Gangqiang Liu

Bose-Einstein Condensation of Long-Lifetime Polaritons in Thermal Equilibrium

The experimental realization of Bose-Einstein condensation (BEC) with atoms and quasiparticles has triggered wide exploration of macroscopic quantum effects. Microcavity polaritons are of particular interest because quantum phenomena such as BEC and superfluidity can be observed at elevated temperatures. However, polariton lifetimes are typically too short to permit thermal equilibration. This has led to debate about whether polariton condensation is intrinsically a nonequilibrium effect. Here we report the first unambiguous observation of BEC of optically trapped polaritons in thermal equilibrium in a high-Q microcavity, evidenced by equilibrium Bose-Einstein distributions over broad ranges of polariton densities and bath temperatures. With thermal equilibrium established, we verify that polariton condensation is a phase transition with a well-defined density-temperature phase diagram. The measured phase boundary agrees well with the predictions of basic quantum gas theory.

Long-range ballistic motion and coherent flow of long-lifetime polaritons

Exciton-polaritons can be created in semiconductor microcavities. These quasiparticles act as weakly interacting bosons with very light mass, of the order of

Direct measurement of polariton–polariton interaction strength

10^{-4}

A new type of half-quantum circulation in a macroscopic polariton spinor ring condensate

times the vacuum electron mass. Many experiments have shown effects which can be viewed as due to a Bose-Einstein condensate, or quasicondensate, of these particles. The lifetime of the particles in most of those experiments has been of the order of a few picoseconds, leading to significant nonequilibrium effects. By increasing the cavity quality, we have made new samples with longer polariton lifetimes. With a photon lifetime on the order of 100-200 ps, polaritons in these new structures can not only come closer to reaching true thermal equilibrium, a desired feature for many researchers working in this field, but they can also travel much longer distances. We observe the polaritons to ballistically travel on the order of one millimeter, and at higher densities we see transport of a coherent condensate, or quasicondensate, over comparable distances. In this paper we report a quantitative analysis of the flow of the polaritons both in a low-density, classical regime, and in the coherent regime at higher density. Our analysis gives us a measure of the intrinsic lifetime for photon decay from the microcavity and a measure of the strength of interactions of the polaritons.

Erratum: Bose-Einstein Condensation of Long-Lifetime Polaritons in Thermal Equilibrium [Phys. Rev. Lett. 118 , 016602 (2017)]

Yongbao Sun,1∗ Yoseob Yoon, Mark Steger, Gangqiang Liu, Loren N. Pfeiffer, Ken West, David W. Snoke,2∗ and Keith A. Nelson Department of Chemistry and Center for Excitonics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Department of Physics, University of Pittsburgh, 3941 O’Hara St., Pittsburgh, PA 15218, USA Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA

Dissipationless Flow and Sharp Threshold of a Polariton Condensate with Long Lifetime

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. 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.We report the observation of vorticity in a macroscopic Bose-Einstein condensate of polaritons in a ring geometry. Because it is a spinor condensate, the elementary excitations are “half vortices” in which there is a phase rotation of π in connection with a polarization vector rotation of π around a closed path. This is clearly seen in the experimental observations of the polarization rotation around the ring. In the ring geometry, a new type of half vortex is allowed in which the handedness of the spin flips from one side of the ring to the other, in addition to the rotation of the linear polarization component; such a state is not allowed in a simply-connected geometry. Theoretical calculation of the energy of this state shows that when many-body interactions are taken into account, it is lower in energy than a simple half vortex. The direction of circulation of the flow around the ring fluctuates randomly between clockwise and counterclockwise from one shot to the next; this corresponds to spontaneous breaking of time-reversal symmetry in the system. These new, macroscopic polariton ring condensates allow for the possibility of direct control of the vorticity of the condensate.

Coherent Flow and Trapping of Polariton Condensates with Long Lifetime

This corrects the article DOI: 10.1103/PhysRevLett.118.016602.