arXiv subjects

Takuya Hatomura

Publications and source records attributed to Takuya Hatomura.

25 records · Page 2Linked to original sources

arXiv2019

Suppressing nonadiabatic transitions during adiabatic generation of highly entangled states in bosonic Josephson junctions

We study suppression of nonadiabatic transitions during adiabatic generation of a cat-like state (a superposition of different size cat states) and a spin squeezed state in a bosonic Josephson junction. In order to minimize the adiabatic error, we use quantum adiabatic brachistochrone, which enables us to track a geometrically efficient path in parameter space under given conditions without requiring additional terms. For creation of a cat-like state, divergence of the quantum geometric tensor associated with gap closing at the critical point is avoided because of the parity conservation. The resulting schedules of parameters are smooth and monotonically decreasing curves. Use of these schedules offers reduction of time to generate both of a cat-like state and a spin squeezed state.

cond-mat.quant-gas↗

arXiv2019

Superadiabatic generation of cat states in bosonic Josephson junctions under particle losses

We investigate a superadiabatic scheme to produce a cat state in a bosonic Josephson junction in absence and presence of particle losses. The generation scheme is based on shortcuts to adiabaticity and strongly relies on the parity conservation. The parity conservation also ensures that the produced state is a superposition of cat states with various sizes, i.e., a "cats state". Parity is also the quantity to be measured in order to utilize the produced state in interferometry. The generation scheme still works even if a number of particle losses during generation are substantial.

cond-mat.quant-gas↗

arXiv2018

Shortcuts to Adiabatic Classical Spin Dynamics Mimicking Quantum Annealing

We propose a simple construction of shortcuts to adiabaticity tracking instantaneous stationary states in classical spin systems without knowing tracked stationary states. In our construction, control fields of counter-diabatic driving are constituted by state-dependent magnetic fields, which can be easily determined with an aid of numerical calculations. Easiness of our construction is a remarkable feature since it is usually a hard task to determine explicit form of required counter-diabatic terms in many-body systems. We also argue that our method can be applied to solve combinatorial optimization problems by considering classical spin dynamics under a time-dependent Hamiltonian, which mimics the procedure of quantum annealing.

quant-ph↗

arXiv2017

Shortcuts to adiabatic cat-state generation in bosonic Josephson junctions

We propose a quantum speedup method for adiabatic generation of cat states in bosonic Josephson junctions via shortcuts to adiabaticity. We apply approximated counter-diabatic driving to a bosonic Josephson junction using the Holstein-Primakoff transformation. In order to avoid the problem of divergence in counter-diabatic driving, we take finite-size corrections into account. The resulting counter-diabatic driving is well-defined over whole processes. Schedules of the counter-diabatic driving consist of three steps; the counter-diabatic driving in the disordered phase, smoothly and slowly approaching the critical point, and the counter-diabatic driving in the ordered phase. Using the counter-diabatic driving, adiabatic generation of cat states is successfully accelerated. The enough large quantum Fisher information ensures that generated cat states are highly entangled.

quant-ph↗

arXiv2017

Shortcuts to adiabaticity in the infinite-range Ising model by mean-field counter-diabatic driving

The strategy of shortcuts to adiabaticity enables us to realize adiabatic dynamics in finite time. In the counter-diabatic driving approach, an auxiliary Hamiltonian which is called the counter-diabatic Hamiltonian is appended to an original Hamiltonian to cancel out diabatic transitions. The counter-diabatic Hamiltonian is constructed by using the eigenstates of the original Hamiltonian. Therefore, it is in general difficult to construct the counter-diabatic Hamiltonian for quantum many-body systems. Even if the counter-diabatic Hamiltonian for quantum many-body systems is obtained, it is generally non-local and even diverges at critical points. We construct an approximated counter-diabatic Hamiltonian for the infinite-range Ising model by making use of the mean-field approximation. An advantage of this method is that the mean-field counter-diabatic Hamiltonian is constructed by only local operators. We numerically demonstrate the effectiveness of this method through quantum annealing processes going the vicinity of the critical point. It is also confirmed that the mean-field counter-diabatic Hamiltonian is still well-defined in the limit to the critical point. The present method can take higher order contributions into account and is consistent with the variational approach for local counter-diabatic driving.

quant-ph↗

arXiv2017

Distribution of eigenstate populations and dissipative beating dynamics in uniaxial single-spin magnets

Numerical simulations of magnetization reversal of a quantum uniaxial magnet under a swept magnetic field [Hatomura, \textit{et al}., \textit{Quantum Stoner-Wohlfarth Model}, Phys. Rev. Lett. \textbf{116}, 037203 (2016)] are extended. In particular, how the "wave packet" describing the time-evolution of the system is scattered in the successive avoided level crossings is investigated from the viewpoint of the distribution of the eigenstate populations. It is found that the peak of the distribution as a function of the magnetic field does not depend on spin-size $S$, which indicates that the delay of magnetization reversal due to the finite sweeping rate is the same in both the quantum and classical cases. The peculiar synchronized oscillations of all the spin components result in the beating of the spin-length. Here, dissipative effects on this beating are studied by making use of the generalized Lindblad-type master equation. The corresponding experimental situations are also discussed in order to find conditions for experimental observations.

cond-mat.mes-hall↗

arXiv2015

Quantum Stoner-Wohlfarth model

The quantum mechanical counterpart of the famous Stoner-Wohlfarth model -- an easy-axis magnet in a tilted magnetic field -- is studied theoretically and through simulations, as a function of the spin-size $S$ in a sweeping longitudinal field. Beyond the classical Stoner-Wohlfarth transition, the sweeping field-induced adiabatic change of states slows down as $S$ increases, leading to a dynamical quantum phase transition. This result is described as a critical phenomenon associated with Landau-Zener tunneling gaps at metastable quasi-avoided crossings. Furthermore, a beating of the magnetization is discovered after the Stoner-Wohlfarth transition. The period of the beating, obtained analytically, arises from a new type of quantum phase factor.

cond-mat.stat-mech↗