arXiv subjects

Yu Shi

Publications and source records attributed to Yu Shi.

At least 235 records · Page 13Linked to original sources

arXiv2012

Exact Theorems Concerning CP and CPT Violations in C=-1 Entangled State of Pseudoscalar Neutral Mesons

Neutral pseudoscalar mesons in an entangled or Einstein-Podolsky-Rosen state are routinely produced in phi and B factories. Based on the peculiar properties of an entangled state, we present some general exact theorems about parameters characterizing CP and CPT violations, by using various asymmetries defined for the correlated decays of the two entangled mesons, which are rigorously calculated.

hep-ph↗

arXiv2011

Field theory of low energy excitations of a mixture of two species of pseudospin-1/2 Bose gases with interspecies spin-exchange

We develop a low energy effective field theory of a mixture of two species of pseudospin-1/2 atoms with interspecies spin-exchange, in addition to density-density interaction. This approach is beyond the single orbital-mode approximation. In a wide parameter regime, it indicates the existence of the four elementary excitations, especially a gapped mode due to interspecies spin-exchange. On the other hand, the spectrum of the effective spin Hamiltonian yielded by the single mode approximation can be obtained by quantizing the homogeneous excitation, which is spin excitation and is the long-wavelength limit of the gapped mode of elementary excitations. These low energy excitations can be experimentally measured by using Bragg spectroscopy.

cond-mat.quant-gas↗

arXiv2011

Binary mixture of pseudospin-1/2 Bose gases with interspecies spin exchange: from classical fixed points and ground states to quantum ground states

We consider the effective spin Hamiltonian describing a mixture of two species of pseudo-spin-1/2 Bose gases with interspecies spin exchange. First we analyze the stability of the fixed points of the corresponding classical dynamics, of which the signature is found in quantum dynamics with a disentangled initial state. Focusing on the case without an external potential, we find all the ground states by taking into account quantum fluctuations around the classical ground state in each parameter regime. The nature of entanglement and its relation with classical bifurcation is investigated. When the total spins of the two species are unequal, the maximal entanglement at the parameter point of classical bifurcation is possessed by the excited state corresponding to the classical fixed point which bifurcates, rather than by the ground state.

cond-mat.quant-gas↗

arXiv2011

Ground states of a mixture of two species of spin-1 Bose gases with interspecies spin exchange in a magnetic field

We consider a mixture of two species of spin-1 atoms with both interspecies and intraspecies spin exchanges in a weak magnetic field. Under the usual single mode approximation, it can be reduced to a model of coupled giant spins. We find most of its ground states. This is a complicated problem of energy minimization, with three quantum variables under constraints, i.e. the total spin of each species and the total spin of the whole mixture, as well as four parameters, including intraspecies and interspecies spin coupling strengths and the magnetic field. The quantum phase diagram is very rich. Compared with the case without a magnetic field, the ground states are modified by a magnetic field, which also modifies the ground state boundaries or introduces new crossover regimes on the phase diagram. Without interspecies spin coupling, the quantum phase transitions existing in absence of a magnetic field disappear when a magnetic field is applied, which leads to crossover regimes in the phase diagram. Under ferromagnetic interspecies spin coupling, the ground states remain disentangled no matter whether there is a magnetic field. For antiferromagnetic interspecies spin coupling, a magnetic field entangles the ground states in some parameter regimes. When the intraspecies spin couplings are both ferromagnetic, the quantum phase transition between antiferromagnetic and zero interspecies spin couplings survives the magnetic field. When the intraspecies spin couplings are both antiferromagnetic, a magnetic field induces new quantum phase transitions between antiferromagnetic and zero interspecies spin couplings.

cond-mat.quant-gas↗

arXiv2011

Polarized entangled Bose-Einstein condensation

We consider a mixture of two distinct species of atoms of pseudospin-1/2 with both intraspecies and Interspecies spin-exchange interactions, and find all the ground stats in a general case of the parameters in the effective Hamiltonian. In general, corresponding to the two species and two pseudo-spin states, there are four orbital wave functions into which the atoms condense. We find that in certain parameter regimes, the ground state is the so-called polarized entangled Bose-Einstein condensation, i.e. in addition to condensation of interspecies singlet pairs, there are unpaired atoms with spins polarized in the same direction. The interspecies entanglement and polarization significantly affect the generalized Gross-Pitaevskii equations governing the four orbital wave functions into which the atoms condense, as an interesting interplay between spin and orbital degrees of freedom.

cond-mat.quant-gas↗

arXiv2011

Three-dimensional quantum phase diagram of the exact ground states of a mixture of two species of spin-1 Bose gases with interspecies spin exchange

We find nearly all the exact ground states of a mixture of two species of spin-1 atoms with both interspecies and intraspecies spin exchanges in absence of a magnetic field. The quantum phase diagram in the three-dimensional parameter space and its two-dimensional cross sections are described. The boundaries where the ground states are either continuous or discontinuous are determined, with the latter identified as where quantum phase transitions take place. The two species are always disentangled if the interspecies spin coupling is ferromagnetic or zero. Quantum phase transitions occur when the interspecies spin coupling varies between antiferromagtic and zero or ferromagnetic while the two intraspecies spin couplings both remain ferromagnetic. On the other hand, by tuning the interspecies spin coupling from zero to antiferromagnetic and then back to zero, one can circumvent the quantum phase transition due to sign change of the intraspecies spin coupling of a single species, which is spin-decoupled with the other species with ferromagnetic intraspecies spin coupling. Overall speaking, interplay among interspecies and two intraspecies spin exchanges significantly enriches quantum phases of spinor atomic gases.

cond-mat.quant-gas↗

arXiv2011

Orbital elementary excitations as probes of entanglement and quantum phase transitions of collective spins in an entangled Bose-Einstein condensate

A mixture of two species of pseudospin-1/2 Bose gases exhibits interesting interplay between spin and orbital degrees of freedom. Expectation values of various quantities of the collective spins of the two species play crucial roles in the Gross-Pitaevskii-like equations governing the four orbital wave functions in which Bose-Einstein condensation occurs. Consequently, the elementary excitations of these orbital wave functions reflect properties of the collective spins. When the coupling between the two collective spins is isotropic, the energy gap of the gapped orbital excitation peaks, while there is a quantum phase transition in the ground state of the effective Hamiltonian of the two collective spins, which have previously been found to be maximally entangled.

cond-mat.quant-gas↗

arXiv2010

Ground States of a Mixture of Two Species of Spinor Bose Gases with Interspecies Spin Exchange

We consider a mixture of two species of spin-1 atoms with interspecies spin exchange, which may cooperate or compete with the intraspecies spin exchanges and thus dramatically affect the ground state. It represents a new class of bosonic gases differing from single-species spinor gases. We determine the exact ground states in several parameter regimes, and study the composite structures by using the generating function method generalized here to be applicable to a mixture of two species of spinor gases. The most interesting phase is the so-called entangled Bose-Einstein condensation (BEC), which is fragmented BEC with quantum entanglement between the two species, and with both interspecies and intraspecies singlet pairs. For comparison, we also apply the generating function method to a mixture of two species of pseudospin-1/2 atoms, for which the total spin quantum number of each species is fixed as half of the atom number, in contrast with the case of spin-1, for which it is a variable determined by energetics. Consequently, singlet pairs in entangled BEC of a pseudospin-1/2 mixture are all interspecies. Interspecies spin exchange leads to novel features beyond those of spinor BEC of a single species of atoms as well as mixtures without interspecies spin exchange.

cond-mat.quant-gas↗

arXiv2010

Entangled Superfluids

We study the condensate dynamics of the so-called entangled Bose-Einstein condensation (EBEC), which is the ground state of a mixture of two species of pseudospin-$\frac{1}{2}$ atoms with interspecies spin-exchange scattering in certain parameter regimes. EBEC leads to four inter-dependent superfluid components, each corresponding to the orbital wave function associated with a spin component of a species. The four superflows have various counter-relations, and altogether lead to a conserved total supercurrent and a conserved total spin supercurrent. In the homogenous case, we also obtain the elementary excitations due to variations of the single-particle orbital wave functions, by exactly solving the generalized time-dependent Bogoliubov equations. There are three gapless Bogoliubov modes and one Klein-Gordon-like gapped mode. The origin of these excitations are also discussed from the perspective of spontaneous breaking of the symmetries possessed by the system.

cond-mat.quant-gas↗

arXiv2009

Geometric Phases of Two Ising-interacting Spins in a Rotating Magnetic Field

We consider how to obtain a nontrivial two-qubit unitary transformation purely based on geometric phases of two spin-1/2's with Ising-like interaction in a magnetic field with a static z-component and a rotating xy-component. This is an interesting problem both for the purpose of measuring the geometric phases and in quantum computing applications. In previous approach, coupling of one of the qubit with the rotating component of field is ignored. By considering the exact two-spin geometric phases, we find that a nontrivial two-spin unitary transformation purely based on Berry phases can be obtained by using two consecutive cycles with opposite directions of the magnetic field and opposite signs of the interaction constant. In the nonadiabatic case, starting with a certain initial state, a cycle in the projected space of rays and thus Aharonov-Anandan phase can be achieved. The two-cycle scheme cancels the total phases, hence any unknown initial state evolves back to itself without a phase factor.

quant-ph↗

arXiv2009

Off-diagonal Long-Range Order and Supersolidity in a Quantum Solid with Vacancies

We consider a lattice of bosonic atoms, whose number N may be smaller than the number of lattice sites M. We study the Hartree-Fock wave function built up from localized wave functios w(\mathbf{r}) of single atoms, with nearest neighboring overlap. The zero-momentum particle number is expressed in terms of permanents of matrices. In one dimension, it is analytically calculated to be α*N(M-N+1)/M, with α=|\int w(\mathbf{r})dΩ|^2/[(1+2a)l], where a is the nearest-neighboring overlap, l is the lattice constant. αis of the order of 1. The result indicates that the condensate fraction is proportional to and of the same order of magnitude as that of the vacancy concentration, hence there is off-diagonal long-range order or Bose-Einstein condensation of atoms when the number of vacancies M-N is a finite fraction of the number of the lattice sites M.

cond-mat.other↗

arXiv2009

Feasibility of Experimental Realization of Entangled Bose-Einstein Condensation

We examine the practical feasibility of the experimental realization of the so-called entangled Bose-Einstein condensation (BEC), occurring in an entangled state of two atoms of different species. We demonstrate that if the energy gap remains vanishing, the entangled BEC persists as the ground state of the concerned model in a wide parameter regime. We establish the experimental accessibility of the isotropic point of the effective parameters, in which the entangled BEC is the exact ground state, as well as the consistency with the generalized Gross-Pitaevskii equations. The transition temperature is estimated. Possible experimental implementations are discussed in detail.

cond-mat.other↗

arXiv2008

Geometric vs. Dynamical Gates in Quantum Computing Implementations Using Zeeman and Heisenberg Hamiltonians

Quantum computing in terms of geometric phases, i.e. Berry or Aharonov-Anandan phases, is fault-tolerant to a certain degree. We examine its implementation based on Zeeman coupling with a rotating field and isotropic Heisenberg interaction, which describe NMR and can also be realized in quantum dots and cold atoms. Using a novel physical representation of the qubit basis states, we construct $π/8$ and Hadamard gates based on Berry and Aharonov-Anandan phases. For two interacting qubits in a rotating field, we find that it is always impossible to construct a two-qubit gate based on Berry phases, or based on Aharonov-Anandan phases when the gyromagnetic ratios of the two qubits are equal. In implementing a universal set of quantum gates, one may combine geometric $π/8$ and Hadamard gates and dynamical $\sqrt{\rm SWAP}$ gate.

quant-ph↗

arXiv2008

CP Measurement in Quantum Teleportation of Neutral Mesons

Quantum teleportation using neutral pseudoscalar mesons shows novel connections between particle physics and quantum information. The projection basis, which is crucial in the teleportation process, is determined by the conservation laws of particle physics, and is different from the Bell basis, as in the usual case. Here we show that one can verify the teleportation process by CP measurement. This method significantly simplifies the high energy quantum teleportation protocol. Especially, it is rigorous, and is independent of whether CP is violated in weak decays. This method can also be applied to general verification of Einstein-Podolsky-Rosen correlations in particle physics.

hep-ph↗

arXiv2006

High Energy Quantum Teleportation Using Neutral Koans

We describe a scheme of stochastic implementations of quantum teleportation and entanglement swapping in terms of neutral kaons. In this scheme, the kaon whose state is to be teleported collides with one of the two entangled kaons in an Einstein-Podolsky-Rosen state. Subsequent detection of the outgoing particles of the collision completes the two-qubit projection on Alice side. There appear novel features, which connects quantum information science with fundamental laws of particle physics.

quant-ph↗

arXiv2006

Superfluidity or supersolidity as a Consequence of Off-diagonal Long-range Order

We present a general derivation of Hess-Fairbank effect or non-classical rotational inertial (NCRI), i.e. the refusal to rotate with its container, as well as the quantization of angular momentum, as consequences of off-diagonal long-range order (ODLRO) in an interacting Bose system. Afterwards, the path integral formulation of superfluid density is re-derived without ignoring the centrifugal potential. Finally and in particular, for a class of variational wavefunctions used for solid helium, treating the constraint of single-valuedness boundary condition carefully, we show that there is no ODLRO and, especially, demonstrate explicitly that NCRI cannot be possessed in absence of defects, even though there exist zero-point motion and exchange effect.

cond-mat.supr-con↗

arXiv2006

Bose-Einstein Condensation with Entangled Order Parameter

We propose a practically accessible non-mean-field ground state of Bose-Einstein condensation (BEC), which occurs in an interspecies two-particle entangled state, and is thus described by an entangled order parameter. A suitably defined entanglement entropy is used as the characterization of the non-mean-field nature, and is found to persist in a wide parameter regime. The interspecies entanglement leads to novel interference terms in the dynamical equations governing the single particle orbital wavefunctions. Experimental feasibility and several methods of probe are discussed. We urge the study of multi-channel scattering between different species of atoms.

cond-mat.other↗

arXiv2004

Entanglement in Relativistic Quantum Field Theory

I present some general ideas about quantum entanglement in relativistic quantum field theory, especially entanglement in the physical vacuum. Here, entanglement is defined between different single particle states (or modes), parameterized either by energy-momentum together with internal degrees of freedom, or by spacetime coordinate together with the component index in the case of a vector or spinor field. In this approach, the notion of entanglement between different spacetime points can be established. Some entanglement properties are obtained as constraints from symmetries, e.g., under Lorentz transformation, space inversion, time reverse and charge conjugation.

hep-th↗