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Quantum Optics Journal of the European Optical Society Part B

Loop coherent states and photon localizability

Edward R. Pike, Sarben Sarkar

Published Aug 1, 1993• 2 citations
Publisher

Abstract

Loop states in quantum optics and their 1-1 correspondence with states in Fock space are described. Owing to their natural encoding of geometrical information and close relation to coherent states it is proposed that a certain subclass of loop coherent states may be used as a probe for localization of photon states.
DOI
10.1088/0954-8998/5/4/005

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We show that simple geometric requirements lead to a unique position operator for the photon. The different components of the position do not commute. We investigate also the problem of maximal localizability and we draw the conclusion that for every closed curve there is a photon state localized on this curve. Note: This paper was published in Bulletin de l’Academie Polonaise de Sciences, Serie des sciences math. astr. et phys., Vol XXI, No 5, 1972, pp. 477-483. The original paper has ”for every closed space-like curve there are exactly two independent photon states corresponding to helicities ±1 localized on this curve. ” The conclusion in this form is incorrect.
You might think that the great success of quantum electrodynamics (QED) would settle the debate on the nature of light to provide a clear view of its behavior, where the photon is regarded as the unit of excitation associated with a quantized mode of the electromagnetic (radiation) œeld. However, Heisenberg’s uncertainty principle tells us that a state of deœnite momentum, energy, and polarization associated with a plane wave used as a basis function of quantization must be completely indeœnite in space and time. It suggests the diµculty of spatial localization of a photon as a particle. In fact, Newton and Wigner showed that a free photon, as a massless particle with spin 1, has no localized states on the basis of natural invariance requirements that localized states for which operators of the Lorentz group apply should be orthogonal to the undisplaced localized states, a§er a translation (Newton and Wigner 1949). According to them, one can obtain a general expression for a position operator for massive particles and for massless particles of spin 0 or 1/2, not for massless particles with œnite spin, which indicates that there is no probability density for the position of the photon, and thus a position-representation wave function cannot be consistently introduced. It has also been shown that photons are not localizable, on the basis of imprimitive representations of the Euclidean group (Wightman 1962). It is now believed that photons are only weakly localizable, although single-photon states with arbitrarily fast asymptotic fallo˜ of energy density exist, and that a lack of strict localizability is directly related to the absence of a position operator for a photon in free space and a positionrepresentation photon wave function (Hawton 1999).
Abstract Through the classic work of Planck, Einstein, and others the particle nature of the photon has been established. In the previous chapter we considered the wave field of the photon. In this chapter we will emphasize more its particle nature and discuss the extent to which a wave function with a probabilistic interpretation is an appropriate description. Once we have the concept of a particle it is natural to ask how localized the state can be. The degree of localizability of photons is of concern in the interpretation of experiments in quantum optics involving weak light sources and non-linear electromagnetic media. If a photon is not to have the structure of a composite particle its quantum states should be describable by translations, rotations, and Lorentz boosts of a particular state and possibly super positions of such states. This is equivalent to a photon being associated with an irreducible representation of the Poincaré group. A brief overview of the general theory of group representations is given in Appendix 3. We will be able to give a complete discussion of the representations of the Poincaré group pertinent to the photon. Our discussion of localizability will be quite rigorous but we will gloss over measure-theoretic niceties. For more mathematical discussions the reader can refer to the works of Mackey (1978) and Wightman (1962). We will finish the chapter by examining the implication of Poincaré invariance for the metric on the space of quantum states of the photon.
Non-classical light sources are now being developed in more and more laboratories. Their noise reduction properties are potentially important for technology. Examples are squeezed and antibunched light sources. It is tempting to picture in some of these processes single photons flying through space. In non-relativistic quantum mechanics it is easy to discuss the properties of particle localisation in a general context without recourse to a particular experimental configuration. A position operator exists and the localised states are just the eigenstates of this operator. The situation in relativistic quantum mechanics (or field theory) is not as straightforward. The photon is relativistic and it has the addi-tional property of being massless. It has no position operator associated with it. Yet, nevertheless, it is possible to pose the question of finding a generic construction for a wave packet for the photon which is localised as far as possible in some (imaginary) box in space. The difficulty of the construction follows from the fact that in an electromagnetic wave the polarisation is perpendicular to the direction of propagation. Thus we need to construct a vector field which is localised and has this transverse property related to the masslessness of the photon.