# Publications by Martin Kiffner

## Variational quantum algorithms for nonlinear problems

Physical Review A American Physical Society **101** (2020) 010301(R)

We show that nonlinear problems including nonlinear partial di↵erential equations can be e- ciently solved by variational quantum computing. We achieve this by utilizing multiple copies of variational quantum states to treat nonlinearities eciently and by introducing tensor networks as a programming paradigm. The key concepts of the algorithm are demonstrated for the nonlinear Schr¨odinger equation as a canonical example. We numerically show that the variational quantum ansatz can be exponentially more ecient than matrix product states and present experimental proof-of-principle results obtained on an IBM Q device.

## Efficient microwave-to-optical conversion using Rydberg atoms

PHYSICAL REVIEW A **99** (2019) ARTN 023832

## Mott polaritons in cavity-coupled quantum materials

New Journal of Physics IOP Publishing (2019)

## Manipulating quantum materials with quantum light (vol 99, 085116, 2019)

PHYSICAL REVIEW B **99** (2019) ARTN 099907

## Manipulating quantum materials with quantum light

Physical Review B American Physical Society **99** (2019) 085116-

We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.

## Collimated UV light generation by two-photon excitation to a Rydberg state in Rb vapor

OPTICS LETTERS **44** (2019) 2931-2934

## Coherent microwave-to-optical conversion via six-wave mixing in Rydberg atoms

Physical Review Letters American Physical Society **120** (2018) 093201

We present an experimental demonstration of converting a microwave field to an optical field via frequency mixing in a cloud of cold 87Rb atoms, where the microwave field strongly couples to an electric dipole transition between Rydberg states. We show that the conversion allows the phase information of the microwave field to be coherently transferred to the optical field. With the current energy level scheme and experimental geometry, we achieve a photon conversion efficiency of \sim 0.3\% at low microwave intensities and a broad conversion bandwidth of more than 4~MHz. Theoretical simulations agree well with the experimental data, and indicate that near-unit efficiency is possible in future experiments.

## A polynomial Ansatz for norm-conserving pseudopotentials.

Journal of physics. Condensed matter : an Institute of Physics journal **30** (2018) 275501-275501

We show that efficient norm-conserving pseudopotentials for electronic structure calculations can be obtained from a polynomial Ansatz for the potential. Our pseudopotential is a polynomial of degree ten in the radial variable and fulfils the same smoothness conditions imposed by the Troullier-Martins method (TM) (1991 Phys. Rev. B 43 1993) where pseudopotentials are represented by a polynomial of degree twenty-two. We compare our method to the TM approach in electronic structure calculations for diamond and iron in the bcc structure and find that the two methods perform equally well in calculations of the total energy. However, first and second derivatives of the total energy with respect to atomic coordinates converge significantly faster with the plane wave cutoff if the standard TM potentials are replaced by the pseudopotentials introduced here.

## Probing microscopic models for system-bath interactions via parametric driving

PHYSICAL REVIEW A **98** (2018) ARTN 012122

## Topological spin models in Rydberg lattices

Applied Physics B Springer Verlag **123** (2017)

We show that resonant dipole–dipole interactions between Rydberg atoms in a triangular lattice can give rise to artificial magnetic fields for spin excitations. We consider the coherent dipole–dipole coupling between np and ns Rydberg states and derive an effective spin-1/2 Hamiltonian for the np excitations. By breaking time-reversal symmetry via external fields, we engineer complex hopping amplitudes for transitions between two rectangular sub-lattices. The phase of these hopping amplitudes depends on the direction of the hop. This gives rise to a staggered, artificial magnetic field which induces non-trivial topological effects. We calculate the single-particle band structure and investigate its Chern numbers as a function of the lattice parameters and the detuning between the two sub-lattices. We identify extended parameter regimes where the Chern number of the lowest band is C=1C=1 or C=2C=2 .

## Terahertz field control of interlayer transport modes in cuprate superconductors

Physical Review B - Condensed Matter and Materials Physics American Physical Society **96** (2017) 064526-

We theoretically show that terahertz pulses with controlled amplitude and frequency can be used to switch between stable transport modes in layered superconductors, modelled as stacks of Josephson junctions. We find pulse shapes that deterministically switch the transport mode between superconducting, resistive and solitonic states. We develop a simple model that explains the switching mechanism as a destablization of the centre of mass excitation of the Josephson phase, made possible by the highly non-linear nature of the light-matter coupling.

## Two-way interconversion of millimeter-wave and optical fields in Rydberg gases

New Journal of Physics Institute of Physics **18** (2016) 093030

We show that cold Rydberg gases enable an efficient six-wave mixing process where terahertz or microwave fields are coherently converted into optical fields and vice versa. This process is made possible by the long lifetime of Rydberg states, the strong coupling of millimeter waves to Rydberg transitions and by a quantum interference effect related to electromagnetically induced transparency (EIT). Our frequency conversion scheme applies to a broad spectrum of millimeter waves due to the abundance of transitions within the Rydberg manifold, and we discuss two possible implementations based on focussed terahertz beams and millimeter wave fields confined by a waveguide, respectively. We analyse a realistic example for the interconversion of terahertz and optical fields in rubidium atoms and find that the conversion efficiency can in principle exceed 90%.

## Two-way interconversion of millimeter-wave and optical fields in Rydberg gases (vol 18, 093030, 2016)

NEW JOURNAL OF PHYSICS **18** (2016) ARTN 109502

## Corrigendum: Two-way interconversion of millimeter-wave and optical fields in Rydberg gases (2016 New J. Phys. 18 093030)

New Journal of Physics IOP Publishing **18** (2016) 109502-109502

## Coherent bidirectional microwave-optical conversion using Rydberg atoms

2016 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2016)

## Quantum mechanical calculation of Rydberg-Rydberg autoionization rates

Journal of Physics B: Atomic, Molecular and Optical Physics Institute of Physics **49** (2016) 204004-

We present quantum mechanical calculations of autoionization rates for two Rubidium Rydberg atoms with weakly overlapping electron clouds. We neglect exchange effects and consider tensor products of independent atom states forming an approximate basis of the two-electron state space. We consider large sets of two-atom states with randomly chosen quantum numbers and find that the charge overlap between the two Rydberg electrons allows one to characterise the magnitude of the autoionization rates. If the electron clouds overlap by more than one percent, the autoionization rates increase approximately exponentially with the charge overlap. This finding is independent of the energy of the initial state.

## Lensing effect of electromagnetically induced transparency involving a Rydberg state

PHYSICAL REVIEW A **92** (2015) ARTN 063824

## Pulse splitting in light propagation through N-type atomic media due to an interplay of Kerr nonlinearity and group-velocity dispersion

PHYSICAL REVIEW A **92** (2015) ARTN 023840

## Few-body bound states of dipole-dipole-interacting Rydberg atoms

PHYSICAL REVIEW A **89** (2014) ARTN 052717

## Single spontaneous photon as a coherent beamsplitter for an atomic matter-wave

ELEVENTH INTERNATIONAL CONFERENCE ON QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTATION (QCMC) **1633** (2014) 25-28