Quantum Battery with Ultracold Atoms: Bosons vs. Fermions

We design a quantum battery made up of bosons or fermions in an ultracold atom setup, described by Fermi-Hubbard (FH) and Bose-Hubbard (BH) models respectively. We compare the performance of bosons as well as fermions and check which can act more efficiently as a quantum battery for a given on-site interaction and temperature of the initial state. The performance of a quantum battery is quantified by the maximum power generated over the time evolution under an on-site charging Hamiltonian. We report that when the initial battery state is in the ground state, fermions outperform bosons in a certain configuration over a large range of on-site interactions which are shown analytically for a smaller number of lattice sites and numerically for a considerable number of sites. Bosons take the lead when the temperature is comparatively high in the initial state for a longer range of on-site interaction. We perform the study of a number of up and down fermions as well as the number of bosons per site to find the optimal filling factor for maximizing the power of the battery. We also introduce disorder in both on-site and hopping parameters and demonstrate that the maximum power is robust against impurities. Moreover, we identify a range of tuning parameters in the fermionic as well as bosonic systems where the disorder-enhanced power is observed.

Detection of unbroken phase of non-Hermitian system via Hermitian factorization surface

In the traditional quantum theory, one-dimensional quantum spin models possess a factorization surface where the ground states are fully separable having vanishing bipartite as well as multipartite entanglement. We report that in the non-Hermitian counterpart of these models, these factorization surfaces either can predict the exceptional points where the unbroken to the broken transition occurs or can guarantee the reality of the spectrum, thereby proposing a procedure to reveal the unbroken phase. We first analytically demonstrate it for the nearest-neighbor rotation-time RT-symmetric XY model with uniform and alternating transverse magnetic fields, referred to as the iATXY model. Exact diagonalization techniques are then employed to establish this fact for the RT-symmetric XYZ model with short- and long-range interactions as well as for the variable-ranged iATXY model. Moreover, we show that although the factorization surface prescribes the unbroken phase of the non-Hermitian model, the bipartite nearest-neighbor entanglement at the exceptional point is nonvanishing.

Randomness Amplification under Simulated PT-symmetric Evolution

PT-symmetric quantum theory does not require the Hermiticity property of observables and hence allows a rich class of dynamics. Based on PT-symmetric quantum theory, various counter-intuitive phenomena like faster evolution than that allowed in standard quantum mechanics, single-shot discrimination of nonorthogonal states has been reported invoking Gedanken experiments. By exploiting open-system experimental set-up as well as by computing the probability of distinguishing two states, we prove here that if a source produces an entangled state shared between two parties, Alice and Bob, situated in a far-apart location, the information about the operations performed by Alice whose subsystem evolves according to PT-symmetric Hamiltonian can be gathered by Bob, if the density matrix is in complex Hilbert space. Employing quantum simulation of PT-symmetric evolution, feasible with currently available technologies, we also propose a scheme of sharing quantum random bit-string between two parties when one of them has access to a source generating pseudo-random numbers. We find evidence that the task becomes more efficient with the increase of dimension.

Decoherence-free mechanism to protect long-range entanglement against decoherence

Quantum spin models with variable-range interactions can exhibit certain quantum characteristics that a short-ranged model cannot possess. By considering the quantum XYZ model whose interaction strength between different sites varies either exponentially or polynomially, we report the creation of long-range entanglement in dynamics both in the absence and presence of system-bath interactions. Specifically, during closed dynamics, we determine a parameter regime from which the system should start its evolution so that the resulting state after quench can produce a high time-averaged entanglement having low fluctuations. Both in the exponential and power-law decays, it occurs when the magnetic field is weak and the interactions in the z-direction are nonvanishing. When part of the system interacts with the bath repeatedly or is attached to a collection of harmonic oscillators along with dephasing noise in the z-direction, we observe that long-range entanglement of the subparts which are not attached with the environment remains constant with time in the beginning of the evolution, known as freezing of entanglement, thereby demonstrating a method to protect long-range entanglement. We find that the frozen entanglement content in any length and the time up to which freezing occurs called the freezing terminal to follow a complementary relation for all ranges of interactions. However, we find that for a fixed range of entanglement, there exists a critical value of interaction length which leads to the maximum freezing terminal.