Model basis states for photons and "empty waves"

From the perspective of physical realism (PR), a photon is a localized entity that carries energy and momentum, and which is surrounded by a wave packet (anempty wave) that is devoid of observable energy or momentum. In creating quantized PR basis states for a photon wave packet, three requirements must be met:(1) The basis states must each carry the frequency of the wave;(2) They must closely resemble the photon, so that e.g. they scatter in the same manner from an optical mirror;(3) They must have infinitesimal energy, linear momentum, and angular momentum. An essentially zero-energy "empty wave" quantum-a "zeron"-is defined which meets these requirements. It is created as an asymmetric single-particle (or single-antiparticle) excitation of the vacuum state, with the "particle" (or "antiparticle") and its associated "hole" (or "antihole") forming a rotational bound state. The photon is reproduced as a symmetric particle-antiparticle excitation of the vacuum state, with the "particle" and "antiparticle" also forming a rotational bound state. The relativistic transformation problem is discussed. A key point in this development is the deduction of the correct equation of motion for a "hole" state in an external electrostatic field.     

Triggered single photons and entangled photons from a quantum dot microcavity

Current quantum cryptography systems are limited by the attenuated coherent pulses they use as light sources: a security loophole is opened up by the possibility of multiple-photon pulses. By replacing the source with a single-photon emitter, transmission rates of secure information can be improved. We have investigated the use of single self-assembled InAs/GaAs quantum dots as such single-photon sources, and have seen a tenfold reduction in the multi-photon probability as compared to Poissonian pulses. An extension of our experiment should also allow for the generation of triggered, polarization-entangled photon pairs. The utility of these light sources is currently limited by the low efficiency with which photons are collected. However, by fabricating an optical microcavity containing a single quantum dot, the spontaneous emission rate into a single mode can be enhanced. Using this method, we have seen 78% coupling of single-dot radiation into a single cavity resonance. The enhanced spontaneous decay should also allow for higher photon pulse rates, up to about 3 GHz. Received 8 July 2001 and Received in final form 25 August 2001     

Phonons and photons in crystals

The simultaneous propagation of phonons and photons in an insulator is discussed from both phenomenological and quantum mechanical points of view. A phenomenological form of the energy of an insulator is first supposed from which is obtained the equation of propagation of modes involving nuclei displacements and an electric field. This equation is then studied, mainly in the vicinity of q=0, in order to show how various limits lead to different types of propagation. The phenomenological equations are then justified from a microscopic point of view. The proof goes in two steps. A linear screened response function of the electrons in a solid is first assumed; one then proves that all the coefficients entering into the phenomenological equations may be obtained from the sole knowledge of this response function and of the charge of the nuclei. The existence of the response function is then justified from a many-body point of view. Finally, the necessary relations between the phenomenological coefficients are proved. Some other possible applications of the microscopic equations are also discussed at the end of the paper.     

Flying qubits and entangled photons

Efficient generation of polarized single photons or entangled photon pairs is crucial for the implementation of quantum key distribution (QKD) systems. Self organized semiconductor quantum dots (QDs) are capable of emitting on demand one polarized photon or an entangled photon pair upon current injection. Highly efficient single‐photon sources consist of a pin structure inserted into a microcavity where single electrons and holes are funneled into an InAs QD via a submicron AlOx aperture, leading to emission of single polarized photons with record purity of the spectrum and non‐classicality of the photons. A new QD site‐control technique is based on using the surface strain field of an AlO_x current aperture below the QD. GaN/AlN QD based devices are promising to operate at room temperature and reveal a fine‐structure splitting (FSS) depending inversely on the QD size. Large GaN/AlN QDs show disappearance of the FSS. Theory also suggests QDs grown on (111)‐oriented GaAs substrates as source of entangled photon pairs.     

Torsion and interaction of polarized photons

The contribution of the interacting vector and pseudovector torsion components to the interaction of polarized photons in an atomic sodium vapor is investigated. Estimates are obtained for the parameters of the gauge-theoretical model of gravitation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 50–55, December, 1991.     

Instability of incoherent photons

A kinetic theory for the modulational instability of incoherent photons with a broad spectrum is presented. Explicit results for the growth rates are obtained in several limiting cases.     

The physics of photons

The intensity of the electromagnetic field is treated using the photon wave function in the usual probabilistic sense, as governed by Maxwell's equations. The dynamic characteristics and symmetry properties are determined. The uncertainty relations are found, from which the degree of localization follows for single particle states.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 46–50, September, 1989.     

Entangled photons and quantum communication

This article reviews the progress of quantum communication that utilizes photonic entanglement. We start with a survey of various methods for generating entangled photons, followed by an introduction of the theoretical principles and the experimental implementations of quantum key distribution. We then move on to a discussion of more involved quantum communication protocols including quantum dense coding, teleportation and quantum communication complexity. After that, we review the progress in free-space quantum communication, decoherence-free subspace, and quantum repeater protocols which are essential ingredients for long-distance quantum communication. Practical realizations of quantum repeaters, which require an interface between photons and quantum memories, are discussed briefly. Finally, we draw concluding remarks considering the technical challenges, and put forward an outlook on further developments of this field.     

Polarization-controlled single photons

Vacuum-stimulated Raman transitions are driven between two magnetic substates of a 87Rb atom strongly coupled to an optical cavity. A magnetic field lifts the degeneracy of these states, and the atom is alternately exposed to laser pulses of two different frequencies. This produces a stream of single photons with alternating circular polarization in a predetermined spatiotemporal mode. MHz repetition rates are possible as no recycling of the atom between photon generations is required. Photon indistinguishability is tested by time-resolved two-photon interference.     

Interference of single photons

The observed interference of single photons is explained without contradiction with the indivisibility principle.     

Orbital and intrinsic angular momentum of single photons and entangled pairs of photons generated by parametric down-conversion

For systems that exhibit a second-order phase transition with a spontaneously broken continuous O(N) symmetry at low temperatures, we give a criterion for judging at which temperature T(K) long-range directional fluctuations of the order field destroy the order when approaching the critical temperature from below. The temperature T(K) lies always significantly below the famous Ginzburg temperature T(G) at which size fluctuations of finite range become important.     

Rest frames for tachyons and photons

A formalism is developed which admits particles faster than light and reference frames faster than light and as fast as light. It is fully consistent with the physical principles of special relativity. The necessity of introducing imaginary quantities does not arise. It does not encounter any difficulties with the principle of causality if it is reasonably interpreted.Dedicated to Prof. E. C. G. Sudarshan, in honor of his receipt of the Padmabushan Award.