December 19, 2014

NICT, in collaboration with the University of Electro-Communications (UEC, President: Dr. Takashi Fukuda), has succeeded in the demonstration of a twin photon source pumped by a 2.5 GHz repetition-rate-tunable frequency comb laser. This photon source has a 30 times higher signal-to-noise ratio and a better performance in quantum interference than the traditional photon source pumped by a 76 MHz laser. This experiment will contribute to further progress of quantum information and quantum communications technology.

The result was published in Scientific Reports (Nature Publishing Group) on December 19, 2014.

Single photons generated from spontaneous parametric down conversion (SPDC) are the most widely used information carriers in the study of quantum information processing and quantum communications. Due to the spontaneous property, however, a dilemma always exists in SPDC: higher pump power is required for higher data rate, while it degrades quantum interference visibility due to unwanted multi-pair emissions, leading to the increase of error rates in quantum key distribution and photonic quantum information processing.

Here the researchers have solved this problem by using a 2.5 GHz repetition-rate-tunable comb laser, which was recently developed in NICT. This laser can operate in relatively low pulse energy, while keeping high average power, thanks to a high repetition rate. The low pulse energy would result in the reduction of the multiple-pair emission. At the same time a high counting rate would be expected owing to the high average power.

In the experiment, the researchers have demonstrated that the photon source at the telecom wavelength pumped by their comb laser operated at 2.5 GHz has a 30 times higher signal-to-noise ratio and a better performance in quantum interference than the nowadays widely used sources, which are usually pumped by a 76 MHz laser.

They compared the performances of their photon source pumped by 76MHz laser to that by 2.5 GHz laser with the experimental setup shown in Figure 1.

A comb laser with a 10-0.625 GHz tunable-repetition-rate at 1,553 nm wavelength is amplified by a high-power erbium-doped fiber amplifier (EDFA), and frequency-doubled by a 10-mm-long type-0 periodically poled lithium niobate wave guide (PPLN-WG). Then, after filtering out 1,553 nm light by short-pass filters, the 776.5 nm laser light from second harmonic generation (SHG) pumps a 30-mm-long type-II periodically poled potassium titanyl phosphate crystal (PPKTP) for SPDC. The downconverted photons, the signal and idler, are separated by a polarization beamsplitter (PBS), and coupled into single mode fibers (SMF) by couplers (SMFC).

SMFs are connected to a fiber beamsplitter (FBS) by two fiber connectors (FC), for the test of Hong-Ou-Mandel (HOM) interference. For comparison, we also introduce a 76 MHz pump laser (Mira900, at 776.5 nm, around 2 ps). An iris is inserted for the 76 MHz laser to control the beam size, so as to make the coincidence counts comparable for both pump lasers.

In this experiment, the researchers have tried both 10 GHz and 2.5 GHz repetition rate, and mainly used 2.5 GHz repetition rate for the pump laser. Their superconducting nanowire single photon detectors (SNSPDs) have a system detection efficiency (SDE) of around 70% with a dark count rate (DCR) less than 1 kcps.

Figure 2: Quantum interference visibilities versus pump powers for 2.5 GHz and 76 MHz lasers

To compare the different performances of the 2.5 GHz and 76 MHz lasers, they performed the quantum interference (called Hong-Ou-Mandel interference) tests at different pump powers. Without subtract any background counts, they compared the raw visibilities of the interference at different pump powers in Figure 2.

It is noteworthy that the visibilities by the 76 MHz laser decrease rapidly when the pump powers increase. In contrast, the visibilities by the 2.5 GHz laser show almost no decrease up to 35 mW, thanks to the high repetition rate and low multi-pair emission. The experimental data are well fitted theoretical predicted lines.

Future Prospects

The system provides a powerful tool box for the single- and two-photon sources at the telecom wavelength, which will pave a way to implementing quantum photonics circuits with a variety of good and low-cost telecom components, and will eventually realize scalable quantum information and communications technology in optical infrastructures.  

Journal: Scientific Reports (Nature Publishing Group), DOI: 10.1038/srep07468

http://www.nature.com/scientificreports

Title: Efficient generation of twin photons at telecom wavelengths with 2.5 GHz repetition-rate-tunable comb laser

Authors: Rui-Bo Jin, Ryosuke Shimizu, Isao Morohashi, Kentaro Wakui, Masahiro Takeoka, Shuro Izumi, Takahide Sakamoto, Mikio Fujiwara, Taro Yamashita, Shigehito Miki, Hirotaka Terai, Zhen Wang and Masahide Sasaki