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arXiv:1204.5499 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Unveiling optical hidden correlations: the illusionist game

G. Brida, Ivo Pietro Degiovanni, Marco Genovese, Alice Meda, Stefano Olivares, Matteo G. A. Paris

Published Oct 15, 2012• 0 citations
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Abstract

Optical correlations represent a resource for the development of technologies with very promising opportunities for future widespread applications.1–14 Here we will show, both theoretically and experimentally, the optical illusionist game, an innovative protocol that exploit correlations between thermal beams of light.15 In the game, an illusionist demonstrates that if two uncorrelated light beams excited in the same state are mixed in a beam splitter (BS), then no correlations arise between them. On the other hand, the presence of the BS can be identified by the illusionist when the public is asked to insert the BS behind the illusionist’s back. Here we unveil the trick and the physics that explain these counterintuitive correlations. 1

DOI
10.1117/12.929439

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Optical correlations represent a resource for the development of technologies with very promising opportunities for future widespread applications.1–14 Here we will show, both theoretically and experimentally, the optical illusionist game, an innovative protocol that exploit correlations between thermal beams of light.15 In the game, an illusionist demonstrates that if two uncorrelated light beams excited in the same state are mixed in a beam splitter (BS), then no correlations arise between them. On the other hand, the presence of the BS can be identified by the illusionist when the public is asked to insert the BS behind the illusionist’s back. Here we unveil the trick and the physics that explain these counterintuitive correlations. 1
Photons and laser beams provide an ideal situation to discuss some of the mysteries of quantum physics. There are many additional fundamental features which are not accessible with a classical system such as an electric current, which include: quantum noise, the uncertainty relation and correlations between different laser beams. Not only is the difference between the quantum behavior of photons and classical waves a curious effect, it is also important in many technical applications and forms the basis of future technologies, such as improved optical sensors or quantum cryptography. Using the example of a simple beam splitter and its effect on a laser beam one can explore these mysterious quantum effect with undergraduate students. We will discuss a systematic series of cases, using the beam splitter, which identify the difference between the quantum and the classical world. We will present the technical details of an experiment suitable for third year students that demonstrate genuine quantum noise effects.
Light, which is composed of discrete quanta, or photons, is one of the most fundamental concepts in physics. Being an elementary entity, the behaviour of photons is governed by the rules of quantum mechanics. The ability to create, manipulate and measure quantum states of light is not only useful in foundational tests of quantum theory, but also in a wide range of quantum technologies – which aim to utilize non-classical properties of quantum systems to perform tasks not possible with classical resources. Only recently has it been possible to control the properties of number states of light, which have a fixed photon-number. Two-photon states are central to testing fundamental physical theories (such as locality and reality) and the implementation of quantum information technologies. The versatility of photon-pair states is en- abled by the potential entanglement properties it can posses. Thus controlling the correlations between photons is crucial to both pure and applied physics. To produce a single photon, a photon-pair state can be used. Detection of one photon indicates its twin’s existence. Many applications, such as optical quantum computation, require pure indistinguishable single photons. Heralding single pho- tons from a photon-pair will, in general, produce single photons in a mixed quantum state due to correlations within the pair. A common approach to creating photon-pairs is through the nonlinear sponta- neous four-wave mixing interaction in optical fibres. This thesis presents a theoreti- cal and experimental implementation of a scheme to tailor the spectral correlations within the pairs. Emphasis is placed on engineering the two-photon state such that they are completely uncorrelated. Spatial entanglement is naturally avoided due to the discrete nature of the optical fibre modes. Spectral correlations are eliminated by careful choice of dispersion characteristics and conditions. The purity of the photons generated by this scheme is demonstrated by means of two-photon inter- ference from independent sources. We measure a purity of (85.9 ± 1.6)% with no spectral filtering, exhibiting the usefulness of this source for quantum technologies and applications.
In general, a pair of uncorrelated Gaussian states mixed in a beam splitter (BS) produces a correlated state at the output. However, when the inputs are identical Gaussian states the output state is equal to the input, and no correlations appear, as the interference had not taken place. On the other hand, since physical phenomena do have observable effects, and the BS is there, a question arises on how to reveal the interference between the two beams. We prove theoretically and demonstrate experimentally that this is possible if at least one of the two beams is prepared in a discordant, i.e., Gaussian correlated, state with a third beam. We also apply the same technique to reveal the erasure of polarization information. Our experiment involves thermal states and the results show that Gaussian discordant states, even when they show a positive Glauber P-function, may be useful to achieve specific tasks.