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[[File:Superdense coding.png|right|frame|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 boolean, and the zeroes at the left hand side represent the [[quantum state]] <math>|0\rangle</math>. See the section
In [[quantum information theory]], '''superdense coding''' (or dense coding) is a [[quantum communication]] protocol to transmit two classical bits of information (i.e., either 00, 01, 10 or 11) from a sender (often called Alice) to a receiver (often called Bob), by sending only one [[qubit]] from Alice to Bob, under the assumption of Alice and Bob pre-sharing an entangled state.<ref name="bennett1992communication">{{cite journal|last1=Bennett|first1=C.|last2=Wiesner|first2=S.|year=1992|title=Communication via one- and two-particle operators on Einstein-Podolsky-Rosen states|journal=Physical Review Letters|volume=69|issue=20|pages=2881–2884|doi=10.1103/PhysRevLett.69.2881|pmid=10046665}}</ref><ref name="NielsenChuang2010">{{cite book|url=https://books.google.com/books?id=-s4DEy7o-a0C|title=Quantum Computation and Quantum Information: 10th Anniversary Edition|last1=Nielsen|first1=Michael A.|last2=Chuang|first2=Isaac L.|date=9 December 2010|publisher=Cambridge University Press|isbn=978-1-139-49548-6|page=97|section=2.3 Application: superdense coding}}</ref> This protocol was first proposed by [[Charles H. Bennett (physicist)|Bennett]] and [[Stephen Wiesner|Wiesner]] in 1992 and experimentally actualized in 1996 by Mattle, Weinfurter, Kwiat and [[Anton Zeilinger|Zeilinger]] using entangled photon pairs.<ref name="NielsenChuang2010" /> By performing one of four [[quantum gate]] operations on the (entangled) qubit she possesses, Alice can prearrange the measurement Bob makes. After receiving Alice's qubit, operating on the pair and measuring both, Bob has two classical bits of information. If Alice and Bob do not already share entanglement before the protocol begins, then it is impossible to send two classical bits using 1 qubit, as this would violate [[Holevo's theorem]].
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