Spin physics, spin-chains, quantum magnonics and entanglement theory

A project of the Theoretical Chemical and Quantum Physics Group


Mr. Muhammad Ahmed, Dr. Jan Jeske, A/Prof. Jared Cole, Prof. Andrew Greentree


C. Müller, M. Marthaler, Prof. Schön: Karlsruhe Institute of Technology

S. Huelga: University of Ulm

L. Hall, L. Hollenberg: University of Melbourne

S. Devitt: NII, Tokyo, Japan

Brief Project Outline

The study of spins and their interactions (be they electron-, nuclear- or pseudo-spins) is one of the central components of quantum mechanics. In recent years, the new fields of quantum computing and quantum information processing have link the physics of spins to the fields of information theory, computing theory and cryptography. In this project, we consider the behaviour of interacting few spin systems to study entanglement, transport and measurement. This has applications to quantum computing, quantum sensing as well as the fundamental theory of quantum mechanics. Specific topics of recent interest include:

  • The interplay between decoherence and entanglement theory, the connection between entanglement quantification and entanglement sudden birth and death.
  • Hamiltonian characterisation and measurement, the extraction of system information (system calibration) from a quantum system.
  • Spin transport and the direct control of single magnons.

Geometric entanglement evolution

Evolution of the absolute geometric entanglement hierarchy within a system consisting to two atom-photon pairs, showing the inter-conversion between different classes of entanglement.
J. Cole, J. Phys. A: Math. Theor. 43 135301 (2010)

Entanglement generation

The maximum entanglement which can be generated by an interaction, governed by its position in the Weyl chamber. An arbitrary two-qubit interaction can always be expressed as a trajectory in the Weyl chamber, giving a graphical method for study the creation and destruction of entanglement during coherent evolution.

Spin-wave guiding via time-varying local magnetic potential. By varying the shape and translational speed of the potential, the spin-wave behaves in a similar manner to the light field in an optic fibre. Provided the translation of the potential is adiabatic enough, the spin-wave can be steered while staying as a coherent excitation.
Makin et al. Phys. Rev. Lett. 108 017207 (2012)

Recent Publications

J. H. Cole, Understanding entanglement sudden death through multipartite entanglement and quantum correlations, J. Phys. A: Math. Theor. 43 135301 (2010)

M. I. Makin, J. H. Cole, C. D. Hill and A. D. Greentree, Spin Guides and Spin Splitters: Waveguide Analogies in One-Dimensional Spin Chains, Phys. Rev. Lett. 108 017207 (2012)

Muhammad H. Ahmed and Andrew D. Greentree, Guided magnon transport in spin chains: Transport speed and correcting for disorder, Phys. Rev. A 91 022306 (2015)

For more information about this project, please contact Jared Cole or Andrew Greentree.

Open quantum systems and decoherence theory

A project of the Theoretical Chemical and Quantum Physics Group


Mr. David Ing, Dr. Nicolas Vogt, Dr. Jan Jeske, A/Prof. Jared Cole


M. Marthaler, G. Schön: Karlsruhe Institute of Technology

S. Huelga, M. Plenio: University of Ulm

C. Müller, T. Stace: University of Queensland

Brief Project Outline

What defines the boundary between the quantum and classical worlds is one of the most fundamental questions of modern physics. Quantum theory has proven to be spectacularly successful at small length scales, short times and very cold temperatures. Yet, in our everyday world the equations of classical physics have a fundamentally different form and interpretation. How do we smoothly swap between these descriptions? - this is one of the central points of decoherence theory. How do we perturb an otherwise phase coherent theory (quantum mechanics) in order to include the effects of the wider environmental, ie. the effects of dephasing and dissipation.

The TCQP group studies this problem in a range of different physical systems and with several different mathematical techniques. Problems of current interest include:

  • Mathematical methods for describing spatial correlations in noise processes and there ramifications of these correlations in quantum system dynamics
  • The cross-over from Markovian to non-Markovian dynamics and the effects of finite bath effects
  • Extensions to the usual decoherence models for more efficient and scalable numerical simulation

A linear chain of spins is an ideal system to study effects due to spatially correlated noise. Here we show the transmission probability of so-called "perfect state transfer" as a function of correlation length of environmental dephasing.  We see that for noise which is correlated on length scales longer than the spin chain, the noise has little effect on the excitation transfer process.

The distribution of charge states in an single-electron transistor evolving as a function of time, computed using stochastic Bloch-Redfield theory. As the system evolves, the statistical distribution of charge states broadens due to the interplay of coherent and incoherent processes which are modelled consistently within a stochastic master equation.

Recent Publications

J. Jeske and J. H. Cole, Derivation of Markovian master equations for spatially correlated decoherence, Physical Review A 87 052138 (2013)

J. Jeske, N. Vogt and J. H. Cole, Excitation and state transfer through spin chains in the presence of spatially correlated noise, Physical Review A 88 062333 (2013)

N. Vogt, J. Jeske and J. H. Cole, Stochastic Bloch-Redfield theory: Quantum jumps in a solid-state environment, Physical Review B 88 174514 (2013)

For more information about this project, please contact Jared Cole.

Superconducting circuit theory for information processing and metrology

A project of the Theoretical Chemical and Quantum Physics Group


Ms. Kelly Walker, Mr. Samuel Wilkinson, Dr. Nicolas Vogt, A/Prof. Jared Cole


M. Marthaler, G. Schön, A. Shnirman: Karlsruhe Institute of Technology

C. Müller: University of Queensland

Brief Project Outline

Electrical circuits operating at sub-Kelvin temperatures display a range of effects, which can only be described by the laws of quantum mechanics. As these circuits can be fabricated "at will", they provide unique opportunities to study quantum effects where a circuit can be designed specifically to study a particular effect. Quantum circuits already find application in the detection of microscopic magnetic and electric fields, ultra-sensitive amplifiers and ultra-fast electronics. This project investigates the behaviour of quantum circuits for both applications in modern technology and to study fundamental physical principles.

These include:

  • Quantum information processing – The generalisation of classical information theory and its use in the design of operation of quantum computers using quantum circuits.
  • Quantum Metrology with Josephson junction arrays – The possible application of linear arrays of Josephson junction for providing a quantum definition of the unit of current, the ampere.
  • Circuit quantum electrodynamics – Using quantum circuits to replicate effects and behaviour observed in conventional quantum optics experiments, including strong-coupling, coherent-incoherent interactions and single-atom/qubit effects.

Phase qubits coupled to a two-level defect

Rabi spectroscopy of a strongly coupled phase qubit/two-level fluctuator system (experiment and theory). Time evolution of the probability (a) to measure the excited qubit state vs. driving frequency, and (b) its Fourier-transform from the experimental data. The resonance frequency of the TLF is indicated by vertical lines. (c) The corresponding numerical calculation for the time evolution (c) and Fourier transform (d). The numerical calculations consist of solving the full density matrix of the system including decoherence using independently measured system parameters.
Lisenfeld et al., Phys. Rev. B 81, 100511(R) (2010)

JJA transport animation

Evolution of the charge distribution within a linear array of Josephson junctions in the deep Coulomb regime. A regular lattice of charges is formed due to the charge-charge interaction within the array

Charge distribution within the array for an example Monte-Carlo instance, as a function of time. At higher voltages (compared to the threshold for conduction) a new mechanism for correlated transport forms, consisting of a quasi-static "quasiparticle gas" through which transport is carrier by movement of Cooper-pair states.
Cole et al., New Journal of Physics 16 063019 (2014)

Recent Publications

P. Bushev, C. Müller, J. Lisenfeld, J. H. Cole, A. Lukashenko, A. Shnirman and A. V. Ustinov, Multi-photon spectroscopy of a hybrid quantum system, Phys. Rev. B 82, 134530 (2010)

M. Marthaler, J. Leppäkangas and J. H. Cole, Circuit-QED analogue of a single-atom injection maser: Lasing, trapping states and multistability, Phys. Rev. B 83, 180505 (2011)

K. A. Walker and J. H. Cole, Correlated charge transport in bilinear tunnel junction arrays, Physical Review B 88 245101 (2013))

J. H. Cole, J. Leppakangas and M. Marthaler, Correlated transport through junction arrays in the small Josephson energy limit: incoherent Cooper-pairs and hot electrons, New Journal of Physics 16 063019 (2014)

For more information about this project, please contact Jared Cole.