F’ = 5) and
|3> (Rydberg nD states) interacting with two laser fields, which is shown in Fig. 1 (a). The
transition |1>→ |2 > is coupled with the weak probe field with wavelength of λp = 852.3 nm,
and another transition |2>→ |3 > is coupled with the intense coupling field with wavelength of
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Received 1 Jul 2009; revised 12 Aug 2009; accepted 13 Aug 2009; published 21 Aug 2009
31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15822
λc = 509 nm - 517 nm. The probe laser is produced by an external-cavity diode laser (Toptica
DL100) with the laser linewidth of ~1MHz, the power of 50µW, and the waist of about
1.2mm (1/e2 radius), which propagates through a room temperature cesium atom vapor cell
with length of 60mm. The coupling laser, produced by a commercial frequency doubled
tunable diode laser system (Toptica TA-SHG110) with the linewidth less than 2MHz, counterpropagates through the cell with a maximum power up to 100mW and a waist of 1.0mm (1/e2
radius). The frequency of the coupling laser is calibrated by the wavelength meter (HFAngstrom WSU-30). The absorption of the weak probe beam is measured by an avalanche
photodiode (Hamamatsu Si APD, S3884).
Fig. 1. (a) Relevant levels scheme used for the experimental demonstration of a ladder-type
EIT. Ωp and Ωc are Rabi frequencies of probe and coupling beam, γ2 and γ3 are delay rates of
level |2> and |3>. (b) Schematic of the experimental setup, where SAS: saturated absorption
spectroscopy; M: reflecting mirror; PBS: polarization prism; BS: beam splitter; ST: second EIT
device used to lock the coupling laser; DM: dichroic mirror.
The probe laser is scanned cross the transition of 6S1/2 (F = 4) to 6P3/2(F’ = 5). The
coupling laser is locked to the transition of 6P3/2(F’ = 5)→nD using EIT signal in second cell
or scanned cross the transition from 6P3/2(F’ = 5) to the nD Rydberg states (the nD-state
hyperfine splitting is not resolved) and relevant transition frequency is taken from Ref [13]. In
order to reduce the effect of the stray magnetic fields, cesium vapor cell is placed in the µmetal shield material. The experimental schematic diagram is shown in Fig. 1(b).
3. Results and discussions
Figure 2 shows the typical saturated absorption spectroscopy (SAS) (a) and the 6S1/2-6P3/2-nD
ladder-type three-level system EIT signal involved 40D5/2 Rydberg atoms (b) with the probe
laser is scanned through the transition of 6S1/2 (F = 4) to 6P3/2 and the intense coupling laser is
locked to the transition of 6P3/2(F’ = 5) to 40D5/2 using the second cell. The power of probe
and coupling laser are 50µW and 60mW, respectively and corresponding EIT linewidth is
measured to be 16MHz.
We can obtain a theoretical prediction for the EIT line shape using an approximate
expression for the susceptibility derived under the condition of a weak probe laser [14]
χ (v)dv = i
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4ℏg 22
ε0
Ωc
)2
2
N (v)dv[γ 2 − i ∆ p − i
v+
]−1 (1)
c
γ 3 − i (∆ p + ∆ c ) − i (ωP − ωc )v / c
ωp
(
Received 1 Jul 2009; revised 12 Aug 2009; accepted 13 Aug 2009; published 21 Aug 2009
31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15823
saturated absorption spectrum
Rydberg atom EIT
0.09
C35
0.08
C45
0.07
T5
(a)
0.06
0.05
0.04
0.03
0.36
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
T4
(b)
-250
0
-125
Probe frequency /MHz
125
250
Fig. 2. A saturated absorption spectroscopy of probe laser (a) and EIT signal of ladder three
levels system of Rydberg atoms 40D5/2 (b), the weak probe beam is scanned through the
transition of 6S1/2 (F = 4)→ 6P3/2 and the coupling laser is locked to the transition of 6P3/2 (F’ =
5)→40D5/2 Rydberg state.
Where Ωp,c, ∆p,c, and ωp,c are Rabi frequencies, detunings and resonance frequencies of the
probe or coupling laser, respectively, and N(v) is the number density of cesium atoms with
velocity v, c is speed of the light, γ2,3 are the decay rates of the intermediate and upper states
in the cascade system. Because the Rydberg states lifetime increase with the principal
quantum number increasing and scaling as n3, the lifetime of the 40D state is much longer
than that of 6P state and measured to be about 39µs [16], the decay rate of 40D state γ3 is
much less than that of 6P state, so we can ignore γ3 in Eq. (1). Keep the detuning of coupling
laser is zero and integrate the imaginary part of Eq. (1) at room temperature and we can obtain
the absorption coefficient through the vapor cell as a function of the probe detuning.
In order to study the effect of the power of coupling laser on the EIT spectra, we keep the
probe laser power fixed to 50µW and change the coupling laser power with neutral attenuation
plates and measure the EIT signals, the results are shown in Fig. 3 (a) for signal magnitude
and (b) for linewidth as the function of coupling laser power. It is clear that the EIT
magnitude increases as the coupling laser power increases at power less than 60mW. However
when the laser power is more than 60mW, the EIT magnitude is no longer increased and tends
to saturate and similar results have been found in reference [14]. In addition, the linewidth of
EIT resonance is also increases as the laser power increases. When the coupling
Fig. 3. 40D EIT signal versus the power of the coupling laser with (a) for the signal magnitude
and (b) for the linewidth.
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31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15824
EIT signal /Arb.U
DROP signal /Arb.U
laser power is 5.2mW, the linewidth of 5.6MHz is obtained, which is close to the natural
linewidth of the intermediate 6P state (5.2MHz). The reasons for the broadening linewidth are
complicated; there are two main factors, one is the power broadening effect and the other is
EIT effect and double resonance optical pumping (DROP) effect.
DROP is the optical pumping phenomenon in the ladder-type atomic system, which is
based on the interaction of atoms with two optical fields that are resonant with two transitions
that share a common state. Kim group [15] investigated the DROP effect in 5S1/2-5P3/2-5D5/2
rubidium atomic ladder-type system, and they obtained the double structure spectrum with the
narrow line for EIT and broad line for DROP. For the ladder-type system, the EIT signals can
be observed in the counter-propagation regime of pump and probe lasers, while the DROP
signals are observed in both the counter-propagation and co-propagation regime [14]. In order
to investigate the DROP and EIT phenomena further, we make the pump and probe lasers copropagation through the cesium vapor cell and measure the absorption of the probe laser.
DROP signal is obtained and compared with the EIT signal. At the condition of the weak
probe laser of 50µW, the DROP signal is much smaller than the EIT signal, as the population
of the intermediate states is very low, so the DROP signal is very small or no DROP signal.
EIT is a nonlinear effect for atomic coherence, there is a larger EIT signal although the power
of probe laser is very small, and result is shown in Fig. 4 with the probe laser is locked to the
transition of 6S1/2(F = 4)→6P3/2(F’ = 5) and the coupling laser is scanned cross the transition
from 6P3/2(F’ = 5) to 50D. The big and small peaks denote the coupling laser resonant with
6P3/2(F’ = 5)→50D5/2 and 6P3/2(F’ = 5)→50D3/2 transitions, respectively. However a larger
DROP signal is obtained when we increase the power of the probe. On the other hand, DROP
affects the EIT spectrum profile, the DROP signal linewidth is limited into the spontaneous
decay rates of 6P and nD levels and that of the EIT is limited into the coherence dephasing
rate of dipole forbidden transitions. For ladder-type EIT, we can obtain the signal with narrow
linewidth by decreasing the probe laser power.
0.012
0.010
0.008
0.006
0.004
0.002
0.000
-0.002
-0.004
-0.006
0.330
0.325
0.320
0.315
0.310
0.305
0.300
0.295
0.290
(a)
(b)
19625.56
19625.58
Wavenumber of coupling laser /cm
-1
Fig. 4. DROP (a) and EIT (b) signals under the condition of the weak probe beam is locked to
the transition of 6S1/2 (F = 4)→ 6P3/2 (F’ = 5) and the coupling laser is scanned through the
transition of 6P3/2 (F’ = 5)→50D Rydberg state.
The fine structure splitting is an important parameter for Rydberg atoms and has been
measured using two-photon absorption and a thermionic diode to detect the ionized Rydberg
rubidium atoms [17]. The fine structure splitting of rubidium atoms was also obtained by laser
excitation and field ionization of ultracold Rydberg atoms [18]. Here we obtain the cesium
atomic nD states fine structure splittings by using the high sensitivity Rydberg atoms EIT
spectroscopy at a room temperature vapor cell. We lock the probe laser to the transition of
6S1/2(F = 4)→6P3/2(F’ = 5) using polarization spectroscopy and tune the coupling laser
frequency cross the 6P3/2(F’ = 5)→40D transition in counter-propagating regime. We measure
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Received 1 Jul 2009; revised 12 Aug 2009; accepted 13 Aug 2009; published 21 Aug 2009
31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15825
the probe absorption signal and obtain the fine structures of 40D. The corresponding excited
wavenumber of coupling laser are 19596.2175 cm−1 for 40D3/2 and 19596.2556 cm−1 for
40D5/2, respectively, which is consistent with the theoretical value that is calculated using
quantum defect theory. The EIT spectra give a measurement of 40D state fine structure
splitting of 0.0381 cm−1.
We change the wavelength of coupling laser for different nD states (n = 39-55) and further
investigate the EIT resonance as well as fine structure. The fine structure splittings of the nD
states are measured experimentally and Fig. 5 shown fine structure splittings as a function of
principal quantum number n. We calculate the fine structure splittings of nD state according to
the quantum defect theory and the result is shown in Fig. 5 with red solid line. It is clear that
the experiment is in agreement with the quantum defect prediction.
nD fine structure splitting/MHz
1400
experimental
theory
1200
1000
800
600
400
38
40
42
44
46
48
50
52
54
56
principal quantum number
Fig. 5. The measured Rydberg D state fine structure splittings as a function of the principal
quantum number and the red solid line is the theoretical calculation by using the quantum
defect theory, the quantum defect of cesium D state is 2.4699 [13].
4. Conclusion
In conclusion, we have demonstrated EIT in ladder-type three levels system involved Rydberg
states (n = 39-55) at the room temperature cesium cell, which providing a direct
nondestructive probe of Rydberg energy levels. The dependences of the EIT linewidth and
magnitude on coupling laser power are investigated and linewidth of 5.6MHz that is near to
the natural linewidth is obtained. The fine structure splittings of nD states are also measured
and the results are in agreement well with the quantum defect theory. Rydberg atom is very
sensitive to external electric field and one important feature is the possibility to tune the
interaction strength by electric field in Forster resonances process [5]. On the other hand,
Rydberg atoms can lead to a giant electro-optic effect based on the polarizable dark states
[12]. This effect will open up the prospect of single particle detection and single photon
entanglement and so on.
Acknowledgements:
Supported by the 973 program (No. 2006CB921603), the National Natural Science
Foundation of China (No. 60678003 and 60778008), the Special Foundation for State Major
Basic Research Program of China (No. 2005CCA06300) and the Scholarship Foundation of
National and Shanxi Province.
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Received 1 Jul 2009; revised 12 Aug 2009; accepted 13 Aug 2009; published 21 Aug 2009
31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 15826