Quantum entanglement just got a whole lot weirder

It isn't just identical particles that can be entangled, but even those with fundamentally different properties interfere with each other.
quantum sensors
This illustration shows the connectedness of two entangled particles. In early 2023, the first demonstration of entanglement between non-identical particles, a positive and negative pion, not only exists, but can be measured, leveraged and utilized to probe the internal structure of atomic nuclei. 
(Credit: Augusto / Adobe Stock)
  • One of the most bizarre quantum phenomena ever discovered is that of quantum entanglement: where two particles both exist in a state where the properties of one depend on the other. 
  • You can't measure a quantum particle's state without determining its properties in the process, "breaking" the entanglement whenever you do so. 
  • Normally seen with identical particles, entanglement has just been demonstrated between particles with opposite charges, and leveraging that property has shown us an atom's nucleus like never before.
Ethan Siegel

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In the quantum Universe, things behave very differently than our common experience would suggest. In the macroscopic world we’re familiar with, any object we can measure appears to have intrinsic properties that are independent of whether we observe it or not. We can measure things like mass, position, motion, duration, etc., without worrying about whether that object is affected by our measurements; reality exists completely independently of the observer. But in the quantum world, that’s demonstrably not true. The act of measuring a system fundamentally changes its properties in an irrevocable way.

One of the weirdest quantum properties of all is entanglement: where multiple quanta have inherent properties that are both indeterminate, but the properties of each one aren’t independent of the other. We’ve seen this demonstrated for photons, electrons, and all sorts of identical particles before, enabling us to test and probe the fundamental and surprising nature of reality. In fact, the 2022 Nobel Prize in Physics was awarded precisely for investigations into this phenomenon.

But in a novel experiment, quantum entanglement has just been demonstrated between different particles for the first time, and already the technique has been used to see an atom’s nucleus like never before.

Illustration of two entangled particles, separated in space and each with indeterminate properties until they are measured. It has been experimentally determined that the neither member of the entangled pair exists in a particular state until the critical moment at which a measurement occurs: the key aspect which enables many modern quantum technologies.
(Credit: Johan Jamestad/The Royal Swedish Academy of Sciences)

In principle, quantum entanglement is a simple idea to understand, and it’s built on the idea of quantum indeterminism. Imagine you pull a ball out of a hat, and there’s a 50/50 chance that the ball has one of two properties.

  • Perhaps it’s color: the ball could be black or white.
  • Perhaps it’s mass: either you pulled out a light ball or a heavy ball.
  • Perhaps it’s which direction it’s spinning: the ball could be “spin up” or “spin down.”