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arXiv:2409.15399

Testing stimulated emission photon directions

Jarek Duda

Published Sep 23, 2024• 0 citations
arXivHTMLPDF

Abstract

While naively laser only causes excitation of external target, e.g. Rabi cycle, STED microscopy or ASE/SASE/SSA demonstrate it can also stimulate its deexitation, however, under uncommon condition of being prepared as excited. These two causalities are governed by absorption-stimulated emission pair of equations, and swap places in perspective of T/CPT symmetry, however, it means photon direction of stimulated emission should be opposite to usually assumed, allowing for negative radiation pressure $\vec{p}=\langle \vec{E}\times \vec{H}\rangle/c$. This article discusses various arguments and proposes simple direct tests to experimentally verify existence of such backward photon trajectories, complementing consequent forward textbook trajectories. Depending on the results, it could lead to many proposed applications like medical, astronomical or 2WQC more symmetric quantum computers. Alternatively, if unsuccessful, it would require macroscopic violation of CPT symmetry, so far tested probably only in microscopic settings.

DOI
10.48550/arxiv.2409.15399

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•Aug 22, 2023•0 citations
While naively laser only causes excitation of external target, e.g. Rabi cycle, STED microscopy or ASE/SASE/SSA demonstrate it can also stimulate its deexitation, however, under uncommon condition of being prepared as excited. These two causalities are governed by absorption-stimulated emission pair of equations, and swap places in perspective of T/CPT symmetry, however, it means photon direction of stimulated emission should be opposite to usually assumed, allowing for negative radiation pressure p⃗=⟨E⃗×H⃗⟩/c\vec{p}=\langle \vec{E}\times \vec{H}\rangle/cp​=⟨E×H⟩/c. This article discusses various arguments and proposes simple direct tests to experimentally verify existence of such backward photon trajectories, complementing consequent forward textbook trajectories. Depending on the results, it could lead to many proposed applications like medical, astronomical or 2WQC more symmetric quantum computers. Alternatively, if unsuccessful, it would require macroscopic violation of CPT symmetry, so far tested probably only in microscopic settings.
Whereas the basic features of stimulated emission are easily seen to hold true for ensembles of dipoles, the same is not always true for a single dipole system. For example, symmetry requires that well localized, bound dipoles emit a dipole field which is isotropic in the plane defined by the dipole vector. Indeed, for this case, nothing in the interaction between the dipole and the field contains information on the propagation direction of the field. These observations seem at odds with the highly directional nature of stimulated emission. The authors therefore test for stimulated emission by an explicit calculation considering both a single Rabi type dipole and a damped dipole whose centre-of-mass coordinates are fixed at a point. Remarkably, despite the symmetrical dipole radiation pattern, stimulated emission into the original stimulating wave is shown to occur exactly. An explanation of this effect is then considered by investigating dipoles whose centre-of-mass coordinates are allowed to vary. In this case, photon recoil of the dipole introduces a directional sense to the interaction. In the extreme opposite and trivial case of a highly delocalized dipole (centre-of-mass momentum eigenstates) stimulated emission again occurs exactly. In all intermediate cases, however, stimulated emission occurs only partially or approximately. A condition imposed on a dipole's spatial extent is established for stimulated emission to occur approximately. Finally, the original bound and localized dipole is seen to be a special limiting case of recoiling dipoles for which stimulated emission occurs exactly.
The physical nature of the backward-stimulated emission can be understood by considering the angular distribution of the total radiation from a set of atomic dipoles driven by an intense laser beam. From the classical point of view, the backward-stimulated emission is simply a reflection from the interface between a dielectric and a gain medium from the mismatch in the imaginary parts of their indices of refraction. For bulk gain structure, backward-stimulated emission is usually too weak to be noticed. However, in the case of a gain layer much thinner than a wavelength or a periodic thin gain structure with a spacing of λ/2, the backward-stimulated emission is enhanced and can be as large as the forward stimulated emission in terms of the amplitude of radiation field.1
In the literature one finds several conflicting accounts of the phase difference of stimulated and spontaneous emission, as well as absorption, with respect to an existing (triggering) electromagnetic field. One of these approaches proposes that stimulated emission and absorption occur in phase and out of phase with their driving field, respectively, whereas spontaneous emission occurs under an arbitrary phase difference with respect to an existing field. It has served as a basis for explaining quantum-mechanically the laser linewidth, its narrowing by a factor of 2 around the laser threshold, as well as its broadening due to amplitude–phase coupling, resulting in Henry’s α-factor. Assuming the validity of Maxwell’s equations, all three processes would, thus, violate the law of energy conservation. In semi-classical approaches, we investigate stimulated emission in a Fabry–Perot resonator, analyze the Lorentz oscillator model, apply the Kramers–Kronig relations to the complex susceptibility, understand the summation of quantized electric fields, and quantitatively interpret emission and absorption in the amplitude–phase diagram. In all cases, we derive that the phase of stimulated emission is 90° in lead of the driving field, and the phase of absorption lags 90° behind the transmitted field. Also spontaneous emission must obey energy conservation, hence it occurs with 90° phase in lead of an existing field. These semi-classical findings agree with recent experimental investigations regarding the interaction of attosecond pulses with an atom, thereby questioning the physical explanation of the laser linewidth and its narrowing or broadening.
Standard one-way quantum computers (1WQC) combine time symmetric unitary evolution, with asymmetric treatment of boundaries: state preparation allows to enforce chosen initial state, however, for the final state measurement chooses a random value instead. As e.g. pull/push, negative/positive pressure, stimulated emission/absorption causing deexcitation/excitation are CPT analogs, and one can be used for state preparation, there should also exist its CPT analog, referred here as CPT(state preparation) - allowing for additional chosen enforcement of the final state, its more active treatment than measurement. It should act similarly to postselection, but through applied physical constraints (instead of running multiple times). Like pumped to |1> prepared state vs its "unpumped" <0| CPT analog, hopefully allowing to construct two-way quantum computers (2WQC) e.g. hydrodynamical, and hopefully photonic, with influenced both initial and final states. If possible, for example for an instance of 3-SAT problem on n variables, we could prepare ensemble of 2^n inputs with Hadamard gates, calculate 3-SAT alternatives for them, and use CPT(state preparation) to enforce outcomes of all these alternatives to '1'. This way hopefully restricting this ensemble to satisfying given 3-SAT problem, in theory allowing to attack NP problems by simultaneously pushing and pulling information through the system for better control.
Standard one-way quantum computers (1WQC) combine time symmetric unitary evolution, with asymmetric treatment of boundaries: state preparation allows to enforce chosen initial state, however, for the final state measurement chooses a random value instead. As e.g. pull/push, negative/positive pressure, stimulated emission/absorption causing deexcitation/excitation are CPT analogs, and one can be used for state preparation, there should also exist its CPT analog, referred here as CPT(state preparation) - allowing for additional chosen enforcement of the final state, its more active treatment than measurement. It should act similarly to postselection, but through applied physical constraints (instead of running multiple times). Like pumped to |1> prepared state vs its “unpumped” <0| CPT analog, hopefully allowing to construct two-way quantum computers (2WQC) e.g. hydrodynamical, and hopefully photonic, with influenced both initial and final states. If possible, for example for an instance of 3-SAT problem on n variables, we could prepare ensemble of 2^n inputs with Hadamard gates, calculate 3-SAT alternatives for them, and use CPT(state preparation) to enforce outcomes of all these alternatives to ‘1’. This way hopefully restricting this ensemble to satisfying given 3-SAT problem, in theory allowing to attack NP problems by simultaneously pushing and pulling information through the system for better control.