The More You Know: Bell States/EPR Pairs

Bell States (named after John S. Bell) or EPR pairs (after Einsetin, Podolski and Rosen Paradox) are the maximally entangled quantum states of a two qubit system (i.e. a quantum mechanical system composed of two interacting two-level subsystems).

In the computational basis \{|00\rangle,|01\rangle,|10\rangle,|11\rangle\} the four maximally entangled Bell states are expressed as



To understand the correlations present in these states consider the Bell state |\Phi^+\rangle=\frac{|00\rangle+|11\rangle}{\sqrt{2}}. If a measurement is performed on the first qubit the result may be 0 or 1, after which the two qubit state collapses to |00\rangle or |11\rangle respectively. Then, a measurement on the second qubit must result in 0 or 1, yielding the same result as the measurement on the first one. That is, the outcomes of the measurements are correlated.

John Bell proved that the correlations present in EPR pairs cannot be accounted for by any classical theory. See this article for more.

Go to the Dictionary of Quantum Information and Quantum Computation.

All text copyright © Marco Vinicio Sebastian Cerezo de la Roca.

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The More You Know: Bell States/EPR Pairs por Marco Vinicio Sebastian Cerezo de la Roca se distribuye bajo una Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.


About marcocerezo

My name is Marco Cerezo, I have a degree in physics from the Universidad Nacional de la Plata (UNLP) in Buenos Aires, Argentina, and I'm a Ph.D student. Although I study in Argentina, I was born and raised in Guatemala, so I consider myself "chapín" at heart. From an early age I discovered how exciting, interesting and fun physics can be, so I decided to devote myself to study this passionate field. This blog is dedicated to all those who want to dig a little into the mysteries and beauty of physics. And in particular, to all those who are interested in studying Quantum Computation and Quantum Information. ---------------------------------------------------------------------------------------------------------- Mi nombre es Marco Cerezo, Licenciado en Física egresado de la Universidad Nacional de la Plata (UNLP) en Buenos Aires, Argentina. Actualmente estoy haciendo mi Doctorado en física. Aunque estudio en Argentina, nací y crecí en Guatemala por lo que me considero chapín de corazón. Desde muy temprana edad descubrí lo apasionante, interesante y divertida que es la física por lo que decidí dedicarme a su estudio. Este blog es está dedicado a todos aquellos que quieren indagar un poco en los misterios y la belleza de la física; y en particular, a aquellos que quieran aprender sobre Computación e Información Cuántica.
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