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.