Superconductivity In Nanowires Research Articles

Modeling the effect of superconductor properties on sensitivity and responsivity of superconducting nanowire single photon detector

Superconducting nanowire single photon detector (SNSPD) is a leading candidate for applications requiring the fundamental limit of light detection at high detection rates. While SNSPD technology employing nanowires from conventional low temperature superconducting detectors is mature with several commercial solutions, other material options with higher transition temperature approaching liquid nitrogen with faster signal responses are actively being explored. In this context, we develop a comprehensive model that predicts the final potential response from an SNSPD incorporating several physical and material aspects. A phase diagram of photon detection is developed that describes the latching phases and the photon sensitivity as a function of biasing current and temperature for both low temperature and high temperature superconductors. On the one hand, while low temperature superconductors are observed to be more sensitive than high temperature superconductors (HTSs) under any given biasing condition, a biasing window for a single photon detection with HTS nanowires is identified. On the other hand, HTS nanowires demonstrate three orders of magnitude faster response times than the low temperature superconductor nanowire at the same biasing condition, making it uniquely suited for several practical applications.

Minimal lateral damage fabrication of high-temperature superconducting nanowires via focused helium ion beam irradiation

Abstract The low temperature superconducting nanowire single-photon detectors have become a key infrared photon counting technology in communication and astronomy applications. However, the constrained physical space of devices demands for high-performance superconducting detectors capable of operation at higher temperature. To date, the fabrication of high temperature superconductor nanowires is still facing seriously uneven lateral damage during ion etching process. In this work, we report a promising fabricating method for high-temperature superconducting YBa2Cu3O7−x (YBCO) nanowires, using a focused helium ion beam to minimize the lateral damage of the cut. Based on the simulation, we designed tangent circles and adjacent isosceles triangles to replace lines in cutting nanowires for reducing the superimposed damage of He+ ions. The lateral damage of single helium ion cut has been reduced with a superimposed damage width decrease from 58.8 nm to 29.7 nm. This work provides a platform for boosting the YBCO nanowires to achieve single photon detection.

Optical response in a high-TcYBCO nanowire

We demonstrate the optical response from high-Tc superconductor nanowires at temperatures above that of liquid nitrogen. Using hard oxide Al2O3 as a mask material, we achieved miniaturized YBa2Cu3O7−δ wires with widths nearing 100 nm, a significant milestone for highly sensitive photodetectors. We investigated the photoresponse of these nanowires at temperatures almost an order of magnitude higher than previously reported, under varying temperature and bias current conditions. Our findings reveal a strong correlation between the optical response and the critical parameters of the superconductor, with peak optical responses close to the critical values of temperature and bias current density. Nanowire response to wavelengths from visible to near-infrared range demonstrated a pronounced absorption maximum at temperatures below Tc. These results lay a solid foundation for the development of high-Tc superconductor nanowire single-photon detectors, understanding of nanoscale high-Tc superconductor structure optical behavior, and advancing the prospects of quantum technologies.

Nonlinear Seebeck and Peltier effects in a Majorana nanowire coupled to leads

Abstract We theoretically study nonlinear thermoelectric transport through a topological superconductor nanowire hosting Majorana bound states (MBSs) at its two ends, a system named as Majorana nanowire (MNW). We consider that the MNW is coupled to the left and right normal metallic leads subjected to either bias voltage or temperature gradient. We focus our attention on the sign change of nonlinear Seebeck and Peltier coefficients induced by mechanisms related to the MBSs, by which the possible existence of MBSs might be proved. Our results show that for a fixed temperature difference between the two leads, the sign of the nonlinear Seebeck coefficient (thermopower) can be reversed by changing the overlap amplitude between the MBSs or the system equilibrium temperature, which are similar to the cases in linear response regime. By optimizing the MBS–MBS interaction amplitude and system equilibrium temperature, we find that the temperature difference may also induce sign change of the nonlinear thermopower. For zero temperature difference and finite bias voltage, both the sign and magnitude of nonlinear Peltier coefficient can be adjusted by changing the bias voltage or overlap amplitude between the MBSs. In the presence of both bias voltage and temperature difference, we show that the electrical current at zero Fermi level and the states induced by overlap between the MBSs keep unchanged, regardless of the amplitude of temperature difference. We also find that the direction of the heat current driven by bias voltage may be changed by weak temperature difference.

Frequency modulated continuous wave and time of flight LIDAR with single photons: a comparison.

In this study, we compare the two prominent Light Detection and Ranging (LIDAR) technologies: Frequency Modulated Continuous Wave (FMCW) and Time of Flight (ToF). By constructing a setup capable of performing both LIDAR methods at the single photon level using a Superconducting Nanowire Single Photon Detector (SNSPD), we compare the accuracy and investigate the dependence of the resulting images and accuracy on the signal power and the corresponding signal to noise ratio. We demonstrate that both LIDAR methods are able to reconstruct 3D environments with a signal-to-noise ratio as low as 0.03. However, the accuracy of FMCW LIDAR is shown to degrade in the low photon regime, while ToF LIDAR accuracy is shown to be stable across the same range. Lastly, we use a median de-noising convolution filter to effectively combat the typical "salt and pepper" noise found in LIDAR images, further enhancing the performance of both methods.

Particle-hole thermalization in a composite superconducting and normal-conducting nanowire.

The mechanisms by which isolated condensed matter systems thermalize is a topic of growing interest. Thermalization is known to be linked to the emergence of chaos in the dynamics of a system. We show that a solid state scattering system, containing superconducting elements, can thermalize scattered states without affecting the degree of entanglement of the scattered states. We consider a composite NSNSNSNSN nanowire composed of Bi_{2}Sr_{2}CaCu_{2}O_{8+x} superconducting segments (S) and normal conducting segments (N). We consider parameter regimes where all current flow is due to tunneling currents that are facilitated by quasibound state resonances inside the SNSNSNS structure. At certain energies, scattered pure states approach ergodicity, even though they remain pure.

Experimental demonstration of SNSPD-based free space optical communication with a high extinction ratio modulator

With the development of space technology, the amount of information transmission required by satellites and various spacecraft has increased exponentially. The use of optical communications is a means for future high-speed communications. Adopting superconducting nanowire single photon detectors (SNSPDs) as a receive detector can be capable of detecting signals on the photon scale, which has great application prospects in the field of deep-space optical communication for faint signal detection. However, the use of SNSPDs and high-peak-power optical pulse signals also place higher demands on the extinction ratio of modulators. Therefore, in this paper, we designed a high extinction ratio optical modulator, which adopts cascaded Mach-Zehnder modulators structure, and the static extinction ratio can be reached 77.5 dB. In addition, an SNSPD-based 3.4 km free space optical communication experiment is successfully carried out by using the modulator and a diffuse reflector plate.

Template‐Free Growth of High‐Temperature Superconductor Nanowires

As devices become ever smaller and more efficient, the crystallochemically controlled synthesis of high‐performance materials that comprise their core has attracted enormous attention. Integration of complex functional materials into the next generation of electronic devices will require exquisite control of anisotropic form, either as nanotubes, nanotapes, or nanowires, yet the easy preparation of abundant quantities of them remains stubbornly challenging. Herein, a template‐free, flux‐mediated growth of vast quantities of three compositions of phase‐pure, high‐temperature superconductor nanowires, including for the first time, nanowires of the technologically important quinternary superconductor Bi2Sr2CaCu2O8+x (B2212) is demonstrated. The results of this work may provide an opportunity to investigate the physics and chemistry of highly anisotropic superconductor nanowires and enable their incorporation into nanoelectronics and energy generation systems.

Ultrafast low-jitter optical response in high-temperature superconducting microwires

We report ultrafast optical response in high-Tc superconductor (YBa2Cu3O7−δ) based microwires operating at 76 K and we find a rise time ∼850 ps and a fall time ∼1250 ps and an upper limit of timing jitter of ∼100 ps, using twice the standard deviation of the fitted data. In our experiment, incident power is proven to be an important factor for a device jitter. At low incident power, a lower rate of hot-spot generation by a smaller number of absorbed photons results in a longer latency time to obtain the required number of hot-spots for superconductor-to-normal transition. The lower hot-spot generation rate also results in larger timing jitter of the device. Whereas, at high incident power, a higher hot-spot generation rate yields shorter latency and smaller timing jitter. These observations agree well with our statistical model. Enhancing the sensitivity of the current device can enable future high-Tc superconductor nanowire single photon detectors, toward the widespread use of ultrafast quantum technologies.

Integration of a superconducting nanowire single-photon detector into a confocal microscope for time-resolved photoluminescence (TRPL)-mapping: Sensitivity and time resolution.

This report highlights the combination of the MicroTime 100 upright confocal fluorescence lifetime microscope with a Single Quantum Eos Superconducting Nanowire Single-Photon Detector (SNSPD) system as a powerful tool for photophysical research and applications. We focus on an application in materials science, photoluminescence imaging, and lifetime characterization of Cu(InGa)Se2 (CIGS) devices intended for solar cells. We demonstrate improved sensitivity, signal-to-noise ratio, and time-resolution in combination with confocal spatial resolution in the near-infrared (NIR) range, specifically in the 1000-1300nm range. The MicroTime 100-Single Quantum Eos system shows two orders of magnitude higher signal-to-noise ratio for CIGS devices' photoluminescence imaging compared to a standard NIR-photomultiplier tube (NIR-PMT) and a three-fold improvement in time resolution, which is now limited by the laser pulse width. Our results demonstrate the advantages in terms of image quality and time resolution of SNSPDs technology for imaging in materials science.

Josephson dc Current through T-Shaped Double-Quantum-Dots Hybridized to Majorana Nanowires

We study quantum interference effects on Josephson current in T-shaped double quantum dots (TDQDs) with one of them (the central dot) is sandwiched between the left and right topological superconductor nanowires hosting Majorana bound states (MBSs). We find that the current’s magnitude is suppressed by the inter-dot coupling that induces the quantum interference effect, with unchanged jump in the current at particular phase difference between the two nanowires from which the Josephson effect arises. The current remains as a sinusoidal function with respective to the phase difference in the presence of quantum interference effect, but with significant reduction. The central broad peak in the curve of the Josephson current versus the QDs’ levels are split in different ways depending on the configurations of the latter. We also find that the impacts of the non-z-axial direction magnetic field, bending angle between the two nanowires and the direct hybridization amplitude between the MBSs on the current all depend on the arrangement of the QDs’ energy levels.

Andreev reflection mediated by Majorana zero modes in T-shaped double quantum dots

We theoretically study the Andreev reflection processes in T-shaped double quantum dots (TDQDs) in terms of the nonequilibrium Green’s function technique. It is considered that one of the TDQDs is coupled to the Majorana zero modes (MZMs) prepared at the ends of a topological superconductor nanowire and simultaneously to one metallic and one superconductor lead. Our numerical results show that the in-gap state originated from the proximity effect due to the superconductor lead being sensitive to the existence of MZMs. The local density of states (LDOS) of the spin-up electrons, which are directly coupled to the MZMs, has a Fano antiresonance at the in-gap state. Meanwhile, the local density of the spin-down electrons, which are free from hybridization to the MZMs due to the helical property of the latter, has a Lorentzian resonance at the same state. The differential Andreev conductance of both the spin directions exhibits Fano-type resonance but with different tails’ directions. The in-gap state is also significantly influenced by the energy level and coupling strength of the other side-coupled dot, as well as the MZM–MZM interaction.

Selective Area Growth of PbTe Nanowire Networks on InP

AbstractHybrid semiconductor–superconductor nanowires are promising candidates as quantum information processing devices. The need for scalability and complex designs calls for the development of selective area growth techniques. Here, the growth of large scale lead telluride (PbTe) networks is introduced by molecular beam epitaxy. The group IV‐VI lead‐salt semiconductor is an attractive material choice due to its large dielectric constant, strong spin‐orbit coupling, and high carrier mobility. A crystal re‐orientation process during the initial growth stages leads to single crystalline nanowire networks despite a large lattice mismatch, different crystal structure, and diverging thermal expansion coefficient to the indium phosphide (InP) substrate. The high quality of the resulting material is confirmed by Hall bar measurements, indicating mobilities up to 5600 cm2 (Vs)−1, and Aharonov–Bohm experiments, indicating a low‐temperature phase coherence length exceeding 21 µm. Together, these properties show the high potential of the system as a basis for topological networks.

Fast and accurate measurement of the polarization-dependent detection efficiency of superconducting nanowire single photon detectors.

Superconducting nanowire single photon detectors (SNSPDs) have been extensively investigated due to their superior characteristics, including high system detection efficiency, low dark count rate and short recovery time. The polarization sensitivity introduced by the meandering-type superconductor nanowires is an intrinsic property of SNSPD, which is normally measured by sweeping hundreds of points on the Poincaré sphere to overcome the unknown birefringent problem of the SNSPD's delivery fiber. In this paper, we propose an alternative method to characterize the optical absorptance of SNSPDs, without sweeping hundreds of points on the Poincaré sphere. It is shown theoretically that measurements on the system detection efficiencies (SDEs) subject to cases of four specific photon polarization states are sufficient to reveal the two eigen-absorptances of the SNSPD. We validate the proposed method by comparing the measured detection spectra with the spectra attained from sweeping points on the Poincaré sphere and the simulated absorption spectra.

Quantum dot scanning tunneling microscopy for Majorana bound states in continuum

We propose a device composed of a quantum dot (QD) connected to a normal metal lead to detect Majorana bound states (MBSs), which are formed at the ends of a topological superconductor nanowire (TSNW) and coupled to the lead with spin-dependent hybridization strengths. The information of the MBSs leaked into the lead is inferred from the spectral function of the QD serving as the tip of a scanning tunneling microscope (STM). It is found that lead–MBSs interaction induces a bound state characterized by an infinitely high peak in the dot’s zero-energy spectral function. The overlap between the two modes of the MBSs turns this bound state into a resonant one, and thus the zero-energy peak is split into three with the height of the central one equaling that in the absence of lead–MBSs coupling. We also find that the MBSs have lower impacts on the additional peak in the dot’s spectral function induced by intradot Coulomb interaction.

Uncovering the Role of Ordering in MoSi Superconducting Nanowire Single Photon Detectors with 4D STEM

Uncovering the Role of Ordering in MoSi Superconducting Nanowire Single Photon Detectors with 4D STEM

Microscopic quantum point contact formation as the electromigration mechanism in granular superconductor nanowires

Granular aluminium is a high kinetic inductance thin film superconductor which, when formed into nanowires can undergo an intrinsic electromigration process. We use a combination of experimental and computational approaches to investigate the role of grain morphology and distribution in granular aluminium thin films, when formed into nanowire constrictions. Treating the granular aluminium film as a network of randomly distributed resistors with parameters motivated by the film microstructure allows us to model the electrical characteristics of the nanowires. This model provides estimates of the dependence of sheet resistance on grain size and distribution, and the resulting device to device variation for superconducting nanowires. By fabricating a series of different length nanowires, we study the electromigration process as a function of applied current, and then compare directly to the results of our computational model. In doing so we show that the electromigration is driven by the formation of quantum point contacts between metallic aluminium grains.

Spin phase regulated spin Josephson supercurrent in topological superconductor

Without applied bias voltage, a superconducting phase difference can drive a charge Josephson supercurrent in a superconductor junction. In analogy, we here theoretically propose a spin phase that intrinsically generates spin Josephson supercurrent, and this spin Josephson effect is studied in a junction of superconducting nanowire (SNW). We show that spin-orbit coupling and magnetic field give rise to spin-triplet superconductivity in the SNW, thus allowing the superfluid of both charge and spin. Next we introduce the concept of a spin phase that can be generated by controls of spin current, magnetic field, or electric field. By the analysis of pairing correlations and Ginzburg-Landau-type theory, it is shown that the spin phase makes spin-up and spin-down $S=1$ Cooper pairs get opposite phases and move oppositely in a Josephson junction, so a dissipationless pure spin current is induced. We also derive the formula that the spin current is equal to the derivative of an Andreev bound state to the spin phase, which is analogous to that of charge Josephson effect. At last, our calculation verifies the existence of spin supercurrent inside the superconducting gap in the topologically nontrivial phase. Our paper provides a view on spin Josephson effect and indicates the potential combination of topological superconductivity and spintronics.

Robust topological superconductivity in weakly coupled nanowire-superconductor hybrid structures

The emergence of topological superconductivity and Majorana bound states in superconductor nanowire structures relies on a large induced topological gap, usually assumed to require strong coupling between superconductor and nanowire. However, strong coupling also brings detrimental effects such as the need for large critical Zeeman fields and the emergence of trivial zero-energy states. This work demonstrates that the aforementioned problems can be drastically mitigated in the weak-coupling regime, while retaining the robust properties of topological superconductivity including large topological gaps.

Probabilistic Energy-to-Amplitude Mapping in a Tapered Superconducting Nanowire Single-Photon Detector.

A spectrum-resolved photon detector is crucial for cutting-edge quantum optics, astronomical observation, and spectroscopic sensing. However, such an ability is rarely obtained because a direct linear conversion from weak single-photon energy to a readable electrical signal above the noise level without causing an avalanche is challenging. Here, we overcame these difficulties by building a probabilistic energy-to-amplitude mapping in a tapered superconducting nanowire single-photon detector and combining a computational reconstruction to obtain equivalent spectral resolving capacity. Distinguished dependence of pulse amplitude distributions on varied input spectra has been observed experimentally. As the energy-to-amplitude mapping is probabilistic, statistical measurements are required. By collecting around a few hundred photons, we have demonstrated wavelength perception over a wide spectral range from 600 to 1700 nm with a resolution of 100 nm. These findings represent a new approach to designing spectrum-sensitive SNSPDs for low-light spectroscopic applications.