Spooky action

In the last blog I mentioned about quantum entanglement. Back in 1927, Niels Bohr and Albert Einstein were debating whether quantum mechanics, however strange, renders the world as it really is. In quantum mechanics a particle’s location, polarization and other properties can be indefinite until the moment they are measured. If a pair of particles is generated such that their total spin is known to be zero, the particles will have opposite spins when they are measured. Furthermore, whatever is the distance between the particles, the measurement of one's spin determines another's. Einstein considered such behavior impossible, as it violated the local realism view of causality, referring to it as "spooky action at a distance". As a conclusion in 1935, Einstein with Boris Podolsky and Nathan Rosen published the famous EPR-paper, which supposedly proved that quantum mechanics could not represent reality. Erwin Schrödinger shortly thereafter published a seminal paper defining and discussing the notion of "entanglement."

In 1964 John Steward Bell devised a creative thought experiment that, if carried out for real, would show whether entanglement is caused by local “hidden variables” – properties that exist before taking a measurement as proposed by Einstein. In the experiment, Alice and Bob, each receive particles from a common source. Each chooses one of several measurement settings, and then records a measurement outcome. Repeated many times, the experiment generates a list of results. Bell realized that quantum mechanics predicts that there will be strange correlations in this data. In 2022, the Physics Nobel prize was awarded for experimental work carried out, over several decades, by Alain Aspect, John Clauser and Anton Zeilinger, showing the predictions true.  This poses a threat to Albert Einstein's theory of special relativity, which states that nothing could travel faster than the speed of light. If entangled particles were separated by a great distance, how could information apparently travel between them instantaneously? The experiments especially challenge "locality"—the intuition that distant objects need a physical mediator to interact and what Einstein had taken for granted conceptually – the existence of definite properties of particles prior to measurement. Others instead think the experiments challenge "realism"—the intuition that there's an objective state of affairs underlying our experience. 

But that's because Bell assumed that quantum particles don't know what measurements they are going to encounter in the future. Retrocausal models propose that Alice's and Bob's measurement choices affect the particles back at the source. This can explain the strange correlations, without breaking special relativity. For instance, recently Huw Price and Ken Wharton have argued that retrocausality makes better sense of the fact that the microworld of particles doesn't care about the difference between past and future - the future might influence the past. However, concrete retrocausal models need still more research. Referring to the blog and blog, Juan Maldacena and Leonard Susskind proposed, in 2013, that wormholes in spacetime are equivalent to quantum entanglement, a conjecture known as ER = EPR. They wrote “We argue that the Einstein Rosen bridge between two black holes is created by EPR-like correlations between the microstates of the two black holes,” In 2019, Jafferis, Gao and Wall found the first concrete realization of ER = EPR, a qubit traversable wormhole. It is possible that a wormhole links every entangled pair of particles in the universe, forging a spatial connection that records their shared histories. Maybe Einstein’s hunch that wormholes have to do with particles was right. This makes to think that the universe is more coupled than expected. 

Entanglement has shot up in perceived importance since physicists discovered in the 1990s that it allows new kinds of computations. Entangling two qubits — quantum objects like particles that exist in two possible states, 0 and 1 — yields four possible states with different likelihoods (0 and 0, 0 and 1, 1 and 0, and 1 and 1). Three qubits make eight simultaneous possibilities, and so on; the power of a “quantum computer” grows exponentially with each additional entangled qubit. Cleverly orchestrate the entanglement, and you can cancel out all combinations of 0s and 1s except the sequence that gives the answer to a calculation. Prototype quantum computers made of a few dozen qubits have materialized in the last couple of years, led by Google’s 54-qubit Sycamore machine.