Photonics experiment resolves quantum paradox

https://phys.org/news/2023-07-photonics-quantum-paradox.html


Jul 5, 2023
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Physics Optics & Photonics
Physics Quantum Physics
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Photonics experiment resolves quantum paradox
by K. W. Wesselink , University of Twente

Photonics experiment resolves quantum paradox
Photonic simulation of quantum equilibration. A closed, many-body quantum system, initialized in a product state and undergoing unitary evolution generated by a Hamiltonian, necessarily remains in a pure state. However, local observables may exhibit a generalized thermalization. Entanglement builds up between sub-systems until, after some time teq, each sub-system appears to have approximately relaxed into a maximum entropy state. The paradigmatic case of a non-Gaussian bosonic state evolving under a quadratic Hamiltonian can be probed via a photonic simulation platform. A fully programmable linear optical chip can provide 'snapshots' of the local and global system dynamics for arbitrary times and interaction ranges by implementing the appropriate unitary U(V)=e−iHt with V ∈ U(m) for m modes. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-38413-9

It seems quantum mechanics and thermodynamics cannot be true simultaneously. In a new publication, UT researchers use photons in an optical chip to demonstrate how both theories can be true at the same time. They recently published their results in the journal Nature Communications.

In quantum mechanics, time can be reversed and information is always preserved. That is, one can always find the previous state of particles. How to reconcile this with the second law of thermodynamics has been a challenging paradox. There, time has a direction and information can also be lost. "Just think of two photographs that you put in the sun for too long, after a while you can no longer distinguish them," explains author Jelmer Renema.

There was already a theoretical solution to this quantum puzzle and even an experiment with atoms, but now the UT researchers have also demonstrated it with photons. "Photons have the advantage that it is quite easy to reverse time with them," explains Renema. In the experiment, the researchers used an optical chip with channels through which the photons could pass. At first, they could determine exactly how many photons there were in each channel, but after that, the photons shuffled positions.

Entanglement of subsystems

"When we looked at the individual channels, they obeyed the laws of thermodynamics and built up disorder. Based on measurements on one channel, we didn't know how many photons were still in that channel, but the overall system was consistent with quantum mechanics," says Renema. The various channels—also known as subsystems—were entangled. The missing information in one subsystem "disappears" to the other subsystem.

Dr. Jelmer Renema is assistant professor in the Adaptive Quantum Optics research group. His team included the research group of Prof. Dr. Jens Eisert of the Freie Universität Berlin, who played an important role in demonstrating the reversibility of the experiment. They recently published their article, titled "Quantum simulation of thermodynamics in an integrated quantum photonic processor," in the journal Nature Communications.

More information: F. H. B. Somhorst et al, Quantum simulation of thermodynamics in an integrated quantum photonic processor, Nature Communications (2023). DOI: 10.1038/s41467-023-38413-9

Journal information: Nature Communications

Related. Looks like they are looking at this in terms of Miles:


Published: 17 August 2023

Charge-state lifetimes of single molecules on few monolayers of NaCl

Katharina Kaiser, Leonard-Alexander Lieske, …Leo Gross Show authors
Nature Communications volume 14, Article number: 4988 (2023) Cite this article

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Abstract
In molecular tunnel junctions, where the molecule is decoupled from the electrodes by few-monolayers-thin insulating layers, resonant charge transport takes place by sequential charge transfer to and from the molecule which implies transient charging of the molecule. The corresponding charge state transitions, which involve tunneling through the insulating decoupling layers, are crucial for understanding electrically driven processes such as electroluminescence or photocurrent generation in such a geometry. Here, we use scanning tunneling microscopy to investigate the decharging of single ZnPc and H2Pc molecules through NaCl films of 3 to 5 monolayers thickness on Cu(111) and Au(111). To this end, we approach the tip to the molecule at resonant tunnel conditions up to a regime where charge transport is limited by tunneling through the NaCl film. The resulting saturation of the tunnel current is a direct measure of the lifetimes of the anionic and cationic states, i.e., the molecule’s charge-state lifetime, and thus provides a means to study charge dynamics and, thereby, exciton dynamics. Comparison of anion and cation lifetimes on different substrates reveals the critical role of the level alignment with the insulator’s conduction and valence band, and the metal-insulator interface state.

Introduction
Single-molecule charge transfer plays a significant role in many areas, from molecular electronics1,2, single-molecule light emission3,4,5,6,7,8,9, photocurrent generation10 to natural processes such as photosynthesis11. Over the past 20 years, the tremendous progress in high-resolution scanning probe microscopy has facilitated the investigation of single molecules with charge state control5,8,12,13,14,15,16,17. In scanning tunneling microscopy (STM), this was enabled by introducing a thin insulating film as a decoupling layer between molecule and metallic substrate, preventing hybridization between molecule and substrate while still allowing charge transfer and preserving a sufficient conductance for STM3,18,19,20. This facilitates, for example, mapping molecular ion resonances, which is based on a transient change in the charge state of the molecule19. For the investigation of molecular electroluminescence in STM-induced luminescence (STML) experiments, the decoupling layer serves two purposes: It reduces luminescence quenching from the metallic substrate3,21,22,23, and, due to the finite lifetime of charged species, it enables an exciton formation mechanism based on subsequent charge transfer from tip and sample7,24.

Such experiments on thin insulating films have in common that at sufficiently high bias voltages, sequential tunneling through a molecular resonance sets in. In this two-step sequential tunneling process, the molecule is transiently charged by a tunneling event between molecule and tip, followed by a tunneling event between molecule and metallic substrate. In almost all cases, the former tunneling event involving the tip is the current-limiting factor, such that little is known about the rate of the second tunneling transition involving the substrate. However, the latter can be critical for the interpretation of experimental results. For example, the aforementioned sequential tunneling process can—depending on the level alignment—lead to the formation of an excited state3,7,8, which can subsequently decay under the emission of a photon. The excitation mechanism is fundamentally different from optical excitation because it entails a two-step process8,9,25, i.e., charging from the tip and subsequent charge transfer to the substrate. Hence, the entire cycle of electroluminescence including the emission of a photon already involves (at least) three transitions with their respective rates. In addition, the creation of the exciton by charge transfer competes with the neutralization of the molecule to its neutral ground state involving even a fourth rate. Thus, the charge-state lifetime of the adsorbed molecule needs to be taken into account in any consideration of dynamics in STML experiments, all-electric pump-probe measurements of excited states24,26,27,28,29,30 as well as yields in photocurrent generation10.

The average elapsed time between charging by tunneling between tip and molecule, and neutralization by charge transfer between molecule and substrate depends on the tunneling probability between molecule and metallic substrate and thus on the thickness of the insulating film15,16,31,32. Although this time is a property of the entire system and occurs in an out-of-equilibrium situation, in the following, we refer to this quantity as the charge-state lifetime.

One way of experimentally determining charge-state lifetimes has been demonstrated for Cl-vacancies in NaCl films of various thicknesses31. Analogously to surface-adsorbed molecules, these defects exhibit electronic resonances and can be transiently charged at sufficiently high bias voltages. At resonance, the system represents a double-barrier tunnel junction with one barrier corresponding to tunneling between tip and defect (vacuum barrier) and the other corresponding to tunneling between defect and metal substrate (NaCl barrier). At tunnel conditions in typical STM experiments, the charging step by tunneling through vacuum is the rate-limiting step and therefore determines the measured current I. In this (usual) regime, the current I(z) increases exponentially with decreasing tip height z. At close distances, however, the time for charging the neutral defect by tunneling through vacuum (τc) can become smaller than the time to discharge the defect through the insulating film (i.e., the charge-state lifetime τd), and thus, I(z) reaches a z-independent saturation current Isat for small z. Specifically, at the first (positive and negative) ion resonance, the doubly-charged state is energetically not available due to Coulomb repulsion, such that in this regime, I is limited by the defect’s charge-state lifetime τd and for small z no longer depends on z22,29,30,31,32. Hence, under these conditions, one can directly deduce τd = qIsat−1, with the elementary charge q, from the saturation current Isat. Since Cl-vacancy states are localized within the top-most NaCl layer, possess an s-wave character as well as strong lateral confinement, and have no occupied state in the relevant energy range, it is not straightforward to generalize the results from vacancies to molecules.

more at link:

https://www.nature.com/articles/s41467-023-40692-1