Quantum telepathy

ost people have probably experienced, or thought they experienced, a telepathic incident, where two people communicate at a long distance without any technology during an episode of intense emotion.

Neurologists think it's an artifact of how the brain works. Psychiatrists think it's a delusion called magical thinking that's associated with high polygenic risk for schizophrenia.^[1] And then there are some weirdos who just have an open mind.^[2] I'll discuss why the concept of polygenic risk is unscientific pseudoscience in a future post. But physicists have a trifecta: quantum supremacy, quantum telepathy,^[3] and quantum magic.^[4]

Quantum supremacy simply means that quantum computers could someday replace, i.e. reign supreme, over classical computers. The other two are magic tricks that involve quantum entanglement, which is where two particles, usually photons, exist in a combined or superposed state irrespective of the distance between them. If the state of one photon is measured, the entanglement collapses and they instantly become two different photons with opposite states. For instance, if the two possible states are |0⟩ and |1⟩, the entangled state would be 1    √2 (|0⟩⊗|1⟩ − |1⟩⊗|0⟩) .

To do quantum telepathy (or pseudo-telepathy), you play a game where the players aren't allowed to communicate with each other, but they can share an entangled state beforehand. The players, A and B, Godzilla and King Kong, Alice and Bob, or whatever, can get an answer using quantum telepathy that can't possibly be obtained any other way.

Deutsch-Jozsa game

One example is the Deutsch-Jozsa game ^[5] where the two players are given different bit strings, say x = 1001 and y = 1100. (We put the name in bold because it's considered to be a vector with four elements: 1, 1, 0, and 0.) Their task is to send back an answer a or b, say 10 and 01. To win, their answers must have a = b only if x = y. This tells the referee R whether the players think x and y are the same or different.

Note that, for short strings, they have a certain probability of guessing right, but for a sufficiently long string x and y, it's impossible to do this classically. A and B are too far away to communicate with each other. The usual solution, hitting the computer with a mallet until it gives the right answer, is not allowed.

For the game, A and B have a certain number of entangled bits, which we call an entangled vector j, which they prepared in advance. Let's call it 1    √n (∑n−1j=0 |j⟩|j⟩), where n is the number of bits in the vector. A and B interact their shared entangled vector with their query vector (x or y) using standard quantum computa­tional calcula­tions. They each calculate a classical result a or b and send it back. Lo and behold, A and B always get the right answer. A and B must have a telepathic link!

Of course, it's just a trick. What's actually happening is that entanglement means whatever A has is guaranteed to become the opposite of whatever B has. Assume, for example, they're using spin qubits. If one of A's photons is “up,” then B's corresponding one will turn out to be “down.” When A uses A's half of the entangled vector by combining it with the query, the other half, held by B, instantaneously and automatically becomes the opposite. So when they combine them with the query, each bit of x and y that is equal gives a ≠ b and vice versa. After a simple bit flip, A's and B's answer is always the same. They have no way of knowing what happened, of course, until the referee checks the result, tells them, and hands them their prize.

In practice, running this game is challenging because noise can easily make A and B lose the game. Noise can come during preparation and storage of the entangled states or in transmission. Some researchers ^[3] think that if they use a qubit quantum dot-cavity to prepare their qubit, it may be easier than an ordinary everyday transmon qubit you might find in your fridge because a quantum dot-cavity in a magnetic field provides a more definitive answer. By that, they mean it emits a photon at one wavelength or the other depending on its state.

Magic squares game

Another example is magic squares game, which was invented by Aravind ^[6][7]. It's played by those same two quantum-crossed lovers A and B whose secret love letters are so popular in cryptography.

In this example, played on a 3×3 grid, the referee specifies a row between 1 and 3 to A and a column between 1 and 3 to A to B. A and B must pick a sequence of 1s or 0s so that the intersecting square is the same, the sum of the column digits must be an odd number, and the sum of the row digits must be an even number. For instance, if the referee specifies 2 and 3, one winning combination is if A picks 101 and B picks 111.

      -   -   1
      1   0   1
      -   -   1 

This is even more boring than the Deutsch-Jozsa game, but classically A and B can win 8 times out of 9. Using their quantum cheating method, they can win more often (though if there's noise, they might win less often). Brassard et al.^[8] point out that telepathy can only be shown by repeated wins, not by a single win, because in this game it's possible to win by chance.

These experiments can get fairly complicated, especially when the physicists start theorizing about them; and the ‘games’ are highly contrived, which means they might have no practical application, but the concept is simple. Physicists say it's a good way of convincing people that quantum entanglement is real. But I suspect the real reason is to flip the bird to all those psychiatrists in the building across the street who kept saying telepathy was just a psychiatric disorder.

may 07 2025