Large-scale Quantum Computers Research Articles

Parallel window decoding enables scalable fault tolerant quantum computation

Large-scale quantum computers have the potential to hold computational capabilities beyond conventional computers. However, the physical qubits are prone to noise which must be corrected in order to perform fault-tolerant quantum computations. Quantum Error Correction (QEC) provides the path for realizing such computations. QEC generates a continuous stream of data that decoders must process at the rate it is received, which can be as fast as 1 μs per QEC round in superconducting quantum computers. If the decoder infrastructure cannot keep up, a data backlog problem is encountered and the computation runs exponentially slower. Today’s leading approaches to quantum error correction are not scalable as existing decoders typically run slower as the problem size is increased, inevitably hitting the backlog problem. Here, we show how to parallelize decoding to achieve almost arbitrary speed, removing this roadblock to scalability. Our parallelization requires some classical feed forward decisions to be delayed, slowing-down the logical clock speed. However, the slow-down is now only polynomial in the size of the QEC code, averting the exponential slowdown. We numerically demonstrate our parallel decoder for the surface code, showing no noticeable reduction in logical fidelity compared to previous decoders and demonstrating the predicted speedup.

Superconducting wide strip photon detector with high critical current bank structure

The single-photon detector is an essential technology in photonic quantum information science and technology. Large-scale photonic quantum computers and quantum networks inevitably require numerous high-performance single-photon detectors. Superconducting nanostrip single-photon detectors (SNSPDs) using around 100-nm-wide nanostrips are promising technologies with high detection efficiency, low dark count, and low jitter, but there has been room for evolution in terms of polarization dependence and productivity. Using wide strips with widths of tens of micrometers provides polarization-independent high detection efficiency and high-yield fabrication using high-throughput photolithography with submicron resolution. However, detecting photons with such wide strips has been challenging due to rapidly increasing intrinsic dark counts caused by the uneven distribution of the superconducting current in the strip. Here, we present a novel superconducting wide strip photon detector (SWSPD) with a high critical current bank (HCCB) structure. This new strip structure suppresses the intrinsic dark counts and provides highly efficient photon detection in the wide strips. We have simultaneously achieved a polarization-independent detection efficiency of over 78% for 1550-nm wavelength photons, a low dark count rate (DCR) of ∼80 cps, and a low jitter of 29.8 ps using a 20-µm-wide SWSPD with the HCCB structure. This result paves the way for a new class of photon detectors using ultra-wide superconducting strips. These photon detectors with excellent productivity and polarization-independent high detection performances would boost the advance of large-scale photonic quantum technologies.

An information-theoretically secure quantum multiparty private set intersection

In the current age of increasing data, Private Set Intersection (PSI) is one of the most prominent and important cryptographic building block. It enables two organizations to determine the intersection of their input sets in such a way that no other information is leaked. In real life, there are situations where more than two parties desire to compute the intersection of their private sets. Herein, we propose a generalized version of PSI, called Multi-party private set intersection (MPSI), where number of involved parties is not restricted to two. Nearly all of the existing MPSI protocols are relying on the hardness problems like integer factorization and discrete logarithm. However, these hard problems are unable to withstand the threat possessed by large scale quantum computers. Thus, there is a need of MPSI protocols which are secure from the quantum attacks and also provide long-term security. Quantum cryptography (QC) is one such tool that addresses this issue. In this work, we develop the first MPSI based on QC. The scheme put-forwarded by us attains long-term security and remains secure against quantum attacks.

Deterministic Bell state measurement with a single quantum memory

Entanglements serve as a resource for any quantum information system and are deterministically generated or swapped by a joint measurement called complete Bell state measurement (BSM). The determinism arises from a quantum nondemolition measurement of two coupled qubits with the help of readout ancilla, which inevitably requires extra physical qubits. We here demonstrate a deterministic and complete BSM with only a nitrogen atom in a nitrogen-vacancy (NV) center in diamond as a quantum memory without relying on any carbon isotopes, which are the extra qubits, by exploiting electron‒nitrogen (14N) double qutrits at a zero magnetic field. The degenerate logical qubits within the subspace of qutrits on the electron and nitrogen spins are holonomically controlled by arbitrarily polarized microwave and radiofrequency pulses via zero-field-split states as the ancilla, thus enabling the complete BSM deterministically. Since the system works under an isotope-free and field-free environment, the demonstration paves the way to realize high-fidelity quantum repeaters for long-haul quantum networks and quantum interfaces for large-scale distributed quantum computers.

Characterization of superconducting through-silicon vias as capacitive elements in quantum circuits

The large physical size of superconducting qubits and their associated on-chip control structures presents a practical challenge toward building a large-scale quantum computer. In particular, transmons require a high-quality-factor shunting capacitance that is typically achieved by using a large coplanar capacitor. Other components, such as superconducting microwave resonators used for qubit state readout, are typically constructed from coplanar waveguides, which are millimeters in length. Here, we use compact superconducting through-silicon vias to realize lumped-element capacitors in both qubits and readout resonators to significantly reduce the on-chip footprint of both of these circuit elements. We measure two types of devices to show that through-silicon vias are of sufficient quality to be used as capacitive circuit elements and provide a significant reduction in size over existing approaches.

Cryo-CMOS modeling and a 600 MHz cryogenic clock generator for quantum computing applications

The development of large-scale quantum computing has boosted an urgent desire for the advancement of cryogenic CMOS (cryo-CMOS), which is a promising scalable solution for the control and read-out interface of quantum bits. In the current work, 180 nm CMOS transistors were characterized and modeled down to 4 K, and the impact of low-temperature transistor performance variations on circuit design was also analyzed. Based on the proposed cryogenic model, a 180 nm CMOS-based 450 to 850 MHz clock generator operating at 4 K for quantum computing applications was presented. At the output frequency of 600 MHz, it achieved < 4.8 ps RMS jitter with 30 mW power consumption (with test buffer), corresponding to a −211.6 dB jitter-power FOM, which is suitable for providing a stable clock signal for the control and readout electronics of scalable quantum computers.

A Decoder for a Lightweight McEliece Cryptosystem Based on Concatenated Codes

Large-scale quantum computers threaten the security of today&#x0027;s public-key cryptography. The McEliece cryptosystem is one of the most promising candidates for post-quantum cryptography. However, the McEliece system has the drawback of large key sizes for the public key. Similar to other public-key cryptosystems, the McEliece system has a comparably high computational complexity. Embedded devices often lack the required computational resources to compute those systems with sufficiently low latency. Hence, those systems require hardware acceleration. Lately, a generalized concatenated code construction was proposed together with a restrictive channel model, which allows for much smaller public keys for comparable security levels. In this work, we propose a hardware decoder suitable for a McEliece system based on these generalized concatenated codes. The results show that those systems are suitable for resource-constrained embedded devices.

Active reset of superconducting qubits using the electronics based on RF switches

Active reset of qubits is a crucial step in achieving quantum algorithms, particularly in fault-tolerant quantum computing, as it allows qubits to be quickly initialized to the ground state. In this paper, we demonstrate active reset of superconducting qubits using the feedback electronics based on RF switches. We are able to reset the qubit to the ground state in 11.412 µs with a success rate of 99.82%, which is better than the natural relaxation method in terms of both time consumption (180 µs) and success rate (99.61%). The demonstrated method facilitates fast and high-fidelity qubit initialization, holding practical significance for large-scale quantum computation.

Fast and perfect state transfer in superconducting circuit with tunable coupler

Abstract In quantum computation and quantum information processing, the manipulation and engineering of quantum systems to suit certain purposes is an ongoing task. One such example is quantum state transfer (QST), an essential requirement for both quantum communication and large-scale quantum computation. Here we engineer a chain of four superconducting qubits with tunable couplers to realize the perfect state transfer (PST) protocol originally proposed in quantum spin networks and successfully demonstrate the efficient transfer of an arbitrary single-qubit state from one end of the chain to the other, achieving a high fidelity of 0.986 in just 25 nanoseconds. This demonstrated QST is readily to extend to larger chain and multi-node configurations, thus serving as a desirable tool for scalable quantum information processing.

Frequency-tunable microwave quantum light source based on superconducting quantum circuits

A non-classical light source is essential for implementing a wide range of quantum information processing protocols, including quantum computing, networking, communication and metrology. In the microwave regime, propagating photonic qubits, which transfer quantum information between multiple superconducting quantum chips, serve as building blocks for large-scale quantum computers. In this context, spectral control of propagating single photons is crucial for interfacing different quantum nodes with varied frequencies and bandwidths. Here a deterministic microwave quantum light source was demonstrated based on superconducting quantum circuits that can generate propagating single photons, time-bin encoded photonic qubits and qudits. In particular, the frequency of the emitted photons can be tuned in situ as large as 200 MHz. Even though the internal quantum efficiency of the light source is sensitive to the working frequency, it is shown that the fidelity of the propagating photonic qubit can be well preserved with the time-bin encoding scheme. This work thus demonstrates a versatile approach to realizing a practical quantum light source for future distributed quantum computing.

Qubit energy tuner based on single flux quantum circuits

A device called the qubit energy tuner (QET), based on single flux quantum (SFQ) circuits, has been proposed for Z control of superconducting qubits. The QET is created by improving flux digital-to-analog converters (flux DACs). It can set the energy levels or frequencies of qubits, particularly flux-tunable transmons, and perform gate operations requiring Z control. The circuit structure of the QET is elucidated, consisting of an inductor loop and flux bias units for coarse or fine-tuning. The key feature of the QET is analyzed to understand how SFQ pulses change the inductor loop current, which provides external flux for qubits. Three simulations were performed to verify QET functionality. The first simulation verified the responses of the inductor loop current to SFQ pulses, showing a relative deviation of approximately 4.259% between the analytical solutions of the inductor loop current and the solutions from the WRSpice time-domain simulation. The second and third simulations, using QuTip, demonstrated how to perform a Z gate and an iSWAP gate using the QET, respectively, with corresponding fidelities of 99.99884% and 99.93906% for only one gate operation to specific initial states. These simulations indicate that the SFQ-based QET could act as an efficient component of SFQ-based quantum–classical interfaces for digital Z control of large-scale superconducting quantum computers.

Development of an SIS Mixer-Based Low-Noise Amplifier Amenable to Josephson Oscillator Pumping

We propose a low-noise and low-power-consumption microwave amplifier using superconductor-insulator-superconductor (SIS) mixers as quasi-particle frequency up- and down-converters pumped by a Josephson array oscillator in the millimeter-wave band. A proof-of-concept study of an amplifier with two W-band waveguide mixers using Nb/Al-AlOx/Nb junctions in a cascade connection demonstrated an average gain of 6 dB and a noise temperature of 12 K from nearly DC to 5 GHz. We also designed and fabricated a Josephson array oscillator at 100 GHz, which consisted of 31 overdamped Josephson junctions placed at each half-wavelength in an Nb microstrip line. The estimated output power of the oscillator was 0.2 μW at 100 GHz, which can be increased by increasing the number of the oscillator junctions. These results suggest that the proposed SIS mixer-based amplifier is suitable for on-chip integrated circuits to be implemented in large-scale multipixel SIS receivers and quantum computers.

An RTL-to-GDSII Flow for Single Flux Quantum Circuits Based on an Industrial EDA Toolchain

Josephson Junction-based superconducting circuits are promising candidates for high-speed digital electronics with dramatically lower power consumption than CMOS, as well as a potential enabler towards the implementation of large-scale quantum computing. In this paper, we will describe an automated flow for the creation of microcontrollers and other digital systems in the single flux quantum (SFQ) technology. Starting with a register-transfer level (RTL) description of the circuit, the flow integrates logic synthesis, technology mapping, timing and logic verification, library cell placement and routing, and completes with a candidate physical design for fabrication. The flow makes use of the same tools employed in leading-edge CMOS. We will examine the challenges specific to the SFQ technology at the different stages in this flow. We will also report on metrics to qualify the resulting physical layout, such as circuit density and timing results.

Adaptive surface code for quantum error correction in the presence of temporary or permanent defects

Whether it is at the fabrication stage or during the course of the quantum computation, e.g. because of high-energy events like cosmic rays, the qubits constituting an error correcting code may be rendered inoperable. Such defects may correspond to individual qubits or to clusters and could potentially disrupt the code sufficiently to generate logical errors. In this paper, we explore a novel adaptive approach for surface code quantum error correction on a defective lattice. We show that combining an appropriate defect detection algorithm and a quarantine of the identified zone allows one to preserve the advantage of quantum error correction at finite code sizes, at the cost of a qubit overhead that scales with the size of the defect. Our numerics indicate that the code's threshold need not be significantly affected; for example, for a certain scenario where small defects repeatedly arise in each logical qubit, the noise threshold is 2.7&amp;#x0025; (versus the defect-free case of 2.9&amp;#x0025;). These results pave the way to the experimental implementation of large-scale quantum computers where defects will be inevitable.

Suppression of Crosstalk in Quantum Computers Based on Instruction Exchange Rules and Duration

Crosstalk is the main noise source in quantum computing devices. In quantum computation, the parallel execution of multiple instructions will cause crosstalk, destroy the quantum state, and cause the program to fail to execute correctly. Overcoming the influence of crosstalk is a key requirement for quantum error correction and large-scale fault-tolerant quantum computing. This paper presents a method to suppress crosstalk in quantum computers based on multiple instruction exchange rules and duration. Several Benchmarks experiments verify the effectiveness of the proposed method. Compared with the prior techniques, the fidelity of the proposed method is improved by 15.97% on average.

Suppression of Crosstalk in Quantum Circuit Based on Instruction Exchange Rules and Duration.

Crosstalk is the primary source of noise in quantum computing equipment. The parallel execution of multiple instructions in quantum computation causes crosstalk, which causes coupling between signal lines and mutual inductance and capacitance between signal lines, destroying the quantum state and causing the program to fail to execute correctly. Overcoming crosstalk is a critical prerequisite for quantum error correction and large-scale fault-tolerant quantum computing. This paper provides an approach for suppressing crosstalk in quantum computers based on multiple instruction exchange rules and duration. Firstly, for the majority of the quantum gates that can be executed on quantum computing devices, a multiple instruction exchange rule is proposed. The multiple instruction exchange rule reorders quantum gates in quantum circuits and separates double quantum gates with high crosstalk on quantum circuits. Then, time stakes are inserted based on the duration of different quantum gates, and quantum gates with high crosstalk are carefully separated in the process of quantum circuit execution by quantum computing equipment to reduce the influence of crosstalk on circuit fidelity. Several benchmark experiments verify the proposed method's effectiveness. In comparison to previous techniques, the proposed method improves fidelity by 15.97% on average.

Approximate error correction scheme for three-dimensional surface codes based reinforcement learning

Quantum error correction technology is an important method to eliminate errors during the operation of quantum computers. In order to solve the problem of influence of errors on physical qubits, we propose an approximate error correction scheme that performs dimension mapping operations on surface codes. This error correction scheme utilizes the topological properties of error correction codes to map the surface code dimension to three dimensions. Compared to previous error correction schemes, the present three-dimensional surface code exhibits good scalability due to its higher redundancy and more efficient error correction capabilities. By reducing the number of ancilla qubits required for error correction, this approach achieves savings in measurement space and reduces resource consumption costs. In order to improve the decoding efficiency and solve the problem of the correlation between the surface code stabilizer and the 3D space after dimension mapping, we employ a reinforcement learning (RL) decoder based on deep Q-learning, which enables faster identification of the optimal syndrome and achieves better thresholds through conditional optimization. Compared to the minimum weight perfect matching decoding, the threshold of the RL trained model reaches 0.78%, which is 56% higher and enables large-scale fault-tolerant quantum computation.

Automated Extraction of Capacitive Coupling for Quantum Dot Systems

Single electrons trapped on semiconductor-defined quantum dots (QDs) are a promising platform for large-scale quantum computing. The authors demonstrate a reliable, automated method to identify the capacitive coupling between QDs. The approach combines machine learning with traditional fitting, to take advantage of the desirable properties of each. Also, analysis of cross capacitance may be used for automatic identification of the spurious QDs that occasionally form during device tuning. These techniques can autonomously flag devices with spurious dots near the operating regime, which is crucial information for reliable tuning for qubit operations.

A Secure Scheme Based on a Hybrid of Classical-Quantum Communications Protocols for Managing Classical Blockchains.

Blockchain technology affords data integrity protection and building trust mechanisms in transactions for distributed networks, and, therefore, is seen as a promising revolutionary information technology. At the same time, the ongoing breakthrough in quantum computation technology contributes toward large-scale quantum computers, which might attack classic cryptography, seriously threatening the classic cryptography security currently employed in the blockchain. As a better alternative, a quantum blockchain has high expectations of being immune to quantum computing attacks perpetrated by quantum adversaries. Although several works have been presented, the problems of impracticality and inefficiency in quantum blockchain systems remain prominent and need to be addressed. First, this paper develops a quantum-secure blockchain (QSB) scheme by introducing a consensus mechanism-quantum proof of authority (QPoA) and an identity-based quantum signature (IQS)-wherein QPoA is used for new block generation and IQS is used for transaction signing and verification. Second, QPoA is developed by adopting a quantum voting protocol to achieve secure and efficient decentralization for the blockchain system, and a quantum random number generator (QRNG) is deployed for randomized leader node election to protect the blockchain system from centralized attacks like distributed denial of service (DDoS). Compared to previous work, our scheme is more practical and efficient without sacrificing security, greatly contributing to better addressing the challenges in the quantum era. Extensive security analysis demonstrates that our scheme provides better protection against quantum computing attacks than classic blockchains. Overall, our scheme presents a feasible solution for blockchain systems against quantum computing attacks through a quantum strategy, contributing toward quantum-secured blockchain in the quantum era.

Heavy tails and pruning in programmable photonic circuits for universal unitaries

Developing hardware for high-dimensional unitary operators plays a vital role in implementing quantum computations and deep learning accelerations. Programmable photonic circuits are singularly promising candidates for universal unitaries owing to intrinsic unitarity, ultrafast tunability and energy efficiency of photonic platforms. Nonetheless, when the scale of a photonic circuit increases, the effects of noise on the fidelity of quantum operators and deep learning weight matrices become more severe. Here we demonstrate a nontrivial stochastic nature of large-scale programmable photonic circuits—heavy-tailed distributions of rotation operators—that enables the development of high-fidelity universal unitaries through designed pruning of superfluous rotations. The power law and the Pareto principle for the conventional architecture of programmable photonic circuits are revealed with the presence of hub phase shifters, allowing for the application of network pruning to the design of photonic hardware. For the Clements design of programmable photonic circuits, we extract a universal architecture for pruning random unitary matrices and prove that “the bad is sometimes better to be removed” to achieve high fidelity and energy efficiency. This result lowers the hurdle for high fidelity in large-scale quantum computing and photonic deep learning accelerators.