Lixing You

Jitter analysis of a superconducting nanowire single photon detector

Jitter is one of the key parameters for a superconducting nanowire single photon detector (SNSPD). Using an optimized time-correlated single photon counting system for jitter measurement, we extensively studied the dependence of system jitter on the bias current and working temperature. The signal-to-noise ratio of the single-photon-response pulse was proven to be an important factor in system jitter. The final system jitter was reduced to 18 ps by using a high-critical-current SNSPD, which showed an intrinsic SNSPD jitter of 15 ps. A laser ranging experiment using a 15-ps SNSPD achieved a record depth resolution of 3 mm at a wavelength of 1550 nm.

Superconducting nanowire single photon detector with on-chip bandpass filter

Dark count rate is one of the key parameters limiting the performance of the superconducting nanowire single photon detector (SNSPD). We have designed a multi-layer film bandpass filter that can be integrated onto the SNSPD to suppress the dark counts contributed by the stray light and blackbody radiation of the fiber. The bandpass filter is composed of 16 SiO(2)/Si bilayers deposited onto the backside of a thermally oxidized Si substrate. The substrate shows an excellent bandpass filter effect and provides a high transmittance of 88% at the central wavelength of the pass band, which is the same as that of the bare substrate. The SNSPDs fabricated on the substrate integrated with the bandpass filter show conspicuous wavelength-sensitive detection efficiency. The background dark count rate is reduced by two orders of magnitude to sub-Hz compared with the conventional SNSPD (a few tens of Hz). The detector exhibits a system detection efficiency of 56% at DCR of 1 Hz, with the measured minimal noise equivalent power reaching 2.0 × 10(-19) W/Hz(1/2).

Measurement-Device-Independent Quantum Key Distribution over Untrustful Metropolitan Network

Quantum cryptography holds the promise to establish an information-theoretically secure global network. All field tests of metropolitan-scale quantum networks to date are based on trusted relays. The security critically relies on the accountability of the trusted relays, which will break down if the relay is dishonest or compromised. Here, we construct a measurement-device-independent quantum key distribution (MDIQKD) network in a star topology over a 200 square kilometers metropolitan area, which is secure against untrustful relays and against all detection attacks. In the field test, our system continuously runs through one week with a secure key rate ten times larger than previous result. Our results demonstrate that the MDIQKD network, combining the best of both worlds --- security and practicality, constitutes an appealing solution to secure metropolitan communications.

Multimode fiber-coupled superconducting nanowire single-photon detector with 70% system efficiency at visible wavelength

We report the development of the multimode fiber-coupled superconducting nanowire single-photon detector with high system detection efficiency at visible wavelength. The detector consists of a 10.5-nm-thick and 150-nm-wide NbN nanowire meander fabricated on a Si substrate with a multilayer dielectric mirror and a quarter wavelength cavity for obtaining high optical absorptance. The meander area was 35 µm in diameter and coupled with the GRIN-lensed multimode optical fiber with a core diameter of 50 µm. The system reached detection efficiency of 70% with dark count rate of 100 Hz at the wavelength of 635 nm, 3 dB roll-off response counting rate of 8.5 Mcps, and timing jitter of 76 ps.

Field Test of Measurement-Device-Independent Quantum Key Distribution

The main type of obstacles of practical applications of quantum key distribution (QKD) network are various attacks on detection. Measurement-device-independent QKD (MDIQKD) protocol is immune to all these attacks, and thus, a strong candidate for network security. Recently, several proof-of-principle demonstrations of MDIQKD have been performed. Although novel, those experiments are implemented in the laboratory with secure key rates less than 0.1 b/s. Besides, they need manual calibration frequently to maintain the system performance. These aspects render these demonstrations far from practicability. Thus, justification is extremely crucial for practical deployment into the field environment. Here, by developing an automatic feedback MDIQKD system operated at a high clock rate, we perform a field test via deployed fiber network of 30 km total length achieving a 16.9 b/s secure key rate. The result lays the foundation for a global quantum network, which can shield from all the detection-side attacks.

High- T c superconductivity in ultrathin Bi 2 Sr 2 CaCu 2 O 8+ x down to half-unit-cell thickness by protection with graphene

High-Tc superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi2Sr2CaCu2O(8+x) (Bi2212) model system, despite extensive studies, the intrinsic superconducting properties at the thinness limit have been difficult to determine. Here, we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage characteristic due to the Dirac nature of the graphene band structure. More interestingly, although the critical temperature remains essentially the same with reduced thickness of Bi2212, the slope of the normal state T-linear resistivity varies by a factor of 4-5, and the sheet resistance increases by three orders of magnitude, indicating a surprising decoupling of the normal state resistance and superconductivity. The developed technique is versatile, applicable to investigate other two-dimensional (2D) superconducting materials.

Single photon detector with high polarization sensitivity

Polarization is one of the key parameters of light. Most optical detectors are intensity detectors that are insensitive to the polarization of light. A superconducting nanowire single photon detector (SNSPD) is naturally sensitive to polarization due to its nanowire structure. Previous studies focused on producing a polarization-insensitive SNSPD. In this study, by adjusting the width and pitch of the nanowire, we systematically investigate the preparation of an SNSPD with high polarization sensitivity. Subsequently, an SNSPD with a system detection efficiency of 12% and a polarization extinction ratio of 22 was successfully prepared.

Improving the timing jitter of a superconducting nanowire single-photon detection system

Low timing jitter is a unique merit of superconducting nanowire single-photon detectors (SNSPDs) for time-correlated applications. Quantitative analysis was performed for the SNSPD system. Aided by an oscilloscope with an optimal signal amplitude, we were able to measure a full width at half-maximum system timing jitter as low as 14.2 ps for a high-switching-current SNSPD using a room-temperature low-noise amplifier. When using a time-correlated single-photon counting module, the system timing jitter was 17.3 ps. The detectors intrinsic timing jitter was estimated at ∼12.0  ps.

Superconducting nanowire single photon detector at 532 nm and demonstration in satellite laser ranging

Superconducting nanowire single-photon detectors (SNSPDs) at a wavelength of 532 nm were designed and fabricated aiming to satellite laser ranging (SLR) applications. The NbN SNSPDs were fabricated on one-dimensional photonic crystals with a sensitive-area diameter of 42 μm. The devices were coupled with multimode fiber (ϕ = 50 μm) and exhibited a maximum system detection efficiency of 75% at an extremely low dark count rate of <0.1 Hz. An SLR experiment using an SNSPD at a wavelength of 532 nm was successfully demonstrated. The results showed a depth ranging with a precision of ~8.0 mm for the target satellite LARES, which is ~3,000 km away from the ground ranging station at the Sheshan Observatory.

Superconducting critical current of a single Cu2O4 plane in a Bi2Sr2CaCu2O8+x single crystal

By feeding current into the topmost Cu2O4 layer of a mesa etched into the surface of a Bi2Sr2CaCu2O8+x (BSCCO) single crystal, we measured its superconducting critical value from a sharp upturn or break in the current-voltage characteristics of the mesa. From this, we estimate the sheet critical current density of a single Cu2O4 plane to be similar to 0.3-0.7 A/cm at 4.5 K, corresponding to the bulk current density of 2-5 MA/cm(2). These values are among the largest ever measured for BSCCO single crystals, thin films and tapes, and we argue that they represent the true intrinsic values of the material.