The armchair quantum physicist

Non-locality versus no-signalling

The year 2024 marks the 60th birthday of John Bell’s famous inequalities. Interestingly enough, the debate about what these inequalities mean and what conclusions we can draw is as lively as ever. Bell’s inequalities distinguish ‘local’ theories and ‘non-local’ theories. Here, we loosely define a ‘local’ theory as a theory where the likelihood of events…
Guessing game with superphotons: a Python simulation

Quantum non-locality is the effect that there can be a strong correlation between events at distant locations. Scientists have performed experiments where they observe these correlations and can exclude any communication or otherwise shared information that causes them. The correlations exist because Nature is fundamentally non-local. In the 1980s, Boris Tsirelson demonstrated that non-locality in…
Information causality as a physical principle

Can we explain fundamental physics if we use ‘information’ as the basic concept instead of the usual concepts like matter and energy? Some physicists argue that this is indeed the case for quantum mechanics. In this post, we will discuss the work of one of these physicists, Marcin Pawlowski. Since 1935, a discussion has been…
Quantum Mechanics as a branch of information theory?

Many people (mainly physicists) consider physics the most fundamental field in science. These physicists may be in for a reality check. Nowadays, arguments from information theory are used as principles to explain fundamental properties of the quantum world. In this post, we explore the work of Marcin Pawłowski and co-workers. They use information theory to…
Entanglement beyond Quantum Mechanics

If something happens in one location, it will influence the probability of something else happening elsewhere. This effect is instantaneous and works over any distance. We cannot explain this effect by some hidden information representing a shared past, but it is proof of the fundamental non-locality of Nature. Quantum mechanics predicts this non-locality, and experimenters…
Popescu-Rohrlich photons

In quantum mechanics, entangled photons show correlations, which can only be explained by what Einstein called ‘Spooky action at a distance.’ Entanglement was ‘discovered’ in the 1930s and first experimentally demonstrated for photons in the 1950s [1]. In the early days, a debate raged in science about whether one could explain these correlations through ‘local…
Tsirelson’s bound: How Nature limits non-locality

Spooky action at a distance”, or more formally non-locality, is one of the (or maybe even the) most intriguing and defining aspects of quantum physics. The idea that our world could be non-local is almost a century old. It started around 1935 when Einstein posed his EPR paradox. Today, science is still exploring the consequences…
Modeling Quantum Optical Experiments in Python

Light has been used in famous experiments in physics, especially on fundamental quantum properties like entanglement. Photons (light particles that appear when describing light with quantum mechanics) have the excellent property that they are not easily affected by the environment, and Nature gives us a few mechanisms to create entanglement between these photons. Photons were…
EPR, Bell’s inequalities, and Alain Aspect’s experiments

In previous posts, we addressed the challenge posted by Einstein on quantum theory in the 1930s [1], the response from John Bell in the 1960s [2], and the experimental evidence in favor of quantum mechanics provided by Alain Aspect in the 1980s [3]. In this post, we evaluate what aspects of quantum physics are challenged…
The experiment that proved Einstein wrong: entangled photons show spooky action at a distance

In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a paper concluding that quantum mechanics was an incomplete theory [1]. Their conclusion was based on a ‘thought experiment’ resulting in contradictions. However, it took nearly half a century to transform this ‘thought experiment’ into a real experiment. When it finally happened, the results were…