Can Entanglement Send Messages Faster Than Light?

• First result sheet
• Second result sheet
• Entanglement link (dashed line)
• Ordinary wire (solid line)

What you will learn

• Explain why the correlation between two entangled particles cannot carry a message.
• Describe the role of an ordinary channel in quantum teleportation.
• State how the failure to send messages connects to causality in relativity.

Before you start

It helps to read What Is Quantum Entanglement? first. This article uses the words correlation, measurement and causality; each is explained as it appears.

Why can’t entanglement send a message?

Entanglement cannot send a message because nobody gets to choose their own measurement result. According to the Stanford Encyclopedia of Philosophy, the second system cannot be steered into a particular quantum state at the experimenter’s whim. With many entangled pairs, the experimenter can only restrict the set of states the second system may end up in.

Try a scenario. Lan wants to signal “yes” to Minh by choosing how to measure her particle. But the Bell’s Theorem entry in the same encyclopedia states that quantum correlations cannot be exploited for superluminal signalling. Minh’s string of results does not let him read Lan’s choice.

The decisive point is that Minh’s local statistics do not reveal Lan’s choice. Certifying randomness from a Bell test requires the test’s assumptions; the statement “no faster-than-light signalling” alone does not imply that every measurement outcome is completely random.

• Dial that chooses how to measure
• Barrier: the choice does not reach the other side
• Results when the first setting is chosen
• Results when the second setting is chosen

What does a message need besides entanglement?

A message needs an extra ordinary channel to report the result. Quantum teleportation is the clearest example, according to the Stanford article on entanglement.

Alice and Bob share a pair of entangled photons, one each. Alice receives a further photon in an unknown polarization state and performs an operation on the two photons she holds. The operation has four random outcomes, and depending on which occurs, Bob’s photon is steered into one of four states.

Bob cannot yet tell which state he holds. Only after Alice communicates the outcome of her operation does Bob know whether his photon is already in the required state or how to transform it. That outcome amounts to two bits of classical information, and the Stanford source says it travels over a classical communication channel such as a telephone line.

In the end, Alice and Bob use their shared entangled state as a quantum communication channel to destroy the state of a photon on Alice’s side and recreate it on Bob’s side. Without those two bits, Bob has nothing he can use.

• Alice's side
• Bob's side
• State to be transferred, at Alice's side
• State recreated at Bob's side
• Ordinary message (two squares)
• Entanglement link (dashed line)

What does this have to do with causality?

The failure to send messages keeps causality in relativity from breaking. According to the Bell’s Theorem entry, the cause–effect relation is taken to point along time: causes precede their effects. Events at spacelike separation, meaning too far apart for light to connect them, are taken to have no temporal order.

The reason is that their order changes with the reference frame. For two such events, different observers assign them different before-and-after orderings. If all reference frames are on a par, a relation that does not hold in every frame has no physical significance. Starting from this Lorentz invariance and the assumption that causes precede their effects, Bell introduces what he calls the principle of local causality.

Shimony spoke of “peaceful coexistence” between special relativity and quantum theory. According to the Stanford entry, in later writings he held that what suggests this peace is the fact that quantum correlations cannot be used to signal faster than light.

The causal question does not close there. The same entry lists one option open to physicists: to accept causal relations between events outside each other’s light cones. Bell’s theorem, in the next article, explains why they have to weigh that option.

• First event
• Second event
• Light paths forming the light cones
• Correlation between the two events (dashed line)
• Time axis and space axis

Real-world example

Imagine Lan and Minh writing their measurement results on two sheets of paper in two cities. Each sheet, taken alone, looks like a random string, and Lan cannot make Minh’s sheet carry a single letter. Only when they phone each other and read the sheets aloud do they see that the two match.

That randomness is useful in its own right. According to the Bell’s Theorem entry, one can design a quantum key distribution protocol that is provably secure on the assumption that the underlying physics, whatever it is, does not permit superluminal signalling.

Common misunderstandings

Measuring one particle does not make the other “receive an order”. The person measuring cannot choose the result on the other side, so cannot use it to send letters.

Quantum teleportation does not send a person or an object instantly. The state of one photon is destroyed on one side and recreated on the other, and an ordinary call is needed before the result is usable.

Being unable to send messages does not mean the two particles are unrelated. The correlation is real; it just is not a signal.

Frequently asked questions

Is entanglement faster than light?

It cannot be used to send messages faster than light. Quantum correlations cannot be exploited for superluminal signalling.

Does quantum teleportation need extra information?

Yes. Bob knows how to use his photon only after Alice reports her operation’s outcome, two bits of classical information over an ordinary channel.

Why is each side’s result random?

In the entangled-pair example, each party has outcomes they cannot control. What prevents signalling is that local statistics do not reveal the distant choice; not every quantum measurement is equally random.

Can entanglement be used for security?

It can, with security that can be proven under the assumption that no signal travels faster than light.

Summary

• Nobody chooses their own measurement result, so the correlation between two entangled particles carries no message.
• Quantum teleportation still needs a report of the outcome over an ordinary channel.
• The absence of signalling keeps causal order intact, but it does not answer every causal question about entangled correlations.