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Entanglement detection via quantum Fisher information in a coupled atom-field system

Safoura Sadat Mirkhalaf

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

We consider a system of finite number of particles collectively interacting with a single mode coherent \nfield inside the cavity. Depending on the strength of the initial field compared to the number of \natoms, we introduce three regimes of weak, intermediate and strong field interaction. The dynamics \nof multi-particle entanglement detected by quantum Fisher information is studied in each regime. \nSpecifically, for the two extreme limits of weak and strong field regimes, analytical results are \nfound. Moreover, we have compared our outcomes with spin squeezing. It is seen that in the weak \nfield regime, spin squeezing and quantum Fisher information coincides. However, by increasing the \ninitial field population toward the strong field regime, spin squeezing compared to quantum Fisher \ninformation gradually decreases and eventually fails in detecting entanglement. In addition, in the \ntwo-atom system, we also consider concurrence. In this case, the quantum Fisher information and \nconcurrence show good agreement in predicting entanglement peaks.

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We consider a system of finite number of particles collectively interacting with a single mode coherent \nfield inside the cavity. Depending on the strength of the initial field compared to the number of \natoms, we introduce three regimes of weak, intermediate and strong field interaction. The dynamics \nof multi-particle entanglement detected by quantum Fisher information is studied in each regime. \nSpecifically, for the two extreme limits of weak and strong field regimes, analytical results are \nfound. Moreover, we have compared our outcomes with spin squeezing. It is seen that in the weak \nfield regime, spin squeezing and quantum Fisher information coincides. However, by increasing the \ninitial field population toward the strong field regime, spin squeezing compared to quantum Fisher \ninformation gradually decreases and eventually fails in detecting entanglement. In addition, in the \ntwo-atom system, we also consider concurrence. In this case, the quantum Fisher information and \nconcurrence show good agreement in predicting entanglement peaks.
We consider a system of finite number of particles collectively interacting with a single-mode coherent field inside a cavity. Depending on the strength of the initial field compared to the number of atoms, we consider three regimes of weak-, intermediate-, and strong-field interaction. The dynamics of multiparticle entanglement detected by quantum Fisher information and spin squeezing are studied in each regime. It is seen that in the weak-field regime, spin squeezing and quantum Fisher information coincide. However, by increasing the initial field population toward the strong-field regime, quantum Fisher information is more effective in detecting entanglement compared to spin squeezing. In addition, in the two-atom system, we also study concurrence. In this case, the quantum Fisher information as a function of time is in good agreement with concurrence in predicting entanglement peaks.
The generation of entanglement in atomic systems plays a central topic in the fields of quantum information storage and processing. Moreover, a special category of entangled states of multi-atom ensembles, spin squeezed states, have been proven to lead to considerable improvement in the sensitivity of precision measurements compared to systems involving uncorrelated atoms. A treatment of entanglement in open systems is, however, incomplete without a precise description of the process of decoherence which necessarily accompanies it. The theory of entanglement and decoherence are the two main topics of this thesis. Methods are described for the generation of strong correlations in large atomic ensembles using either cavity quantum electrodynamics or measurement outcome conditioned quantum dynamics. Moreover, the description of loss of entanglement resulting from the coupling to a noise reservoir (electromagnetic vacuum) is explored. A spin squeezing parameter is used throughout this thesis as both a measure of entanglement strength and as an indication of the sensitivity improvement above the so-called standard quantum limit (sensitivity obtained with uncorrelated particles) in metrology. The first scheme considered consists of a single mode cavity field interacting with a collection of atoms for which spin squeezing is produced in both resonant and off-resonant regimes. In the resonant case, transfer of squeezing from a field state to the atoms is analyzed, while in the off-resonant regime squeezing is produced via an effective nonlinear interaction (one-axis twisting Hamiltonian). A second, more experimentally realistic case, is one involving the interaction of free space atoms with laser pulses; a projective measurement of a source field originating from atomic fluctuations provides a means of preparing atomic collective states such as spin squeezed and Schrodinger cat states. A new unravelling is proposed, that employs the detection of photon number in a single output channel and is capable of producing mesoscopic Schrodinger cat states in a single step. As a first step to understanding the role of cooperative decoherence, of importance in the case of dense pencil-shaped atomic ensembles, the collective spin decoherence of a two multilevel atom system is derived. This calculation is also relevant to entanglement loss for two qubits manipulated using with reading/writing pulses. Finally, a scheme is proposed in which arbitrarily strong entanglement can be produced in a four-wave mixing setup, where the preparation of the atoms in a dark state limits the decoherence to negligible values.
A clear relationship between the quantum version of Fisher information and the bipartite system entanglement has been explored recently. Known existing results for some quantum systems having strongly nonlocal correlations are still limited. In this paper, based on the interaction between a two-qubit and squeezed field, we investigate the correlation between the dynamical behavior of qubit-qubit entanglement and the quantum Fisher information (QFI). We focus on the effect of a number of photon multiplicity, low and high squeezing regime of the field on the dynamics of the QFI and entanglement quantifier. We show how the QFI and entanglement quantifier can be affected by the number of photon multiplicity and squeeze parameter during the time evolution of cases of absence and presence of dissipation effect. Our results exhibit the correlation between the entanglement quantifiers and the QFI with respect to the appearance of collapse and revival phenomena of the atomic inversion.
We investigate the link between quantum Fisher information (QFI) and quantum entanglement for a scheme of two atoms that interact with two non-degenerate modes in the absence and presence of intrinsic decoherence effect. The explicit form of the density matrix approach to the dissipative systems is given when the cavity-field is in generalized coherent states. The dynamical behavior of the QFI is compared with the atom–atom concurrence entanglement. We examine the effect of the dipole–dipole interaction and intrinsic decoherence on the dynamical properties of the QFI, concurrence and variance entropy squeezing. It is found that the atom–atom concurrence and the QFI can be controlled by the intrinsic decoherence and the dipole–dipole interaction.
We study the competing effects of collective generalized measurements and interaction-induced scrambling in the dynamics of an ensemble of spin-1/2 particles at the level of quantum trajectories. This setup can be considered as analogous to the one leading to measurement-induced transitions in quantum circuits. We show that the interplay between collective unitary dynamics and measurements leads to three regimes of the average Quantum Fisher Information (QFI), which is a witness of multipartite entanglement, as a function of the monitoring strength. While both weak and strong measurements lead to extensive QFI density (i.e., individual quantum trajectories yield states displaying Heisenberg scaling), an intermediate regime of classical-like states emerges for all system sizes where the measurement effectively competes with the scrambling dynamics and precludes the development of quantum correlations, leading to sub-Heisenberg-limited states. We characterize these regimes and the crossovers between them using numerical and analytical tools, and discuss the connections between our findings, entanglement phases in monitored many-body systems, and the quantum-to-classical transition.
We investigate spin squeezing (SS) and the quantum Fisher information (QFI) for the Jaynes–Cummings–Dicke (JCD) model in a two-component atomic Bose–Einstein condensate (BEC) inside an optical cavity. Analytical expressions for spin squeezing and the reciprocal of the quantum Fisher information per particle (RMQFI) are derived using the frozen spin approximation. It is shown that in the superradiant phase near the critical point, maximum squeezing and maximum quantum entanglement occur and thus the critical point emerges as a useful resource for precision measurements. In the presence of decoherence and particle loss, we show that gradually with time, even though the ability of squeezing and entanglement generation are weakened, yet significant amounts are still present which can be relevant to quantum information processing and precision spectroscopy.