A question of reality (Vol. 51, No. 5)

Spooky action at a distance: Bell’s Theorem in sketch form (Figure 12 in the paper). Credit: Reinhold Bertlmann (1988). Original presented to Bell on the occasion of his 60th birthday by R. Bertlmann

John Stewart Bell’s eponymous theorem and inequalities set out, mathematically, the contrast between quantum mechanical theories and local realism. They are used in quantum information, which has evolving applications in security, cryptography and quantum computing.

The distinguished quantum physicist John Stewart Bell (1928-1990) is best known for the eponymous theorem that proved current understanding of quantum mechanics to be incompatible with local hidden variable theories. Thirty years after his death, his long-standing collaborator Reinhold Bertlmann of the University of Vienna, Austria, has reviewed his thinking in a paper for EPJ H, ‘Real or Not Real: That is the question’. In this historical and personal account, Bertlmann aims to introduce his readers to Bell’s concepts of reality and contrast them with some of his own ideas of virtuality.

R. Bertlmann, Real or Not Real: that is the question, Eur. Phys. J. H 45, 205–236 (2020)

PLED polymers evolve characteristically during operation (Vol. 51, No. 5)

Waves localised around the time detect

Molecular dynamics simulations have shown that the mysteriously high efficiency of polymer LEDs arises from interactions between triplet excitons in their polymer chains, and unpaired electrons in their molecular impurities.

Polymer LEDs (PLEDs) are devices containing single layers of luminescent polymers, sandwiched between two metal electrodes. They produce light as the metal layers inject electrons and holes into the polymer, creating distortions which can combine to form two different types of electron-hole pair: either light-emitting ‘singlets’, or a non-emitting ‘triplets’. Previous theories have suggested that the ratio between these two types should be around 1:3, which would produce a light emission efficiency of 25%. However, subsequent experiments showed that the real value can be as high as 83%. We found that this higher-than-expected efficiency can be reached through interactions between triplet excitons, and impurities embedded in the polymer.

Y D Wang, J J Liu, Y X Liu, X R Wang, Y Meng, Dynamic Recombination of Triplet Exciton with Trapped Counterion in Conjugated Polymers, Eur. Phys. J. B 93, 173 (2020)

Effects of time defects in modulated systems (Vol. 51, No. 5)

Waves localised around the time detect

The spatial periodicity in crystals induces energy band-gaps. Similarly, time modulated systems possess momentum band-gaps. Is there a temporal analogue to the localised edge modes induced by topological defects in spatial crystals?

We show that in a vertically vibrated liquid with a pi-shift in the excitation as a time defect, waves grow exponentially before the defect and decay exponentially after. Because of causality and non-energy conservation, this apparent time localisation must, in fact, be interpreted as a permutation of band-gap modes. However, as such, time defects provide an original way to explore these gaps.

G. d’Hardemare, A. Eddi and E. Fort, Probing Floquet modes in a time periodic system with time defects using Faraday instability, EPL 131, 24007 (2020)

Avoiding environmental losses in quantum information systems (Vol. 51, No. 5)

Transitioning states in diamond imperfections

Through new techniques for generating ‘exceptional points’ in quantum information systems, researchers have minimised the transitions through which they lose information to their surrounding environments.

Recently, researchers have begun to exploit the effects of quantum mechanics to process information in some fascinating new ways. One of the main challenges faced by these efforts is that systems can easily lose their quantum information as they interact with particles in their surrounding environments.

To understand this behaviour, researchers in the past have used advanced models to observe how systems can spontaneously evolve into different states over time – losing their quantum information in the process. We have discovered how robust initial states can be prepared in quantum information systems, avoiding any unwanted transitions extensive time periods.

R Ramírez, M Reboiro , D Tielas, Exceptional Points from the Hamiltonian of a hybrid physical system: Squeezing and anti-Squeezing, Eur. Phys. J. D 74, 193 (2020)