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Fortschritte der Physik

Why Two-Photon but Not Two Photons?

Yanhua H. Shih, Dmitry Strekalov, T. D. Pittman, Morton H. Rubin

Published Nov 1, 1998• 4 citations
Publisher

Abstract

In his famous book, The Principles of Quantum Mechanics, Dirac stated that “... photon ... interferes only with itself. Interference between two different photons never occurs.” However, two-photon interference has been demonstrated experimentally. Was Dirac wrong? We report an new experiment in this paper. It demonstrated that two-photon interference can not be considered the interference of two photons. “Two-photon” is not two photons. Two-photon is a single entity, which is just like a photon. We may call it biphoton. Dirac was correct. Biphoton interferes only with itself.

DOI
10.1002/(sici)1521-3978(199811)46:6/8<627::aid-prop627>3.0.co;2-1

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In his famous book, The Principles of Quantum Mechanics, Dirac stated that “... photon ... interferes only with itself. Interference between two different photons never occurs.” However, two-photon interference has been demonstrated experimentally. Was Dirac wrong? We report an new experiment in this paper. It demonstrated that two-photon interference can not be considered the interference of two photons. “Two-photon” is not two photons. Two-photon is a single entity, which is just like a photon. We may call it biphoton. Dirac was correct. Biphoton interferes only with itself.
Two-photon interferometry started to play an important role in quantum optics since 1980’s. In that time, Dirac was criticized to be mistaken because he stated in his book, The Principles of Quantum Mechanics , that “photon only interferes with itself. Interference between two different photons never occurs”. Two-photon interference was considered as the interference between two photons. In fact, neither classical theory nor quantum theory suggested the interference between different photons. Classical theory views optical interference the result of coherent superposition between electromagnetic waves. The two-photon interference picture provided reasonable interpretation to that observation too: under the condition of complete overlapping, when the two biphoton amplitudes superpose in phase the joint-detection of the two distant photodetectors reaches its maximum counting rate as the sign of “correlation”. When the optical path difference is increased from zero to unbalanced values, a coincidence curve of “dip” is observed.
Summary form only given. The superposition principle is at the heart of quantum mechanics and plays the central role in interference phenomena in quantum mechanics. Quantum interference differs from classical interference because of particle nature of light. In a quantum mechanical picture, interference occurs because there are indistinguishable ways of an event to occur. Whenever which-path information is given to the system, interference disappears. We have experimentally demonstrated that two-photon interference occurs for biphoton amplitudes arising from two well-separated pump pulses.
This article reviews the historical and conceptual evolution of quantum physics, challenging P.A.M. Dirac’s famous dictum, “Each photon then interferes only with itself. Interference between two different photons never occurs.” We trace the progression from the early experiments by Forrester et al. and Magyar & Mandel, which first questioned this notion, to the advent of Spontaneous Parametric Down-Conversion (SPDC) following the invention of the laser and nonlinear optics. The article elucidates how D. N. Klyshko’s quantum description of SPDC laid the groundwork for the landmark Shih-Alley and Hong-Ou-Mandel (HOM) experiments, which demonstrated two-photon quantum interference and entanglement. Finally, we address the common misconception that photons must physically overlap at a beam splitter for two-photon interference to occur. We illustrate that quantum interference arises solely from the indistinguishability of probability amplitudes leading to a joint detection event, even in configurations where the photons never locally interact at a beam splitter.
We report an interference experiment in which the two-photon entangled state interference cannot be pictured in terms of the overlap and bunching of two individual photons on a beamsplitter. We also demonstrate that two-photon interference, or photon bunching effect on a beamsplitter, does not occur if the two-photon Feynman amplitudes are distinguishable, even though individual photons do overlap on a beamsplitter. Therefore, two-photon interference cannot be viewed as interference of two individual photons, rather it should be viewed as two-photon or biphoton interfering with itself. The results may also be useful for studying decoherence management in entangled two-qubit systems as we observe near complete restoration of quantum interference after the qubit pairs, generated by a femtosecond laser pulse, went through certain birefringent elements.