An an exactly solvable quantum model is found to sample the evolution towards the sudden and complete loss of observability. For the purpose we choose an N-level system. While the time runs from 0 to 1, the process (leading to the collapse) is controlled by a toy-model Hamiltonian H and by a unitary-evolution guaranteeing minimally anisotropic (i.e., unique) Hilbert-space metric. The process of the degeneracy of the real N-plet of energy levels is studied without the usual assumption of adiabaticity. The initial Hamiltonian is diagonal, the initial metric is chosen as identity. Once the system reaches the observability horizon, the metric becomes singular (of rank one) while the end-point Hamiltonian acquires the canonical Jordan-block form (i.e., it loses its diagonalizability). An optimal measure of the distance from the final catastrophe is finally found in a universal, exact formula for the spectrum of the metric.
An an exactly solvable quantum model is found to sample the evolution towards the sudden and complete loss of observability. For the purpose we choose an N-level system. While the time runs from 0 to 1, the process (leading to the collapse) is controlled by a toy-model Hamiltonian H and by a unitary-evolution guaranteeing minimally anisotropic (i.e., unique) Hilbert-space metric. The process of the degeneracy of the real N-plet of energy levels is studied without the usual assumption of adiabaticity. The initial Hamiltonian is diagonal, the initial metric is chosen as identity. Once the system reaches the observability horizon, the metric becomes singular (of rank one) while the end-point Hamiltonian acquires the canonical Jordan-block form (i.e., it loses its diagonalizability). An optimal measure of the distance from the final catastrophe is finally found in a universal, exact formula for the spectrum of the metric.
An an exactly solvable quantum model is found to sample the evolution towards the sudden and complete loss of observability. For the purpose we choose an N-level system. While the time runs from 0 to 1, the process (leading to the collapse) is controlled by a toy-model Hamiltonian H and by a unitary-evolution guaranteeing minimally anisotropic (i.e., unique) Hilbert-space metric. The process of the degeneracy of the real N-plet of energy levels is studied without the usual assumption of adiabaticity. The initial Hamiltonian is diagonal, the initial metric is chosen as identity. Once the system reaches the observability horizon, the metric becomes singular (of rank one) while the end-point Hamiltonian acquires the canonical Jordan-block form (i.e., it loses its diagonalizability). An optimal measure of the distance from the final catastrophe is finally found in a universal, exact formula for the spectrum of the metric.
A unitary-evolution process leading to an ultimate collapse and to a complete loss of observability alias quantum phase transition is studied. A specific solvable N−state model is considered, characterized by a non-stationary non-Hermitian Hamiltonian. Our analysis uses quantum mechanics formulated in Schrödinger picture in which, in principle, only the knowledge of a complete set of observables (i.e., operators Λj) enables one to guarantee the uniqueness of the related physical Hilbert space (i.e., of its inner-product metric Θ). Nevertheless, for the sake of simplicity, we only assume the knowledge of just a single input observable (viz., of the energy-representing Hamiltonian H≡Λ1). Then, out of all of the eligible and Hamiltonian-dependent “Hermitizing” inner-product metrics Θ=Θ(H), we pick up just the simplest possible candidate. Naturally, this slightly restricts the scope of the theory, but in our present model, such a restriction is more than compensated for by the possibility of an alternative, phenomenologically better motivated constraint by which the time-dependence of the metric is required to be smooth. This opens a new model-building freedom which, in fact, enables us to force the system to reach the collapse, i.e., a genuine quantum catastrophe as a result of the mere conventional, strictly unitary evolution.
To the existing list of alternative formulations of quantum mechanics, a new version of the non-Hermitian interaction picture is added. What is new is that, in contrast to the more conventional non-Hermitian model-building recipes, the primary information about the observable phenomena is provided not only by the Hamiltonian but also by an additional operator with a real spectrum (say, R(t)) representing another observable. In the language of physics, the information carried by R(t)≠R†(t) opens the possibility of reaching the exceptional-point degeneracy of the real eigenvalues, i.e., a specific quantum phase transition. In parallel, the unitarity of the system remains guaranteed, as usual, via a time-dependent inner-product metric Θ(t). From the point of view of mathematics, the control of evolution is provided by a pair of conjugate Schrödiner equations. This opens the possibility od an innovative dyadic representation of pure states, by which the direct use of Θ(t) is made redundant. The implementation of the formalism is illustrated via a schematic cosmological toy model in which the canonical quantization leads to the necessity of working with two conjugate Wheeler-DeWitt equations. From the point of view of physics, the “kinematical input” operator R(t) may represent either the radius of a homogeneous and isotropic expanding empty Universe or, if you wish, its Hubble radius, or the scale factor a(t) emerging in the popular Lemaitre-Friedmann-Robertson-Walker classical solutions, with the exceptional-point singularity of the spectrum of R(t) mimicking the birth of the Universe (“Big Bang”) at t=0.
The bound-state spectrum of a Hamiltonian H is assumed real in a non-empty domain of physical values of parameters. This means that for these parameters, H may be called crypto-Hermitian, i.e. made Hermitian via an ad hoc choice of the inner product in the physical Hilbert space of quantum bound states (i.e. via an ad hoc construction of the operator Θ called the metric). The name quantum catastrophe is then assigned to the N -tuple-exceptional-point crossing, i.e. to the scenario in which we leave the domain along such a path that at the boundary of , an N -plet of bound-state energies degenerates and, subsequently, complexifies. At any fixed N ⩾ 2, this process is simulated via an N × N benchmark effective matrix Hamiltonian H . It is being assigned such a closed-form metric which is made unique via an N -extrapolation-friendliness requirement. This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to 'Quantum physics with non-Hermitian operators'.
Abstract In this series of works, we study exactly solvable non-unitary time evolutions in one-dimensional quantum critical systems ranging from quantum quenches to time-dependent drivings. In this part I, we are motivated by the recent works of Kontsevich and Segal (2021 arXiv: 2105.10161 ) and Witten (2021 arXiv: 2111.06514 ) on allowable complex spacetime metrics in quantum field theories. In general, such complex spacetime metrics will lead to non-unitary time evolutions. In this work, we study the universal features of such non-unitary time evolutions based on exactly solvable setups. Various physical quantities including the entanglement Hamiltonian and entanglement spectrum, entanglement entropy, and energy density at an arbitrary time can be exactly solved. Due to the damping effect introduced by the complex time, the excitations in the initial state are gradually damped out in time. The non-equilibrium dynamics exhibit universal features that are qualitatively different from the case of real-time evolutions. For instance, for an infinite system after a global quench, the entanglement entropy of the semi-infinite subsystem will grow logarithmically in time, in contrast to the linear growth in a real-time evolution. Moreover, we study numerically the time-dependent driven quantum critical systems with allowable complex spacetime metrics. It is found that the competition between driving and damping leads to a steady state with an interesting entanglement structure.
Quantum phase transition is interpreted as an evolution, at the end of which a parameter-dependent Hamiltonian H(g) loses its observability. In the language of mathematics, such a quantum catastrophe occurs at an exceptional point of order N (EPN). Although the Hamiltonian H(g) itself becomes unphysical in the limit of g→gEPN, it is shown that it can play the role of an unperturbed operator in a perturbation-approximation analysis of the vicinity of the EPN singularity. Such an analysis is elementary at N≤3 and numerical at N≥5, so we pick up N=4. We demonstrate that the specific EP4 degeneracy becomes accessible via a unitary evolution process realizable inside a parametric domain Dphysical, the boundaries of which are determined non-numerically. Possible relevance of such a mathematical result in the context of non-Hermitian photonics is emphasized.