Runtime optimization of the quantum computing within a given computational resource is important to achieve practical quantum advantage. In this paper, we propose a runtime reduction protocol for the lattice surgery, which utilizes the soft information corresponding to the logical measurement error. Our proposal is a simple two-step protocol: operating the lattice surgery with the small number of syndrome-measurement cycles and reexecuting it with full syndrome-measurement cycles in cases where the timelike soft information suggests the presence of logical errors. We first discuss basic features of the timelike complementary gap as the concrete example of the timelike soft information based on numerical results. Then, we study average runtime of sequences of commutable logical Pauli measurements. We find that our protocol surpasses an existing runtime reduction protocol called temporally encoded lattice surgery (TELS) when the length of the sequence is small. In addition, we confirm that the combination of our protocol and the TELS protocol can reduce the runtime further, over 50% in comparison to the naive serial execution of the lattice surgery. The proposed protocol in this paper can be applied to any quantum computing architecture based on the lattice surgery, and we expect that this will be one of the fundamental building blocks of runtime optimization to achieve practical scale quantum computing.

We introduce STAR-magic mutation, an efficient protocol for implementing logical rotation gates on early fault-tolerant quantum computers. This protocol judiciously combines two of the latest state preparation protocols: transversal multi-rotation protocol and magic state cultivation. It achieves a logical rotation gate with a favorable error scaling of 𝒪(θ_L^2(1-Θ(1/d))p_ph), while requiring only the ancillary space of a single surface code patch. Here, θ_L is the logical rotation angle, p_ph is the physical error rate, and d is the code distance. This scaling marks a significant improvement over the previous state-of-the-art, 𝒪(θ_L p_ph), making our protocol particularly powerful for implementing a sequence of small-angle rotation gates, like Trotter-based circuits. Notably, for θ_L ≲ 10^-5, our protocol achieves a two-order-of-magnitude reduction in both the execution time and the error rate of analog rotation gates compared to the standard T-gate synthesis using cultivated magic states. Building upon this protocol, we also propose a novel quantum computing architecture designed for early fault-tolerant quantum computers, dubbed “STAR ver. 3". It employs a refined circuit compilation strategy based on Clifford+T+ϕ gate set, rather than the conventional Clifford+T or Clifford+ϕ gate sets. We establish a theoretical bound on the feasible circuit size on this architecture and illustrate its capabilities by analyzing the spacetime costs for simulating the dynamics of quantum many-body systems. Specifically, we demonstrate that our architecture can simulate biologically-relevant molecules or lattice models at scales beyond the reach of exact classical simulation, with only a few hundred thousand physical qubits, even assuming a realistic error rate of p_ph=10^-3.

Tin vacancy (SnV) centers in diamond are promising solid state qubits for integrated quantum photonics. Here, we fabricate and characterize a diamond on Al2O3 dual taper waveguide structure containing SnV centers, demonstrating optical coupling between the diamond nanobeam and the underlying Al2O3 waveguide. The devices are realized using a bilayer fabrication approach compatible with wafer scale lithography. Clear guided SnV- emission is observed in all optically active devices, indicating effective optical coupling in the integrated structure. These results demonstrate a scalable fabrication approach toward integrating diamond color centers with photonic waveguides.

In near-term quantum computations that do not employ error correction, noise can proliferate rapidly, corrupting the quantum state and making results unreliable. These errors originate from both decoherence and control imprecision. The latter can manifest as coherent noise that is especially detrimental. Here, we study the impact of coherent errors and their mitigation under standard error-reduction techniques, both theoretically and experimentally on a trapped-ion quantum computer. As a representative case study, we implement a range of Grover's algorithm circuits containing up to 10 qubits and 26 two-qubit gates. We demonstrate the effectiveness of randomized compiling (RC) and algorithm error detection (ED), where the latter is realized via post-selection on ancillary qubits that ideally return to the ground state at the end of each circuit. Our results highlight a synergetic effect: combining RC and ED yields the largest reductions in errors, indicating that these methods can work together to extend the capabilities of near-term quantum devices for moderately deep circuits.

The variational quantum eigensolver (VQE) algorithm has positioned quantum chemistry calculation methods as promising applications for noisy intermediate-scale quantum (NISQ) devices. In NISQ devices, where the quantum noise is considerable, it is desirable to have an ansatz for the quantum circuit that is practical to implement and capable of achieving high chemical accuracy. Hardware-efficient Ansätze (HEA), such as the RyRz linear ansatz (RLA) and its modified form that preserves physical quantitiesthe symmetry preserving ansatz (SPA)are constructed using quantum gates that can be implemented easily on a quantum computer. However, the extent to which HEA can accurately yield the electronic state energies of actual molecules and the number of quantum gates required to achieve chemical accuracywhich, in practical cases, should be within 1 kcal/mol of the exact energy valueare not well understood. In this study, we aimed to gain a detailed understanding of these aspects by performing noiseless simulations to obtain the ground and low-lying excited state using high-depth HEA quantum circuits for several molecules, including LiH, H2O, BeH2, CH4, and N2. From the results, we demonstrated that symmetry preserving HEA, such as SPA, can achieve accurate computational results that maintain CCSD-level chemical accuracy by increasing the number of layers. We quantitatively analyze how the expressibility and entangling capability of RLA and SPA quantum circuits evolve with circuit depth. Our results reveal that while increased layers generally expand accessible Hilbert space and entanglement, differences in their limitations highlight the importance of considering physically allowed entanglement for performance of a parametric quantum circuit in VQE. We also studied the potential energy surface of the dissociation of these molecules and found that quantum circuits using SPA can capture static electron correlation, which is challenging to address with classical single-reference quantum chemistry methods such as CCSD. These results from SPA demonstrate its potential to represent electronic states, as it achieves highly accurate results with fewer gate operations compared to physically inspired quantum circuits such as the Unitary Coupled Cluster (UCC) method. This study establishes a possible path for the application of quantum devices in solving quantum chemistry problems.

Achieving quantum speedups in practical tasks remains challenging for current noisy intermediate-scale quantum (NISQ) devices. These devices always encounter significant obstacles such as inevitable physical errors and the limited scalability of current near-term algorithms. Meanwhile, assuming a typical architecture for fault-tolerant quantum computing (FTQC), realistic applications inevitably require a vast number of qubits, typically exceeding 10(6), which seems far beyond near-term realization. In this work, to bridge the gap between the NISQ and FTQC eras, we propose an alternative approach to achieve practical quantum advantages on early-FTQC devices. Our framework is based on partially fault-tolerant logical operations to minimize spatial overhead and avoids the costly distillation techniques typically required for executing non-Clifford gates. To this end, we develop a space-time efficient state preparation protocol to generate an ancillary non-Clifford state consumed for implementing an analog rotation gate with an arbitrary small angle theta and a remarkably low worst-case error rate below O(|theta|P-ph), where P-ph is the physical error rate. Furthermore, we propose several error suppression schemes tailored to our preparation protocol, which are essential to minimize the overhead for mitigating errors. Based on this framework, we present several promising applications that leverage the potential of our framework, including the Trotter simulation and quantum phase estimation (QPE). Notably, we demonstrate that our framework allows us to perform the QPE for an (8 x 8)-site Hubbard model with fewer than 6.8 x 10(4) qubits and an execution time of 10.6 days (or 14 min with full parallelization) under P-ph = 10(-4), which is significantly faster than recent classical estimation with tensor network techniques (density matrix renormalization group and projected entangled pair states).

Simulating thermal-equilibrium properties at finite temperature is crucial for studying quantum many-body systems. Quantum computers are expected to enable us to simulate large systems at finite temperatures, overcoming challenges faced by classical computers, such as the sign problem of the quantum Monte Carlo technique. Conventional methods suitable for fault-tolerant quantum computing (FTQC) devices are designed for studying large-scale quantum many-body systems but require a large number of ancilla qubits and a deep quantum circuit with many basic gates, making them unsuitable for the early stage of the FTQC era, when the availability of qubits and quantum gates is limited. In this paper, we propose a method suitable for quantum devices in this early stage to calculate the thermal-equilibrium expectation value of an observable at finite temperatures. Our proposal, named the Markov chain Monte Carlo with sampled pairs of unitaries (MCMC-SPU) algorithm, involves sampling simple quantum circuits and generating the corresponding statistical ensembles. This approach addresses the issues of resource demand and the decay in probability associated with postselection of measurement outcomes on ancilla qubits. We validate our proposal with a numerical simulation of the one-dimensional transverse-field Ising model as an illustrative example.

Achieving practical quantum speedup with limited resources is a crucial challenge in both academic and industrial communities. To address this, a partially fault-tolerant quantum computing architecture called "space-time efficient analog rotation quantum computing architecture" (STAR architecture) been recently been proposed. This architecture focuses on minimizing resource requirements while maximizing the precision of non-Clifford gates, essential for universal quantum computation. However, nondeterministic processes such as the repeat-until-success (RUS) protocol and state injection can introduce significant computational overhead. Therefore, optimizing the logical circuit to minimize this overhead by using efficient fault-tolerant operations is essential. This paper presents an efficient method for simulating the time evolution of the two-dimensional (2D) Hubbard-model Hamiltonian, a promising application of the STAR architecture. We present two techniques, the parallel-injection protocol and adaptive injectionregion updating, to reduce unnecessary time overhead specific to our architecture. By integrating these with the existing fermionic SWAP (fSWAP) technique, we develop an efficient Trotter-based time-evolution operation for the 2D Hubbard model. Our analysis reveals an acceleration of over 10 times compared to naive serial compilation. Based on this optimized compilation, we estimate the computational resources required for the quantum phase estimation of the 2D Hubbard model. For devices with a physical error rate of pphys = 10-4, we estimate that approximately 6.2 x 104 physical qubits are required to achieve faster ground-state energy estimation of the 8 x 8 Hubbard model compared to classical computation.

Abstract Dielectric barrier discharge (DBD) plasma actuators are devices that actively control the airflow using nonequilibrium atmospheric-pressure plasmas, showing promise for practical applications in the field of aerospace engineering. Numerous studies have revealed the dynamics of surface discharge and the process of generating electrohydrodynamic (EHD) force in detail. The performance of DBD plasma actuators has improved continuously over the past 20 years. However, there is a need for further improvement in EHD force generation to enable the practical applications of DBD plasma actuators. In this review, we provide insights that contribute to the development of a high-performance DBD plasma actuator by reviewing previous studies focused on revealing the surface discharge and EHD force generation processes. The foundations of the discharge process in DBD plasma actuators are briefly described from the perspectives of experiments and numerical simulations. We also reviewed various strategies for improving EHD force generation by optimizing the geometric structure and the applied voltage waveform as well as by controlling the surface charge accumulation. Improving EHD force generation and its efficiency is a fundamental research area to realize the practical applications of a novel active airflow control device that uses nonequilibrium plasmas.

Low-loss visible-light photonic circuits are crucial for high-performance photonic quantum processors. By using aluminum oxide (Al2O3) for its low visible-light absorption, we achieved waveguides exhibiting an exceptionally low propagation loss (1.39 dB/cm for the transverse electric mode) at red-light wavelengths. Directional coupler beam splitters fabricated using this platform exhibited good controllability of the optical splitting ratios. Furthermore, we fabricated a half beam splitter, which is an essential component of entangled photon generation in quantum optics. These results represent a significant advance toward developing low-loss photonic circuits, paving the way for improved performance in photonic quantum processors.

Coherence between multiple low-frequency components latent in the flow fields characterizes the nonlinear aspects of fluid dynamics. This study reveals the existance of the distinct frequency components and their interaction relation of the classical Mode A of the cylinder wake. Primaries are one-third of the Karman vortex shedding frequency (third-subharmonic) and bubble pumping, known as the previous study. However, when the spanwise domain size in numerical simulations is sufficiently large, their interaction is obscured by the presence of numerous frequency components. To address this, we introduce a process in which distinct frequency components gradually emerge by starting with a small spanwise domain size and then gradually increasing it from 3.3D to 4.7D, where D represents the diameter of the cylinder. From 3.3D to 3.5D, only the vortex shedding frequency harmonics are present. Third-subharmonic frequency appeared ranging from 3.5D to 3.7D. Bispectral mode decomposition reveals that the harmonics of the third-subharmonic frequency govern the flow in this domain size. The bubble pumping is emergence in the flow fields between 3.7D and 3.8D. The frequency component after this emergence is not only the harmonics of bubble pumping and periodic nature is disrupt. Nonlinear interactions between bubble pumping, the Karman vortex, and the third-subharmonic component complicate the temporal behavior of the flow field. Utilizing the constraint of the spanwise domain size, our approach effectively reveals the interaction relationship between frequency components inherent a flow field with a significant number of frequency components.

The realization of fault-tolerant quantum computers hinges on the construction of high-speed, high-accuracy, real-time decoding systems. The persistent challenge lies in the fundamental trade-off between speed and accuracy: efforts to improve the decoder's accuracy often lead to unacceptable increases in decoding time and hardware complexity, while attempts to accelerate decoding result in a significant degradation in logical error rate. To overcome this challenge, we propose a novel framework, decoder switching, which balances these competing demands by combining a faster, soft-output decoder ("weak decoder") with a slower, high-accuracy decoder ("strong decoder"). In usual rounds, the weak decoder processes error syndromes and simultaneously evaluates its reliability via soft information. Only when encountering a decoding window with low reliability do we switch to the strong decoder to achieve more accurate decoding. Numerical simulations suggest that this framework can achieve accuracy comparable to, or even surpassing, that of the strong decoder, while maintaining an average decoding time on par with the weak decoder. We also develop an online decoding scheme tailored to our framework, named double window decoding, and elucidate the criteria for preventing an exponential slowdown of quantum computation. These findings break the long-standing speed-accuracy trade-off, paving the way for scalable real-time decoding devices.

We derive a phase-averaged representation of transient flows based on the eigenmodes of a data-driven linear operator that approximates the Navier-Stokes dynamics. In performing phase averaging, it is assumed that, at each instant during the transient evolution, the eigenmode amplitude remains invariant, while only the complex phase angle differs among distinct realizations of the transient process. From this modal-phase perspective, the linear operator is defined as the best-fit operator that represents phase-different transient evolutions. By introducing a time-varying dynamic mode decomposition with a phase-control strategy formulated from this modal-phase perspective, time-varying eigenmodes are extracted from numerical simulations. In this formulation, the transient process is decomposed into time-varying eigenmodes, phase-shift angles, and amplitude coefficients. Furthermore, by averaging the Navier-Stokes equations over the phase-shift angle, a frequency-domain form of the equations can be derived at any given instant, assuming that the phase-shift angle is time-independent. This frequency-domain representation reveals the instantaneous energy budget and the presence of energy transfer through triadic interactions. The proposed analysis is demonstrated using a canonical example of two-dimensional flow around a circular cylinder transitioning from a steady to an unsteady state. The time-varying dynamic mode decomposition with phase control is shown to capture the transient evolution of the frequency components accurately. In addition, the temporal evolution of the energy budget and transfer distribution reveals that transient growth processes exhibit different time-dependent characteristics of energy transfer, even in cylinder flows at Reynolds numbers that eventually lead to a periodic state.

Simulation of quantum many-body systems is one of the most promising applications of quantum computers. It is crucial to efficiently implement the time-evolution operator as a quantum circuit to execute such simulations on near-term quantum computing devices with limited computational resources. However, standard approaches such as Trotterization sometimes require a deep quantum circuit, which is hard to implement on near-term quantum computers. Here, we propose a hybrid quantum-classical algorithm, called local subspace variational quantum compilation (LSVQC), for compiling the time-evolution operator of quantum many-body systems. The LSVQC performs a variational optimization to reproduce the action of the target time-evolution operator within a physically reasonable subspace. The optimization is performed for small local subsystems based on the Lieb-Robinson bound, which allows us to execute the cost function evaluation using small-scale quantum devices and/or classical computers. We demonstrate the validity of the LSVQC algorithm through numerical simulations of a simple spin-lattice model and an effective model of a parent compound of cuprate superconductors, Sr2CuO3, constructed by the ab initio downfolding method. It is shown that the LSVQC achieves a 95% reduction of the circuit depth for simulating quantum many-body dynamics compared to the Trotterization at best while maintaining the same computational accuracy. We also demonstrate that the restriction to a subspace leads to a substantial reduction of required resources and improved accuracy compared to the case of considering the entire Hilbert space. Furthermore, we estimate the gate count needed to execute the quantum simulations using the LSVQC on near-term quantum computing architectures in the noisy intermediate-scale or early fault-tolerant quantum computing era. Our estimation suggests that the acceptable physical gate error rate for the LSVQC can be about one order of magnitude larger than that for the Trotterization.

Developing a parametrized reduced-order model (ROM) that can describe the flow field over a wide range of parameters, e.g., Reynolds number, Mach number, and different shapes of obstacles such as airfoils has attracted attention. The reliable parametrized ROM can be applied to optimal design, active flow control, and uncertainty quantification. In this study, a robust parametric ROM was developed by using a proper orthogonal decomposition (POD) and a Galerkin projection-based ROM with a focus on a flow field around a cylinder. It is well known that the subspace spanned by the POD modes varies with the parameter and is optimal only for the training point. As a result, a conventional ROM fails to reproduce the flow field for different parameters. Moreover, a simple interpolation of the POD modes fails to estimate the subspace for a parameter that is not contained in the dataset. Contrary to this, a Grassmann manifold interpolation method successfully estimates the subspace and reproduces the flow field based on the estimated subspace by using the conventional Galerkin projection-based ROM. The result obtained in this study indicates that the manifold interpolation method is a powerful tool to develop a robust parametric ROM to realize the fast computation of complex flow fields.

Numerical simulations were conducted to investigate dynamic instability of a Japanese lift-type reentry capsule, which is named H-II transfer vehicle Recovery Vehicle (HRV), at Mach 1.2 by comparing two capsule shapes of an original HRV model and a model with a different shoulder angle of 10°. The 10° model has a thicker aft-body with a smaller shoulder angle than the original model. A previous pitch-direction One-Degree Of Freedom (1DOF) experiment revealed that the original model was dynamically stable, whereas the 10° model was dynamically unstable at Mach 1.2. So as to reproduce the same trends as the experiment, a high-order numerical scheme and zonal detached eddy simulation were employed. In fixed angle simulations, we found that the static longitudinal stability of the 10° model is higher than that of the original model in contrast with the observed dynamic stability. 1DOF free oscillation simulations successfully reproduced the same trends as the experiment. We elucidated that the main factor of the dynamic instability of the HRV-type capsule is a hysteresis of a boundary layer separation on the upper side of the capsule. The hysteresis is induced by a variation of momentum in a boundary layer due to capsule motion. We also found that for HRV-type capsules, the lower the static stability within the statically stable range, the higher the dynamic stability might be. In addition, on the lower side, a sign of the work exerted by surrounding flow is different between the original model and the 10° model. A dynamic mode decomposition analysis indicated that the vortex structures on the capsule wake might induce this difference.

Highly accurate quantum state transfer and remote entanglement between superconducting fixed-frequency qubits have not yet been realized. In this study, we calculate the characteristics of a transmission path with a 1- or 0.25 m superconducting coaxial cable and use the characteristics to perform time evolution simulations of quantum state transfer and remote entanglement between superconducting fixed-frequency qubits. We find that remote entanglement, or half-quantum state transfer, can achieve a high fidelity >99% even in the presence of a qubit frequency detuning caused by manufacturing fluctuations, while a small qubit frequency detuning substantially reduces the efficiency of quantum state transfer. Quantum circuit simulations modeling proposed remote entanglement demonstrate that teleportation of a logical qubit with a 3x3 surface code, as an example of computation using remote entanglement, attains nearly the same fidelity as in-node computation for both 1- and 0.25 m cable lengths.