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Optics Letters

Fine-tuning of a squeezed state across the hyperfine transition in a cesium D 1 line

Yan Zhang, Yitian Wang, Tianyu Yu, Juan Yu, Chao Liu, Dan Wang, Haitao Zhou, Junxiang Zhang

Published Feb 17, 2026• 0 citations
Publisher

Abstract

Squeezed states tuned to alkali atomic transitions are essential for developing atom–light quantum interfaces. We experimentally demonstrate a frequency-tunable quadrature squeezed state across the hyperfine transition of F =4(6 2 S 1/2 )→ F ′ =3(6 2 P 1/2 ) of a cesium ( 133 Cs) D 1 line at 894.6 nm. Using a compact semi-monolithic optical parametric oscillator (OPO) operating below threshold, we achieve up to −4.1 dB squeezing below the standard quantum limit. When reconfigured as an optical parametric amplifier (OPA), the system generates bright squeezed light that can be finely tuned over a 50 MHz range, which is verified using electromagnetically induced transparency (EIT) spectroscopy. This portable and tunable quantum light source provides a promising platform for cesium-based quantum interfaces in emerging quantum networks.

DOI
10.1364/ol.588377

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Squeezed states tuned to alkali atomic transitions are essential for developing atom–light quantum interfaces. We experimentally demonstrate a frequency-tunable quadrature squeezed state across the hyperfine transition of F =4(6 2 S 1/2 )→ F ′ =3(6 2 P 1/2 ) of a cesium ( 133 Cs) D 1 line at 894.6 nm. Using a compact semi-monolithic optical parametric oscillator (OPO) operating below threshold, we achieve up to −4.1 dB squeezing below the standard quantum limit. When reconfigured as an optical parametric amplifier (OPA), the system generates bright squeezed light that can be finely tuned over a 50 MHz range, which is verified using electromagnetically induced transparency (EIT) spectroscopy. This portable and tunable quantum light source provides a promising platform for cesium-based quantum interfaces in emerging quantum networks.
We report the experimental generation of a squeezed vacuum at frequencies ranging from 2.5 kHz to 200 kHz that is resonant on the cesium D2 line by using a below-threshold optical parametric oscillator (OPO). The OPO is based on a periodically-poled KTiOPO 4 (PPKTP) crystal that is pumped using a bow-tie four-mirror ring frequency doubler. The phase of the squeezed light is controlled using a quantum noise locking technique. At a pump power of 115 mW, maximum quadrature phase squeezing of 3.5 dB and anti-squeezing of 7.5 dB are detected using a home-made balanced homodyne detector. This squeezed vacuum at an atomic transition in the kilohertz range is an ideal quantum source for quantum metrology of enhancing measurement precision, especially for ultra-sensitive measurement of weak magnetic fields when using a Cs atomic magnetometer in the audio frequency range.
The non-classical light resonance on the cesium D1 (894.6 nm) line has important applications in solid-state quantum information networks due to its unique advantages. The cesium D1 line has a simplified hyperfine structure and can be used to realize a light-atom interface. In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D1 line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal. However, the squeezing level is relatively low, and the tunability that has practical significance for squeezed light has not been further investigated. Theoretically, the increase of the transmittance of output mirror and the decrease of the intra-cavity loss of the OPO can improve the squeezing level. Here, we use super-polished and optimal coating cavity mirrors to improve the nonlinear process in OPO. We prepare 447.3 nm blue light from 894.6 nm fundamental light by a second harmonic generation cavity (SHG). The SHG is a two-mirror standing-wave cavity with a PPKTP crystal as the nonlinear medium. The power of generated blue laser is 32 mW when the incident infrared power is 120 mW. Using the blue light to pump an OPO, we achieve quadrature squeezed vacuum light at cesium D1 line. The OPO is a two-mirror standing-wave cavity with a PPKTP crystal. The threshold of OPO is reduced to 28 mW. The squeezing level of generated quadrature squeezed vacuum light is increased to 3.3 dB when the pump power is 15 mW. Taking into account the overall detection efficiency, the actual squeezing reaches 5.5 dB. We inject a weak signal beam into the OPO cavity to act as an optical parametric amplifier (OPA), and test the tunability of squeezzed light. The blue light and the squeezed light are tuned by using a low-frequency triangular wave signal to scan the Ti: sapphire laser. Gradually increasing the amplitude of the scanning triangle wave signal, the generated bright squeezed light can be continuously tuned over a range around 80 MHz without losing the stability of the whole system. The generated squeezed light offers the possibility for the efficient coupling between the non-classical source and solid medium in the process of quantum interface.
A frequency tunable source of squeezed light has been developed for application to a variety of problems in atomic physics. The basic experimental arrangement consists of a subthreshold optical parametric oscillator (OPO) that is pumped by the frequency-doubled output of a titanium-sapphire laser. Squeezed light with continuous tunability of 2 GHz has been generated with a directly observed nonclassical noise reduction of 5.0 dB relative to the vacuumstate limit in a balanced homodyne detector (5.2 dB when corrected for detector noise). Although our source is potentially broadly tunable over the noncritical phase-matching bandwidth of potassium niobate, current research has concentrated on operation at 852 nm for excitation of the D2 line of atomic cesium. Relative to the usual quantum limit associated with vacuumstate fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in FM saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB after correction for detector noise.
A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.
Abstract. We report on the generation of more than 5 dB of vacuum squeezed light at the Rubidium D1 line (795 nm) using periodically poled KTiOPO4 (PPKTP) in an optical parametric oscillator. We demonstrate squeezing at low sideband frequencies, making this source of non-classical light compatible with bandwidth limited atom optics experiments. When PPKTP is operated as a parametric amplifier, we show a noise reduction of 4 dB stably locked within the 150 kHz-500 kHz frequency range. This matches the bandwidth of Electromagnetically Induced Transparency (EIT) in Rubidium hot vapour cells under the condition of large information delay. Squeezed light at the Rubidium D1 line 2 The control of quantum states of light is of great interest for quantum communication purposes. Many quantum information protocols rely on the possibility of coherently delaying and storing the information carried by a laser beam. This can for example be achieved using Electromagnetically Induced Transparency (EIT). Experimental and theoretical studies have shown that information can be delayed within a narrow bandwidth when light interacts with atoms in a lambda configuration [2, 3, 4, 5]. One step further is the delay of quantum information, carried by squeezed
We report on the generation of more than 5 dB of vacuum squeezed light at the Rubidium D1 line (795 nm) using periodically poled KTiOPO4_{4}4​ (PPKTP) in an optical parametric oscillator. We demonstrate squeezing at low sideband frequencies, making this source of non-classical light compatible with bandwidth limited atom optics experiments. When PPKTP is operated as a parametric amplifier, we show a noise reduction of 4 dB stably locked within the 150 kHz-500 kHz frequency range. This matches the bandwidth of Electromagnetically Induced Transparency (EIT) in Rubidium hot vapour cells under the condition of large information delay.