We developed a four-channel superconducting nanowire single-photon detector system based on a Gifford– McMahon cryocooler. All channels showed a system detection efficiency (DE) (at a dark-count rate) higher than 16% at wavelength, and the best channel showed a system DE of 21% and 30% at 1550 and wavelength, respectively.
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In recent times, multichannel superconducting nanowire single-photon detector (SNSPD ) systems based on closed-cycle cryocoolers have been recognized as promising instruments in the field of optical quantum information technology; this is because SNSPDs deliver a good performance and are capable of continuous and stable operation without any liquid cryogen. At present, the type of SNSPDs mostly used in the multichannel systems are single-layer nanowire devices. They typically showed a practical system detection efficiency (DE) of 1–3% at a wavelength, a low dark-count rate (DCR) of , and excellent timing jitter of [2, 3]. Although they have been successfully employed in quantum key distribution (QKD) [2, 4, 5, 6, 7], further improvement in the system performance, especially in the system DE, is desired.
An effective method of improving the system DE is enhancement of the photoabsorption coefficient by integrating an optical cavity structure with the SNSPD device (OC-SNSPD) . Moreover, efficient optical coupling to the meander nanowire area simultaneously is crucial, and a primary concern is how to implement the OC-SNSPDs in a practical multichannel system. Although the successful implementation of a single OC-SNSPD in a practical closed-cycle cryocooler systems has been reported [9, 10], the development of multichannel systems has not yet been reported. Because the BB84  protocol, which is currently a matured protocol with complete security certification, requires four channels at the receiver side , it is necessary to have four channels in the system for use in these applications. In this Letter, we report the development of a practical four-channel OC-SNSPD system with higher system DE than 16% at a wavelength of . We present a newly developed compact fiber-coupled packaging technique using lenses and describe their system DE and DCR performances.
The devices used in this study were patterned onto 4-nm-thick NbN thin films on MgO substrates . We fabricated 80-nm-wide NbN meander nanowires, covering an area of with a filling factor of 62.5%. The superconducting critical temperature and critical current density of nanowires were and , respectively. An optical cavity structure, consisting of a 100-nm-thick Au mirror and a 250-nm-thick SiO cavity, were covered on the meander nanowire area. The thicknesses of the Au mirror and SiO cavity were designed for wavelengths of .
Figure 1a shows the schematic layout of the fiber-coupled packaging for OC-SNSPDs. This compact fiber-coupled packaging technique was modified from the one used for single-layer SNSPD [2, 3], which is simple and has high reliability. A fiber ferrule was fixed to the fiber- holding block in advance by using an adhesive so that the distance from the exit end to the rear surface of the OC-SNSPD chip was at low temperature. OC-SNSPD chips were mounted on chip-mounting blocks, which had a through hole at the center of the chip-mounting area. An MU-type fiber ferrule was inserted through this hole from the rear. The fiber-holding block was joined to the chip-mounting block from the rear, and the two blocks were accurately aligned so that the incident light spot illuminated the center of the meander area. The dimensions of the packaging blocks that could be used for OC-SNSPDs are almost of the same size [15 mm (length) × 15 mm (width) × 10 mm (thickness)] as those used for single-layer SNSPD chips, as shown in Fig. 1b. Hence, OC-SNSPD packaging blocks were installed in the multichannel Gifford–McMahon (GM) cryocooler system without any modification, which can simultaneously cool six SNSPD packages to with a thermal fluctuation range of .
To achieve efficient optical coupling, the light beam waist on the meander nanowire area must be smaller than the size of the nanowire area. Since the OC-SNSPDs have to be illuminated from the rear side through the substrate, small-gradient index (GRIN) lenses were used to reduce the beam waist at a distance from the exit end. To embed lenses into the compact packages, GRIN lenses with a diameter of , which is equal to the clad diameter of a single-mode (SM) optical fiber, are directly fusion spliced to the end of the optical fiber. Because the fiber-spliced lenses were inserted into the MU fiber ferrule, the shape of the end of fiber did not change at all from that without lenses, as shown in Fig. 1c. The NA and length of the two lenses are chosen so that the focal length is equal to the appropriate distance in the packaging and the beam waist becomes as small as possible. As a result, the beam waist () was estimated to be on the meander nanowire area, when the distance between the exit end and the substrate is and the thickness of the MgO substrate is . This beam waist is sufficiently small to allow efficient optical coupling with the meander nanowire area of .
To verify the effectiveness of the fiber-spliced GRIN lenses, we measured the system DE versus the DCR of the OC-SNSPD device, with and without the lenses, as shown in Fig. 2. Here, the system DE is defined as the ratio of the output count rate and the input photon flux to the system. Since the SNSPD devices have polarization dependences , the polarization properties of input photons were optimized to maximize the DE . The system DE (at a DCR of ) of a device without lenses remained a small value of 2.8%; this was because the beam waist expanded on the meander nanowire area, thereby resulting in poor optical coupling. With focusing by GRIN lenses, system DE was much improved to 21%. Figure 3a shows system DE versus DCR, and Fig. 3b shows the system DE and DCR versus bias current normalized by superconducting critical current at wavelengths of 1310 and . A maximum system DE reached 40% and 28%, at 1310 and wavelength, respectively, at the DCR of several thousand hertz, where the bias current was just below . This implies the optical cavity also worked efficiently at wavelength, and the device DE at became higher than that at .
By using compactly packaged OC-SNSPDs, we built up a four-channel SNSPD system. Figure 4 shows the system DE versus DCR of the four-channel OC-SNSPDs at a wavelength of . Needless to say, all the channels can operate simultaneously without any time gating, and hence, our OC-SNSPD can be used in QKD protocols and other quantum optical applications. The system DE of all the channels show similar dependencies on the DCR and exceed 16% and 20% at DCRs of 100 and , respectively, which are significantly higher than those of standard devices . Although the system DE varies from channel to channel, we believe values of all the channels can exceed those achieved in the present study by improving the yield of nanowire uniformity. The performance of the packaged OC-SNSPDs does not show any significant change during several thermal cycles. These results indicate that our packaging technique exhibits high stability against thermal cycling.
As a future study, an effective optical coupling to smaller devices should be considered, because miniaturization of device size are certainly effective for further improvement of both device DE and response speed [14, 15]. It should be noted that this will be possible by utilizing a combination of fiber-spliced GRIN lenses and a substrate thickness-reduction technique . For example, if the thickness of the MgO substrate is reduced to and the GRIN lenses are optimally designed, the beam waist can be reduced to , thus making it possible to achieve efficient optical coupling to smaller devices. Although system DE is still limited by the filling factor of the meander nanowire area, new structures to focus input photons into nanowire, such as nanograting structure, would be effective for further improvement .
In conclusion, we have developed a four-channel SNSPD system using OC-SNSPD devices. To achieve efficient optical coupling in the multichannel system, a compact fiber-coupled packaging technique that employs GRIN lenses has been developed. The beam waist on the meander nanowire area has been successfully reduced for achieving effective optical coupling to devices with an area of . All the channels showed a system DE higher than 16%, and the best device showed 21% at a DCR of and wavelength of . These DE values are comparable to that using a low-temperature nanostage , are significantly higher than those of standard multichannel SNSPD systems using a compact packaging technique [2, 3, 4], and definitely make a great impact to the QKD and various applications.
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