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Contributed by Loren N. Pfeiffer, January 13, 2015 (sent for review November 25, 2014; reviewed by Peter Littlewood)
Significance
Polaritons are propagating states in certain solid-state systems that couple directly to light signals. This work gives a clear observation of quantized circulation of a polariton condensate in a ring; spontaneous quantized circulation is one of the key tests of true superfluidity. The quantized circulation seen here is a new type that is only possible in a spinor condensate in a ring geometry. Because polariton condensates can be made relatively easily in solid-state systems that can operate up to room temperature, the door is open to all kinds of superfluid effects of light in optical communications.
Abstract
We report the observation of coherent circulation in a macroscopic Bose–Einstein condensate of polaritons in a ring geometry. Because they are spinor condensates, half-quanta are allowed in where there is a phase rotation of π in connection with a polarization vector rotation of π around a closed path. This half-quantum behavior is clearly seen in the experimental observations of the polarization rotation around the ring. In our ring geometry, the half-quantum state that we see is one in which the handedness of the spin flips from one side of the ring to the other side in addition to the rotation of the linear polarization component; such a state is allowed in a ring geometry but will not occur in a simply connected geometry. This state is lower in energy than a half-quantum state with no change of the spin direction and corresponds to a superposition of two different elementary half-quantum states. The direction of circulation of the flow around the ring fluctuates randomly between clockwise and counterclockwise from one shot to the next; this fluctuation corresponds to spontaneous breaking of time-reversal symmetry in the system. This type of macroscopic polariton ring condensate allows for the possibility of direct control of the circulation to excite higher quantized states and the creation of Josephson junction tunneling barriers.
Footnotes
- ↵1To whom correspondence may be addressed. Email: snoke{at}pitt.edu or loren{at}princeton.edu.
Author contributions: G.L. and D.W.S. designed research; G.L. performed research; L.N.P. and K.W. fabricated new semiconductor/optical structures; G.L., D.W.S., and A.D. analyzed data; and G.L. and D.W.S. wrote the paper.
Reviewers included: P.L., University of Chicago.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1424549112/-/DCSupplemental.
Quantum circulation in a polariton ring condensate
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