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Two-atom bunching

Lindsay J. LeBlanc

Published Jan 1, 2015• 0 citations
OpenAlex

Abstract

The Hong–Ou–Mandel effect, whereby two identical quantum particles launched into the two input ports of a 'beam-splitter' always bunch together in the same output port, has now been demonstrated for helium-4 atoms. See Letter p.66

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The quantum theory has introduced physicists to two major counter-intuitive concepts. On the one hand, there is wave-particle duality, expressing the idea that objects normally described as particles can also behave as waves, while entities primarily described as waves, such as light, can behave as particles. This revolutionary idea nev- ertheless relies on concepts borrowed from classical physics, either waves or particles evolving in an ordinary space-time. On the other hand, entanglement can lead to interferences between the amplitudes of multi-particle states, which are described via Hilbert space and have no classical counter-part.This fundamental feature has of course been strikingly demonstrated by the violation of Bell’s inequalities. There is however, a conceptually simpler situation in which the interference between two-particle amplitudes entails a behavior impossible to describe by any classical model. This is the celebrated Hong Ou and Mandel experiment, in which two photons arriving simultaneously in the input channels of a beam-splitter always emerge together in one of the output channels. This effect has been extensively used to characterize the quality of non- classical light sources.Our group in Palaiseau has recently realized a Hong-Ou-Mandel experiment using metastable helium atoms [1]. A schematic diagram of the apparatus is shown in Fig. 1. We monitor the cross correlation signal between the two output ports ⟨nˆcnˆd⟩ as a function of the time t3 of application of the beam splitter. At this time we observe a 65% reduction in the cross correlation. This degree of supression cannot be explained by either a wavelike or particlelike description of the atoms.
When two indistinguishable bosons interfere at a beam splitter, they exit through the same output port. This quantum effect, known as Hong-Ou-Mandel interference, underpins many protocols in quantum information. It also generalizes to larger numbers of identical particles, for which interference produces characteristic many-body patterns. So far, experiments have observed the many-particle regime mainly in photonic platforms with unavoidable loss, and atomic realizations have remained challenging. Here we demonstrate Hong-Ou-Mandel interference with up to 12 indistinguishable neutral atoms in a system with negligible loss. Single-particle counting reveals parity oscillations, a bunching envelope and genuine multipartite entanglement, which are defining features of the multiparticle Hong-Ou-Mandel effect. Using the generated quantum states, we demonstrate metrological sensitivities that scale with the number of particles according to the Heisenberg limit. Our technique can be extended to larger ensembles, with wide-ranging applications from high-precision atom interferometry to multiparticle Bell tests.
Hong-Ou-Mandel interference is one of the clearest signatures of quantum behaviour: two identical photons meeting at a beam splitter always leave together. Here we demonstrate that this symmetry can be broken. Using a programmable integrated photonic processor, we interfere imbalanced two-photon states on a variable beam splitter and show that the output state of quantum interference can be continuously tuned, reaching the extreme case where destructive interference suppresses bunching at one output port. The asymmetry creates a forbidden bunching channel: photons may exit together at one port or separately at both ports, but cannot emerge together at the other port.
The celebrated Hong - Ou - Mandel effect is one of the simplest illustrations of two-particle interference, and is unique to the quantum realm. In the original experiment, two photons arriving simultaneously in the input channels of a beam-splitter were observed to always emerge together in one of the output channels. I will discuss the realisation of a closely analogous experiment with atoms instead of photons. This opens the prospect of setting up experiments to violate Bell's inequalities involving mechanical observables of massive particles, such as momentum, using methods inspired by quantum optics. Our work also demonstrates a new way to produce and benchmark twin-atom pairs that are of interest for quantum information processing and quantum simulation.