Einstein famously called it “spooky action at a distance”; the notion that obtaining information about one particle in an entangled pair provides information about the other, no matter how far they are apart. Often called one of the trickiest ideas in science, quantum entanglement has given rise to a host of arguments and experiments — from possibly dead cats to many-worlds concepts and quantum time tweaks — that further complicate the issue, making it even harder to unpack and understand.
Let’s simplify the spooky and help settle the score with a quick look at the quantum condition.
State of the Union
Understanding quantum entanglement starts with the quantum state. All photons — the ideal particle for quantum questions since they can be reliably split into entangled pairs — exist simultaneously in multiple states. It’s called quantum superposition: The idea that all states are potentially possible until a photon is observed or measured.
Simple, right? Not so fast. When it comes to entangled pairs, measuring one tells you something about the other: For example, if the first one you measure spins up, its pair must spin down — no matter where it’s located — giving rise to Einstein’s spooky action statement. But here’s the thing: We’re already familiar with this framework.
Science Alert likens it to finding one glove of a set. If it’s a right-handed glove you know the other has to be left-handed, even if you can’t see it to confirm the observation. If you’ve misplaced the pair and you’re searching the house, the gloves are in superpositional state; they could be either left- or right-handed. Once you find one, however, you collapse this condition and confirm the configuration of both.
Quantum entanglement isn’t quite this easy; while knowing some states provides 100% certainty about the action of the paired particle, most offer only a probability, making another state only more or less likely for the entangled entity. Still, this means the more scientists learn about one particle, the greater their body of knowledge about the other — instantaneously — no matter where it’s located in the universe. But this also poses a problem, since it seemingly violates the notion that nothing — not even information — can travel faster than the speed of light.
What You See — And What You Get
The action of observation is fundamental for science. Measurement allows for greater accuracy and understanding, but in a quantum entanglement scenario, measurement seemingly has an impact on the properties of particles themselves. This doesn’t dovetail with classic conceptualizations of the world, or what physicist John Bell called “realism” — that the properties of objects remain consistent regardless of observation.
But even before the deep dive into quantum mechanics, issues were cropping up around the edges of classical construction: Heisenberg’s uncertainty principle states that as the position of a particle is more precisely determined, momentum predictions suffer a precision loss — and vice versa. At first glance, this makes it seem like the act of observation changes the behavior of the particle itself; instead, it simply precludes our ability to obtain all knowledge simultaneously.
Quantum entanglement poses the same problem. If particles exist in “real” states, their properties shouldn’t be affected by measurement or observation; knowing something about one of a pair shouldn’t tell you anything about the other. To test this theory, scientists sent pairs of photons toward detectors located a significant distance apart. If both had the same polarization, they should reach the detectors. If not, at least one would be blocked. Under a quantum view of the universe, entangled pairs should show higher polarization correlations than if they’re governed by classical conditions — and that’s exactly what happens.
Some criticism has suggested that scientists’ decision about the angle of approach to each detector wasn’t entirely random, in effect imposing outside, unknown constraints on the experiment. To solve this issue, one team used a random measurement setting based on the wavelength of light detected from high-redshift quasars, effectively removing any human hubris. The results? Higher state correlation than would be expected under classical models.
As noted by Scientific American, the idea of entanglement offers potential benefits for information security. Scientists at the University of Bristol have now created and deployed an eight node quantum network that spans 17 kilometers and empowers secure communication using what’s called quantum key distribution (QKD).
Consider those most classic of communicators: Alice and Bob. In a QKD model, Alice and Bob use a pair of entangled photons to create a private key. Both randomly perform a set of measurements, compare the results and select the measurement subset that gives them the same results as the basis for their quantum key.
Scaling this up has proven problematic, however, since adding “Charlie” to the mix without having him go through Bob to get to Alice required separate links to both. While this only requires three links for three end nodes, the volume increases rapidly — eight-node networks need 28 links and 100-node networks require 4,950. The Bristol team solved this problem by creating a central source model that allows direct connection between any two nodes without the need to pass through other users, in turn making spooky security both possible and potentially profitable.
Closing the Quantum Loophole
Ultimately, the quantum conundrum comes down to free will. The idea that measuring one particle property seemingly influences the probability of its paired partners’ state isn’t an easy thing for humans to handle, since it seems to violate the core concepts of our universe: Information can’t travel faster than light, and measurement shouldn’t preclude specific properties for either particle pair. But just like the case of the missing glove, there’s no compulsion in collapsing the quantum state — instead, more information about preexisting conditions has simply been confirmed.
Spooky? Undoubtedly. Solid science? Absolutely.