232 citations · 1.3k across the 64 of their papers we have counts for
25 papers · 1 filter
Quantum metrology with generalized cat states
Mamiko Tatsuta, Yuichiro Matsuzaki, Akira Shimizu
We show a general relationship between a superposition of macroscopically distinct states and sensitivity in quantum metrology. Generalized cat states are defined by using an index…
Anomalous Enhancement of Entanglement Entropy in Nonequilibrium Steady States Driven by Zero-Temperature Reservoirs
Hideaki Hakoshima, Akira Shimizu
We investigate the size scaling of the entanglement entropy (EE) in nonequilibrium steady states (NESSs) of a one-dimensional open quantum system with a random potential. It models…
Conversion of Thermal Equilibrium States into Superpositions of Macroscopically Distinct States
Mamiko Tatsuta, Akira Shimizu
A simple procedure for obtaining superpositions of macroscopically distinct states is proposed and analyzed. We find that a thermal equilibrium state can be converted into such a s…
Necessity of Superposition of Macroscopically Distinct States for Quantum Computational Speedup
Akira Shimizu, Yuichiro Matsuzaki, Akihisa Ukena
For quantum computation, we investigate the conjecture that the superposition of macroscopically distinct states is necessary for a large quantum speedup. Although this conjecture…
Protected Rabi oscillation induced by natural interactions among physical qubits
Naoaki Kokubun, Akira Shimizu
For a system composed of nine qubits, we show that natural interactions among the qubits induce the time evolution that can be regarded, at discrete times, as the Rabi oscillation…
Visualization of superposition of macroscopically distinct states
Tomoyuki Morimae, Akira Shimizu
We propose a method of visualizing superpositions of macroscopically distinct states in many-body pure states. We introduce a visualization function, which is a coarse-grained quas…
Macroscopic entanglement in Quantum Computation
Akihisa Ukena, Akira Shimizu
We investigate macroscopic entanglement of quantum states in quantum computers, where we say a quantum state is entangled macroscopically if the state has superposition of macrosco…
Detection of Macroscopic Entanglement by Correlation of Local Observables
Akira Shimizu, Tomoyuki Morimae
We propose a correlation of local observables on many sites in macroscopic quantum systems. By measuring the correlation one can detect, if any, superposition of macroscopically di…
Quantum Zeno effect by general measurements
K. Koshino, A. Shimizu
It was predicted that frequently repeated measurements on an unstable quantum state may alter the decay rate of the state. This is called the quantum Zeno effect (QZE) or the anti-…
Anomalous quantum states in finite macroscopic systems
Akira Shimizu, Takayuki Miyadera, Akihisa Ukena
We consider finite macroscopic systems, i.e., systems of large but finite degrees of freedom, which we believe are poorly understood as compared with small systems and infinite sys…
Correlations of observables in chaotic states of macroscopic quantum systems
Ayumu Sugita, Akira Shimizu
We study correlations of observables in energy eigenstates of chaotic systems of a large size . We show that the bipartite entanglement of two subsystems is quite strong, wherea…
Appearance and Stability of Anomalously Fluctuating States in Shor's Factoring Algorithm
Akihisa Ukena, Akira Shimizu
We analyze quantum computers which perform Shor's factoring algorithm, paying attention to asymptotic properties as the number L of qubits is increased. Using numerical simulations…
Quantum Zeno Effect for Exponentially Decaying Systems
Kazuki Koshino, Akira Shimizu
The quantum Zeno effect -- suppression of decay by frequent measurements -- was believed to occur only when the response of the detector is so quick that the initial tiny deviation…
Quantum Zeno and anti-Zeno effects by indirect measurement with finite errors
Kazuki Koshino, Akira Shimizu
We study the quantum Zeno effect and the anti-Zeno effect in the case of `indirect' measurements, where a measuring apparatus does not act directly on an unstable system, for a rea…
Stability of quantum states of finite macroscopic systems against classical noises, perturbations from environments, and local measurements
Akira Shimizu, Takayuki Miyadera
We study the stability of quantum states of macroscopic systems of finite volume V, against weak classical noises (WCNs), weak perturbations from environments (WPEs), and local mea…
Stability of quantum states of finite macroscopic systems
Akira Shimizu, Takayuki Miyadera, Akihisa Ukena
We study the stabilities of quantum states of macroscopic systems, against noises, against perturbations from environments, and against local measurements. We show that the stabili…
Decoherence of anomalously-fluctuating states of finite macroscopic systems
Akira Shimizu, Takayuki Miyadera, Akihisa Ukena
In quantum systems of a macroscopic size V, such as interacting many particles and quantum computers with many qubits, there exist pure states such that fluctuations of some intens…
Fragility of Thermodynamically-Abnormal Ground States of Finite Systems
Takayuki Miyadera, Akira Shimizu
We study a general macroscopic quantum system of a finite size, which will exhibit a symmetry breaking if the system size goes to infinity, when the system interacts with an enviro…
Robustness of Wave Functions of Interacting Many Bosons in a Leaky Box
Akira Shimizu, Takayuki Miyadera
We study the robustness, against the leakage of bosons, of wave functions of interacting many bosons confined in a finite box, by deriving and analyzing a general equation of motio…
A Fundamental Limit of Measurement Imposed by the Elementary Interactions
Akira Shimizu
Quantum information theory is closely related to quantum measurement theory because one must perform measurement to obtain information on a quantum system. Among many possible limi…
Time evolution of condensed state of interacting bosons with reduced number fluctuation in a leaky box
Akira Shimizu, Jun-ichi Inoue
We study the time evolution of the Bose-Einstein condensate of interacting bosons confined in a leaky box, when its number fluctuation is initially (t=0) suppressed. We take accoun…
Quantum Langevin equations for semiconductor light-emitting devices and the photon statistics at a low-injection level
Hiroshi Fujisaki, Akira Shimizu
From the microscopic quantum Langevin equations (QLEs) we derive the effective semiconductor QLEs and the associated noise correlations which are valid at a low-injection level and…
Quantum Optics in Dispersive and Absorptive Media
Akira Shimizu, Teruaki Okushima, Kazuki Koshino
Using microscopic models in which both photons and excitons are treated as microscopic degrees of freedom, we discuss polaritons of two cases: One is the case when excitonic parame…
Quantum Measurement and Fluctuations in Nanostructures
Akira Shimizu
Measurement and fluctuations are closely related to each other in quantum mechanics. This fact is explicitly demonstrated in the case of a quantum non-demolition photodetector whic…