Homework: Tensor products and the partial trace
Georg Maringer
Thursday 19
thApril, 2018
Let|Ψi = √1
2|0i ⊗ |2i+√1
6|1i ⊗(|0i+i|1i+|2i) ∈ C2⊗C3
where {|0i,|1i}and{|0i,|1i,|2i}are ONBs ofC2andC3, respectively.
1) Expressρ12 =|ΨihΨ| as a matrix in a suitable orthonormal basis ofC2⊗C3.
2) Compute the reduced density operators (partial traces) of the first system (qubit) and the second system (qutrit).
3) Compute the eigenvalues of the reduced density operators.
4) Compute the Schmidt-decomposition of the of the state
ψ= √1
3(|0i ⊗ |1i+|1i ⊗ |0i − |1i ⊗ |1i) (0.1) Hint: The existence of the Schmidt decomposition |ψi = ∑ri=1√
λi|eii ⊗ |fji was shown in the presentation. It was also shown thatspec(tr2|ψihψ|) = {λ1. . .λr} = spec(tr1|ψihψ|) = {λ1, . . .λr}.
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