Quantum Teleportation Protocol

Overview

Quantum teleportation transfers an unknown quantum state from one place to another without sending the physical particle that carries it. The sender, Alice, and the receiver, Bob, share a pair of entangled qubits. Alice performs a joint measurement on her half of the pair and the qubit to be sent, then tells Bob two classical bits. Bob applies a correction that depends on those bits, and his qubit becomes an exact copy of the original state, while Alice's original is destroyed.

Despite the science-fiction name, nothing travels faster than light and no matter is moved. Teleportation needs both a shared entangled pair and a classical communication channel, and it is a basic building block for quantum networks, quantum repeaters, and gate operations inside quantum computers.

The problem it solves

Moving a fragile, unknown quantum state to a distant location without physically transporting the particle itself.

History and background

The protocol was proposed in 1993 by six researchers, Bennett, Brassard, Crépeau, Jozsa, Peres, and Wootters, in Physical Review Letters. It was inspired by earlier work on superdense coding and by the question of how to transmit quantum information with limited resources. The first experiments came in 1997, when groups in Innsbruck, led by Anton Zeilinger, and Rome used photons to teleport a qubit, and in 1998 Furusawa and colleagues teleported a continuous-variable state of light.

Distances grew over the next decades. In 2012 Zeilinger's group teleported over 143 kilometers between two Canary Islands, and in 2017 a Chinese team led by Jian-Wei Pan teleported a photon from the ground to the Micius satellite more than 1,400 kilometers away. Teleportation is also used between solid-state qubits, ions, and atoms, and experiments have teleported quantum gates between separate modules to link small quantum processors. Zeilinger shared the 2022 Nobel Prize in Physics for work that includes entanglement and teleportation experiments.

How it works

Uses a pre-shared entangled pair plus a measurement and classical message to reconstruct the original quantum state at the destination, destroying the original in the process (consistent with the no-cloning theorem).

A qubit state has two complex numbers that cannot be copied or measured without disturbing them, because of the no-cloning theorem. Teleportation avoids this by never measuring the state itself. Instead, Alice's measurement asks a question about the relationship between her unknown qubit and her half of the entangled pair, which reveals nothing about the state but moves its information into Bob's qubit in a scrambled form.

After the Bell measurement, Bob's qubit holds the original state up to one of four simple transformations: nothing, a bit flip, a phase flip, or both. Which one depends on the random outcome of Alice's measurement. Without Alice's two classical bits, Bob's qubit looks completely random, which is why teleportation cannot be used to send information faster than light.

The protocol also shows that entanglement is a resource. One shared entangled pair and two classical bits are exactly enough to send one qubit, a trade-off that is built into the theory of quantum communication.

Step by step

1. Alice and Bob share an entangled pair in a Bell state, with Alice holding qubit A and Bob holding qubit B. Alice has a third qubit, C, in the unknown state to be sent.
2. Alice applies a controlled-NOT gate with qubit C as control and qubit A as target, then a Hadamard gate on qubit C. This changes the basis of her two qubits to the Bell basis.
3. Alice measures qubits C and A in the computational basis, obtaining two classical bits, each equally likely to be 0 or 1.
4. Alice sends the two bits to Bob over a classical channel.
5. Bob applies a correction to qubit B: if the second bit is 1 he applies an X gate, and if the first bit is 1 he applies a Z gate.
6. Bob's qubit is now in the original state. Alice's qubits hold random results, and the original state no longer exists at her end.

A worked example

Suppose Alice wants to send the state 0.6 times the state 0 plus 0.8 times the state 1. After her measurement, there are four equally likely outcomes. If she gets bits 00, Bob's qubit is already in the correct state. If she gets 01, Bob's qubit has the bit flipped, so he applies X. If 10, it has the sign flipped on the 1 component, so Bob applies Z. If 11, he applies both X and Z.

In each case the final state is exactly 0.6 times the state 0 plus 0.8 times the state 1, and the measurement results give no information about the numbers 0.6 and 0.8. An eavesdropper who intercepts the two classical bits learns nothing about the state.

Complexity and resources

What hardware it needs

• A source of high-fidelity entangled pairs, such as spontaneous parametric down-conversion for photons or local gates for matter qubits.
• Bell-state measurement capability, which is hard for photons and easier with gates on matter qubits.
• Fast classical communication and feed-forward to apply the correction.

Applications

• Quantum repeaters and quantum networks, which extend entanglement across long distances by teleportation and entanglement swapping.
• Gate teleportation, used in fault-tolerant quantum computing to apply gates without exposing data to noise.
• Distributed quantum computing, linking small processors into larger ones by teleporting qubits or gates between modules.
• Experimental tests of quantum mechanics and entanglement.

Limitations and caveats

• It requires distributing entanglement first, which is difficult over long distances because photons are lost in fibers.
• Photonic Bell measurements can succeed only part of the time without extra resources.
• Fidelity of the teleported state depends on the entangled resource and on noise.

Common misconceptions

• It does not send matter or information faster than light. The classical bits must still be transmitted.
• It does not copy the state. The original is destroyed by Alice's measurement, in agreement with the no-cloning theorem.
• It is not a way to transport people or objects.

How it compares

• Superdense coding is the reverse trade: one entangled pair and one qubit send two classical bits.
• Direct transmission of a photon is simpler but suffers from loss, while teleportation can combine with repeaters.
• Entanglement swapping is teleportation applied to half of an entangled pair.

Real-world impact

Foundational to quantum networking and the long-term vision of a quantum internet; experimentally demonstrated over increasing distances, including via satellite.

Why it matters

Teleportation is the basic tool for moving quantum information in a network. A future quantum internet, and modular quantum computers made of many linked chips, rely on it.

It also appears inside processors: gate teleportation lets a fault-tolerant machine apply difficult operations using prepared resource states, which is central to practical error-corrected designs.

Frequently asked questions

Keep learning

• Related algorithm: BB84 Quantum Key Distribution
• Related algorithm: E91 Quantum Key Distribution
• Related algorithm: Quantum Error-Correcting Codes (Shor Code, Surface Code)
• Related algorithm: Quantum Secret Sharing

This page is written for educational purposes. Complexity statements describe the best known results at the time of review (September 30, 2026), and research in this area moves quickly, so check recent papers before relying on a specific bound.

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