Search

The coexistence of quantum and classical light in the same fiber link is extremely desired in developing quantum communication. It has been implemented for different quantum information tasks, such as classical light coexisting with polarization-entangled photons at telecom O-band, and with quantum signal based quantum key distribution (QKD). In this work, we demonstrate the coexistence of energy-time entanglement based QKD and fiber optical communication at the telecom C-band. The property of noise from the classical channel is characterized with classical light at different wavelengths. With the largest noise, i.e., the worst case, the properties of energy-time entanglement are measured at different fiber optical communication rates. By measuring the two-photon interference of energy-time entanglement, our results show that a visibility of 82.01$\pm$1.10\% is achieved with a bidirectional 20 Gbps fiber optical communication over 40 km. Furthermore, by performing the BBM92 protocol for QKD, a secret key rate of 245 bits per second could be generated with a quantum bit error rate of 8.88\% with the coexisted energy-time entanglement.~Our demonstration paves the way for developing the infrastructure for quantum networks compatible with fiber optical communication., Comment: 6 pages, 3 figures

We report a compact, scalable, and high-performance superconducting nanowire single-photon detector (SNSPD) array by using a multichannel optical fiber array-coupled configuration. For single pixels with an active area of 18 um in diameter and illuminated at the telecom wavelength of 1550 nm, we achieved a pixel yield of 13/16 on one chip, an average system detection efficiency of 69% at a dark count rate of 160 cps, a minimum timing jitter of 74 ps, and a maximum count rate of ~40 Mcps. The optical crosstalk coefficient between adjacent channels is better than -60 dB. The performance of the fiber array-coupled detectors is comparable with a standalone detector coupled to a single fiber. Our method is promising for the development of scalable, high-performance, and high-yield SNSPDs., Comment: 8 pages, 12 figures (including Supplementary Material)

The magnetic penetration depth (λ) of a superconductor is an important parameter which connects the macroscopic electrodynamics with the microscopic mechanism of superconductivity. High-accuracy measurement of λ is of great significance for revealing the pairing mechanism of superconductivity and exploring the applications of superconductors. Among various methods used to measure λ of superconducting films, the two-coil mutual inductance (MI) technique has been widely adopted due to its high precision and simplicity. In this paper, we start with introducing the principle of MI technique and pointing out that its accuracy is mainly limited by the uncertainties in the geometric parameters (e.g. the distance between two coils) and the leakage flux around the film edge. On this basis, we build a homemade transmission-type MI device with a delicate design to achieve high-accuracy. Two coils are fixed by a single-crystal sapphire block machined with high precisions to minimize the uncertainty in geometry. As a result, the reproducibility in induced voltage measured with sample remounted is better than 4%. Besides, the flux leakage around the film edge is accurately determined by measuring a thick Nb film and Nb foils. The voltage induced by leakage flux is only around 1% of that measured in the normal state. Therefore, the absolute value of λ can be accurately extracted after flux leakage subtraction and normalization. It is shown that the error of the measured λ is less than 10% for a typical superconducting film with a thickness of 100 nm and a penetration depth of 150 nm. Furthermore, the performance of our apparatus is tested on epitaxial NbN films with thickness of 6.5 nm. The results show that the low temperature variation of superfluid density is well described by the dirty s-wave BCS theory, and at temperatures close to Tc, the superfluid density decrease drastically, owing to the Berezinski-Kosterlitz-Thouless transition transition. Moreover, the zero-temperature magnetic penetration depth and the superconducting energy gap extracted from the fitting parameters are both consistent with the reported values. Our device provides an ideal platform for carrying out detailed studies of the dependence of λ on temperature, chemical composition and epitaxial strain, etc. It could also be utilized to characterize other parameters of superconductor such as the critical current density, and when combined with the ionic liquid gating technique, our device offers an efficient route for revealing the microscopic mechanism of superconductivity.

In order to investigate the characteristics of the package and fiber coupling of a Superconducting Nanowire Single-photon Detector(SNSPD),the SNSPD was successfully built up in our laboratory.Experiments show that the quantum efficiencies of the detector under different light wavelengths are 6%@1310nm and 3%@1550nm when the dark count rate is 100 Hz.Then,as the distance(gap) between SNSPD and optical fiber would be changed due to the thermal stress during cooling,and would cause a misalignment,two kinds of fiber coupling methods to modulate the gap at room temperature were proposed.Finally,by altering the wavelength of the input light,the exact values of the gap at different temperatures were measured and then the influence of temperature on the gap were figured out by an experiment.The experimental results indicate that decreases of the gap for the two packages are 4.1 μm and 17 μm when the temperature dereases 270 K.Based on the analysis of package material and structure,the gaps at both room and low temperature were calculated theoretically.The result fits well with that of the experiments and it may offer some references to the design of new package and fiber coupling in the future.

The injection efficiency of spin-polarized quasi-particles in a superconducting Y-Ba-Cu-O (YBCO) film is investigated by means of current injection into the Nd0.7Sr0.3MnO3/SrTiO3/YBa2Cu3O7-δ heterostructures. We have successfully prepared six injection junction windows in the YBCO thin film strip, which are all 56 µm wide, but 80 µm, 40 µm, 20 µm, 10 µm, 5 µm, and 2 µm long, respectively. At 16 K, our YBCO thin film strip (80 nm thick) has Jc = 2×105A/cm2. Under the spin-polarized injection current of Iinj = 0.5 mA, with decreasing injection junction length L, injection efficiency η (η≡ΔJc/ΔJinj; ΔJc: the suppression of critical current density due to finite ΔJinj) increases gradually, until L is not more than 20 µm. In that case, η hardly varies with the change of L, reaching up to about 6. We think that these results are related to the spin diffusion length of spin-polarized quasi-particles in the superconducting thin film. In our heterostructures, the YBCO thin film keeps good superconductivity and the injection junction windows are of proper size, therefore, we have obtained relatively high injection efficiency of spin-polarized quasi-particles.

A YBa2Cu3O7-δ thin film is modified by a probe electric field of an atomic force microscope to form a ridge with the width of only a grain cell. The modification varies with the operation parameters of the bias voltage, the moving velocity of the probe and the ambient humidity. Energy dispersive spectroscopy analysis shows only oxygen deficiency in the modified YBCO thin film. As a result, the suppressed superconductivity was found in the junction crossing the ridge.

A new method to study the transient detection efficiency (DE) and pulse amplitude of superconducting nanowire single photon detectors (SNSPD) during the current recovery process is proposed — statistically analyzing the single photon response under photon illumination with a high repetition rate. The transient DE results match well with the DEs deduced from the static current dependence of DE combined with the waveform of a single-photon detection event. This proves that static measurement results can be used to analyze the transient current recovery process after a detection event. The results are relevant for understanding the current recovery process of SNSPDs after a detection event and for determining the counting rate of SNSPDs.

In fabricating $\mathrm{Bi_2Sr_2CaCu_2O_{8+��}}$ intrinsic Josephson junctions in 4-terminal mesa structures, we modify the conventional fabrication process by markedly reducing the etching rates of argon ion milling. As a result, the junction number in a stack can be controlled quite satisfactorily as long as we carefully adjust those factors such as the etching time and the thickness of the evaporated layers. The error in the junction number is within $\pm 1$. By additional ion etching if necessary, we can controllably decrease the junction number to a rather small value, and even a single intrinsic Josephson junction can be produced., to bu published in Jpn. J. Appl. Phys., 43(7A) 2004