Superconducting Circuits for Quantum Information: An Outlook

A quantum computer consists of quantum bits (qubits) and a control processor that acts as an interface between the programmer and the qubits. As qubits are very sensitive to noise, they rely on continuous error correction to maintain the correct state. Current proposals rely on software-managed error correction and require large instruction bandwidth, which must scale in proportion to the number of qubits. While such a design may be reasonable for small-scale quantum computers, we show that instruction bandwidth tends to become a critical bottleneck for scaling quantum computers. In this paper, we show that 99.999% of the instructions in the instruction stream of a typical quantum workload stem from error correction. Using this observation, we propose QuEST (Quantum Error-Correction Substrate), an architecture that delegates the task of quantum error correction to the hardware. QuEST uses a dedicated programmable micro-coded engine to continuously replay the instruction stream associated with error correction. The instruction bandwidth requirement of QuEST scales in proportion to the number of active qubits (typically < < 0.1%) rather than the total number of qubits. We analyze the effectiveness of QuEST with area and thermal constraints and propose a scalable microarchitecture using typical Quantum Error Correction Code (QECC) execution patterns. Our evaluations show that QuEST reduces instruction bandwidth demand of several key workloads by ftve orders of magnitude while ensuring deterministic instruction delivery. Apart from error correction, we also observe a large instruction bandwidth requirement for fault tolerant quantum instructions (magic state distillation). We extend QuEST to manage these instructions in hardware and provide additional reduction in bandwidth. With QuEST, we reduce the total instruction bandwidth by eight orders of magnitude. CCS CONCEPTS • Computer systems organization → Quantum computing;

This thesis investigates fundamental properties of Josephson junctions embedded in microwave circuits, and an application arising from this hybrid approach. We used the versatility of superconducting coplanar DC bias cavities to extract previously inaccessible information on phase coherent and subgap mechanisms of graphene Josephson junctions. Chapter 1 gives an introduction to the technology of Josephson field effect transistors, among which graphene junctions show promise for future improvements in quantum computation. Together with an overview of the Josephson effect in superconducting-semiconducting systems, we introduce the concept of coplanar DC bias cavities for probing Josephson junctions at gigahertz frequencies. In chapter 2, we describe the experimental methods developed for carrying out the subsequent measurements. We include details on fabrication, material properties and measurement setup. Results of graphene Josephson junctions embedded in DC bias microwave resonators are presented in chapters 3 and 4. By following the resonance frequency and losses of the circuit, we are able to extract the junctions’ Josephson inductance and subgap resistance. Studying the nonlinear power and bias current response reveals further information on the underlying loss mechanisms and current phase relation. We turn to an application of our hybrid bias cavity – Josephson junction devices to detect small, low-frequency currents in chapter 5. Our device is competitive with state-of-the-art techniques for microwave radiation detection and, with minor modifications, should be able to outperform existing technologies by orders of magnitude. Finally, we conclude the presented work in chapter 6 and provide an outlook on potential future research.

In this paper we propose a novel error correction scheme/architecture specially tailored for polyhedral memories which: (i) allows for the formation of long codewords without interfering with the memory architecture/addressing mode/data granularity and (ii) make use of codecs located on a dedicated tier of the 3D memory stack. For a transparent error correction process we propose an online memory scrubbing policy that performs the error detection and correction decoupled from the normal memory operation. To evaluate our proposal we consider as a case study a 4-die 4-MB polyhedral memory and simulate various data width codes implementations. The simulations indicate that our proposal outperforms state of the art single error correction schemes in terms of error correction capability, being able to diminish the Word Error Rates (WER) by many orders of magnitude, e.g., WER from 10−10 to 10−21 are achieved for bit error probabilities between 10−4 and 10−6, while requiring less redundancy overhead. The scrubbing mechanism hides the codec latency and provides up to 10% and 25% write and read latency reductions, respectively. In addition, by relocating the encoders/decoders from the memory dies to a dedicated one a 13% footprint reduction is obtained and parallel energy effective scrubbing can be enabled, which results in further WER reductions.

Abstract: Quantum computing is on the brink of transforming computation, cryptography, artificial intelligence, and materials science, with quantum computing chips at the core of this revolution. "Quantum Computing Chips: Advances in Superconducting and Topological Qubits" provides an in-depth exploration of the latest advancements in quantum hardware, focusing on superconducting and topological qubits, two of the most promising approaches for scalable, fault-tolerant quantum computing. The book examines the fundamental principles of quantum computing, qubit architectures, and fabrication techniques, highlighting how Josephson junctions, transmon qubits, and Majorana fermions contribute to quantum logic operations. It delves into quantum chip integration, error correction strategies, hybrid quantum-classical computing, and emerging quantum networking technologies, offering insights into how industry leaders such as Google, IBM, and Microsoft are advancing quantum processor development. The book also explores the commercialization, industrial impact, and policy challenges of quantum computing chips, discussing applications in cryptography, AI acceleration, quantum simulation, and financial modeling. Through technical analysis, case studies, and expert insights, this book serves as a comprehensive resource for scientists, engineers, researchers, and technology leaders navigating the rapidly evolving quantum computing landscape. Keywords: Quantum computing, superconducting qubits, topological qubits, Josephson junctions, transmon qubits, Majorana fermions, non-Abelian anyons, quantum error correction, quantum chip fabrication, cryogenic quantum systems, hybrid quantum-classical computing, quantum networking, quantum supremacy, quantum cryptography, quantum AI acceleration, quantum materials science, fault-tolerant quantum computing, scalable quantum processors, quantum circuit design, quantum gate fidelity, quantum simulation, IBM quantum computing, Google quantum computing, Microsoft quantum computing, quantum industry, quantum economy, quantum policy, quantum innovation.

The actual environments under which robots are going to operate from now on are complex and sometimes unstable unlike the arranged environments in factories. It is becoming increasingly necessary for the robots to be able to cope with complexities by maintaining a symbiotic relationship with man who are behaving in mental world in various manners and life styles. This is not a task for the distant future but has already been posing daily problems in the fields of computer information communications that reach an international networked society. In this world, there is already a limitation to top-down controls based on hierarchical knowledge and guidance instructions. Rather, the situations indicate that this world should be taken as a complex adaptive system that requires recognition of the environments based on an autonomy of the system, adaptation and learning by behavior, and formation of orders according to self-organization, through a bottom-up approach. This is a considerable task to be tackled from now on if it is desired that robots are to play an active role in various parts of society in the near future. In considering this problem, it is important to learn from nature, ecological systems, and life systems. In recent years, new academic research fields have been generated that imitate the information processing functions of creatures such as recognition, evolution and adaptation, then reproduce these information processing functions in CG's and robots, and apply them to science and technology. This has been posing large topics relating to the autonomy, adaptation, learning and evolution of complex systems and finally to the creativity of a living body system. The key concept in this case is a chaos edge system called by Langton. In a complex system like a life, there is a self-organization marginal level of a certain scale, above which information is scattered, and below which information is fixed like a crystal. An integration is progressed and emerged in these systems. This characteristic is called the chaos edge system. Along with the outbreak of research in this new field, this has given substantial influence to the research into conventional fuzzy theories, neural networks, and intelligence systems such as artificial brains. Furthermore, along with the development of calculation methods called genetic algorithms which have been learned from the adaptive evolution of creatures, chaos, fuzzy, neuro, AI and GA are all directed towards the structuring of a new intelligent system. We cannot take our attention off the research in this new field. In order for robots to maintain a symbiotic relationship with man in daily life, the new development relating to the complexities of the creatures as described above is necessary. We already open every year an intelligent system symposium which aims at the merging and new developments of fuzzy, neuro and AI. At last year's FAN'95, we opened an organized session whose theme was chaos and self-organization systems. This special article has been organized now to high-light the papers announced at this OS.It has been possible to take up a wide range of issues including ecological systems, living systems, chaos, robots and the mind. I would like to express my thanks to those who have found time in their busy lives to contributed their papers to this special article . There are many more papers that were presented at the symposium and only a function of them were included in this article because of the limitation of space. I look forward to organizing a similar article again for this book, and it will be my great pleasure if readers can understand the latest situation of the research in this field from this issue. Finally, my thanks are also due to Dr. Tetsuro Yabuta (NTT) and Dr. Tadashi Iokibe (Meidensha) who helped in editing this article.

Quantum synchronizable codes (in short QSCs) are a special type of quantum error correcting codes (ECCs) which are useful to correct bit error, phase error, and misalignment in the block of synchronization. In this review paper, two classes of cyclic codes are converted into classes of QSCs. The QSCs with the greatest resistance to misalignment are Calderbank–Shor–Steane QECCs. Quantum information necessitates error correction, and quantum technologies are employed for safe communication, machine learning, and chemical analysis. Due to the non-cloning theorem, decoherence and the difficulty of measuring quantum states, quantum information processing is more challenging than classical information processing. Due to the high frequency of faults, a quantum computer needs an error detection and correction mechanism. To identify quantum information unit bounds in the absence of block synchronization, quantum information boundaries must be located using coding theory in quantum computing and communication systems and this prevents decoherence and correct errors while synchronizing qubits. Also, this method generates synchronizable and error-correcting quantum codes.

Abstract: The rapid advancement of quantum computing is largely driven by innovations in quantum chip architectures, with superconducting and topological qubits emerging as the most promising candidates for scalable and fault-tolerant quantum processors. Superconducting qubits, built using Josephson junctions, have demonstrated significant progress in coherence time, gate fidelity, and quantum error correction, making them the backbone of leading quantum processors such as IBM’s Eagle and Google’s Sycamore. Meanwhile, topological qubits, based on Majorana zero modes (MZMs) and non-Abelian anyons, offer intrinsic error resistance, reducing the computational overhead required for fault-tolerant quantum operations. This chapter explores the fundamental principles, fabrication techniques, and scalability challenges associated with these next-generation quantum chips, highlighting key advancements in hybrid quantum architectures, quantum error mitigation, and cryogenic quantum-classical interfacing. Additionally, it examines the geopolitical and ethical considerations of quantum supremacy, along with the transformative impact of next-gen quantum chips on AI, cryptography, materials science, and optimization problems. As research accelerates, the path toward scalable, fault-tolerant quantum computing is becoming clearer, paving the way for a future where quantum processors outperform classical supercomputers in solving the world’s most complex challenges. Keywords: Quantum chips, superconducting qubits, topological qubits, Josephson junctions, Majorana zero modes, quantum error correction, hybrid quantum architectures, fault-tolerant quantum computing, cryogenic electronics, quantum-classical interfacing, scalable quantum processors, AI-driven quantum computing, quantum supremacy, post-quantum cryptography, nanofabrication, quantum hardware innovation

Using high temperature superconducting thin films, we designed and fabricated Josephson junctions associated with a log-periodic toothed trapezoid antenna. Microwave self-radiation from the Josephson junction was measured by using both a superheterodyne receiver at the center frequency of 22 GHz and an external waveguide system. In the case of junction incorporated with an antenna structure, there was a significant increase of power by an order of magnitude with respect to a junction without antenna. Under the 22 GHz irradiation, we observed distinctive Shapiro steps in the I-V measurement, which allowed as estimating the received power of the junction. The received power of the junction with an antenna exhibited as a large increase as that of a self-radiation. Our results indicate that a Josephson junction with an antenna is very effective for the microwave coupling between Josephson junction and free space. The combined device can be used either for a high power microwave source or for a high sensitive sensor.

The dynamics of fluxons in long Josephson junctions is a well-known example of soliton physics and allows for studying highly nonlinear relativistic electrodynamics on a microscopic scale. Such fluxons are supercurrent vortices that can be accelerated by bias current up to the Swihart velocity, which is the characteristic velocity of electromagnetic waves in the junction. We experimentally demonstrate slowing down relativistic fluxons in Josephson junctions whose bulk superconducting electrodes are replaced by thin films of a high kinetic inductance superconductor. Here, the amount of magnetic flux carried by each supercurrent vortex is significantly smaller than the magnetic flux quantum Φ0. Our data show that the Swihart velocity is reduced by about one order of magnitude compared to conventional long Josephson junctions. At the same time, the characteristic impedance is increased by an order of magnitude, which makes these junctions suitable for a variety of applications in superconducting electronics.

We designed and fabricated HTS Josephson junctions associated with a log-periodic toothed trapezoid antenna. Microwave self-radiation from the Josephson junction was measured by using both a superheterodyne receiver at the center frequency of 22 GHz and an external waveguide system. In the case of junction integrated with an antenna, there was a significant increase of power by an order of magnitude in compared to a junction without antenna. Under the 22 GHz irradiation, we observed distinctive Shapiro steps in the I-V measurement and estimated the received power of the junction. Since the received power of the Josephson junctions with an antenna exhibited as a large increase as that of a self-radiation, The HTS Josephson junctions with an antenna are very effective for the microwave coupling between Josephson junction and free space. The combined devices can be used both for a high power microwave source and for a high sensitive sensor.

Abstract: This study investigates the feasibility of exploiting macroscopic quantum tunnelling (MQT) in superconducting circuits as a constructive resource for fault-tolerant quantum computing. While MQT has historically been treated as a decoherence pathway associated with metastable escape and switching events, recent advances in circuit engineering suggest that controlled access to tunnelling dynamics can be integrated into protected qubit modalities and error-mitigating architectures. The present framework synthesizes quantum escape theory, circuit Hamiltonian models, and contemporary fault-tolerance criteria to propose an experimentally realistic pathway for harnessing MQT-driven transitions as calibrated, monitorable events that can support syndrome extraction and bias-preserving logical operations. A methods blueprint is developed that combines cryogenic device characterization, time-resolved switching statistics, and noise spectroscopy with scalable surface-code-inspired performance projections. Using literature-aligned representative device parameters, the results illustrate how barrier engineering and bias-point optimization can reshape the MQT rate landscape while maintaining coherence metrics compatible with microsecond-scale error-correction cycles. The analysis indicates that decoupling the tunnelling coordinate from dominant low-frequency noise sources and implementing fast, quantum-limited readout may enable an error budget in which MQT transitions become part of a controlled operational envelope rather than a catastrophic failure mode. The study concludes with design rules, limitations, and a roadmap for validating MQT-assisted fault-tolerant primitives in next-generation superconducting quantum processors. Keywords: macroscopic quantum tunnelling; superconducting circuits; fluxonium; fault tolerance; surface code; noise spectroscopy; Josephson junctions; quantum error correction.

<p indent="0mm">Recently, the achievement, entitled “lifetime enhancement for a logic qubit by bosonic encoding error correction”, has been selected as one of “The Ten Major Advances in Chinese Science 2023”. Here, we will introduce briefly the importance of this work. It is known that quantum states are fragile due to decoherence induced by environment. Error will happen on a qubit, resulting in continuously changes of the parameters for this qubit, such as amplitude and phase parameters. The aim of quantum error correction is to amend those errors by using various quantum error correction codes, in which a logic qubit can be represented by several physical qubits. It should be noted that the quantum error correction can be realized by correcting a discrete set of errors, such as bit-flip error, phase-flip error or both of them. On the other hand, types of errors occurred depend on specific platforms of quantum computation. For example, photon loss is more inclined to occur in photonic systems. In this case, the approach of bosonic encoding of quantum error correction is more efficient. The work “lifetime enhancement for a logic qubit by bosonic encoding error correction” is for such a scenario. The challenge of quantum error correction is that operation itself is not perfect which may incur new errors. So it is actually difficult to demonstrate that we can benefit from quantum error correction. The achievement of lifetime enhancement by bosonic encoding error correction succeeds in beating this break-even point. The experiments are performed by circuit quantum electrodynamics on a superconducting processor with a qubit and a coupled resonate cavity. The logic qubit is realized by the bosonic mode of the cavity, while the superconducting qubit plays the role of the ancillary qubit in error correction. The initial states are prepared by the superconducting qubit, and are transferred to the logic qubit in the cavity. Then the error correction can be performed based on detection of photon loss by the superconducting qubit. The results show that the lifetime of the logic qubit increases from <sc>694 μs</sc> to <sc>805 μs</sc> with 16% enhancement. Besides high quality devices, the success of this experiment depends on the feedback control to detect and correct the error of photon loss in a short time. The result of beating the break-even point of bosonic encoding error correction is for one logic qubit. In the near future, it is hopeful to implement logic gates, such as single-qubit rotation gate and two-qubit gate, on a couple of logic qubits by bosonic encoding. The importance of the results will depend on whether higher fidelity can be achieved compared with those of gates with physical qubits. In this way, the advantage of the logic qubits for quantum computation is presented. It will be significant that the approach of bosonic encoding with a superconducting processor is scalable to hundreds of logic qubits, which may take a few years to realize. Our aim is to realize a fault-tolerant universal quantum computer based on a large quantity of high-precision logic qubits. The work “lifetime enhancement for a logic qubit by bosonic encoding error correction” is an important step toward this aim.

Digital information based on the laws of quantum mechanics promisses powerful new ways of computation and communication. However, quantum information is very fragile; inevitable errors continuously build up and eventually all information is lost. Therefore, realistic large-scale quantum information processing requires the protection of quantum bits (qubits) against errors. In this thesis we present the experimental implementation of quantum error correction protocols based on spins in diamond. In such protocols, a quantum state is protected against errors by encoding in multiple qubits. Errors can be detected and corrected by measurement of correlations, so-called stabilizer-measurements, on these qubits.The experimental work presented in this thesis employs multiple spins in diamond as qubits to explore and implement error correction protocols. The nitrogen-vacancy (NV) centre in diamond is a lattice defect consisting of a nitrogen atom (N) and a vacancy (V) on two adjacent diamond lattice sites. This defect effectively results in an electronic spin that can be addressed as a qubit. The spin state can be manipulated by microwave fields and optically read out. At liquid helium temperatures (cryogenic temperature, ~4 K = -269 C), the NV electron spin provides high-fidelity single-shot readout and long coherence times.The NV centre is surrounded by naturally available (1.1% abundance) nuclear C13 spins. As the number of spins that are close enough to the NV centre to be strongly coupled is limited, we employ the weakly coupled nuclear spins in the spin bath of the NV centre. Using dynamical decoupling techniques these nuclear spins can be detected via the NV electron spin through the hyperfine interaction. The nuclear spins are long-lived and robust against optical excitation of the NV electron spin, which can make these spins a robust quantum register for quantum error correction.In Ch. 4 we demonstrate universal control over multiple of such weakly coupled nuclear C13 spins in the environment of the NV centre at ambient temperatures. We demonstrate initialization, control and read-out of individual nuclear spins. Finally, we implement a quantum error correction protocol by encoding a quantum state in the NV electron spin and two nuclear spins. Errors are detected by un-encoding the quantum state back to the electron spin and correction via a double controlled operation.For universal fault-tolerant quantum computations it is essential that the quantum information remains encoded at all times. In Ch. 5 we present multiple rounds of quantum error correction and active feedback on a continuously encoded qubit at cryogenic temperatures. A quantum state is protected by encoding in three weakly coupled spins. Errors are detected via high-fidelity non-demolition readout of the NV electron spin and actively corrected using fast classical electronics. We demonstrate that an actively error-corrected qubit is robust against phase flip errors and show that a superposition state can live longer than the best physical qubit in the encoding.The presented methods and results can be extended to a range of future experiments. In Ch. 6 we propose the implementation of five-qubit quantum error correction, the smallest code to correct for general single-qubit errors on the physical qubits in the encoding, by extending the experimental methods as developed in Chs. 4a5. Besides the exploration and development of larger error correction protocols and fault-tolerant quantum computing, the presented quantum register based in spins in diamond can be employed as a quantum node and combined with recent advances in the realization of quantum entanglement over large distances to form quantum networks. These networks can be used to study both fundamental questions as well as future applications in quantum information technology.

Large-scale quantum computers will inevitably need quantum error correction (QEC) to protect information against decoherence. Given that the overhead of such error correction is often formidable, autonomous quantum error correction (AQEC) proposals offer a promising near-term alternative. AQEC schemes work by transforming error states into excitations that can be efficiently removed through engineered dissipation. The recently proposed AQEC scheme by Li , called the Star code, can autonomously correct or suppress all single qubit error channels using two transmons as encoders with a tunable coupler and two lossy resonators as a cooling source. The Star code requires only two-photon interactions and can be realized with linear coupling elements, avoiding experimentally challenging higher-order terms needed in many other AQEC proposals, but needs carefully selected parameters to achieve quadratic improvements in logical states' lifetimes. Here, we theoretically and numerically demonstrate the optimal parameter choices in the Star code. We further discuss adapting the Star code to other planar superconducting circuits, which offers a scalable alternative to single qubits for incorporation in larger quantum computers or error correction codes. Published by the American Physical Society 2024

Self-radiation and sensing of microwave using a Josephson junction coupled to a frequency-independent antenna