By Thomas HornigoldJul 10 2018

*Natali art/Shutterstock*

**Einstein was not a fan of quantum mechanics – which is ironic, because his work laid the foundations for the field of study. When Planck first realized that energy was quantized – it came in discrete little packets, rather than flowing continuously like a fluid – Einstein realized that this would necessitate a complete rewriting of the laws of physics. **

Yet the conclusions and the phenomena that were discovered in the years that followed – particles that behaved like waves, or that had no fundamentally determined position whatsoever – led to conclusions that were at odds with the way that Einstein viewed the universe.

The classical world held deep promise for believers in rationality. It promised that, if you were smart enough – if you discovered all of the laws of physics, and simply took the appropriate measurements and made the appropriate calculations – you could, in principle, know everything.

You could precisely trace a line between cause and effect: you could extrapolate the state of the Universe backwards to its origins, or forward to its end, with absolute certainty. The quantum world, on the other hand, seemed to argue that this could not be done. That, in fact, the Universe – and subatomic particles – had no “certain state” until they were observed, at which point they had some probability of ‘collapsing’ into certainty. Einstein, famously, felt that “God does not play dice with the Universe.”

And so, amongst others, he attempted to come up with theories that contradicted the interpretation of quantum mechanics that had become mainstream in the physics community. Most famous amongst these is the Einstein-Podolsky-Rosen paradox.

This idea rests on a phenomenon known as quantum entanglement. Take a classic example – an electron positron pair that are generated in such a way that their total spin (along the z axis) is known to be zero.

Our understanding of spin as a quantum number suggests that, when you take a measurement of the electron’s spin, you “collapse its wavefunction” – thus forcing the spin to be either up or down. If you take a measurement of the electron spin along a particular axis, you will find that it’s either down or up with a 50% probability of either outcome.

For tradition’s sake, let’s say that Alice is in Alpha Centurai while Bob is in Copenhagen. We produce these particles and send one to Alice and one to Bob. Now, when Alice measures the spin on her particle, she finds it to be up. This means that Bob must measure spin down for the total spin to be zero.

When Alice collapsed the entangled wavefunction by measuring her particle, what happened to Bob’s particle? It seems as if information is instantly transmitted across light years from Alice to Bob, telling his particle’s spin to point downwards.

Perhaps this phenomenon could be exploited to allow for faster-than-light communication – and, in any case, it violates a physics principle called “locality”, which states that physical changes to a system should be caused by things that are nearby – for example, interactions between particles that are close together. Anything else, Einstein said, would be “spooky action at a distance.”

Faster-than-light communication, if it were possible, would wreak havoc with the laws of physics. Special relativity sets light-speed as the ultimate speed limit for information transmission in the Universe – and with good reason.

Without this, causality – the sequence of cause and effect – is ruined. If faster-than-light communication is possible, some observers in certain reference frames will see the signal travel back in time – and perhaps arrive at its destination before it is sent. Such a phenomenon, that would effectively allow for time-travel of signals, would present all kinds of knotty paradoxes that physics would struggle to resolve.

Luckily, it turns out that there are several reasons that this system cannot be used for faster than light communication – and laboratory tests, as well as a massive multiplayer video game have thoroughly examined various loopholes that might allow you to imagine using quantum entanglement to communicate some substantive information.

The first thing to understand is that the outcome of Alice’s measurement makes no immediate difference to Bob. Even if his particle is collapsed into a particular state, he has no way of knowing this without making a measurement. The process of measuring an entangled particle that has already collapsed into a definite state, and the process of measuring an entangled particle that has not yet collapsed are identical.

So if Bob sees that his particle is spin-down, he has no way of knowing whether this is because Alice already measured hers to be spin-up, or if he has collapsed the entangled wavefunction himself. The second measurement still appears random. Communicating information about the first measurement from Alice to Bob will be limited by light speed.

What if Alice and Bob agree in advance on who will measure their particles first, and when the particles will be measured? Again, this is no good for faster-than-light communication: Alice has no control over the outcome of the first measurement, which is still a 50-50 chance. She cannot intentionally use it to transmit a bit of data, a zero or a one, a yes or a no to Bob. Instead, regardless of what Alice does, Bob’s perception of his measurement and situation will be the same.

This is clearly a simplification of the complex mathematics behind the EPR paradox and other, related questions of quantum entanglement. Quantum entanglement does allow for “spooky” correlations between measurements across vast distances. The moment that Alice measures her particle as having spin up, she knows that Bob will measure his to have spin down. In a sense, she has instantaneous information about something that has not happened yet – and that could occur many millions of miles away.

But this information cannot be used to communicate across those miles – and the no-communication theorem shows mathematically that Bob cannot tell the difference between Alice’s measurements and his own, random measurement (or, indeed, whether Alice has measured her entangled particle at all.) This has been confirmed by many experiments using entangled “Bell pairs” of the kind we’ve described here.

Nevertheless, quantum entanglement could still have fascinating uses for quantum cryptography. Given that measuring an entangled system affects all parts of the system, and breaks down the coherence of that system, you can set up systems of entangled particles that can allow you to come up with a quantum “key” by measuring certain subsets of entangled particles.

Alice and Bob would then be able to detect whether someone has “eavesdropped” on their communications and discovered the key, because these measurements would change the statistics of the system in a detectable way.

In most of these systems, essentially what happens is that Alice measures some subset of her entangled particles. She can then communicate to Bob information about the results of his particle measurements – the private key. Bob can then, at any time, check whether anyone has attempted to access the key by measuring his particles; if the statistics have changed from Alice’s information, then someone is attempting to eavesdrop.

Many of the developments in quantum computing and quantum cryptography focus on these quantum locks and quantum keys. A secure key allows for information to be decrypted and encrypted safely. But to communicate information, they are still limited by the speed of light: there still needs to be channel of information between the two that’s at lightspeed. It’s just more secure.

Given that time-travelling signals and violating causality would introduce paradoxes that we don’t know how to resolve, it’s all for the best that entanglement can’t allow faster-than-light communication. At the same time, though, given the extreme lengths that stock traders will go to in order to shave a few milliseconds off their communication times, it’s something of a shame. If it worked, physicists could become billionaires at light-speed – or beyond.

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