Why Does Teleportation Need a Classical Channel?

• Alice's side, holding the state to be moved
• Bob's side, not yet holding the state
• Ordinary message (solid arrow)
• Two squares: two bits of information
• Entanglement link (dashed line)

What you will learn

• Describe the three parts of teleportation: the entangled pair, the sender’s operation and the classical message.
• Explain why, without the classical message, the receiver cannot use their photon.
• State what teleportation does not do.

Before you start

It helps to read What Is Quantum Entanglement? first. Other terms are explained as they appear.

Where does teleportation start?

Teleportation starts with an entangled pair that two people share. According to the Stanford Encyclopedia of Philosophy, Alice and Bob share an entangled pure state, for example two photons entangled in polarization. Alice holds one photon of the pair and Bob holds the other.

Alice then receives an extra photon in a polarization state nobody knows. That is the state to be “moved” to Bob’s side.

The Stanford source says the idea was first proposed in 1993 by Bennett and colleagues.

• Alice's frame
• Bob's frame
• State to be moved (triangle)
• Alice's entangled photon
• Bob's entangled photon
• Entanglement link (dashed line)

What does Alice do, and what does Bob end up with?

Alice performs an operation on the two photons she holds, and Bob’s photon lands in one of four states. According to the Stanford source, the operation has four possible random outcomes, and Bob’s photon is steered into the state that goes with the outcome.

That state is either exactly the state to be moved or related to it in a definite way. Bob cannot tell which, because Alice’s result is random.

Alice and Bob need their previously shared entangled pair for this protocol. The classical message is not a complete description of the unknown state: it tells Bob which correction to apply. Reading Alice’s photon and describing it by telephone is not a replacement for entanglement.

• Alice's operation
• The four possible outcomes
• The four possible states of the photon on Bob's side

Why is a classical channel needed?

Because Bob can only fix his photon once he knows which outcome Alice got. According to the Stanford source, only after Alice communicates the outcome of her operation to Bob does Bob know whether his photon already has the right state or how to transform it locally.

That outcome takes just two bits of classical information, since the operation has four outcomes. The two bits travel over an ordinary channel, the kind a telephone line provides.

Without that message, Bob holds a photon in one of four states and does not know which.

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. The state itself remains unknown to both of them.

• State destroyed on Alice's side
• Two-bit message over the ordinary channel
• State recreated on Bob's side

Real-world example

Picture Alice in Hanoi and Bob in Hue, who shared a pair of entangled photons earlier. Alice performs her operation and gets one of four results. She phones Bob and reads out the two matching bits. Once he hears them, Bob knows whether to leave his photon alone or transform it.

This is a made-up example to show the order of the steps, not a description of a particular device.

Stanford describes experiments on the ground and between the ground and a satellite. Distances quoted in a source are reported results, not automatically current records.

In the motion model below, the spheres are systems that already exist; color and pointer direction only illustrate state. The small block on the connection represents the classical message, not a photon or transported matter.

Common misunderstandings

Teleportation does not send a person or an object from one place to another. According to the Stanford source, it destroys the state of a photon on one side and recreates it on the other.

It does not happen through the entangled pair alone. Bob can use his photon only after receiving two bits from Alice over an ordinary channel.

After teleportation, the original state is still unknown to both Alice and Bob. They cannot “read” it.

Frequently asked questions

Is quantum teleportation instant?

Not through the entangled pair alone. Bob has to wait for Alice to report her operation’s outcome over an ordinary channel before he can use his photon.

How much classical information does teleportation take?

Two bits, because Alice’s operation has four outcomes. These bits specify a correction, not a complete description of the photon state.

Who proposed quantum teleportation?

According to the Stanford source, Bennett and colleagues first proposed the idea in 1993.

Do long-distance demonstrations remove the classical channel?

No. Whether on the ground or through a satellite, Bob still needs the measurement result to determine the correction.

Summary

• The shared entangled pair handles the quantum state, while Alice reports her operation’s outcome over a classical channel.
• Alice’s outcome is one of four random possibilities and takes two bits to report.
• Teleportation destroys a state on one side and recreates it on the other, and no object is sent.