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What is density matrix of one on two entangled qubits?

  1. May 18, 2008 #1
    Let we have two qubits A and B. First qubit has eigenstates |A0> and |A1>, and second has |B0> and |B1>.

    Let them be in the entangled state, described with vector

    c1 * |A0> * |B0> + c2 * |B0> * |B1>|

    where c1 and c2 are complex numbers with |c1|^2 + |c2|^2 = 1.

    Then what is density matrix, describing only first qubit separately? Is this matrix 1x1 or 2x2?
     
  2. jcsd
  3. May 18, 2008 #2
    Is the correct answer is 1x1 matrix

    |c1|^2 * |A0> * <A0| + |c2|^2 * |A1> * <A1|

    ?
     
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