In recent years, the entanglement spectra of quantum states have been identified to be highly valuable for improving our understanding on many problems in quantum physics, such as classification of topological phases, symmetry-breaking phases, and eigenstate thermalization, etc. However, it remains a major challenge to fully characterize the entanglement spectrum of a given quantum state. An outstanding problem is whether the difficulty is intrinsically technical or fundamental? Here using the tools in computational complexity, we perform a rigorous analysis to pin down the counting complexity of entanglement spectra of (i) states generated by polynomial-time quantum circuits, (ii) ground states of gapped 5-local Hamiltonians, and (iii) projected entangled-pair states (PEPS). We prove that despite the state complexity, the problems of counting the number of sizable elements in the entanglement spectra all belong to the class $\mathsf{\# P}$ -complete, which is as hard as calculating the partition functions of Ising models. Our result suggests that the absence of an efficient method for solving the problem is fundamental in nature, from the point of view of computational complexity theory.In recent years, the entanglement spectra of quantum states have been identified to be highly valuable for improving our understanding on many problems in quantum physics, such as classification of topological phases, symmetry-breaking phases, and eigenstate thermalization, etc. However, it remains a major challenge to fully characterize the entanglement spectrum of a given quantum state. An outstanding problem is whether the difficulty is intrinsically technical or fundamental? Here using the tools in computational complexity, we perform a rigorous analysis to pin down the counting complexity of entanglement spectra of (i) states generated by polynomial-time quantum circuits, (ii) ground states of gapped 5-local Hamiltonians, and (iii) projected entangled-pair states (PEPS). We prove that despite the state complexity, the problems of counting the number of sizable elements in the entanglement spectra all belong to the class $\mathsf{\# P}$-complete, which is as hard as calculating the partition functions of Ising models. Our result suggests that the absence of an efficient method for solving the problem is fundamental in nature, from the point of view of computational complexity theory.

arXiv:1906.00324Jun 2, 20190 citations

New supersymmetry-generated complex potentials with real spectra

Óscar Rosas-Ortiz, Octavio Castaños, Dieter Schuch

A new form to construct complex superpotentials that produce real energy spectra in supersymmetric quantum mechanics is presented. This is based on the relation between the nonlinear Ermakov equation and a second order differential equation of the Schrödinger type. The superpotentials so constructed are characterized by the Ermakov parameters in such a way that they are always complex-valued and lead to non-Hermitian Hamiltonians with real spectra, whose eigenfunctions form a bi-orthogonal system. As applications we present new complex supersymmetric partners of the free particle that are P T -symmetric and can be either periodic or regular (of the Pöschl–Teller form). A new family of complex oscillators with real frequencies that have the energies of the harmonic oscillator plus an additional real eigenvalue is introduced.A new form to construct complex superpotentials that produce real energy spectra in supersymmetric quantum mechanics is presented. This is based on the relation between the nonlinear Ermakov equation and a second order differential equation of the Schrödinger type. The superpotentials so constructed are characterized by the Ermakov parameters in such a way that they are always complex-valued and lead to non-Hermitian Hamiltonians with real spectra, whose eigenfunctions form a bi-orthogonal system. As applications we present new complex supersymmetric partners of the free particle that are P T -symmetric and can be either periodic or regular (of the Pöschl–Teller form). A new family of complex oscillators with real frequencies that have the energies of the harmonic oscillator plus an additional real eigenvalue is introduced.

arXiv:1505.05197Journal of Physics A Mathematical and TheoreticalOct 12, 201545 citations

Quantum Computational Complexity

John Watrous

Springer eBooksNov 2, 201147 citations

Line-shape analysis of double-quantum multidimensional coherent spectra

Bachana Lomsadze, Steven T. Cundiff

Double-quantum two-dimensional (2D) coherent spectroscopy (MDCS) is a powerful optical method that is used to study the optical properties of atomic and complex molecular systems and semiconductor materials. Double-quantum 2D spectra and particularly the peak line shapes on the spectra can also provide information about many-body interactions. We model 2D spectra by solving the optical Bloch equations and show the effects of correlation between coupled resonances, which also explains the discrepancies between the experimental results reported by multiple groups.Double-quantum two-dimensional (2D) coherent spectroscopy (MDCS) is a powerful optical method that is used to study the optical properties of atomic and complex molecular systems and semiconductor materials. Double-quantum 2D spectra and particularly the peak line shapes on the spectra can also provide information about many-body interactions. We model 2D spectra by solving the optical Bloch equations and show the effects of correlation between coupled resonances, which also explains the discrepancies between the experimental results reported by multiple groups.

arXiv:2007.15080Physical Review AOct 15, 202032 citations

Space-Bounded Quantum Complexity

John Watrous

Journal of Computer and System SciencesOct 1, 199959 citations

\mathsf{\# P}

-complete, which is as hard as calculating the partition functions of Ising models. Our result suggests that the absence of an efficient method for solving the problem is fundamental in nature, from the point of view of computational complexity theory.