One-dimensional waveguide coupled to multiple qubits: Photon-photon correlations

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For a one-dimensional (1D) waveguide coupled to two or three qubits, we show that the photon-photon correlations have a wide variety of behavior, with structure that depends sensitively on the frequency and on the qubit-qubit separation L. We study the correlations by calculating the second-order correlation function g 2 (t) in which the interference among the photons multiply scattered from the qubits causes rich structure. In one case, for example, transmitted and reflected photons are both bunched initially, but then become strongly anti-bunched for a long time interval. We first calculate the correlation function g2(t) including non-Markovian effects and then show that a much simpler Markovian treatment, which can be solved analytically, is accurate for small qubit separation. As a result, the non-classical properties of microwaves in a 1D waveguide coupled to many superconducting qubits with experimentally accessible separation L could be readily explored with our approach.





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Fang, YLL, H Zheng and HU Baranger (2014). One-dimensional waveguide coupled to multiple qubits: Photon-photon correlations. EPJ Quantum Technology, 1(1). p. 3. 10.1140/epjqt3 Retrieved from

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Harold U. Baranger

Professor of Physics

The broad focus of Prof. Baranger's group is quantum open systems at the nanoscale, particularly the generation of correlation between particles in such systems. Fundamental interest in nanophysics-- the physics of small, nanometer scale, bits of solid-- stems from the ability to control and probe systems on length scales larger than atoms but small enough that the averaging inherent in bulk properties has not yet occurred. Using this ability, entirely unanticipated phenomena can be uncovered on the one hand, and the microscopic basis of bulk phenomena can be probed on the other. Additional interest comes from the many links between nanophysics and nanotechnology. Within this thematic area, our work ranges from projects trying to nail down realistic behavior in well-characterized systems, to more speculative projects reaching beyond regimes investigated experimentally to date.

Correlations between particles are a central issue in many areas of condensed matter physics, from emergent many-body phenomena in complex materials, to strong matter-light interactions in quantum information contexts, to transport properties of single molecules. Such correlations, for either electrons or bosons (photons, plasmons, phonons,…), underlie key phenomena in nanostructures. Using the exquisite control of nanostructures now possible, experimentalists will be able to engineer correlations in nanosystems in the near future. Of particular interest are cases in which one can tune the competition between different types of correlation, or in which correlation can be tunably enhanced or suppressed by other effects (such as confinement or interference), potentially causing a quantum phase transition-- a sudden, qualitative change in the correlations in the system.

My recent work has addressed correlations in both electronic systems (quantum wires and dots) and photonic systems (photon waveguides). We have focused on 3 different systems: (1) qubits coupled to a photonic waveguide, (2) quantum dots in a dissipative environment, and (3) interfaces between graphene and a superconductor, particularly when graphene is in the quantum Hall state. The methods used are both analytical and numerical, and are closely linked to experiments.

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