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## Main Question or Discussion Point

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?

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?