Testing stimulated emission photon directions

TL;DR

This work investigates whether stimulated emission can generate backward-propagating photons under CPT symmetry by proposing STED-like macro-scale tests and exploring synchrotron and antenna realizations. It develops a CPT-based framework in which absorption and stimulated emission swap roles under reverse bias, predicting opposite photon directions and delays and suggesting observable signs in the delay $\Delta t$ and in negative radiation pressure. It details concrete experimental tests, including removing forward optical isolators and applying reverse bias, with delay relations such as $\Delta t=(d\pm l)/c$, to distinguish backward trajectories. The findings point to potential transformative applications in two-way photonic computing, imaging, and therapeutics, while highlighting that a null result would challenge macroscopic CPT symmetry and prompt further foundational studies.

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.

Figures (15)

• Figure 1: Top: recent widely commented experiment by Steinberg group (backwardnegative) has observed medium response (measured phase of 85Rb) before and after the applied impulse, proposing "photon can spend a negative amount of time in an atom cloud" explanation, which does not seem physical, however, theoretical explanation for this article uses forward/backward-in-time propagators negexp. Bottom: as discussed here, intuitive explanation of such forward/backward propagators, like in two-state vector formalism tsvf, could come from absorption/stimulated emission being CPT analogs, hence should have opposite delay.
• Figure 2: Top: CPT symmetry says that if there exists "laser causes excitation" scenario, also "laser causes deexciation" should exist (needs reversing bias: voltage). Being CPT symmetry analogs, they should have opposite photon direction and delay sign - we would like to verify it experimentally and apply if successful. For example in https://en.wikipedia.org/wiki/STED_microscopy diode lasers cause both. Bottom: proposed STED-like test of this photon direction(s): continuous laser excites fluorescent dye being approximately 3-level medium, and impulse laser causes its deexcitation through stimulated emission - its photon direction can be found from $\Delta t=(d\pm l)/c$ delay between laser impulse and observed reduced intensity by fast photodetector focused on dye spontaneous emission. Textbooks assumption would mean $'+'$ sign here, while CPT symmetry requires $'-'$ sign instead, hence this is also macroscopic test of CPT symmetry, complementing dozes of earlier microscopic tests CPTdata. Applying reverse bias for depletion laser as in Fig. \ref{['negtemp']} brings hope for STED avoiding photobleaching.
• Figure 3: Top: swirl-like wave behind marine propeller carries energy, momentum, and angular momentum like photon, also allowing to perform time-symmetry-like transformation by just reversing rotation, getting "pulling" photons able to pull energy from excited resonator. EM is mathematically close to hydrodynamics (e.g. EMh), suggesting to analogously search for EM "pulling photons" using e.g. synchrotron radiation: coming from just circulating electron - we could also reverse by just changing electron direction, what should analogously reverse $\vec{p}=\langle \vec{E}\times \vec{H}\rangle/c$ radiation pressure. T/CPT symmetry also switches shown absorption and stimulated emission equations, allowing to estimate strength of the effect. Practically it could be realized e.g. by betatron, synchrotron, free electron laser (FEL) with target positioned for reversed electron trajectory. Bottom: for lower frequencies there might be used analogous to marine propeller e.g. spring-like antenna powered with impulse: reversing $\vec{p}=\langle \vec{E}\times \vec{H}\rangle/c$ radiation pressure for reversed time ($t\to -t, H\to -H$), or used impulse electric potential $V\to -V$ - hopefully allowing both to excite resonator of receiver as in wireless charging, but also speedup it deexcitation/relaxation for reversed impulse.
• Figure 4: In commercially available e.g. erbium-doped optical fiber amplifiers (EDFA, or simpler semiconductor SOA, etc.) erb there is used backward ASE (amplified spontaneous emission): increased emission from pumped doped fiber, also toward laser e.g. in bASE, exactly as in the sought CPT analog of "laser causes excitation" effect, usually removed by forward optical isolators. Top: Analogously to STED-like test, we can verify this photon direction by measuring delay between the impulse and reduced population of excited atoms of external target, e.g. by monitoring its spontaneous emission. For this purpose, the shown standard forward optical isolator should be removed, or rotated to backward position. Bottom: simplified setting from bASE seems much more practical for tests: of delay (opposite if CPT analog), separation (of forward/backward action), transparency (forward blocked by $N_1$, so backward should be by usually much lower $N_2$) - allowing for potential future applications. As in Fig. \ref{['negtemp']}, it seems crucial to use reverse bias for the laser, in practice (e.g. backwardnegativebASEsuperr) probably included in varying/impulse signal.
• Figure 5: Usually system has tendency to get to the ground state, emitting photon, which e.g. excites some atom, according to absorption equation. However, getting reversed tendency should increase amplitude (probability) of shown symmetric reversed Feynman diagram: causing absorption of photon, emitted by some atom - according to symmetric stimulated emission equation. Beside previous based on synchrotron radiation, diagram shows two examples of obtaining such revered tendency: upper uses membrane GFP (green fluorescent protein) which can act as light-driven protein pump gfp, in reversed proton gradient would get tendency to absorb photons, what might be used e.g. by biological organisms for thermoregulation. The bottom is analogously applied reversed bias/potential/electron gradient for light/laser diode, what is also applied for light detectorsrLED, but should actually also stimulate emission - e.g. used as absorber in superr clearly helps with generation of photon impulse. Impulse laser sources often have e.g. sinc-like shape, so should also include reversed bias, what might explain observations in backwardnegative. Backward ASE in bASE probably also uses both forward and reverse bias in varying signal, if successful would be perfect for presented applications. Also e.g. Fig. \ref{['testmin']}, \ref{['erb']} tests should rather use reverse bias. Alternatively, change of $N_2$ above/below thermal $N_2 \sim N_1 \exp(-\beta (E_2-E_1))$ of Boltzmann distribution should also increase amplitude of Feynman diagram with emission/absorption there, and absortpion/emission in the target.