A frequency-stabilized pump laser for wavelength
conversion in long distance quantum communication
Kohei Ikeda, Yusuke Hisai, Tomoyuki Horikiri, Kazumichi Yoshii, Hideo Kosaka, Feng-Lei Hong
Department of Physics, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
ikeda-kohei-tm@ynu.jp

Abstract: We demonstrate a compact frequency-stabilized laser at 1.06 m forwavelength conversion. The achieved frequency stability is sufficient to connect thenitrogen-vacancy center in diamond and the telecommunication wavelength for longdistance quantum communication.
1. Introduction
Quantum communication has been expected to realize unconditionally secure communications. For long
distance quantum communication, development of quantum repeaters is necessary. In the quantum repeatersystem, quantum memories are used to transfer the quantum states of propagating photons to electron or
nuclear spin states of the memory material.
The wavelengths of the photons depend on the quantum memories, which lie around visible range. On
the other hand, the photons at the telecommunication wavelength are suitable for low loss propagation in
optical fibers. Therefore, to realize long distance quantum communication based on the quantum repeaters,
visible-to-telecommunication wavelength conversion is inevitable and has been intensively studied [1, 2]. In
the wavelength conversion process, a pump laser fulfilling energy conservation is used. For efficient
coupling with the linewidth of quantum memories, frequency stabilization of the pump laser is necessary
for long term stable operation. In the case of Nitrogen-vacancy (NV) center in diamond, one of the mostpromising quantum memory, the wavelength of photons is 637.2 nm and the linewidth is around 10 MHz.
In this paper, we demonstrate the frequency stabilization of a compact pump laser (1064 nm) by using
Doppler-free spectroscopy of molecular iodine, aiming to control the linewidth and the drift of the laser
frequency under 10 MHz. The pump laser used in the present experiment is compact and low cost and emits
high power of hundreds of mW. Therefore, the laser meets the requirements of the wavelength conversion
in the experiment of the NV centers in diamond. However, the laser contains no piezoelectric elements andvery few input ports for frequency control. By using the driving current of pump diode in the solid-statelaser as a control port, we successfully locked the laser frequency without using any external frequency
modulators such as an acousto-optic modulator (AOM). This has resulted a compact pump laser system forloaded in the quantum repeater nodes.

2. Experimental setup
Figure 1 shows a schematic diagram of the compact iodine-stabilized laser. This laser consists of a laser
controller and a laser head, in which a diode laser pumps a solid-state laser. The dimensions of the compact
laser are 95 mm  28 mm  37 mm (length  width  thickness). A periodically-poled potassium titanyl
phosphate (PPKTP) crystal was used for second harmonic generation at 532 nm. The 532 nm beam wasseparated from the fundamental light by a dichroic mirror. The fundamental light can be used for possible
applications or laser frequency measurement. On the other hand, the 532 nm beam was sent to a saturatedabsorption spectrometer with a 40 cm-long iodine cell. The spectroscopy of molecular iodine wasperformed based on the third-harmonic technique by applying a frequency modulation on the compact laser

Fig. 1. Setup of a compact frequency-stabilized laser. Optical and electrical paths are shown as solid and dashed lines,respectively. ISO: Isolator, PPKTP: Periodically poled potassium titanyl phosphate, DM: Dichroic mirror, PBS: Polarization
beam splitter, PD: Photodetector, LO: Local oscillator, LPF: Low-pass filter.

[3]. The laser beam transmitted through the iodine cell (pump beam) was reflected back to the cell by usinga plane mirror. The reflected beam (probe beam) was overlapped with the pump beam. A quarter wave plate(/4) was set between the iodine cell and the mirror, which rotated the polarization of the probe beam by
90° so that the probe beam was reflected by a polarization beam splitter (PBS) and sent to a photodetector(PD).
Usually, the third-harmonic technique is operated with a relatively high modulation frequency ofhundreds of kHz to obtain a low-noise signal. However, this laser does not accept such a high-speedmodulation. Therefore, we aimed to experimentally overcome this challenge. The original laser frequency
was modulated by adding a sinusoidal signal of 2.6 kHz to the injection current of the pump laser in thesolid-state laser and the obtained signal was demodulated at 7.8 kHz. Then we successfully obtained a third
derivative signal of molecular iodine with a good signal-to-noise ratio. This signal was fed back to theinjection current through a servo controller.

3. Results
We observed the hyperfine transitions of the R(86)33-0 line of molecular iodine at 532 nm. The cold-finger
temperature of the iodine cell was held at 7 °C, corresponding to an iodine pressure of 8 Pa. The
temperature of the cell body was matched to a controlled room temperature of 24 °C. The modulation width
was set at  = 1 MHz which is shown in Figure 2(a). The frequency of the compact laser was locked to oneof the hyperfine transitions, a10. To evaluate the stability of the laser frequency, we measured the beatfrequency between the compact laser and an iodine-stabilized Nd:YAG laser using the fundamental light.
The frequency stability of the iodine-stabilized Nd:YAG laser is smaller than 10-13 at an averaging time  of
1 s [4]. Therefore, the frequency stability of the observed beat frequency is mainly dominated by thecompact laser. Figure 2(b) shows the Allan standard deviation calculated from the measured beatfrequencies. The dashed line shows the frequency stability requirement from the linewidth of the NV
centers in diamond ( 1/10 of the linewidth). The frequency stability of a free-running compact laser is 1.6
× 10-9 at = 1 s corresponding to frequency stability of 4.5 × 104 Hz in the laser frequency and gettingworse as the averaging time increases. The frequency stability of the stabilized compact laser is 2.6 × 10-12at = 1 s and reached 8.7 × 10-13 at = 100 s, corresponding to frequency stabilities of 0.72 kHz and 0.25
kHz, respectively. The achieved laser frequency stability is about three orders of magnitude better than therequirement of the NV centers in diamond.

RBW: 3 kHz~ 1 MHz

Fig. 2. (a) Spectra of the beat frequency between iodine-stabilized Nd:YAG laser and the compact laser. (b) Allan standard
deviation calculated from measured beat frequency between iodine-stabilized Nd:YAG laser and the compact laser

4. References[1] Rikizo Ikuta, et al., "Frequency down-conversion of 637 nm light to the telecommunication band for non-classical light emitted
from NV centers in diamond," Opt. Express 22, 11205-11214 (2014)[2] Sebastian Zaske, et al., "Efficient frequency downconversion at the single photon level from the red spectral range to thetelecommunications C-band," Opt. Express 19, 12825-12836 (2011)[3] Takumi Kobayashi, et al., "Compact iodine-stabilized laser operating at 531 nm with stability at the 10−12 level and using a coinsized laser module," Opt. Express 23, 20749-20759 (2015)[4] Feng-Lei Hong, et al., "Frequency reproducibility of an iodine-stabilized Nd:YAG laser at 532 nm," Opt. Communication 235,377-385 (2004)