We study squeezing and entanglement in superposition of a class of nonlinear spin coherent states. We analyze these properties as a function of the coherence and superposition parameters, number of qubits and the Hamiltonians involved, by presenting analytical and numerical results. We specifically observe that a subclass of these states, the even non-linear cat states are maximally entangled and squeezed, while another related subclass, the odd non-linear cat states, are entangled but not squeezed at all.
IntroductionThe spin coherent states were introduced by Radcliff [1], in similarity to the bosonic coherent states and since, their generation, application and properties have been studied extensively [1][2][3][4][5]. Similarly, spin squeezed states, with all their variations; have been the focus of intensive investigations due to their relevance in interferometers, precision spectroscopy [5, 6] and entanglement studies [7][8][9][10][11][12][13][14][15][16][17][18][19]. The state is said to be spin squeezed if the uncertainty of one of the component in a direction perpendicular to the mean spin, is smaller than the standard quantum limit [1]. The spin squeezed states may be generated by application of nonlinear Hamiltonians to appropriate states [4,[12][13][14][15][16][17][18][19][20] or by superposition of spin states [18][19][20]. Ordinary coherent states are neither squeezed nor entangled; however, the production of squeezed and entangled states by application of non-linear Hamiltonians to spin coherent states [4,8,14,21], and through their superposition, for example cat states, have also been considered [18][19][20]. Several classes of nonlinear coherent states have also been introduced and their properties and applications have been studied [22][23][24][25]. Entangled and squeezed spin coherent states have many applications in quantum information, and in the final analysis, they are based on the correlations in the system, though different kinds are involved; therefore, one may expect some kind of interrelationship between them. In this article we intend to study squeezing and entanglement in a general superposition of nonlinear spin coherent states. We organize the rest of the paper as follows. In section 2 a class of nonlinear spin coherent states (NSCS) and their superpositions (SNSCS) are considered. Squeezing properties of (SNSCS) are analyzed in section 3. The squeezing properties of the (NSCS) have been reported recently [21]. Entanglement properties of NSCS and SNSCS are studied in sections 4 and 5 respectively. Finally section 6 is devoted to discussion and conclusions.
Nonlinear spin coherent states and their superpositionWe consider a system of N spin 1 2 in a coherent state that is described by [1] á ñ + á ñ (ˆˆ)ˆ( ) j J J J J 2 27 x x x z y 2 2 2 2 2 3 J. Phys. Commun. 2 (2018) 055001 A Naji et al