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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Beyond the Heisenberg uncertainty

Milena D’Angelo, Yanhua H. Shih

Published Oct 19, 2004• 0 citations
Publisher

Abstract

Following Einstein, Podolsky, and Rosen (EPR) original argument, we derive a pair of inequalities for the uncertainties in the sum of the momenta and in the difference of the positions of an entangled two-particle system. We prove that only entangled systems can satisfy both EPR-inequalities, in violation of the limits imposed by classical statistics. We propose an operational approach that allows applying the EPR inequalities to two-photon systems, as well. In particular, we demonstrate a scheme that allows implementing the EPR gedanken-experiment and verifying the EPR inequalities on systems of pairs of photons. We report the experimental results obtained in this kind of quantum interference and imaging experiment: SPDC two-photon system satisfies both EPR inequalities. The experimental verification of the EPR inequalities represents a practical way to experimentally distinguish entanglement from classical correlation in momentum and/or position variables for systems of two particles/photons. We emphasize the practical consequences of the EPR inequalities: only entanglement allows one to go beyond the limitations imposed by Heisenberg uncertainty on systems of classically correlated particles/photons. In this context we review a recent experiment of two-photon diffraction and quantum lithography.

DOI
10.1117/12.561455

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Following Einstein, Podolsky, and Rosen (EPR) original argument, we derive a pair of inequalities for the uncertainties in the sum of the momenta and in the difference of the positions of an entangled two-particle system. We prove that only entangled systems can satisfy both EPR-inequalities, in violation of the limits imposed by classical statistics. We propose an operational approach that allows applying the EPR inequalities to two-photon systems, as well. In particular, we demonstrate a scheme that allows implementing the EPR gedanken-experiment and verifying the EPR inequalities on systems of pairs of photons. We report the experimental results obtained in this kind of quantum interference and imaging experiment: SPDC two-photon system satisfies both EPR inequalities. The experimental verification of the EPR inequalities represents a practical way to experimentally distinguish entanglement from classical correlation in momentum and/or position variables for systems of two particles/photons. We emphasize the practical consequences of the EPR inequalities: only entanglement allows one to go beyond the limitations imposed by Heisenberg uncertainty on systems of classically correlated particles/photons. In this context we review a recent experiment of two-photon diffraction and quantum lithography.
We report a quantum interference and imaging experiment which allows identifying the entanglement in momentum and position variables of a two-photon system. The measurements show indeed that the uncertainties in the sum of momenta and in the difference of positions of the entangled two-photon satisfy both EPR inequalities $\ensuremath{\Delta}({k}_{1}+{k}_{2})<\mathrm{min}(\ensuremath{\Delta}{k}_{1},\ensuremath{\Delta}{k}_{2})$ and $\ensuremath{\Delta}({x}_{1}\ensuremath{-}{x}_{2})<\mathrm{min}(\ensuremath{\Delta}{x}_{1},\ensuremath{\Delta}{x}_{2})$. These two inequalities, together, represent a nonclassicality condition. Our measurements provide a direct way to distinguish between quantum entanglement and classical correlation in continuous variables for systems of pairs of photons.
An entangled EPR two-particle system is characterized by Δ(p 1 + p 2) = 0 and Δ(x 1 - x 2) = 0, simultaneously, with the momentum and position of each individual particle completely undefined. Statistically, however, the uncertainty of the correlation measurements on the two individual particles must obey Δ(p1 + p 2) > Δp 1,2 and Δ(x 1 - x 2) > Δx 1,2. While one sees the measurements of p 1 + p 2 and x 1 - x 2 of the two individual particles satisfying the EPR δ-function, and believes the classical statistical inequality, one might easily be trapped into concluding either there is a violation of the uncertainty principle or there exists action-at-a-distance. Through the analysis of three typical EPR experiments, we show that the measurement of p 1 + p 2 and x 1 - x 2 for an entangled system can indeed violate the classical statistical inequality. Based on the experimental results, we emphasize, again, the surprising physics of quantum coherent two-particle superposition. Although questions regarding fundamental issues of quantum theory still exist, quantum entanglement has brought us a novel concept in non-local timing and positioning measurements with ultra-high accuracy, even beyond the classical limit.
Since the early days of quantum mechanics, physicists have been puzzled by the counterintuitive consequences of entanglement. Quantum imaging is one of the most intriguing effects exhibiting the typical nonlocal behavior of entangled states and has in fact stimulated an intense debate regarding fundamental issues of quantum theory. Recently, particular attention has been given to the possibility of simulating quantum imaging classically, that is, without entanglement. The study of quantum imaging has also lead to useful practical applications such as high precision nonlocal timing-positioning and quantum lithography, in which measurements may achieve resolution even beyond the classical limits. In this paper, we first introduce the concept of quantum imaging and emphasize its peculiar quantum nature through a set of inequalities derived from the historical argument of Einstein, Podolsky and Rosen (EPR). The EPR inequalities are the quantitative formulation of the very different physics governing entangled and separable systems. We show that two-photon imaging may achieve its fundamental limit only through nonlocal measurements realized on entangled photon pairs. Furthermore, we show that quantum imaging may be exploited to overcome the Rayleigh diffraction limit; this is the basic idea behind quantum lithography. Finally, we discuss the possibility of extending the coherent two-photon imaging typical of entangled photon pairs to incoherent coincidence imaging, by employing chaotic light. From an applicative viewpoint, chaotic radiation may represent the ideal candidate for mimicking quantum ghost imaging, provided adequate detection schemes are developed.
One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Even though there are still open questions regarding some fundamental issues related to entangled systems, quantum entanglement has started to play important roles in practical applications. Quantum imaging is one of the hot topics. Quantum imaging has many interesting features which are useful for different applications. For example, quantum imaging can be nonlocal, which is useful for secure two-dimensional information transfer. Quantum imaging can reach a much higher spatial resolution compared with classical imaging, even beyond the diffraction limit. This is useful for lithography and other microsystems fabrication technology. The super-resolution does not represent a violation of the uncertainty principle, it is just a quantum multi-particle phenomenon. Can quantum imaging be simulated classically? This question is closely related to the concerns of Einstein-Podolsky-Rosen of 1935. An attempted answer is given based on the analysis of a recent experiment of biphoton imaging-interference/diffraction.