Even with quantum entanglement, there’s no faster-than-light communication

One of the most fundamental rules of physics, undisputed since Einstein first laid it out in 1905, is that no information-carrying signal of any type can travel through the Universe faster than the speed of light. Particles, either massive or massless, are required for transmitting information from one location to another, and those particles are mandated to travel either below (for massive) or at (for massless) the speed of light, as governed by the rules of relativity. You might be able to take advantage of curved space to allow those information-carriers to take a short-cut, but they still must travel through space at the speed of light or below.

Since the development of quantum mechanics, however, many have sought to leverage the power of quantum entanglement to subvert this rule. Many clever schemes have been devised in a variety of attempts to transmit information that “cheats” relativity and allows faster-than-light communication after all. Although it’s an admirable attempt to work around the rules of our Universe, every single scheme has not only failed, but it’s been proven that all such schemes are doomed to failure. Even with quantum entanglement, faster-than-light communication is still an impossibility within our Universe. Here’s the science of why.

Flipping a coin should result in a 50/50 outcome of getting either heads or tails. If two ‘quantum’ coins are entangled, however, measuring the outcome of one of the coins (heads or tails) can provide you with information to do better than random guessing when it comes to the state of the other coin. However, that information can only be transmitted, from one coin to the other, at light speed or slower.

Conceptually, quantum entanglement is a simple idea. You can start by imagining the classical Universe and one of the simplest “random” experiments you could perform: conducting a coin flip. If you and I each have a fair coin and flip it, we’d each expect that there’s a 50/50 chance of each of us getting heads and a 50/50 chance that each of us would get tails. Your results and my results should not only be random, they should be independent and uncorrelated: whether I get heads or tails should still have 50/50 odds irrespective of what you get with your flip.

But if this isn’t a classical system after all, and a quantum one instead, it’s possible that your coin and my coin will be entangled. We might each still have a 50/50 chance of getting heads or tails, but if you flip your coin and measure heads, you’ll instantly be able to statistically predict to better than 50/50 accuracy whether my coin was likely to land on either heads or tails. This is the big idea of quantum entanglement: that there are correlations between the two entangled quanta that means if you actually measure the quantum state of one of them, the other one’s state isn’t instantly determined, but rather some probabilistic information can be gleaned about it.

By creating two entangled photons from a pre-existing system and separating them by great distances, we can ‘teleport’ information about the state of one by measuring the state of the other, even from extraordinarily different locations. Interpretations of quantum physics that demand both locality and realism cannot account for a myriad of observations, but multiple interpretations all appear to be equally good.

How does this work, conceptually?

In quantum physics, there exists a phenomenon known as quantum entanglement, which is where you create more than one quantum particle — each with its own individual quantum state — where something important about the sum of both states together is known. It’s as though there’s an invisible thread connecting these two quanta (or, if two coins were entangled according to the laws of quantum mechanics, your coin and my coin), and when one of us makes a measurement about the coin we have, we can instantly know something about the state of the other coin that goes beyond our familiar “classical randomness.”

Although this sounds like purely theoretical work, it’s been within the realm of experiment for many decades. We’ve created pairs of entangled quanta (photons, to be specific) that are then carried away from one another until they’re separated by large distances, then we have two independent measurement apparatuses that tell us what the quantum state of each particle is. We make those measurements as close to simultaneously as possible, and then get together to compare our results. These experiments are so profound that research following these lines was awarded a share of the 2022 Nobel Prize in physics.

The best possible local realist imitation (red) for the quantum correlation of two spins in the singlet state (blue), insisting on perfect anti-correlation at zero degrees, perfect correlation at 180 degrees. Many other possibilities exist for the classical correlation subject to these side conditions, but all are characterized by sharp peaks (and valleys) at 0, 180, 360 degrees, and none has more extreme values (+/-0.5) at 45, 135, 225, 315 degrees. These values are marked by stars in the graph, and are the values measured in a standard Bell-CHSH type experiment. The quantum and classical predictions can be clearly discerned, and were identified at a variety of angles way back in 1972 with Stuart Freedman’s PhD thesis.

What we find, perhaps surprisingly, is that the results for your coin and my coin (or, if you prefer, your photon’s spin and my photon’s spin) are correlated with one another! We’ve now separated two photons by distances of hundreds of kilometers before making those critical measurements and then measuring their quantum states within nanoseconds of one another. If one of those photons has spin +1, the other one’s state can be predicted to about a 75% accuracy, rather than the standard 50% you would have classically expected from knowing it’s either +1 or -1.

Moreover, that information about the other particle’s spin can be known instantaneously, rather than waiting for the other measurement apparatus to send us the results of that signal, which would take about a millisecond. It seems, on the surface, that we can know some information about what’s going on at the other end of the entangled experiment not only faster than light, but at least tens of thousands of times faster than the speed of light. Does this mean that information is actually being transmitted at speeds faster than the speed-of-light?

If two particles are entangled, they have complementary wavefunction properties, and measuring one determines properties of the other. If you create two entangled particles or systems, however, and measure how one decays before the other decays, you should be able to test for whether time-reversal symmetry is conserved or violated.

On the surface, it might appear that information really is being communicated at speeds faster-than-light. For example, you might attempt to concoct an experiment that obeys the following setup:

• You prepare a large number of entangled quantum particles at one (source) location.
• You transport one set of the entangled pairs a long distance away (to the destination) while keeping the other set of entangled particles at the source.
• You have an observer at the destination look for some sort of signal, and force their entangled particles into either the +1 state (for a positive signal) or a -1 state (for a negative signal).
• Then, you make your measurements of the entangled pairs at the source, and determine with better than 50/50 likelihood what state was chosen by the observer at the destination.

If this setup worked, you really would be able to know whether the observer at the distant destination forced their entangled pairs into either the +1 or the -1 state, simply by measuring your own particle pairs after the entanglement was broken from afar.

The wave pattern for electrons passing through a double slit, one-at-a-time. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Regardless of the interpretation, quantum experiments appear to care whether we make certain observations and measurements (or force certain interactions) or not.

This seems like a great setup for enabling faster-than-light communication. All you need is a sufficiently prepared system of entangled quantum particles, an agreed-upon system for what the various signals will mean when you make your measurements, and a pre-determined time at which you’ll make those critical measurements. From even light-years away, you can instantly learn about what was measured at a destination by observing the particles you’ve had with you all along.

But is this right?

It’s an extremely clever scheme for an experiment, but one that doesn’t actually pay off in any way. When you, at the original source where the particle pairs were entangled and created, go to make these critical measurements, you’ll discover something extremely disappointing: your results simply show 50/50 odds of being in the +1 or -1 state. It’s as though the actions of the distant observer, forcing their member of the entangled pairs to be in either the +1 or the -1 state, had no effect on your experimental results at all. The results are identical to what you’d expect had there never been any entanglement at all.

Schematic of the third Aspect experiment testing quantum non-locality. Entangled photons from the source are sent to two fast switches that direct them to polarizing detectors. The switches change settings very rapidly, effectively changing the detector settings for the experiment while the photons are in flight. Different settings, puzzlingly enough, result in different experimental outcomes.

Where did our plan fall apart? It was at the step where we had the observer at the destination make an observation and try to encode that information into their quantum state, where we had previously stated, “You have an observer at the destination look for some sort of signal, and force their entangled particles into either the +1 state (for a positive signal) or a -1 state (for a negative signal).”

When you take that step — forcing one member of an entangled pair of particles into a particular quantum state — that action not only breaks the entanglement between the two particles, but it doesn’t break the entanglement and determine what that particle’s properties were; it breaks the entanglement and places it into a new state that doesn’t care about which state (+1 or -1) would have been “determined” from making a fair measurement.

That is to say, the other member of the entangled pair is completely unaffected by this “forcing” action, and its quantum state remains random, as a superposition of +1 and -1 quantum states. What you’ve done by “forcing” one member of the entangled particles into a specific state is completely break the correlation between the measurement results. The state you’ve “forced” the destination particle into is now 100% unrelated to the quantum state of the source particle.

A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. You can combine principles like the quantum eraser with the double-slit experiment and see what happens if you keep or destroy, or look at or don’t look at, the information you create by measuring what occurs at the slits themselves.

The only way that this problem could be circumvented is if there existed some way of making a quantum measurement that actually forced a particular outcome. (Note: this is not something permitted within the presently-known laws of physics.)

If you could do this, then someone at the destination could conduct observations — for example, learning whether a planet they were visiting were inhabited or not — and then use some unknown process to:

• measure their quantum particle’s state,
• where the outcome will turn out to be +1 if the planet is inhabited,
• or -1 if the planet is uninhabited,
• and thereby enable the source observer with the entangled pairs to instantaneously figure out whether this distant planet is inhabited or not.

Unfortunately, the results of a quantum measurement are unavoidably random; you cannot encode a preferred outcome into a quantum measurement.

Even by taking advantage of quantum entanglement, it should be impossible to do better than random guessing when it comes to knowing what goes on at the other end of an entanglement experiment, regardless if it’s about photon spins, coin flipping, or trying to know what cards the dealer’s hand holds.

As quantum physicist Chad Orzel has written, there is a big difference between making a measurement (where the entanglement between pairs is maintained) and forcing a particular result — which itself is a change of state — followed by a measurement (where the entanglement is not maintained). If you want to control, rather than simply measure, the state of a quantum particle, you’ll lose your knowledge of the full state of the combined system as soon as you make that change-of-state operation happen.

Quantum entanglement can only be used to gain information about one component of a quantum system by measuring the other component so long as the entanglement remains intact. What you cannot do is create information at one end of an entangled system and somehow send it over to the other end. If you could somehow make identical copies of your quantum state, faster-than-light communication would be possible after all, but this, too, is forbidden by the laws of physics.

If you could somehow take a quantum state and make an identical copy of it, it might be possible to concoct a faster-than-light communication scheme. However, a valid no-cloning theorem was proven back in the 1970s and 1980s by multiple independent parties, as the act of attempting to even measure a quantum state (to know what it is) fundamentally changes the outcome.

There’s an awful lot that you can do by leveraging the bizarre physics of quantum entanglement, such as by creating a quantum lock-and-key system that’s virtually unbreakable with purely classical computations. But the fact that you cannot copy or clone a quantum state — as the act of merely reading the state fundamentally changes it — is the nail-in-the-coffin of any workable scheme to achieve faster-than-light communication with quantum entanglement. Many aspects of quantum entanglement, which itself is a rich field of research, were recognized in the 2022 Nobel Prize in physics.

There are a lot of subtleties associated with how quantum entanglement actually works in practice, but the key takeaway is this: there is no measurement procedure you can undertake to force a particular outcome while maintaining the entanglement between particles. The result of any quantum measurement is unavoidably random, negating this possibility. As it turns out, God really does play dice with the Universe, and that’s a good thing. No information can be sent faster-than-light, allowing causality to still be maintained for our Universe.