title: Coherent zero-state and pi-state in an exciton-polariton condensate array.
abstract: The effect of quantum statistics in quantum gases and liquids results in observable collective properties among many-particle systems. One prime example is Bose-Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose-Einstein condensate is generally defined as a macroscopic occupation of a single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The long-range spatial coherence leads to the existence of phase-locked multiple condensates in an array of superfluid helium, superconducting Josephson junctions or atomic Bose-Einstein condensates. Under certain circumstances, a quantum phase difference of pi is predicted to develop among weakly coupled Josephson junctions. Such a meta-stable pi-state was discovered in a weak link of superfluid 3He, which is characterized by a 'p-wave' order parameter. The possible existence of such a pi-state in weakly coupled atomic Bose-Einstein condensates has also been proposed, but remains undiscovered. Here we report the observation of spontaneous build-up of in-phase ('zero-state') and antiphase ('pi-state') 'superfluid' states in a solid-state system; an array of exciton-polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton-polariton condensates.
authors.affiliation: E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA. cwlai@stanford.edu
authors.forename: C W
authors.initials: CW
authors.lastname: Lai
authors.forename: N Y
authors.initials: NY
authors.lastname: Kim
authors.forename: S
authors.initials: S
authors.lastname: Utsunomiya
authors.forename: G
authors.initials: G
authors.lastname: Roumpos
authors.forename: H
authors.initials: H
authors.lastname: Deng
authors.forename: M D
authors.initials: MD
authors.lastname: Fraser
authors.forename: T
authors.initials: T
authors.lastname: Byrnes
authors.forename: P
authors.initials: P
authors.lastname: Recher
authors.forename: N
authors.initials: N
authors.lastname: Kumada
authors.forename: T
authors.initials: T
authors.lastname: Fujisawa
authors.forename: Y
authors.initials: Y
authors.lastname: Yamamoto