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arXiv:2005.09647

Von Neumann Entropy from Mean Spin Vector

Ram Narayan Deb

Published May 19, 2020• 0 citations
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Abstract

We show for a general pure entangled state of two two-level atoms, the von Neumann entropy of the partial traces can be directly measured from the magnitude of the mean spin vector of a single atom of the pair. We emphasize the fact that the von Neumann entropy of the partial traces for such a system can be obtained without knowing the exact form of the quantum state of the two atoms, if we have the value of the magnitude of the mean spin vector of a single atom of the pair. Mean spin vector, used in the context of spin squeezing and spectroscopic squeezing in population spectroscopy, is experimentally measurable and provides an exact measure of von Neumann entropy of the partial traces for such a system. The idea developed in this paper can be used in the context of other quantum mechanical two level systems as the algebra of two level systems can be described by that of spin- $\frac{1}{2}$ particles.

DOI
10.48550/arxiv.2005.09647

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We show for a general pure entangled state of two two-level atoms, the von Neumann entropy of the partial traces can be directly measured from the magnitude of the mean spin vector of a single atom of the pair. We emphasize the fact that the von Neumann entropy of the partial traces for such a system can be obtained without knowing the exact form of the quantum state of the two atoms, if we have the value of the magnitude of the mean spin vector of a single atom of the pair. Mean spin vector, used in the context of spin squeezing and spectroscopic squeezing in population spectroscopy, is experimentally measurable and provides an exact measure of von Neumann entropy of the partial traces for such a system. The idea developed in this paper can be used in the context of other quantum mechanical two level systems as the algebra of two level systems can be described by that of spin- 12\frac{1}{2}21​ particles.
We study the von Neumann entropy of the partial trace of a system of 2 two-level atoms (qubits) in a dispersive cavity where the atoms are interacting collectively with a single-mode electromagnetic field in the cavity. We make a contrast of this entanglement entropy with the spin squeezing property of the system. We find a close relationship between this von Neumann entropy and the spin squeezing of the system. We observe that this entanglement entropy attains its maximum value when the spin squeezing in the system goes to its maximum and is zero when the spin squeezing in the system vanishes.
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We study particles with spins 12\frac{1}{2}21​ and 111, and define their entropy, a measure of randomness over the observable spin values given a spin state. Specifying the degrees of freedom of a particle spin state does not provide the knowledge of the spin value in all three dimensional directions. This randomness is formally rooted on the non-commutative properties of the spin operators Sx,Sy,SzS_x, S_y, S_zSx​,Sy​,Sz​. Our proposed spin-entropy is a measure of the randomness of all three dimensional directions of a spin. This spin-entropy, in contrast to von Neumann entropy, may have non-zero values for pure states. The proposed spin-entropy when applied to Bell states, which are pure entangled states of two fermions, yields local minima values. This suggests that decoherence and disentanglement increases this entropy. The proposed entropy may be useful for the understanding of phenomena that explore the information content of spin systems, including quantum computational processes.
The aim of this work is to compute the entanglement entropy of real and virtual particles by rewriting the generating functional of ${\ensuremath{\phi}}^{4}$ theory as a mean value between states and observables defined through the correlation functions. Then the von Neumann definition of entropy can be applied to these quantum states and in particular, for the partial traces taken over the internal or external degrees of freedom. This procedure can be done for each order in the perturbation expansion showing that the entanglement entropy for real and virtual particles behaves as ln(m0)\mathrm{ln}({m}_{0})ln(m0​). In particular, entanglement entropy is computed at first order for the correlation function of two external points showing that mutual information is identical to the external entropy and that conditional entropies are negative for all the domain of m0{m}_{0}m0​. In turn, from the definition of the quantum states, it is possible to obtain general relations between total traces between different quantum states of a ${\ensuremath{\phi}}^{r}$ theory. Finally, discussion about the possibility of taking partial traces over external degrees of freedom is considered, which implies the introduction of some observables that measure space-time points where an interaction occurs.
We study the entanglement feature of the ground state of a system composed of spin 1 and 1/2 parts. The concurrence vector is shown to be consistent with the measurement of von Neumann entropy for such system. In the light of the ground state degeneracy, we suggest a average concurrence to measure the entanglement of Hilbert subspace. The entanglement property of both a general superposition as well as the mixture of the degenerate ground states are discussed by means of average concurrence and the negativity respectively.