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{{multiple issues|{{more footnotes|date=August 2013}}
{{refimprove|date=August 2013}}}}
{{Use American English|date=January 2019}}
[[File:Video superdense coding.ogg|right|thumb|Schematic video demonstrating individual steps of superdense coding. A message consisting of two bits (in video these are (1, 0)) is sent from station A to station B using only a single particle. This particle is a member of an entangled pair created by source S. Station A at first applies a properly chosen operation to its particle and then sends it to station B, which measures both particles in the Bell basis. The measurement result retrieves the two bits sent by station A.]]
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== Overview ==
[[File:Superdense coding.png|right|thumb|When the sender and receiver share a Bell state, two classical bits can be packed into one qubit. In the diagram, lines carry [[qubit]]s, while the doubled lines carry classic [[bit]]s. The variables b<sub>1</sub> and b<sub>2</sub> are classic
Suppose [[Alice and Bob|Alice]] wants to send two classical bits of information (00, 01, 10, or 11) to Bob using [[Qubit|qubits]] (instead of classical [[Bit|bits]]). To do this, an entangled state (e.g. a Bell state) is prepared using a Bell circuit or gate by Charlie, a third person. Charlie then sends one of these qubits (in the Bell state) to Alice and the other to Bob. Once Alice obtains her qubit in the entangled state, she applies a certain quantum gate to her qubit depending on which two-bit message (00, 01, 10 or 11) she wants to send to Bob. Her entangled qubit is then sent to Bob who, after applying the appropriate quantum gate and making a [[Measurement in quantum mechanics|measurement]], can retrieve the classical two-bit message. Observe that Alice does not need to communicate to Bob which gate to apply in order to obtain the correct classical bits from his projective measurement.
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