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arXiv:1805.10033 · Physical Review A

No-signaling principle and quantum brachistochrone problem in PT -symmetric fermionic two- and four-dimensional models

Alireza Beygi, S. P. Klevansky

Published Aug 3, 2018• 9 citations
arXivHTMLPDF

Abstract

Fermionic systems differ from bosonic ones in several ways, in particular the time-reversal operator $\mathcal{T}$ is odd, ${\mathcal{T}}^{2}=\ensuremath{-}\mathbb{1}$. For $\mathcal{PT}$-symmetric bosonic systems, the no-signaling principle and the quantum brachistochrone problem have been studied to some degree, both of them controversially. In this paper, we apply the basic methods proposed for bosonic systems [Y. Lee et al., Phys. Rev. Lett. 112, 130404 (2014); C. M. Bender et al., Phys. Rev. Lett. 98, 040403 (2007)] to fermionic two- and four-dimensional $\mathcal{PT}$-symmetric Hamiltonians and obtain several surprising results: We find, in contrast to the bosonic case, that the no-signaling principle is upheld for two-dimensional fermionic Hamiltonians; however, the $\mathcal{PT}$ symmetry is broken. In addition, we find that the time required for the evolution from a given initial state, the spin up, to a given final state, the spin down, is a constant, independent of the parameters of the Hamiltonian, under the eigenvalue constraint. That is, it cannot, as in the bosonic case, be optimized. We do, however, also find a dimensional dependence: Four-dimensional $\mathcal{PT}$-symmetric fermionic Hamiltonians considered here again uphold the no-signaling principle, but it is not essential that the $\mathcal{PT}$ symmetry be broken. The symmetry is, however, broken if the measure of entanglement is conserved. In the four-dimensional systems, the evolution time between orthogonal states is dependent on the parameters of the Hamiltonian, with the conclusion that it again can be optimized and approach zero under certain circumstances. However, if we require the conservation of entanglement, the transformation time between these two states becomes the same constant as that found in the two-dimensional case, which coincides with the minimum time for such a transformation to take place in the Hermitian case.

DOI
10.1103/physreva.98.022105

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Fermionic systems differ from bosonic ones in several ways, in particular the time-reversal operator T\mathcal{T}T is odd, T2=\ensuremath−1{\mathcal{T}}^{2}=\ensuremath{-}\mathbb{1}T2=\ensuremath−1. For PT\mathcal{PT}PT-symmetric bosonic systems, the no-signaling principle and the quantum brachistochrone problem have been studied to some degree, both of them controversially. In this paper, we apply the basic methods proposed for bosonic systems [Y. Lee et al., Phys. Rev. Lett. 112, 130404 (2014); C. M. Bender et al., Phys. Rev. Lett. 98, 040403 (2007)] to fermionic two- and four-dimensional PT\mathcal{PT}PT-symmetric Hamiltonians and obtain several surprising results: We find, in contrast to the bosonic case, that the no-signaling principle is upheld for two-dimensional fermionic Hamiltonians; however, the PT\mathcal{PT}PT symmetry is broken. In addition, we find that the time required for the evolution from a given initial state, the spin up, to a given final state, the spin down, is a constant, independent of the parameters of the Hamiltonian, under the eigenvalue constraint. That is, it cannot, as in the bosonic case, be optimized. We do, however, also find a dimensional dependence: Four-dimensional PT\mathcal{PT}PT-symmetric fermionic Hamiltonians considered here again uphold the no-signaling principle, but it is not essential that the PT\mathcal{PT}PT symmetry be broken. The symmetry is, however, broken if the measure of entanglement is conserved. In the four-dimensional systems, the evolution time between orthogonal states is dependent on the parameters of the Hamiltonian, with the conclusion that it again can be optimized and approach zero under certain circumstances. However, if we require the conservation of entanglement, the transformation time between these two states becomes the same constant as that found in the two-dimensional case, which coincides with the minimum time for such a transformation to take place in the Hermitian case.
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Recently Bender, Brody, Jones and Meister found that in the quantum brachistochrone problem the passage time needed for the evolution of certain initial states into specified final states can be made arbitrarily small, when the time-evolution operator is taken to be non-Hermitian but PT-symmetric. Here we demonstrate that such phenomena can also be obtained for dissipative systems. We conclude that the effect of a tunable passage time can be attributed to the non-Hermitian nature of the time-evolution operator rather than to its PT-symmetry.
In a recent paper, Bender and Klevansky [Phys. Rev. A 84, 024102 (2011)] considered PT\mathcal{PT}PT-symmetric matrix representations for fermionic operator algebras of the form ${\ensuremath{\xi}}^{2}={\overline{\ensuremath{\xi}}}^{2}=0$, $\ensuremath{\xi}\overline{\ensuremath{\xi}}+\overline{\ensuremath{\xi}}\ensuremath{\xi}=\ensuremath{\varepsilon}1$, where $\overline{\ensuremath{\xi}}$ is the PT\mathcal{PT}PT transform of $\ensuremath{\xi}$. They constructed such algebras for $\ensuremath{\varepsilon}=\ensuremath{-}1$ and established that it is not possible to construct a matrix representation for the standard fermionic algebra ($\ensuremath{\varepsilon}=1$). Bender and Klevansky used the formalism developed by Jones-Smith and Mathur [Phys. Rev. A 82, 042101 (2010)] which extends PT\mathcal{PT}PT-symmetric quantum mechanics to the case of odd time-reversal symmetry (fermionic case). By using the same formalism, we show that PT\mathcal{PT}PT-symmetric matrix representations exist for both standard ($\ensuremath{\varepsilon}=1)$ and abnormal ($\ensuremath{\varepsilon}=\ensuremath{-}1)$ fermionic algebras if one takes $\overline{\ensuremath{\xi}}$ as adjoint $\ensuremath{\xi}$ with respect to the CPT\mathcal{CPT}CPT and PT\mathcal{PT}PT inner products, respectively. This general result is illustrated for the example of a typical quaternionic four-level model by an explicit construction of the fermionic creation and annihilation operators which satisfy all the criteria of PT\mathcal{PT}PT quantum mechanics for the odd time-reversal symmetry.
We study the quantum brachistochrone problem for a spin-1 system in a magnetic field of constant absolute value. Such a system gives us the possibility to examine in detail the statement that the state vectors realizing evolution with the minimal time of passage evolve along the subspace spanned by the initial and final state vectors [Carlini et al., Phys. Rev. Lett. 96, 060503 (2006); Brody and Hook, J. Phys. A 39, L167 (2006)]. Considering an explicit example, we show the existence of a quantum brachistochrone with the minimal possible time; however, the state vector we study leaves the subspace spanned by the initial and final state vectors during evolution. This is the result of our choice of a more constrained Hamiltonian than the one assumed in the general quantum brachistochrone problem. It is worth noting that such an evolution, being more complicated, is time optimal but with larger time than in the general case. This might be important for experiments, where a general Hamiltonian with all the allowed parameters is difficult to implement, but a constrained one, depending on the magnetic field, can be realized. However, for the preconstrained Hamiltonian not all final states are accessible. The present result does not contradict the general statement of the quantum brachistochrone problem, but gives additional insight into possible realization of the time-optimal passage.
Recently the quantum brachistochrone problem has been considered for non-Hermitian PT\cal PTPT-symmetric quantum system, and it has been shown that the optimal evolution time required to transform a given initial state ∣ψi>|\psi_i>∣ψi​> into specific final state ∣ψf>|\psi_f>∣ψf​> can be made arbitrary small. Here we study the similar problem for generic non-Hermitian Hamiltonian and find that the same answer can be obtained. We show that at the exceptional point the distance between ∣ψi>|\psi_i>∣ψi​> and ∣ψf>|\psi_f>∣ψf​> becomes virtually vanishing and quantum tunneling occurring at the exceptional point yields the shortest timed required to evolve from the initial state ∣ψi>|\psi_i>∣ψi​> into the final state ∣ψf>|\psi_f>∣ψf​>. These results may have applications in holonomic quantum computation and quantum information processing.