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Deep reinforcement learning is considered an effective technology in quantum optimization and can provide strategies for optimal control of complex quantum systems. More precise measurements require simulation control at multiple experimental stages. Based on this, we improved a multi-objective deep reinforcement learning method in mathematical convex optimization theory for multi-process quantum optimal control optimization. By setting the single-process quantum control optimization result as a multi-objective optimization truncation threshold and reward function transfer strategy, we finally gave a global optimal solution that considers multiple influencing factors, rather than a local optimal solution that only targets a certain error. This method achieved excellent computational results on superconducting qubits. Optimum control of multi-process quantum computing can be achieved only by regulating the microwave pulse parameters of superconducting qubits, and such a set of global parameter values and control strategies are given. [ABSTRACT FROM AUTHOR]

Abstract In the present manuscript, we introduce a quantum system of two superconducting qubits (S–Qs) interacting with a quantized field under the influence of the Kerr nonlinear medium and Ising interaction. We formulate the Hamiltonian of the quantum model and determine the density operator of whole quantum system as well as quantum subsystems. We examine the dynamics of the quantumness measures for subsequent times including the S–Qs entanglement, S–Qs-field entanglement and quantum Fisher information in relation to the system parameters. Finally, we display the connection among the measures of quantumness during the time evolution.

In the present manuscript, we introduce a quantum system of two superconducting qubits (S–Qs) interacting with a quantized field under the influence of the Kerr nonlinear medium and Ising interaction. We formulate the Hamiltonian of the quantum model and determine the density operator of whole quantum system as well as quantum subsystems. We examine the dynamics of the quantumness measures for subsequent times including the S–Qs entanglement, S–Qs-field entanglement and quantum Fisher information in relation to the system parameters. Finally, we display the connection among the measures of quantumness during the time evolution. [ABSTRACT FROM AUTHOR]

As a valid tool for solving ground state problems, imaginary time evolution (ITE) is widely used in physical and chemical simulations. Different ITE-based algorithms in their quantum counterpart have recently been proposed and applied to some real systems. We experimentally realize the variational-based quantum imaginary time evolution (QITE) algorithm to simulate the ground state energy of hydrogen (H2) and lithium hydride (LiH) molecules in a superconducting qubit system. The H2 molecule is directly simulated using the 3-qubit circuit with unitary-coupled clusters (UCC) ansatz. We also combine QITE with the cluster mean-field (CMF) method to obtain an effective Hamiltonian. The LiH molecule is correspondingly simulated using the 3-qubit circuit with hardware-efficient ansatz. For comparison, the LiH molecule is also directly simulated using the 4-qubit circuit with UCC ansatz at the equilibrium point. All the experimental results show a convergence within 4 iterations, with high-fidelity ground state energy obtained. For a more complex system in the future, the CMF may allow further grouping of interactions to obtain an effective Hamiltonian, then the hybrid QITE algorithm can possibly simulate a relatively large-scale system with fewer qubits. [ABSTRACT FROM AUTHOR]

Motivated by recent realizations of two-dimensional (2D) superconducting-qubit lattices, we propose a protocol to simulate Hofstadter butterfly with synthetic gauge fields in superconducting circuits. Based on the existing 2D superconducting-qubit lattices, we construct a generalized Hofstadter model on zigzag lattices, which has a fractal energy spectrum similar to the original Hofstadter butterfly. By periodically modulating the resonant frequencies of qubits, we engineer a synthetic gauge field to mimic the generalized Hofstadter Hamiltonian. A spectroscopic method is used to demonstrate the Hofstadter butterfly from the time evolutions of experimental observables. We numerically simulate the dynamics of the system with realistic parameters, and the results show a butterfly spectrum clearly. Our proposal provides a promising way to realize the Hofstadter butterfly on the latest 2D superconducting-qubit lattices and will stimulate the quantum simulation of novel properties induced by magnetic fields in superconducting circuits. [ABSTRACT FROM AUTHOR]

[Display omitted] Complex quantum electronic circuits can be used to design noise-protected qubits, but their complexity may exceed the capabilities of classical simulation. In such cases, quantum computers are necessary for efficient simulation. In this work, we demonstrate the use of variational quantum computing on a transmon-based quantum processor to simulate a superconducting quantum electronic circuit and design a new type of qubit called "Plasmonium", which operates in the plasmon-transition regime. The fabricated Plasmonium qubits show a high two-qubit gate fidelity of 99.58(3)%, as well as a smaller physical size and larger anharmonicity compared to transmon qubits. These properties make Plasmonium a promising candidate for scaling up multi-qubit devices. Our results demonstrate the potential of using quantum computers to aid in the design of advanced quantum processors. [ABSTRACT FROM AUTHOR]

We propose an experimentally feasible scheme for a source of itinerant microwave photons. It also features a quantum router that controls the direction of quantum microwave signals. This scheme consists of two superconducting qubits coupled to the transmission line in different regimes (strong and dispersive coupling). The dispersive coupling induces a shift in the resonance frequencies of coupled resonators, which depend on the dispersively coupled qubit state. We use this qubit to switch the direction of photon emission so that one output channel is blocked and the other is open, and vice versa, by changing the qubit state. We tune the bare frequencies of resonators and other system parameters to obtain a high efficient routing of photons. To demonstrate it, we implement a theoretical model of the proposed scheme and use the Jaynes–Cummings approach to describe the photon-matter interaction. We formulate a time-dependent wave function to study the behaviour of the proposed system. We derive and numerically solve a set of equations of motion governing the evolution of a single-photon wave packet. As a result, it is shown that the quantum signal can be routed in one of the two channels with high efficiency—nearly 90%. [ABSTRACT FROM AUTHOR]

We present protocols to generate quantum entanglement on nonlocal magnons in hybrid systems composed of yttrium iron garnet (YIG) spheres, microwave cavities and a superconducting (SC) qubit. In the schemes, the YIGs are coupled to respective microwave cavities in resonant way, and the SC qubit is placed at the center of the cavities, which interacts with the cavities simultaneously. By exchanging the virtual photon, the cavities can indirectly interact in the far-detuning regime. Detailed protocols are presented to establish entanglement for two, three and arbitrary N magnons with reasonable fidelities. [ABSTRACT FROM AUTHOR]

The quantum gate is the foundation of quantum computation and simulation in superconducting circuits. In recent years, quantum systems with three energy levels have attracted much attention. Therefore, it is essential to study the protocol of the quantum‐state transfer in qutrits. Herein, the all‐microwave schemes are studied to realize the state transfer and quantum gates. First, the ∣fg>–∣ge> transition is designed in a superconducting circuit with a relatively large coupling. We measured the Stark shift in cases where the experimental parameters did not satisfy the dispersive condition. Then, by carefully calibrating the pulse parameters, the regular Rabi oscillation and the state transfer are realized between the second excited state of two qubits via stimulated Raman adiabatic passage. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate the efficacy of the universally robust dynamical decoupling (URDD) sequence to preserve multipartite maximally entangled quantum states on a cloud-based quantum computer via the IBM platform. URDD is a technique that can compensate for experimental errors and simultaneously protect the state against environmental noise. To further improve the performance of the URDD sequence, phase randomization (PR) as well as correlated PR (CPR) techniques are added to the basic URDD sequence. The performance of the URDD sequence is quantified by measuring the entanglement in several noisy entangled states (two-qubit triplet state, three-qubit Greenberger–Horne–Zeilinger (GHZ) state, four-qubit GHZ state and four-qubit cluster state) at several time points. Our experimental results demonstrate that the URDD sequence is successfully able to protect noisy multipartite entangled states and its performance is modestly improved by adding the PR and CPR sequences. [ABSTRACT FROM AUTHOR]

One significant source of decoherence in superconducting circuits is known as two-level systems (TLSs), found in amorphous oxide layers. These circuits can, however, also be utilized as spectral and temporal TLS probes. Comprehensive investigations on the physics of TLSs are now possible thanks to recent advancements in superconducting qubits. Here, we simultaneously measure the tunable Xmon qubit decoherence time as well as the resonance frequency for more than 3 days to investigate stochastic fluctuations. Time-domain Allan deviation and frequency-domain power spectral density analysis indicate that two TLSs in near resonance with the qubit are responsible for the fluctuations. From the extracted oscillation in T 1 decay, we locate the two TLSs near the junctions. [ABSTRACT FROM AUTHOR]

[Display omitted] Classifying many-body quantum states with distinct properties and phases of matter is one of the most fundamental tasks in quantum many-body physics. However, due to the exponential complexity that emerges from the enormous numbers of interacting particles, classifying large-scale quantum states has been extremely challenging for classical approaches. Here, we propose a new approach called quantum neuronal sensing. Utilizing a 61-qubit superconducting quantum processor, we show that our scheme can efficiently classify two different types of many-body phenomena: namely the ergodic and localized phases of matter. Our quantum neuronal sensing process allows us to extract the necessary information coming from the statistical characteristics of the eigenspectrum to distinguish these phases of matter by measuring only one qubit and offers better phase resolution than conventional methods, such as measuring the imbalance. Our work demonstrates the feasibility and scalability of quantum neuronal sensing for near-term quantum processors and opens new avenues for exploring quantum many-body phenomena in larger-scale systems. [ABSTRACT FROM AUTHOR]

Quantum teleportation plays a milestone role in recent quantum information science and technologies. On the other hand, superconducting (SC) qubits characterized by their potential flexibility, tunability, scalability and robust control, all arising from their possible coupling to external fields, are of considerable attention in the quantum information field of research. Motivated from the above-mentioned facts, in this paper we aim to propose a teleportation scheme for an unknown entangled state of two SC qubits from Alice’s lab to Bob’s lab. To do this task, we introduce a scheme using SC qubits wherein the couplings of SC qubits with each other and with LC resonators are simply tunable by applying appropriate external magnetic fields. Due to the necessity of an appropriate entangled channel in the teleportation processes, we first implement the Hadamard and CNOT gates on three SC qubits, which are initially prepared in their vacuum states, by which we are able to generate the GHZ states that will be used as the entangled channel in our proposal. In the continuation, an effective Hamiltonian of the interaction between two SC qubits is considered via appropriately implementing the external fields in Alice’s lab. Next, two interactions between LC resonators and SC qubits are performed beyond the rotating wave approximation to eliminate the undesired states. Then, to achieve the main purpose of paper, the result of some proper measurements on LC resonators and SC qubits in Alice’s lab is classically informed to Bob. At last, by applying phase gate and Pauli-Z (σz) gate on SC qubits in Bob’s lab, the unknown entangled state of the two SC qubits in Alice’s lab is appropriately teleported to the state of SC qubits in Bob’s lab. Interestingly, it is finally observed that, the goal of the paper is successfully accessed with maximum possible fidelity, 1, very clearly above the classical limit, and satisfactorily acceptable value of success probability 0.5. Our work constitutes a significant step towards the realization of quantum repeaters for quantum communications and broadens the tool set for quantum information processing with SC qubits and circuits. [ABSTRACT FROM AUTHOR]

Phononic crystals are a promising platform for the study of quantum acoustodynamics. In a recent experiment, the interaction of a superconducting quantum bit with modes of a phononic crystal has been demonstrated. The field of these modes is localized in a compact area, providing high values of the coupling constant with the qubit. However, the Q-factor of phononic crystal modes is strongly limited (∼ 1050) due to a phonon emission from the crystal ends. For further use of phononic crystals in research in the field of quantum acoustodynamics, it is desirable to overcome this limitation in the quality factor. In this work, we have proposed a structure consisting of a phononic crystal placed between the Bragg mirrors. Our simulations predict that the Q-factor in such a structure can reach (∼ 100000). We demonstrate experimental results in which this structure has a Q-factor (∼ 60,000), which is 60 times higher than that of an acoustic crystal of the same size. [ABSTRACT FROM AUTHOR]

The accidental discovery of mercury's zero resistance at temperatures lower than 4.2 K which took place in 1911 by the Dutch physicist Heike Kamerlingh Onnes in his laboratory at the University of Leiden, appeared to be one of the greatest breakthroughs of physics of all time. It has led to the creation of an entirely new field within physics called superconductivity; this attracted many of the finest minds in physics whose work in this area produced no less than six Nobel Prizes to date. Zero resistance, together with the expulsion of magnetic fields which was discovered many years later, are the two unique and intriguing properties of superconductors which puzzled scientists' brains for a proper theoretical explanation of the observed phenomena. However in 1935, the phenomenological theory proposed by Fritz and Heinz London (known as the London theory) was the first success in the field, which was followed in the 1950s by another phenomenological theory put forward by Vitaly Ginzburg and Lev Landau. Despite this, a satisfactory microscopic theory for superconductivity had to wait until 1957 when John Bardeen, Leon Cooper and John Robert Schrieffer proposed their theory, which was nicknamed the BCS theory in their honour. The more recent discovery of the cuprate high temperature superconductors (HTS) in 1986 gave a new momentum to the field and intensified the search for room temperature superconductors which continues to this day. While this quest is under way, and new theories of superconductivity are being developed, physicists, material scientists and engineers are using superconductors to establish new technologies and build machines, devices and tools with unprecedented properties. Today superconductors are widely used in healthcare, particle accelerators, ultrasensitive instrumentation and microwave engineering and they are being developed for use in many other areas as well. In this review, we will trace the history of superconductors and provide a brief overview into some of the recent applications of superconductivity. [ABSTRACT FROM AUTHOR]

The quantum gate, as the foundation of quantum computation, depends on the architecture of the quantum chip, especially in superconducting systems. The parametric modulation to the flux bias is an important protocol to realize the multi‐qubit gate, which has some intrinsic advantages. Similar to other two‐qubit operations, parametric gates leave the extra phase in single‐qubit space, which is required to be corrected. In this article, the relation between the phases of the modulated pulse and the applied microwaves is experimentally characterized. By adjusting the envelope shape and phase of the applied pulses, the extra phase of the one qubit generated by the two‐qubit gate is suppressed via a simple and fast Ramsey‐like experiment. The tomography results demonstrate that this protocol is an effective calibrating method for future large‐scale quantum circuit manipulation. [ABSTRACT FROM AUTHOR]

We examine the temporal comportment of formation entanglement and quantum coherence in a quantum system made up of two superconducting charge qubits (SC-Qs), in the case of two different classes of nonlinear field. The results discussed the impact role of time-dependent coupling (T-DC) and dipole-dipole interaction (D-DI) on the temporal comportment of quantum coherence and entanglement in the ordinary and nonlinear field. In addition, we show that the main parameters of the quantum model affect the entanglement of formation and the coherence of the system in a similar way. [ABSTRACT FROM AUTHOR]

We propose to utilize two parity-protected qubits which are built based on superconductor/topological-insulator/superconductor (SC/TI/SC) Josephson junction to implement a parity-spin superconducting qubit. The SC/TI/SC Josephson junctions have identical Josephson potential, which is robust against fabrication variations and guarantees the reliable $\cos2\phi$ energy-phase relation for implementing a parity-protected qubit. By viewing the even and odd parity ground states of a single parity-protected qubit as spin- $\frac{1}{2}$ states, we construct the logic qubit states using the total parity odd subspace of two parity-protected qubits, refereed to parity-spin qubit. This parity-spin qubit exhibits robustness against charge noise, similar to a singlet-triplet (S-T) qubit’s immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and is significantly suppressed. Benefiting from the simultaneous protection from charge and flux noise, we demonstrate an enhancement of both T _1 and T _2 coherence times. Our work presents a TI-based approach to engineer symmetry-protected superconducting qubits.

In a model consisting of two superconducting charge qubits (SC-Qs), we study how the Josephson energy (JE) and temperature have an influence on the coherence, the degree of mixedness, and the nonclassical correlation. In this example, we show how the quantumness metrics change depending on the JE and the temperature. We also show that the number of quantifiers can be controlled by appropriate selection of the JE of the SC-Q and temperature effect. Additionally, we determine the optimal conditions required to enhance and preserve the coherence and maintain the level of coherence in the existence of temperature variations. We observe important quantum phenomena, i.e., coherence trapping and the quantum correlation trapping. Furthermore, we explain the variations in the coherence based on the degree of mixedness in the superconducting qubit state.

We investigate the dynamical behavior of local quantum Fisher information, linear entropy, fidelity, and entanglement of formation for a quantum scheme of two superconducting charge qubits in the presence of a L -deformed field. The effects of time-dependent (t–d) coupling and dipole–dipole (d–d) interaction on the temporal behavior of these four information quantifiers in the presence and absence of L -deformation effects are considered. We show that their dynamical behavior can be controlled by the d–d interactions and t–d coupling, for linear and L-deformed fields. Moreover, we clarify the link between the four quantifiers during the dynamics. The results obtained emphasize that in the presence and absence of t–d coupling, the proposed quantumness is very sensitive to the nonlinearity of the field but in the presence of t–d coupling it is only sensitive to the d–d interaction. We also identify new views to use and understand the nature of this nonlinearity through the behavior of the quantum quantifiers in systems of two superconducting qubits. Our observations have potential implications for the application of this phenomenon in quantum optics and information processing. [ABSTRACT FROM AUTHOR]

We present a new quantum adiabatic theorem that allows one to rigorously bound the adiabatic timescale for a variety of systems, including those described by originally unbounded Hamiltonians that are made finite-dimensional by a cutoff. Our bound is geared towards the qubit approximation of superconducting circuits and presents a sufficient condition for remaining within the 2n -dimensional qubit subspace of a circuit model of n qubits. The novelty of this adiabatic theorem is that, unlike previous rigorous results, it does not contain 2n as a factor in the adiabatic timescale, and it allows one to obtain an expression for the adiabatic timescale independent of the cutoff of the infinite-dimensional Hilbert space of the circuit Hamiltonian. As an application, we present an explicit dependence of this timescale on circuit parameters for a superconducting flux qubit and demonstrate that leakage out of the qubit subspace is inevitable as the tunnelling barrier is raised towards the end of a quantum anneal. We also discuss a method of obtaining a 2n×2n effective Hamiltonian that best approximates the true dynamics induced by slowly changing circuit control parameters. This article is part of the theme issue 'Quantum annealing and computation: challenges and perspectives'. [ABSTRACT FROM AUTHOR]

In systems of two coupled superconducting qubits (SQ), we examine the time evolution of parameter estimation, fidelity, and entropy squeezing under the influence of intrinsic decoherence. We show how the physical parameters of the model can be used to control the evolution of these quantum quantifiers. The impacts of the two-qubit coupling are investigated. We demonstrate that the amount of Fisher information can yield a steady value that illustrates the Fisher-information trapping phenomenon. Furthermore, we explore the dynamics of the fidelity of the SQ state and explain the dependence of the parameter estimation precision on the evolution of the system state. In addition, we investigate the squeezing behavior of SQ-system entropy in the context of decoherence and discuss its dependence on the parameter estimation precision. The results obtained show how the consideration of the physical model here in terms of quantum measurements can be beneficial for description and implementation of realistic experiments under optimum conditions. [ABSTRACT FROM AUTHOR]

We study experimentally and theoretically the transfer of population between the ground state and the second excited state in a transmon circuit by the use of superadiabatic stimulated Raman adiabatic passage (saSTIRAP). We show that the transfer is remarkably resilient against variations in the amplitudes of the pulses (scaling errors), thus demostrating that the superadiabatic process inherits certain robustness features from the adiabatic one. In particular, we provide new evidence of a plateau that appears at high values of the counterdiabatic pulse strength, which goes beyond the usual framework of saSTIRAP. This article is part of the theme issue 'Shortcuts to adiabaticity: theoretical, experimental and interdisciplinary perspectives'. [ABSTRACT FROM AUTHOR]

Gatemons are superconducting qubits resembling transmons, with a gate-tunable semiconducting weak link as the Josephson element. Here, we report a gatemon device featuring an aluminum microwave circuit on a Ge/SiGe heterostructure embedding a Ge quantum well. Owing to the superconducting proximity effect, the high-mobility two-dimensional hole gas confined in this well provides a gate-tunable superconducting weak link between two Al contacts. We perform Rabi oscillation and Ramsey interference measurements, demonstrate the gate-voltage dependence of the qubit frequency, and measure the qubit anharmonicity. We find relaxation times T 1 up to 119 ns, and Ramsey coherence times T 2 * up to 70 ns, and a qubit frequency gate-tunable over 3.5 GHz. The reported proof-of-concept reproduces the results of a very recent work [Sagi et al. Nat. Commun. 2024, 15, 6400] using similar Ge/SiGe heterostructures, thereby validating a novel platform for the development of gatemons and parity-protected cos(2ϕ) qubits.

This article presents a cryo-CMOS quantum controller IC for superconducting qubits. The proposed globally synchronized clock system internally generates different local oscillator (LO) frequencies using multiple phase-locked loops (PLLs) driven by a common reference clock. It provides flexibility in spectral management as well as scalability for expansion to a large-scale quantum controller. The test chip includes two PLLs, four pulse modulator channels, and two receiver channels. Implemented chip in 40-nm CMOS shows full functionalities at 3.5 K. The designed pulse modulator circuits are verified with the specifications for expected fidelity of 99.99%. [ABSTRACT FROM AUTHOR]

Quasiparticles, especially the ones near the Josephson junctions in the superconducting qubits, are known as an important source of decoherence. By injecting quasiparticles into a quantum chip, we characterized the diffusion feature by measuring the energy relaxation time and the residual excited-state population of a transmon qubit. From the extracted transition rates, we phenomenologically modeled the quasiparticle diffusion in a superconducting circuit that contained "hot" nonequilibrium quasiparticles in addition to low-energy ones. [ABSTRACT FROM AUTHOR]

Quantum computing promises an exponentially higher computational power than classical computers; although all the building blocks have become available, certain constraints still prevent quantum advantage. The fundamental challenge in building a practical quantum computer is integrating thousands of highly coherent qubits with the control and readout electronics. The need for a high‐coherence qubit drives the effort for quantum error correction algorithms to create fault‐tolerant quantum systems. Error correction becomes tangible in a quantum processor only in large numbers of qubits. Thus, the other challenge is reducing the number of physical interconnects (coaxial lines) between the quantum–classical interface and bulky room‐temperature electronics. To interface thousands of qubits, interconnects can be reduced by bringing the control and readout electronics near the quantum processor. Cryogenic complementary metal–oxide–semiconductor (CMOS) technology has been an ideal candidate for this purpose. Integrated control and readout at cryogenic temperatures require low power dissipation circuit designs and techniques such as frequency‐division multiplexing (FDM) due to the finite cooling power of a dilution refrigerator. Herein, an overview of each building block in a superconducting quantum computer is provided, focusing on scalability. Furthermore, this article is concluded with an outlook discussing current challenges and future directions for the scalable superconducting control and readout. [ABSTRACT FROM AUTHOR]

Quantum computing promises an exponentially higher computational power than classical computers; although all the building blocks have become available, certain constraints still prevent quantum advantage. The fundamental challenge in building a practical quantum computer is integrating thousands of highly coherent qubits with the control and readout electronics. The need for a high‐coherence qubit drives the effort for quantum error correction algorithms to create fault‐tolerant quantum systems. Error correction becomes tangible in a quantum processor only in large numbers of qubits. Thus, the other challenge is reducing the number of physical interconnects (coaxial lines) between the quantum–classical interface and bulky room‐temperature electronics. To interface thousands of qubits, interconnects can be reduced by bringing the control and readout electronics near the quantum processor. Cryogenic complementary metal–oxide–semiconductor (CMOS) technology has been an ideal candidate for this purpose. Integrated control and readout at cryogenic temperatures require low power dissipation circuit designs and techniques such as frequency‐division multiplexing (FDM) due to the finite cooling power of a dilution refrigerator. Herein, an overview of each building block in a superconducting quantum computer is provided, focusing on scalability. Furthermore, this article is concluded with an outlook discussing current challenges and future directions for the scalable superconducting control and readout.

Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited due to the distinct properties of the classical magnetization, that is manipulated in spintronics, and quantum bits, that are utilized in quantum information science. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science. Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose–Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on some of the challenges and opportunities in quantum magnonics. [ABSTRACT FROM AUTHOR]

Quantum information processing systems rely on a broad range of microwave technologies and have spurred development of microwave devices and methods in new operating regimes. Here we review the use of microwave signals and systems in quantum computing, with specific reference to three leading quantum computing platforms: trapped atomic ion qubits, spin qubits in semiconductors, and superconducting qubits. We highlight some key results and progress in quantum computing achieved through the use of microwave systems, and discuss how quantum computing applications have pushed the frontiers of microwave technology in some areas. We also describe open microwave engineering challenges for the construction of large-scale, fault-tolerant quantum computers.

High-fidelity initialization, manipulation, and measurement of qubits are important in quantum computing. For the Google’s Sycamore processor, the gate fidelity of single- and two-qubit logic operations has improved to>99.6%, whereas single-shot measurement fidelity remains at the level of 97%, which severely limits the application of the superconducting approach to large-scale quantum computing. The current measurement scheme relies on the dispersive interaction between the qubit and the readout resonator, which was proposed back in 2004. However, the measurement fidelity is limited by the trade-off between the state separation and relaxation time of the two-level system. Recently, an exciting phenomenon was observed experimentally, wherein the separation-decay limit could be alleviated by exploiting the cascade decay nature of the higher levels; however, the mechanism and effectiveness of this phenomenon are still unclear. Herein, we present a theoretical tool to extract different types of errors in high-level states encoding dispersive measurement. For the realistic parameters of Google’s Sycamore processor, the use of state |2〉 is sufficient to suppress 92% of the decay readout error on average, where the total readout error is dominated by the background thermal excitation. We also show counter-intuitively that, the assistance of high-level states is effective in the measurement of logic 0, where there is no decay process.

Superconducting qubits utilize the strong non-linearity of Josephson junctions. Control over the Josephson nonlinearity, either by a current bias or by the magnetic flux, can be a valuable resource that brings tunability in the hybrid system consisting of superconducting qubits. To enable such a control, here we incorporate a fast-flux line for a frequency tunable transmon qubit in 3D cavity architecture. We investigate the flux-dependent dynamic range, relaxation from unconfined states, and the bandwidth of the flux-line. Using time-domain measurements, we probe the transmon's relaxation from higher energy levels after populating the cavity with ≈ 2.1 × 10 4 photons. For the device used in the experiment, we find a resurgence time corresponding to the recovery of coherence to be 4.8 μ s . We use a fast-flux line to tune the qubit frequency and demonstrate the swap of a single excitation between cavity and qubit mode. By measuring the deviation in the transferred population from the theoretical prediction, we estimate the bandwidth of the flux line to be ≈ 100 MHz, limited by the parasitic effect in the design. These results suggest that the approach taken here to implement a fast-flux line in a 3D cavity could be helpful for the hybrid devices based on the superconducting qubit. [ABSTRACT FROM AUTHOR]

Herein, a two‐qubit system is experimentally simulated through a four‐level superconducting circuit. By mapping the energy levels of the two‐qubit system to a noncomposite system, some two‐qubit gates with designed microwave pulses, such as Hadamard and CNOT gates, are implemented. Furthermore, the Shannon entropy of the simulated two‐qubit system is measured and the log 2 uncertainty relation is verified, which describes that the noncomposite single‐qudit system possesses hidden quantum correlations. [ABSTRACT FROM AUTHOR]

One significant source of decoherence in superconducting circuits is known as two-level systems (TLSs), found in amorphous oxide layers. These circuits can, however, also be utilized as spectral and temporal TLS probes. Comprehensive investigations on the physics of TLSs are now possible thanks to recent advancements in superconducting qubits. Here, we simultaneously measure the tunable Xmon qubit decoherence time as well as the resonance frequency for more than 3 days to investigate stochastic fluctuations. Time-domain Allan deviation and frequency-domain power spectral density analysis indicate that two TLSs in near resonance with the qubit are responsible for the fluctuations. From the extracted oscillation in T1 decay, we locate the two TLSs near the junctions.

[Display omitted] To ensure a long-term quantum computational advantage, the quantum hardware should be upgraded to withstand the competition of continuously improved classical algorithms and hardwares. Here, we demonstrate a superconducting quantum computing systems Zuchongzhi 2.1, which has 66 qubits in a two-dimensional array in a tunable coupler architecture. The readout fidelity of Zuchongzhi 2.1 is considerably improved to an average of 97.74%. The more powerful quantum processor enables us to achieve larger-scale random quantum circuit sampling, with a system scale of up to 60 qubits and 24 cycles, and fidelity of F XEB = (3.66 ± 0.345) × 10 - 4 . The achieved sampling task is about 6 orders of magnitude more difficult than that of Sycamore [Nature 574, 505 (2019)] in the classic simulation, and 3 orders of magnitude more difficult than the sampling task on Zuchongzhi 2.0 [arXiv:2106.14734 (2021)]. The time consumption of classically simulating random circuit sampling experiment using state-of-the-art classical algorithm and supercomputer is extended to tens of thousands of years (about 4.8 × 10 4 years), while Zuchongzhi 2.1 only takes about 4.2 h, thereby significantly enhancing the quantum computational advantage. [ABSTRACT FROM AUTHOR]

Quantum error correction is an essential ingredient for universal quantum computing. Despite tremendous experimental efforts in the study of quantum error correction, to date, there has been no demonstration in the realisation of universal quantum error-correcting code, with the subsequent verification of all key features including the identification of an arbitrary physical error, the capability for transversal manipulation of the logical state and state decoding. To address this challenge, we experimentally realise the [5, 1, 3] code, the so-called smallest perfect code that permits corrections of generic single-qubit errors. In the experiment, having optimised the encoding circuit, we employ an array of superconducting qubits to realise the [5, 1, 3] code for several typical logical states including the magic state, an indispensable resource for realising non-Clifford gates. The encoded states are prepared with an average fidelity of |$57.1(3)\%$| while with a high fidelity of |$98.6(1)\%$| in the code space. Then, the arbitrary single-qubit errors introduced manually are identified by measuring the stabilisers. We further implement logical Pauli operations with a fidelity of |$97.2(2)\%$| within the code space. Finally, we realise the decoding circuit and recover the input state with an overall fidelity of |$74.5(6)\%$|⁠ , in total with 92 gates. Our work demonstrates each key aspect of the [5, 1, 3] code and verifies the viability of experimental realisation of quantum error-correcting codes with superconducting qubits. [ABSTRACT FROM AUTHOR]

Optimal two-qubit operation is of significance to quantum information processing. An efficient scheme is proposed for realizing the shortcut to adiabatic two-qubit state swap in a superconducting circuit quantum electrodynamics (QED) via effective drivings. Two superconducting qutrits are coupled to a common cavity field and individual classical drivings. Based on two Gaussian-type Rabi drivings, two-qubit state swap can be adiabatically implemented within a reduced three-state system. To speed up the operation, these two original Rabi drivings are modified in the framework of shortcuts to adiabaticity, instead of adding an extra counterdiabatic driving. Moreover, owing to a shorter duration time, the decoherence effects on the accelerated quantum operation can be mitigated significantly. The strategy could offer an optimized method to construct fast and robust quantum operations on superconducting qubits experimentally. [ABSTRACT FROM AUTHOR]

Interest is growing in the development of quantum sensing based on the principles of quantum mechanics, such as discrete energy levels, quantum superposition, and quantum entanglement. Superconducting flux qubits are quantum two-level systems whose energy is sensitive to a magnetic field. Therefore, they can be used as high-sensitivity magnetic field sensors that detect the magnetization of a spin ensemble. Since the magnetization depends on temperature and the magnetic field, the temperature can be determined by measuring the magnetization using the flux qubit. In this study, we demonstrated highly sensitive temperature sensing with high spatial resolution as an application of a magnetic field sensor using the quantum coherence of a superconducting flux qubit. By using a superconducting flux qubit to detect the temperature dependence of the polarization ratio of electron spins in nano-diamond particles, we succeeded in measuring the temperature with a sensitivity of 1.3 µ Kµ $\sqrt{\textrm{Hz}}^{-1}$ at T = 9.1 mK in the submicrometer range.

Quasiparticles, especially the ones near the Josephson junctions in the superconducting qubits, are known as an important source of decoherence. By injecting quasiparticles into a quantum chip, we characterized the diffusion feature by measuring the energy relaxation time and the residual excited-state population of a transmon qubit. From the extracted transition rates, we phenomenologically modeled the quasiparticle diffusion in a superconducting circuit that contained “hot” nonequilibrium quasiparticles in addition to low-energy ones.

Geometric phases are only dependent on evolution paths but independent of evolution details so that they possess some intrinsic noise-resilience features. Based on different geometric phases, various quantum gates have been proposed, such as nonadiabatic geometric gates based on nonadiabatic Abelian geometric phases and nonadiabatic holonomic gates based on nonadiabatic non-Abelian geometric phases. Up to now, nonadiabatic holonomic one-qubit gates have been experimentally demonstrated with superconducting transmons, where the three lowest levels are all utilized in operation. However, the second excited state of transmons has a relatively short coherence time, which results in a decreased fidelity of quantum gates. Here, we experimentally realize Abelian-geometric-phase-based nonadiabatic geometric one-qubit gates with a superconducting Xmon qubit. The realization is performed on the two lowest levels of an Xmon qubit and thus avoids the influence from the short coherence time of the second excited state. The experimental result indicates that the average fidelities of single-qubit gates can be up to 99.6% and 99.7% characterized by quantum process tomography and randomized benchmarking. [ABSTRACT FROM AUTHOR]

Gatemon qubits are the electrically tunable cousins of superconducting transmon qubits. In this work, we demonstrate the full coherent control of a gatemon qubit based on hole carriers in a Ge/Si core/shell nanowire, with the longest coherence times in group IV material gatemons to date. The key to these results is a high-quality Josephson junction obtained using a straightforward and reproducible annealing technique. We demonstrate that the transport through the narrow junction is dominated by only two quantum channels, with transparencies up to unity. This novel qubit platform holds great promise for quantum information applications, not only because it incorporates technologically relevant materials, but also because it provides new opportunities, like an ultrastrong spin-orbit coupling in the few-channel regime of Josephson junctions.

Optimal coherent control of many qubits is critical to quantum information processing. Here, an efficient scheme is proposed for implementing state swap and creating an entanglement with two superconducting qubits in a circuit quantum electrodynamics (QED). Two qutrits of Cooper‐pair box (CPB) circuits are placed into a cavity field and are driven by individual classical microwaves. In the two‐photon resonance with a large detuning, each CPB is reduced into a qubit effectively. Within a composite system composed of qubit states and cavity photons, two‐qubit state swap and entanglement can be fast obtained using the technique of shortcuts to adiabaticity with counterdiabatic drivings. Benefited from the shorter duration times and less sensitivity to timing errors and parameter imperfections, high‐fidelity quantum operations can be performed with the accessible decoherence rates. The present protocol could offer a feasible route towards optimized operations on superconducting qubits in a circuit QED. [ABSTRACT FROM AUTHOR]

Two superconducting (SC) qubits interacting with a radiation field in the coherent state (CS) in the presence of Stark shift are investigated. The density matrix is expressed in terms of the wave function to describe the proposed system. Physical properties, such as the geometric phase, linear entropy, and negativity are used to describe the behavior of the system. The effects of the Stark shift and the initial state on the linear entropy, geometric phase, and nonclassical correlation are discussed. Thermal and magnetic field interactions are also examined during the time evolution of the two SC qubits. By raising the temperature and decreasing the qubit–qubit entanglement, the nonclassical connection between the CS field and the two SC qubits is strengthened. Moreover, the Stark shift considerably affects the dynamical and physical properties of both types of entanglement. [ABSTRACT FROM AUTHOR]

The dc current-voltage characteristics of small Josephson junctions reveal features that are not observed in larger junctions, in particular, a switch to the finite voltage state at current values much less than the expected critical current of the junction and a finite resistance in the nominally superconducting regime. Both phenomena are due to the increased sensitivity to noise associated with the small capacitance of the Josephson junction and have been extensively studied a few decades ago. Here I focus on the current bias dependence of the differential resistance of the junction at low current bias in the nominally superconducting regime, using a quantum Langevin equation approach that enables a physically transparent incorporation of the noise environment of the junction. A similar approach might be useful in modeling the sensitivity of superconducting qubits to noise in the microwave regime. • Josephson junctions form the heart of superconducting qubits, one of the leading platforms for quantum computing. • Noise significantly affects the dc current-voltage characteristics of Josephson junctions. • dc current-voltage characteristics of Josephson junctions provides insights into factors affecting coherence in superconducting qubits. [ABSTRACT FROM AUTHOR]