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An atomic Hong-Ou-Mandel experiment (Orale)

Raphael Lopes, Almazbek Imanaliev, Alain Aspect, Marc Cheneau, Christoph Ira Westbrook, Denis Boiron

Published Mar 17, 2015• 0 citations
OpenAlex

Abstract

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.

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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.
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.
We have developed an atom interferometer to observe two-atom interference of a momentum entangled state. The setup is similar to one used recently in our laboratory to observe the Hong Ou Mandel (HOM) effect for atoms [1], but instead of using two input and two output modes on a beam splitter, we use four input and four output modes. A diagram is shown below. The configuration is essentially that proposed in Ref. [2]. We observe correlations consistent with the presence of an entangled state. An improved setup will allow a test Bell’s inequalities for mechanical degrees of freedom of massive particles.[1] R. Lopes, et al., Nature 520, 66 (2015).[2] M. Horne, A. Shimony, and A. Zeilinger, Phys. Rev. Lett. 62, 2495 (1989).
Two-photon interference effects arise because photons are indistinguishable particles. In the wellknown Hong-Ou-Mandel (HOM) effect, the transmission of two photons at a beam splitter interferes destructively with the reflection of both photons, requiring both photons to "bunch up" by leaving the beam splitter on the same side. Here, we show that the interference between locally propagating photons and photons exchanged by a mode swap can be implemented by post-selecting spatially separated photon outputs of a four-path interferometer. Even though the photons detected at spatially separated locations must have travelled along paths that never met up at the same beam splitter, the Hong-Ou-Mandel effect can be observed in correlations between the output ports that originate from the association of detection events with non-local output modes defined by the two single photon inputs. Local phase shifts can be used to map out non-classical correlations between the photons detected at different output locations, clarifying the role of linear optics in generating entanglement between spatially separated photons. Our work thus establishes a fundamental relation between multiphoton interference and entanglement, opening the door to new possibilities in optical quantum technologies.
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.
The emergence of Quantum Optics, in the second half of the 20th century, was closely related to landmark experiments, which were permitted by the development of single photon detection and correlation measurements. Photon correlation techniques allowed experimentalists to demonstrate effects closely linked to two-photon amplitudes interference with entangled states, a phenomenon emblematic of the second quantum revolution. In the Atom Optics group at Institut d'Optique, we revisit these experiments with photons replaced by metastable Helium atoms, which can be detected individually. After the Hanbury Brown Twiss and the Hong Ou Mandel experiments, we aim at realizing a test of Bell's inequalities with atoms entangled in momentum, i.e., in external degrees of freedom sensitive to mass.