Paper 1, Section I, 10D10 D Introduce the 2 -qubit states

Quantum Information and Computation | Part II, 2019

∣βxz⟩=(ZzXx)⊗I(∣00⟩+∣11⟩2)\left|\beta_{x z}\right\rangle=\left(Z^{z} X^{x}\right) \otimes I\left(\frac{|00\rangle+|11\rangle}{\sqrt{2}}\right)

where XX and ZZ are the standard qubit Pauli operations and x,z∈{0,1}x, z \in\{0,1\}.

(a) For any 1-qubit state ∣α⟩|\alpha\rangle show that the 3 -qubit state ∣α⟩C∣β00⟩AB|\alpha\rangle_{C}\left|\beta_{00}\right\rangle_{A B} of system CABC A B can be expressed as

∣α⟩C∣β00⟩AB=12∑x,z=01∣βxz⟩CA∣μxz⟩B|\alpha\rangle_{C}\left|\beta_{00}\right\rangle_{A B}=\frac{1}{2} \sum_{x, z=0}^{1}\left|\beta_{x z}\right\rangle_{C A}\left|\mu_{x z}\right\rangle_{B}

where the 1 -qubit states ∣μxz⟩\left|\mu_{x z}\right\rangle are uniquely determined. Show that ∣μ10⟩=X∣α⟩\left|\mu_{10}\right\rangle=X|\alpha\rangle.

(b) In addition to ∣μ10⟩=X∣α⟩\left|\mu_{10}\right\rangle=X|\alpha\rangle you may now assume that ∣μxz⟩=XxZz∣α⟩\left|\mu_{x z}\right\rangle=X^{x} Z^{z}|\alpha\rangle. Alice and Bob are separated distantly in space and share a ∣β00⟩AB\left|\beta_{00}\right\rangle_{A B} state with AA and BB labelling qubits held by Alice and Bob respectively. Alice also has a qubit CC in state ∣α⟩|\alpha\rangle whose identity is unknown to her. Using the results of part (a) show how she can transfer the state of CC to Bob using only local operations and classical communication, i.e. the sending of quantum states across space is not allowed.

(c) Suppose that in part (b), while sharing the ∣β00⟩AB\left|\beta_{00}\right\rangle_{A B} state, Alice and Bob are also unable to engage in any classical communication, i.e. they are able only to perform local operations. Can Alice now, perhaps by a modified process, transfer the state of CC to Bob? Give a reason for your answer.

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