arXiv subjects

Yasuhiro Tokura

Publications and source records attributed to Yasuhiro Tokura.

At least 19 recordsLinked to original sources

arXiv2026

Real-time dynamics of triplet-resonant tunneling driven by nonequilibrium phonons

Driven nonequilibrium systems can host emergent functionalities beyond equilibrium, but real-time access to excited-state dynamics remains limited. Here we report real-time measurements of phonon-driven charge and spin dynamics in excited states of a double quantum dot. Under phonon irradiation, resonant inter-dot tunneling emerges at triplet resonance. Time-resolved charge sensing reveals that the resonant inter-dot tunneling is strongly modified by spin blockade. For weaker inter-dot coupling, the nonequilibrium phonon environment generates a unidirectional transport cycle along the phonon density gradient.

cond-mat.mes-hall↗

arXiv2026

Anonymous quantum sensing robust against state preparation errors

Networked quantum sensors have several applications such as the mapping of magnetic fields. When the magnetic fields are biomagnetic ones, i.e., they contain some private information, the information of from who non-zero magnetic fields occur has to be protected from eavesdroppers. Anonymous quantum sensing keeps it secret by estimating amplitudes of the magnetic fields without disclosing the positions of non-zero magnetic fields. In this paper, we propose an anonymous quantum sensing protocol that is robust against any independent noise in state preparations. To this end, we devise a quantum state verification protocol for a superposition of Greenberger-Horne-Zeilinger and Dicke states and combine it with the original protocol of anonymous quantum sensing. Our verification protocol can decide whether the fidelity between the ideal and actual states is high or low more efficiently than the direct fidelity estimation. Since the original protocol of anonymous quantum sensing cannot correctly estimate the amplitudes of the magnetic fields under state preparation errors, our results would improve the performance of anonymous quantum sensing in realistic situations.

quant-ph↗

arXiv2025

Thermal conductance at the superradiant phase transition in the quantum Rabi model

The quantum Rabi model exhibits a superradiant phase transition when the coupling becomes strong, even though it involves only two components: a two-level atom and a single bosonic mode. This phase transition is referred to as a finite-component quantum phase transition, in contrast to conventional phase transitions in many-body systems. In this Letter, we investigate heat transport across an atom embedded in bosonic modes, modeled by the quantum Rabi model, between two thermal baths. We found a manifestation of the superradiant phase transition in the thermal conductance, which represents the linear response to a temperature bias. Our Letter can be helpful for the development of quantum heat devices utilizing controllable finite-component quantum phase transitions.

cond-mat.mes-hall↗

arXiv2025

A New Scaling Function for QAOA Tensor Network Simulations

With the rapid development of quantum computers in recent years, the importance of performance evaluation in quantum algorithms has been increasing. One method that has gained attention for performing this evaluation on classical computers is tensor networks. Tensor networks not only reduce the computational cost required for simulations by using approximations but are also deeply connected to entanglement. Entanglement is one of the most important elements for the quantum advantages of quantum algorithms, but the direct relationship between quantum advantages and entanglement remains largely unexplored. Tensor networks are promising as a means to address this question. In this study, we focus on the entanglement in the Quantum Approximate Optimization Algorithm (QAOA). This study aims to investigate entanglement in QAOA by examining the relationship between the approximation rates of tensor networks and the performance of QAOA. Specifically, we actually perform tensor network simulations of QAOA on a classical computer and extend the study of the scaling relations presented in previous research. We have discovered that scaling relations hold even when entanglement entropy is used as the vertical axis. Furthermore, by analyzing the results of the numerical calculations, we propose a new function for the scaling relation. Additionally, we discovered interesting relationships regarding the behavior of entanglement in QAOA during our analysis. This research is expected to provide new insights into the theoretical foundation of the scaling relations presented in previous studies.

quant-ph↗

arXiv2025

Energy exchange and fluctuations between a dissipative qubit and a monitor under continuous measurement and feedback

Continuous quantum measurement and feedback induce energy exchange between a dissipative qubit and a monitor even in the steady state, as a measurement backaction. Using the Lindblad equation, we identified the maximum and minimum values of the steady-state energy flow as the measurement and feedback states vary, and we demonstrate the qubit cooling induced by these processes. Turning our attention to quantum trajectories under continuous measurement and feedback, we observe that the energy flow fluctuates around the steady-state values. We reveal that the fluctuations are strongly influenced by the measurement backaction, distinguishing them from the standard Poisson noise typically observed in electronic circuits. Our results offer potential application in the development of quantum refrigerators controlled by continuous measurement and feedback, and provide deep insight into quantum thermodynamics from the perspective of fluctuation.

quant-ph↗

arXiv2024

Electric dipole spin resonance with linear and cubic spin-orbit interaction

We consider the electric dipole spin resonance (EDSR) with using the spin-orbit interaction (SOI) in GaAs and Ge based quantum dots formed in a quantum well. We use Schrieffer-Wolff transformation and rotating frame to derive the effective Hamiltonian of EDSR. We treat the couplings of the orbital motion with the environment with Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation. We found that the cubic SOI makes the Rabi frequency non-linear with the applied microwave amplitude. At the same time, the fidelity of the spin manipulations becomes worse since the residual spin-orbital couplings induces spin relaxation.

cond-mat.mes-hall↗

arXiv2024

Heat flow from a measurement apparatus monitoring a dissipative qubit

We investigate the heat flow of a qubit coupled to heat baths under continuous quantum measurement. In the steady-state limit, we show that heat always flows from the measurement apparatus into the qubit regardless of the measured qubit state and derive lower and upper bounds for the heat current between the qubit and the measurement apparatus. Furthermore, we study the transient dynamics of the heat current and the excess heat during the transient regime.

quant-ph↗

arXiv2023

Universal Scaling Bounds on a Quantum Heat Current

We derive new bounds on a heat current flowing into a quantum $L$-particle system coupled with a Markovian environment. By assuming that a system Hamiltonian and a system-environment interaction Hamiltonian are extensive in $L$, we show that the absolute value of the heat current scales at most as $Θ(L^3)$ in a limit of large $L$. Also, we present an example that saturates this bound in terms of scaling: non-interacting particles globally coupled with a thermal bath. However, the construction of such system requires many-body interactions induced by the environment, which may be difficult to realize with the current technology. To consider more feasible cases, we focus on a class of system where any non-diagonal elements of the noise operator (derived from the system-environment interaction Hamiltonian) become zero in the system energy basis, if the energy difference is beyond a certain value $ΔE$. Then, for $ΔE = Θ(L^0)$, we derive another scaling bound $Θ(L^2)$ on the absolute value of the heat current, and the so-called superradiance belongs to a class to saturate this bound. Our results are useful to evaluate the best achievable performance of quantum-enhanced thermodynamic devices, which contain far-reaching applications for such as quantum heat engines, quantum refrigerators and quantum batteries.

quant-ph↗

arXiv2023

Thermodynamics of Computation for CMOS NAND Gate

Understanding how much energy is needed and dissipated as heat for a given computational system and for a given program is a physically interesting and practically important problem. However, the thermodynamic costs of computational systems are only partially understood. In this paper, we focus on a specific logic gate, the CMOS NAND gate, operating in the sub-threshold region and analyze the dissipated heat from two aspects. One is the general Landauer bound, which is the change in entropy of the computational system, and the other is a cost that depends on the difference between the initial and steady-state distributions of the system. We find that the general Landauer bound is the same order for different inputs to the gate, but that the another cost has partially different order due to the difference between the initial and steady-state distributions over output logical states. We also investigate the interplay between the costs, time scale, and reliability of the process and find that for different inputs, there is not always a trade-off between reliability and dissipation of computations.

cond-mat.stat-mech↗

arXiv2023

Anonymous estimation of intensity distribution of magnetic fields with quantum sensing network

A quantum sensing network is used to simultaneously detect and measure physical quantities, such as magnetic fields, at different locations. However, there is a risk that the measurement data is leaked to the third party during the communication. Many theoretical and experimental efforts have been made to realize a secure quantum sensing network where a high level of security is guaranteed. In this paper, we propose a protocol to estimate statistical quantities of the target fields at different places without knowing individual value of the target fields. We generate an enanglement between $L$ quantum sensors, let the quantum sensor interact with local fields, and perform specific measurements on them. By calculating the quantum Fisher information to estimate the individual value of the magnetic fields, we show that we cannot obtain any information of the value of the individual fields in the limit of large $L$. On the other hand, in our protocol, we can estimate theoretically any moment of the field distribution by measuring a specific observable and evaluated relative uncertainty of $k$-th ($k=1,2,3,4$) order moment. Our results are a significant step towards using a quantum sensing network with security inbuilt.

quant-ph↗

arXiv2023

Phonon-mediated spin dynamics in a two-electron double quantum dot under a phonon temperature gradient

We have theoretically studied phonon-mediated spin-flip processes of electrons in a GaAs double quantum dot (DQD) holding two spins, under a phonon temperature gradient over the DQD. Transition rates of inter-dot phonon-assisted tunnel processes and intra-dot spin-flip processes involving spin triplet states are formalized by the electron-phonon interaction accompanied with the spin-orbit interaction. The calculations of the spin-flip rates and the occupation probabilities of the spin-states in the two-electron DQD with respect to the phonon temperature difference between the dots are quantitatively consistent with our previous experiment. This theoretical study on the temperature gradient effect onto spins in coupled QDs would be essential for understanding spin-related thermodynamic physics.

cond-mat.mes-hall↗

arXiv2022

Heat transport through a two-level system under continuous quantum measurement

We study the backaction of quantum measurements on heat transport through a two-level system by considering the continuous quantum measurement onto an eigenstate of the two-level system. For the nonselective measurement, the backaction appears as a dephasing effect on the two-level system. We formulate the heat current under the selective measurement with a stochastic master equation and show that the cross-correlation between the measurement outcomes and the heat current contains information on the backaction. We expect that our findings can be verified by using a platform of superconducting circuits.

cond-mat.mes-hall↗

arXiv2022

Quantum battery based on superabsorption

A quantum battery is a device where an energy is charged by using a quantum effect. Here, we propose a quantum battery with a charger system composed of $N$ qubits by utilizing a collective effect called a superabsorption. Importantly, the coupling strength between the quantum battery and charger system can be enhanced due to an entanglement. While the charger time scales as $Θ\left(N^{-1/2}\right)$ by applying a conventional scheme, we can achieve a charging time $Θ\left(N^{-1}\right)$ in our scheme. Our results open the path to ultra-fast charging of a quantum battery.

quant-ph↗

arXiv2021

Coexistence of parallel and series current paths in parallel-coupled double quantum dots in nonlinear transport regime

We investigated the electron transport properties of parallel-coupled double quantum dot (DQD) devices under magnetic fields. When a low magnetic field was applied, electron tunneling through parallel-coupled DQDs was observed. Under a high magnetic field, we observed both electron tunneling through parallel- and series-coupled DQDs under nonlinear transport conditions. In addition, the Pauli spin blockade was observed, indicating tunneling through the series-coupled DQDs. We attribute these behaviors to the magnetic-field-induced changes in the tunnel-couplings that allow the coexistence of the current paths of the parallel and series configurations.

cond-mat.mes-hall↗

arXiv2021

Real-time observation of charge-spin cooperative dynamics driven by a nonequilibrium phonon environment

Quantum dots are recognized as a suitable platform for studying thermodynamic phenomena involving single electronic charges and spins in nano-scale devices. However, such a thermodynamic system is usually driven by electron reservoirs at different temperatures, not by a lattice temperature gradient. We report on experimental observations of charge-spin cooperative dynamics in transitions of two-electron spin states in a GaAs double quantum dot located in a non-equilibrium phonon environment. Enhancements in the spin-flip processes are observed, originating from phonon excitation combined with the spin-orbit interaction. In addition, due to the spatial gradient of phonon density between the dots, the spin-flip rate during an inter-dot electron tunnel from a hot to a cold dot is more enhanced than in the other direction, resulting in accumulation of parallel spin states in the double dot.

cond-mat.mes-hall↗

arXiv2021

Anonymous quantum sensing

A lot of attention has been paid to a quantum-sensing network for detecting magnetic fields in different positions. Recently, cryptographic quantum metrology was investigated where the information of the magnetic fields is transmitted in a secure way. However, sometimes, the positions where non-zero magnetic fields are generated could carry important information. Here, we propose an anonymous quantum sensor where an information of positions having non-zero magnetic fields is hidden after measuring magnetic fields with a quantum-sensing network. Suppose that agents are located in different positions and they have quantum sensors. After the quantum sensors are entangled, the agents implement quantum sensing that provides a phase information if non-zero magnetic fields exist, and POVM measurement is performed on quantum sensors. Importantly, even if the outcomes of the POVM measurement is stolen by an eavesdropper, information of the positions with non-zero magnetic fields is still unknown for the eavesdropper in our protocol. In addition, we evaluate the sensitivity of our proposed quantum sensors by using Fisher information when there are at most two positions having non-zero magnetic fields. We show that the sensitivity is finite unless these two (non-zero) magnetic fields have exactly the same amplitude. Our results pave the way for new applications of quantum-sensing network.

quant-ph↗

arXiv2020

Microwave spectroscopy of spin-orbit coupled states: energy detuning versus interdot coupling modulation

We study the AC field induced current peaks of a spin blockaded double quantum dot with spin-orbit interaction. The AC field modulates either the interdot tunnel coupling or the energy detuning, and we choose the AC field frequency range to induce two singlet-triplet transitions giving rise to two current peaks. We show that for a large detuning the two current peaks can be significantly stronger when the AC field modulates the tunnel coupling, thus making the detection of the spin-orbit gap more efficient. We also demonstrate the importance of the time dependence of the spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2020

Probing the singlet-triplet splitting in double quantum dots: Implications of the ac field amplitude

We consider a double quantum dot whose energy detuning is controlled by an ac electric field. We demonstrate an energy configuration for which the ac-induced current flowing through the double dot directly probes the spin-orbit anticrossing point for small ac field amplitudes. On the contrary, as the ac amplitude increases a current antiresonance is formed, and the direct information about the spin-orbit interaction is lost. This result indicates that a large ac amplitude is not necessarily advantageous for the spectroscopy of spin-orbit coupled two-spin states. Moreover, we investigate the ac-induced current peaks versus the ac amplitude and show a current suppression when the ac field forms spin blocked states. This effect gives rise to a characteristic pattern for the current which can be controlled at will by tuning the ac amplitude. Our results can be explored by performing electronic transport measurements in the spin blockade regime.

cond-mat.mes-hall↗