Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK.
SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK.
Nat Commun. 2018 Oct 3;9(1):4062. doi: 10.1038/s41467-018-06436-2.
Due to their driven-dissipative nature, photonic quantum fluids present new challenges in understanding superfluidity. Some associated effects have been observed, and notably the report of nearly dissipationless flow for coherently driven microcavity-polaritons was taken as a smoking gun for superflow. Here, we show that the superfluid response-the difference between responses to longitudinal and transverse forces-is zero for coherently driven polaritons. This is a consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, while a normal component exists at finite pump momentum, the remainder forms a rigid state that is unresponsive to either longitudinal or transverse perturbations. Interestingly, the total response almost vanishes when the real part of the excitation spectrum has a linear dispersion, which was the regime investigated experimentally. This suggests that the observed suppression of scattering should be interpreted as a sign of this new rigid state and not a superfluid.
由于其驱动耗散性质，光子量子流体在理解超流性方面带来了新的挑战。已经观察到一些相关的效应，特别是对于相干驱动微腔极化激元近乎无耗散的流动的报告，被视为超流的有力证据。在这里，我们表明，相干驱动极化激元的超导响应——即对纵向和横向力的响应之间的差异——为零。这是由外部相位锁定引起的带隙激发谱的结果。此外，虽然在有限的泵动量下存在正常分量，但其余部分形成一个刚性状态，对纵向或横向扰动都没有响应。有趣的是，当激发谱的实部具有线性色散时，总响应几乎为零，这是实验研究的范围。这表明，观察到的散射抑制应该被解释为这种新的刚性状态的标志，而不是超流的标志。
