arXiv subjects

Jun Suzuki

Publications and source records attributed to Jun Suzuki.

122 records · Page 7Linked to original sources

arXiv2013

Creation of excitations from a uniform impurity motion in the condensate

We investigate a phenomenon of creation of excitations in the homogenous Bose-Einstein condensate due to an impurity moving with a constant velocity. A simple model is considered to take into account dynamical effects due to motions of the impurity. Based on this model, we show that there can be a finite amount of excitations created even if velocity of the impurity is below Landau's critical velocity. We also show that the total number of excitations scales differently for large time across the speed of sound. Thus, our result dictates the critical behavior across Landau's one and validates Landau's institution to the problem. We discuss how Landau's critical velocity emerges and its validity within our model.

cond-mat.quant-gas↗

arXiv2012

Symmetric coupling of four spin-1/2 systems

We address the non-binary coupling of identical angular momenta based upon the representation theory for the symmetric group. A correspondence is pointed out between the complete set of commuting operators and the reference-frame-free subsystems. We provide a detailed analysis of the coupling of three and four spin-1/2 systems and discuss a symmetric coupling of four spin-1/2 systems.

quant-ph↗

arXiv2011

Long-lived qubit from three spin-1/2 atoms

A system of three spin-1/2 atoms allows the construction of a reference-frame-free (RFF) qubit in the subspace with total angular momentum $j=1/2$. The RFF qubit stays coherent perfectly as long as the spins of the three atoms are affected homogeneously. The inhomogeneous evolution of the atoms causes decoherence, but this decoherence can be suppressed efficiently by applying a bias magnetic field of modest strength perpendicular to the plane of the atoms. The resulting lifetime of the RFF qubit can be many days, making RFF qubits of this kind promising candidates for quantum information storage units. Specifically, we examine the situation of three $^6\textrm{Li}$ atoms trapped in a $\textrm{CO}_2$-laser-generated optical lattice and find that, with conservatively estimated parameters, a stored qubit maintains a fidelity of 0.9999 for two hours.

quant-ph↗

arXiv2010

Radiation from accelerated impurities in Bose-Einstein condensate

We investigate radiation spectra arising from accelerated point-like impurities in the homogeneous Bose-Einstein condensate. A general formula for the radiation spectrum is obtained in the integral form as a function of given impurity trajectory. The Planckian spectrum is obtained for a special accelerated motion, which is shown to be unphysical. Non-Planckian spectrum is found in the case of a uniformly accelerated impurity. We compare our result with similar settings as discussed in other quantum many-body systems.

quant-ph↗

arXiv2010

Entanglement detection from interference fringes in atom-photon systems

A measurement scheme of atomic qubits pinned at given positions is studied by analyzing the interference pattern obtained when they emit photons spontaneously. In the case of two qubits, a well-known relation is revisited, in which the interference visibility is equal to the concurrence of the state in the infinite spatial separation limit of the qubits. By taking into account the super-radiant and sub-radiant effects, it is shown that a state tomography is possible when the qubit spatial separation is comparable to the wavelength of the atomic transition. In the case of three qubits, the relations between various entanglement measures and the interference visibility are studied, where the visibility is defined from the two-qubit case. A qualitative correspondence among these entanglement relations is discussed. In particular, it is shown that the interference visibility is directly related to the maximal bipartite negativity.

quant-ph↗

arXiv2010

Encoding many qubits in a rotor

We propose a scheme for encoding many qubits in a single rotor, that is, a continuous and periodic degree of freedom. A key feature of this scheme is its ability to manipulate and entangle the encoded qubits with a single operation on the system. We also show, using quantum error-correcting codes, how to protect the qubits against small errors in angular position and momentum which may affect the rotor. We then discuss the feasibility of this scheme and suggest several candidates for its implementation. The proposed scheme is immediately generalizable to qudits of any finite dimension.

quant-ph↗

arXiv2008

Symmetric construction of reference-frame-free qudits

By exploiting a symmetric scheme for coupling $N$ spin-1/2 constituents (the physical qubits) to states with total angular momentum $N/2-1$, we construct rotationally invariant logical qudits of dimension $d=N-1$. One can encode all qudit states, and realize all qudit measurements, by this construction. The rotational invariance of all relevant objects enables one to transmit quantum information without having aligned reference frames between the parties that exchange the qudits. We illustrate the method by explicit constructions of reference-frame-free qubits and qutrits and, for the qubit case, comment on possible experimental implementations.

quant-ph↗

arXiv2006

Raw-data attacks in quantum cryptography with partial tomography

We consider a variant of the BB84 protocol for quantum cryptography, the prototype of tomographically incomplete protocols, where the key is generated by one-way communication rather than the usual two-way communication. Our analysis, backed by numerical evidence, establishes thresholds for eavesdropping attacks on the raw data and on the generated key at quantum bit error rates of 10% and 6.15%, respectively. Both thresholds are lower than the threshold for unconditional security in the standard BB84 protocol.

quant-ph↗

arXiv2006

Critical Velocities for Energy Dissipation from Periodic Motions of Impurity in Bose-Einstein Condensates

A phenomenon of energy dissipation in Bose-Einstein condensates is studied based on a microscopic model for the motion of impurity. Critical velocities for onset of energy dissipation are obtained for periodic motions, such as a dipole-like oscillation and a circular motion. The first example is similar to a series of MIT group experiments settings where the critical velocity was observed much below the Landau critical velocity. The appearance of the smaller values for the critical velocity is also found in our model, even in the homogeneous condensate in the thermodynamic limit. This suggests that the landau criterion be reexamined in the absence of quantized vortices in the bulk limit.

cond-mat.stat-mech↗

arXiv2005

Quantum Electrodynamics in a Uniform Magnetic Field

A systematic formalism for quantum electrodynamics in a classical uniform magnetic field is discussed. The first order radiative correction to the ground state energy of an electron is calculated. This then leads to the anomalous magnetic moment of an electron without divergent integrals. Thorough analyses of this problem are given for the weak magnetic field limit. A new expression for the radiative correction to the ground state energy is obtained. This contains only one integral with an additional summation with respect to each Landau level. The importance of this formalism is also addressed in order to deal with quantum electrodynamics in an intense external field.

hep-th↗

arXiv2005

Motion of Classical Impurities in the Homogeneous Bose-Einstein Condensate

Motion of classical point-like impurities in the homogeneous Einstein condensate of bosons is studied in the framework of second quantization method. A toy model is proposed and its general solution within the Bogoliubov approximation is obtained. The effective Minkowski space-time structure arises naturally in this non-relativistic quantum many-body system in the low energy regime. This is shown to be true in this model. Several examples are discussed in order to illustrate our model. The homogeneous condensate produces an effective Yukawa type attractive force between impurities sitting in condensate. Landau's criterion is naturally derived in a case of linear motion of impurity. The analytic expressions for spectra of Bogoliubov excitations produced by the accelerated motions of impurities are obtained. A quick look at the analytic expression reveals that the spectrum of gapless excitations emitted by the linearly accelerated impurity {\it is not thermal}. If the homogeneous condensate is the physically correct model for Minkowski space-time then it follows that the apparent thermal response of the simple linearly accelerated detector models may be the result of improper regularization.

gr-qc↗

arXiv2005

Model for Motions of Impurities in Bose-Einstein Condensates

A model for classical impurities moving in Bose-Einstein Condensate (BEC) is proposed in the framework of quantum field theory and is solved within the Bogoliubov approximation at zero temperature. Several formulae are obtained for physical quantities such as the occupation number, the dissipation, the free energy, and the depletion. To illustrate this model two examples are studied; an impurity moving with i) a constant velocity and ii) a constant acceleration. Landau's criterion for energy dissipation accompanying the motion of impurities due to creation of quasi-particles in a superfluid, which was proposed on the basis of purely phenomenological argument, is obtained in the frame work of an exact quantum mechanical model.

cond-mat.stat-mech↗

arXiv2004

Model for Entangled States with Spin-Spin Interaction

A system consisting of two neutral spin 1/2 particles is analyzed for two magnetic field perturbations: 1) an inhomogeneous magnetic field over all space, and 2) external fields over a half space containing only one of the particles. The field is chosen to point from one particle to the other, which results in essentially a one-dimensional problem. A number of interesting features are revealed for the first case: the singlet, which has zero potential energy in the unperturbed case, remains unstable in the perturbing field. The spin zero component of the triplet evolves into a bound state with a double well potential, with the possibility of tunneling. Superposition states can be constructed which oscillate between entangled and unentangled states. For the second case, we show that changes in the magnetic field around one particle affect measurements of the spin of the entangled particle not in the magnetic field nonlocally. By using protective measurements, we show it is possible in principle to establish a nonlocal interaction using the two particles, provided the dipole-dipole potential energy does not vanish and is comparable to the potential energy of the particle in the external field.

quant-ph↗

arXiv2003

Quantum Mechanics in a Rotating Frame

The rotating frame is considered in quantum mechanics on the basis of the position dependent boost relating this frame to the non rotating inertial frame. We derive the Sagnac phase shift and the spin coupling with the rotation in the non relativistic limit by a simple treatment. By taking the low energy limit of the Dirac equation with a spin connection, we obtain the Hamiltonian for the rotating frame, which gives rise to all the phase shifts discussed before. Furthermore, we obtain a new phase shift due to the spin-orbit coupling.

quant-ph↗