A free-standing copper compressible microinterconnect (CMI) for cryogenic applications was fabricated, flip-chip bonded, and its electrical and mechanical characteristics were analyzed in this letter. To address the increasing demand for reliable off-chip interconnects in cryogenic electronic packaging, we present a pressure-contact-based 3D copper CMI that exhibits linear elastic behavior, requires a relatively low bonding force, and offers reworkability. Ex-situ resistance measurements at room temperature and in-situ measurements from 300 K down to 14 mK were performed on a flip-chip bonded CMI array, revealing up to a 39.6% reduction in resistance upon cool down. Furthermore, a mechanistic investigation of the flip-chip bonding process was conducted, including force–displacement measurements, along with force–stress analysis using finite element analysis (FEA), to verify robustness.

Josephson parametric amplifiers (JPAs) have become an important component of superconducting applications. Simulating these circuits is vital for optimizing their performance, including their gain and stability. Previously, simulating JPAs has been inefficient due to the limits of using SPICE-based simulators that work in the time domain. JPAs are better suited to frequency domain simulations over a broad frequency range and parameter space. We present JPA simulations using the recently released flux-aware Josephson junction (JJ) and inductor models in Keysight Advanced Design Systems (ADS). We simulate two different flux-driven JPAs: one based on an RF SQUID, and another based on a DC SQUID. We efficiently run harmonic balance simulations over a wide range of circuit parameters. We compare the use of RF and DC SQUIDs in these JPAs by analyzing their simulation results. This work provides a foundation for future SQUID-based JPA simulations in Keysight ADS.

Disorder in quantum many-body systems can drive transitions between ergodic and non-ergodic phases, yet the nature–and even the existence–of these transitions remains intensely debated. Using a two-dimensional array of superconducting qubits, we study an interacting spin model at finite temperature in a disordered landscape, tracking dynamics both in real space and in Hilbert space. Over a broad disorder range, we observe an intermediate non-ergodic regime with glass-like characteristics: physical observables become broadly distributed and some, but not all, degrees of freedom are effectively frozen. The Hilbert-space return probability shows slow power-law decay, consistent with finite-temperature quantum glassiness. In the same regime, we detect the onset of a finite Edwards-Anderson order parameter and the disappearance of spin diffusion. By contrast, at lower disorder, spin transport persists with a nonzero diffusion coefficient. Our results show that there is a transition out of the ergodic phase in two-dimensional systems.

Disorder-induced phenomena in quantum many-body systems pose significant challenges for analytical methods and numerical simulations at relevant time and system scales. To reduce the cost of disorder-sampling, we investigate quantum circuits initialized in states tunable to superpositions over all disorder configurations. In a translationally-invariant lattice gauge theory (LGT), these states can be interpreted as a superposition over gauge sectors. We observe localization in this LGT in the absence of disorder in one and two dimensions: perturbations fail to diffuse despite fully disorder-free evolution and initial states. However, Rényi entropy measurements reveal that superposition-prepared states fundamentally differ from those obtained by direct disorder sampling. Leveraging superposition, we propose an algorithm with a polynomial speedup in sampling disorder configurations, a longstanding challenge in many-body localization studies.

Quantum error correction (QEC) is the primary strategy for protecting a quantum computer from the environment1,2. The prerequisite of QEC is that errors must remain sufficiently rare, which requires perpetually adapting the control parameters of the computer to the drifting environmental conditions. The current solution to this problem is to terminate the entire quantum computation for recalibration, but it is incompatible with the long runtimes of future quantum algorithms3,4. Here we address this challenge by unifying calibration with computation. We grant the QEC process5-11 a dual role: its error-detection events are not only used to correct the logical quantum state but are also repurposed as a learning signal, teaching a reinforcement learning agent12-16 to continuously steer the control parameters and stabilize the quantum system during computation. We experimentally demonstrate this framework on a Willow superconducting processor, improving the logical stability of the surface code 3.5-fold against injected drift. By synthesizing our full suite of technological advances, we achieve record performance of the surface and colour codes, with average logical error per cycle of 7.72(9) × 10-4 and 8.19(14) × 10-3, respectively. Numerical simulations of large codes with tens of thousands of control parameters confirm the scalability of our RL framework, revealing an optimization speed that is independent of system size. This work thus enables a new paradigm: a quantum computer that learns from its errors and never stops computing.

Cryogenic computing systems often require communication between single flux quantum (SFQ) circuits at 4 K and CMOS circuits at a higher temperature stage. Interface circuits such as a 4JL (four-junction logic) gate and a SQUID (superconducting quantum interference device) stack have previously been used for SFQ-CMOS data links, but they have only been used for systems contained at 4 K. When communicating with CMOS circuits at a higher temperature stage, these interface circuits must be implemented in accordance with a holistic analysis of the entire cryogenic computing system. We have conducted an analysis in which we compare the impact of a 4JL gate and a SQUID stack on a system designed for the HYPRES ICE-T. For our analysis, we use the CORE e4 RSFQ microprocessor as the SFQ circuitry at 4 K. We provide guidelines for deciding between a 4JL gate and a SQUID stack as superconductor-semiconductor interface circuits in hybrid computing systems with multiple temperature stages.

Understanding how interacting particles approach thermal equilibrium is a major challenge of quantum simulators1,2. Unlocking the full potential of such systems towards this goal requires flexible initial state preparation, precise time evolution and extensive probes for final state characterization. Here we present a quantum simulator comprising 69 superconducting qubits that supports both universal quantum gates and high-fidelity analogue evolution, with performance beyond the reach of classical simulation in cross-entropy benchmarking experiments. This hybrid platform features more versatile measurement capabilities compared with analogue-only simulators, which we leverage here to reveal a coarsening-induced breakdown of Kibble-Zurek scaling predictions3 in the XY model, as well as signatures of the classical Kosterlitz-Thouless phase transition4. Moreover, the digital gates enable precise energy control, allowing us to study the effects of the eigenstate thermalization hypothesis5-7 in targeted parts of the eigenspectrum. We also demonstrate digital preparation of pairwise-entangled dimer states, and image the transport of energy and vorticity during subsequent thermalization in analogue evolution. These results establish the efficacy of superconducting analogue-digital quantum processors for preparing states across many-body spectra and unveiling their thermalization dynamics.

A custom, multi-stage, FPGA-based testing platform is proposed for signal integrity testing of superconducting interonnects at gigabit-speeds (6 Gb/s) with emulated, amplified single flux quantum (SFQ) pulses at cryogenic temperatures (4 K). Results for superconducting Nb-striplines will be shown utilizing this platform and characterization for the cryogenic performance of the FPGA's transceivers will be shown. Performance and implementation results demonstrate the viability of complex, CMOS technologies at cryogenic temperatures, providing scalable control and readout options for superconductor-based technologies.

The promise of fault-tolerant quantum computing is challenged by environmental drift that relentlessly degrades the quality of quantum operations. The contemporary solution, halting the entire quantum computation for recalibration, is unsustainable for the long runtimes of the future algorithms. We address this challenge by unifying calibration with computation, granting the quantum error correction process a dual role: its error detection events are not only used to correct the logical quantum state, but are also repurposed as a learning signal, teaching a reinforcement learning (RL) agent to continuously steer the physical control parameters and stabilize the quantum system during the computation. We experimentally demonstrate this framework on a Willow superconducting processor, improving the logical stability of the surface code 3.5-fold against injected drift. By synthesizing our full suite of technological advances, including RL fine-tuning of the entire system and near-optimal decoding, we achieve record performance of the surface and color codes, with average logical error per cycle of ε_L=7.72(9)×10^-4 and ε_L=8.19(14)×10^-3 respectively. Simulations of surface codes up to distance-15 with tens of thousands control parameters confirm the scalability of our RL framework, revealing an optimization speed that is independent of the system size. This work thus enables a new paradigm: a quantum computer that learns from its errors and never stops computing.

A remarkable characteristic of quantum computing is the potential for reliable computation despite faulty qubits. This can be achieved through quantum error correction, which is typically implemented by repeatedly applying static syndrome checks, permitting correction of logical information. Recently, the development of time-dynamic approaches to error correction has uncovered new codes and new code implementations. In this work, we experimentally demonstrate three time-dynamic implementations of the surface code, each offering a unique solution to hardware design challenges and introducing flexibility in surface code realization. First, we embed the surface code on a hexagonal lattice, reducing the necessary couplings per qubit from four to three. Second, we walk a surface code, swapping the role of data and measure qubits each round, achieving error correction with built-in removal of accumulated non-computational errors. Finally, we realize the surface code using iSWAP gates instead of the traditional CNOT, extending the set of viable gates for error correction without additional overhead. We measure the error suppression factor when scaling from distance-3 to distance-5 codes of Λ_35,hex = 2.15(2), Λ_35,walk = 1.69(6), and Λ_35,iSWAP = 1.56(2), achieving state-of-the-art error suppression for each. With detailed error budgeting, we explore their performance trade-offs and implications for hardware design. This work demonstrates that dynamic circuit approaches satisfy the demands for fault-tolerance and opens new alternative avenues for scalable hardware design.

A remarkable characteristic of quantum computing is the potential for reliable computation despite faulty qubits. This can be achieved through quantum error correction, which is typically implemented by repeatedly applying static syndrome checks, permitting correction of logical information. Recently, the development of time-dynamic approaches to error correction has enabled different codes and implementations that do not rely on static syndrome measurements. Here we experimentally demonstrate three time-dynamic implementations of the surface code, each offering a distinct solution to hardware design challenges faced by surface code realizations. First, we embed the surface code on a hexagonal lattice, reducing the necessary couplings per qubit from four to three. Second, we walk a surface code, swapping the role of data and measure qubits each round, achieving error correction with built-in removal of accumulated non-computational errors. Finally, we realize the surface code using iSWAP gates instead of the traditional CNOT, extending the set of viable gates for error correction without additional overhead. We measure the error suppression factor when scaling from distance-3 to distance-5 codes of Lambda 35,hex = 2.15(2), Lambda 35,walk = 1.69(6) and Lambda 35,iSWAP = 1.56(2), achieving state-of-the-art error suppression for each. Our work demonstrates that dynamic circuit approaches meet the demands for fault tolerance and enable alternative strategies for scalable hardware design.

This study investigated the thermal cycling reliability of surface mount assemblies with Quad Flat No-Lead (QFN) and Ceramic Quad Flat J-leads (CQFJ) packages under cryogenic thermal cycling conditions. The investigation included both experimental and modeling components. The experimental procedure was conducted in two steps: controlled thermal cycling (125 cycles) in a Delta Design chamber 400K to 95K ( 127(circle)C to -178(circle)C) followed by direct immersion (125 cycles) in liquid LHe 105K to 5K (168(circle)C to -268(circle)C). During thermal cycling, the resistance of the solder joints and wire bonds in the test structure daisy chains was monitored to detect resistance increases and opens. Finite element modeling (ANSYS) was used to compare with the experimental results. The findings contribute to the database of cryogenic packaging reliability necessary for the use of cold electronics in cryogenic, lunar, and other space missions.

Inthe application of quantum information transduction with superconducting (SC) qubits, SC resonators play a phenomenal role in magnon-photon coupled systems. We present NbN (T-c = 10.8 K) superconducting microstrip resonators tailored for hybrid magnon-photon coupled systems. Our resonators exhibit remarkable coupling capability, enabling rigorous probing of magnon-photon interactions. With a fundamental frequency of 1.4 GHz and a second harmonic of 2.8 GHz, these resonators offer extensive frequency coverage, facilitating the identification and characterization of a wide spectral range. It exhibits a strong magnon-microwave photon coupling with a sphere of yttrium iron garnet (YIG). The average gyromagnetic ratio (gamma/2 pi) and coupling strength (g/2 pi) achieved are 2.7 MHz/Oe and 11.14 MHz, respectively. The NbN SC resonators developed in this work address challenges associated with probing magnon-photon coupling in strong magnetic fields and are expected to serve as a unique platform for studying magnon-photon coupled hybrid devices.

Fault-tolerant quantum computing requires a universal gate set, but the necessary non-Clifford gates represent a significant resource cost for most quantum error correction architectures. Magic state cultivation offers an efficient alternative to resource-intensive distillation protocols; however, testing the proposal's assumptions represents a challenging departure from quantum memory experiments. We present an experimental study of magic state cultivation on a superconducting quantum processor. We implement cultivation, including code-switching into a surface code, and develop a fault-tolerant measurement protocol to bound the magic state fidelity. Cultivation reduces the error by a factor of 40, with a state fidelity of 0.9999(1) (retaining 8

Digital communication between temperature stages is a critical part of superconducting electronics systems. Specifically, enabling clean, low-loss communication between single flux quantum (SFQ) circuits at 4 K and CMOS circuitry and memory at higher temperature stages (such as 50 K) can allow for significantly more system memory than what current superconducting devices and memories allow. An amplifier for SFQ pulses, such as a four-junction logic (4JL) gate, must also be included at the beginning of the data link. We have simulated such a data link in Pathwave ADS, which uses four-junction logic (4JL) gate characteristics that were initially simulated in JoSIM. ANSYS HFSS was used to generate the S-parameters used in ADS. Eye diagrams generated from the ADS simulation results are analyzed. The eye diagrams from the data link give an eye width of 77.2 ps and an eye height of 2.16 mV in the worst case, compared to an eye width of 94.3 ps and an eye height of 4.24 mV from the 4JL gate. These simulations will enable future hardware implementations of SFQ to CMOS memory data links.

Thermocompression bonding of a superconductive flexible cable and a non-superconductive flexible cable was performed to a single multi-chip module (MCM) test vehicle using a unique bonding approach. Custom tooling was designed and machined to retrofit an existing flip chip bonder to perform the atypical assembly demonstration. 59 of 61 connections were achieved of the non-superconductive Cu cable. Of the measurable connections down to 5.5 K, 11 of 11 connections were maintained. The superconductive Nb cable demonstrated full connectivity at room temperature. Of the measured connections down to 5 K, an average onset superconducting transition temperature of 8.80 K was observed. Techniques demonstrated provide options beyond conventional bonding approaches for cryogenic interconnects.

Hybrid magnonic systems have emerged as a promising direction for information propagation with preserved coherence. Due to high tunability of magnons, their interactions with microwave photons can be engineered to probe novel phenomena based on strong photon-magnon coupling. Improving the photon-magnon coupling strength can be done by tuning the structure of microwave resonators to better interact with the magnon counterpart. Planar resonators have been explored due to their potential for on-chip integration, but only common modes from stripline-based resonators have been used. Here, we present a microwave spiral resonator supporting the spoof localized surface plasmons (LSPs) and implement it to the investigation of photon-magnon coupling for hybrid magnonic applications. We showcase strong magnon-LSP photon coupling using a ferrimagnetic yttrium iron garnet sphere. We discuss the dependence of the spiral resonator design to the engineering capacity of the photon mode frequency and spatial field distributions, via both experiment and simulation. By the localized photon mode profiles, the resulting magnetic field concentrates near the surface dielectrics, giving rise to an enhanced magnetic filling factor. The strong coupling and large engineering space render the spoof LSPs an interesting contender in developing novel hybrid magnonic systems and functionalities.

Superconducting resonators have previously been used for magnon-photon coupling, which facilitates quantum information transduction. In this work, we demonstrate robust superconducting Nb resonators that show closely spaced resonances with high quality (Q) factors. The fundamental frequencies ranging from 440 MHz to 668 MHz with second harmonics at 870 MHz and 1.3 GHz, offer extensive frequency coverage. They maintain resonance integrity by exhibiting useful Q-factors for many harmonics, even in magnetic fields up to 1 T at 1.7 K. These resonators provide a versatile platform for investigating magnon-photon interactions in hybrid magnonic systems, overcoming limitations of previous designs and offering the potential for broad frequency coverage and application in quantum information studies.

Josephson-CMOS hybrid memory leverages the high speed and low power operation of single-flux quantum logic and the high integration densities of CMOS technology. One of the commonly used type of interface circuits in Josephson-CMOS memory is a Suzuki stack, which is a latching high-voltage driver circuit. Suzuki stack circuits are typically powered by an AC bias voltage that has several limitations such as synchronization and coupling effects. To address these issues, a novel DC-biased Suzuki stack circuit is proposed in this paper. As compared to a conventional AC-biased Suzuki stack circuit, the proposed DC-biased design can provide similar output voltage levels and parameter margins, approximately two times higher operating frequency, and three orders of magnitude lower heat load of bias cables.

Engineered dissipative reservoirs have the potential to steer many-body quantum systems toward correlated steady states useful for quantum simulation of high-temperature superconductivity or quantum magnetism. Using up to 49 superconducting qubits, we prepared low-energy states of the transverse-field Ising model through coupling to dissipative auxiliary qubits. In one dimension, we observed long-range quantum correlations and a ground-state fidelity of 0.86 for 18 qubits at the critical point. In two dimensions, we found mutual information that extends beyond nearest neighbors. Lastly, by coupling the system to auxiliaries emulating reservoirs with different chemical potentials, we explored transport in the quantum Heisenberg model. Our results establish engineered dissipation as a scalable alternative to unitary evolution for preparing entangled many-body states on noisy quantum processors.