Quantum hydrodynamics of a single particle

Watching a single particle behave like a wave Pioneering experimental observations of the quantum interference of single particles vividly illustrate wave-particle duality, and point the way towards manipulating individual excitations for quantum computing. An international team including Vincenzo Ardizzone and co-workers at CNR Nanotec in Lecce, Italy, generated single photons by directing a pulsed laser onto a nano-sized crystal called a quantum dot. These photons then interacted with excitons in a semiconductor cavity to produce hybrid particles called polaritons, which show great promise as switchable quantum bits (qubits) in future computers. Most interestingly, the researchers observed a single-polariton scattering off a defect in the cavity, leaving wave-like fringes where the particle's incoming wavefunction interfered with the scattered parts. This observation represents the first two-dimensional mapping of wave-particle duality for a single quantum particle, and is reminiscent of the famous double-slit experiments. Semiconductor devices are strong competitors in the race for the development of quantum computational systems. In this work, we interface two semiconductor building blocks of different dimensionalities with complementary properties: (1) a quantum dot hosting a single exciton and acting as a nearly ideal single-photon emitter and (2) a quantum well in a 2D microcavity sustaining polaritons, which are known for their strong interactions and unique hydrodynamic properties, including ultrafast real-time monitoring of their propagation and phase mapping. In the present experiment, we can thus observe how the injected single particles propagate and evolve inside the microcavity, giving rise to hydrodynamic features typical of macroscopic systems despite their genuine intrinsic quantum nature. In the presence of a structural defect, we observe the celebrated quantum interference of a single particle that produces fringes reminiscent of wave propagation. While this behavior could be theoretically expected, our imaging of such an interference pattern, together with a measurement of antibunching, constitutes the first demonstration of spatial mapping of the self-interference of a single quantum particle impinging on an obstacle.