This paper describes a novel strategy to combine laser direct write (LDW) and optical lithography (I-line) to fabricate 200 mm waferscale superconducting multi-chip modules (S-MCM) for interconnecting multiple active superconducting electronics chips based on single flux quantum (SFQ) logic for next generation cryogenic processing systems. The packaging roadmap includes the development of S-MCM (48 mm x 48 mm) using a nearly full I-line reticles, followed by reticle stitching to fabricate the largest possible stitched S-MCM (96 mm x 96 mm) using a four mask/layer process. The stitching process starts with sequential exposure of multiple I-line photomasks, with small overlap (stitched area), to realize larger combined circuit areas for design- critical S-MCM layers with minimum linewidths of 0.8-1 gm. The packaging roadmap further extends the S-MCM size utilizing laser direct write (LDW) lithography to make wider (> 1 gm) features such as fan-out circuits, extending the stitched circuit area to include the entire 200 mm wafer as a single S-MCM. Process control monitors (PCM) include snake/comb test structures, critical dimension (CD) cells, transmission lines, daisy chains etc. Niobium-indium-based microbump technology was developed to demonstrate full-size (20 x 20 mm 2 ) SFQ flip-chips on an S-MCM with low 4 K interconnect resistance (50-100 μΩ) at the SFQ chip to S-MCM interface. Confocal micrographs show a uniform niobium-indium microbump-based interconnect network and X-ray images show the desired deformation of niobiumindium microbumps after flip-chip bonding. Full-size (20x20mm 2 ) flip-chip daisy chains with 10k and 100k bumps maintained high Nb critical current post-bonding (> 50 mA), demonstrating a viable platform for building larger superconducting computing systems.

Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region's contribution to the resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal–air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric etch and find that it reduced losses from the substrate–air interface, thereby improving the quality factor.

Improving the performance of superconducting qubits and resonators generally results from a combination of materials and fabrication process improvements and design modifications that reduce device sensitivity to residual losses. One instance of this approach is to use trenching into the device substrate in combination with superconductors and dielectrics with low intrinsic losses to improve quality factors and coherence times. Here we demonstrate titanium nitride coplanar waveguide resonators with mean quality factors exceeding two million and controlled trenching reaching 2.2 $\mu$m into the silicon substrate. Additionally, we measure sets of resonators with a range of sizes and trench depths and compare these results with finite-element simulations to demonstrate quantitative agreement with a model of interface dielectric loss. We then apply this analysis to determine the extent to which trenching can improve resonator performance.

Superconducting single-flux-quantum-based (SFQ) digital integrated circuits (ICs) are a promising candidate for high-speed and ultralow energy dissipation computing systems. Circuits based on several versions of SFQ-type logic, RQL, and AQFP logic have been demonstrated with complexities reaching up to a few tens of thousands of gates. Packaging many superconducting ICs using microbump (15µm or less) technology and performing high throughput, nearly lossless data transfer between various superconducting and/or CMOS chips are highly desirable for a hybrid superconducting computer architecture, but this density has not yet been demonstrated. An efficient way to achieve this goal is to couple the chips through a passive superconductive multichip module (S-MCM) that distributes information between integrated circuits utilizing lossless superconducting transmission lines. Here we show implementation of such a superconducting base using well-defined impedance lines to couple multiple ICs to enable a cryogenic integration approach for possible future hybrid superconducting computing systems. The use of indium-based microbumps to form interconnects between an S-MCM and superconducting ICs, and its electrical performance, are discussed. Optimized microbumps were used to fabricate interconnections on a large S-MCM by using thermocompression(TC) bonding. From 17200 to 68800 microbumps fabricated in a 5 mm × 5 mm active area of an S-MCM, having pitches ranging from 35 µm to 15 µm, were bonded with a superconducting IC to create a daisy chain structure. In addition, a large active area (10 mm × 10 mm) flip-chip having continuous niobium daisy chains with up to 77,500 bumps with 15 µm bump diameter and 35 µm pitch was also demonstrated. As a case study, an optimized microbump interconnect construction for attaching 16 superconducting chips with a large 32 mm × 32 mm superconducting base was fabricated and tested at room temperature and 4.2 K to study the structural and electrical integrity. Josephson junctions and niobium lines on integrated superconducting chips and the superconducting base maintained their I-V characteristics, which allows the design of building blocks for a superconducting computing system.

We present an athermalized design and performance analysis of a robust imaging system used to couple light from an input fiber to a superconducting nanowire single photon detector.

In this work, we demonstrate saturated photon detection efficiency with narrow NbN nanowires and SNAPs to boost the signal-to-noise ratio, and we demonstrate a stabilizing choke inductance that is part of the optically-active area.

The Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between a lunar-orbiting satellite and ground stations on Earth with error-free downlink data rates up to 622 Mb/s utilizing an optical receiver based on photon-counting superconducting nanowires and operating near 1550 nm.

Superconducting circuits comprising SNSPDs placed in parallel—superconducting nanowire avalanche photodetectors, or SNAPs—have previously been demonstrated to improve the output signal-to-noise ratio (SNR) by increasing the critical current. In this work, we employ a 2-SNAP superconducting circuit with narrow (40 nm) niobium nitride (NbN) nanowires to improve the system detection efficiency to near-IR photons while maintaining high SNR. Additionally, while previous 2-SNAP demonstrations have added external choke inductance to stabilize the avalanching photocurrent, we show that the external inductance can be entirely folded into the active area by cascading 2-SNAP devices in series to produce a greatly increased active area. We fabricated series-2-SNAP (s2-SNAP) circuits with a nanowire length of 20 μm with cascades of 2-SNAPs providing the choke inductance necessary for SNAP operation. We observed that (1) the detection efficiency saturated at high bias currents, and (2) the 40 nm 2-SNAP circuit critical current was approximately twice that for a 40 nm non-SNAP configuration.

Core-shell PbS-CdS quantum dots enhance the peak external quantum efficiency of shortwave-infrared light-emitting devices by up to 50-100-fold (compared with core-only PbS devices). This is more than double the efficiency of previous quantum-dot light-emitting devices operating at wavelengths beyond 1 μm, and results from the passivation of the PbS cores by the CdS shells against in situ photoluminescence quenching.

The understanding of the photophysics of visible-emitting colloidal nanocrystals (NCs) has long been aided by single-molecule studies of their emission. Until recently, no suitable detection technologies have existed for corresponding studies of shortwave-infrared (SWIR) emitters. Now, the use of superconducting nanowire single-photon detectors (SNSPDs) enables the detailed study of SWIR NC emission dynamics at the single-emitter level. Here, we report a detailed analysis of the emission dynamics of individual InAs/CdZnS NCs emitting in the SWIR region. We observe blinking akin to the type A and type B blinking previously observed in visible-emitting CdSe NCs. We determine the intrinsic radiative lifetime of several InAs/CdZnS NCs and find examples ranging from 50-200 ns, indicative of a quasi-type-II electronic structure. We also measure g(0)((2)) for several of these NCs and find that their biexciton emission quantum yields vary from <1% up to 43%.

We describe a number of methods that have been pursued to develop superconducting nanowire single-photon detectors (SNSPDs) with attractive overall performance, including three systems that operate with >70% system detection efficiency and high maximum counting rates at wavelengths near 1550 nm. The advantages and tradeoffs of various approaches to efficient optical coupling, electrical readout, and SNSPD design are described and contrasted. Optical interfaces to the detectors have been based on fiber coupling, either directly to the detector or through the substrate, using both single-mode and multimode fibers with different approaches to alignment. Recent advances in electrical interfaces have focused on the challenges of scalability and ensuring stable detector operation at high count rates. Prospects for further advances in these and other methods are also described, which may enable larger arrays and higher-performance SNSPD systems in the future. Finally, the use of some of these techniques to develop fully packaged SNSPD systems will be described and the performance available from these recently developed systems will be reviewed. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

United States. Dept. of Defense. Assistant Secretary of Defense for Research & Engineering (Air Force Contract FA8721-05-C-0002)

Superconducting nanowire single photon detectors (SNSPDs) have separately demonstrated high efficiency, low noise, and extremely high speed when detecting single photons. However, achieving all of these simultaneously has been limited by detector subtleties and tradeoffs. Here, we report an SNSPD system with <80 ps timing resolution, kHz noise count rates, and 76% fiber-coupled system detection efficiency in the low-flux limit at 1550 nm. We present a model for determining the detection efficiency penalty due to the detection recovery time, and we validate our method using experimental data obtained at high count rates. We demonstrate improved performance tradeoffs, such as 68% system detection efficiency, including losses due to detector recovery time, when coupled to a Poisson source emitting 100 million photons per second. Our system can provide limited photon number resolution, continuous cryogen-free operation, and scalability to future imaging and GHz-count-rate applications.

Superconducting materials provide a unique opportunity for single-photon detectors. The combination of the strong non-linearity present in the superconducting-metal transition and in Josephson junctions; the unique electrical property of zero resistance; the fast relaxation processes present in many superconducting materials; and the easily engineered optical absorptance of metals results in a system that can be adapted to many photo-detection requirements. As a result of these material features, a variety of detector families have emerged in recent years based on superconducting nanowires, tunnel junctions, weak thermal links, and kinetic-inductive resonators. The detectors variously provide high speed single-photon detection; high-sensitivity in the infrared, optical, UV, or even x-ray wavelengths; and high efficiency. The resulting applications include quantum and classical high-data-rate communication, biological imaging, LIDAR, and VLSI circuit evaluation among others.

We present a quantitative measurement of the number of trapped carriers combined with a measurement of exciton quenching to assess limiting mechanisms for current losses in PbS-quantum-dot-based photovoltaic devices. We use photocurrent intensity dependence and short-wave infrared transient photoluminescence and correlate these with device performance. We find that the effective density of trapped carriers ranges from 1 in 10 to 1 in 10 000 quantum dots, depending on ligand treatment, and that nonradiative exciton quenching, as opposed to recombination with trapped carriers, is likely the limiting mechanism in these devices.