Suppose we have an ensemble {pX (x), ρx} of density operators and a POVMwith elements {Λx} that should identify the states ρx with high probability, i.e., we would likeTr {Λxρx} to be as high as possible. The expected success probability of the POVM is then∑xpX (x) Tr {Λxρx} . (4.99)Suppose that there exists some operator τ such thatτ ≥ pX (x)ρx , (4.100)where the condition τ ≥ pX (x)ρx is the same as τ − pX (x)ρx ≥ 0 (i.e., that the operatorτ − pX (x)ρx is a positive semi-definite operator). Show that Tr {τ } is an upper bound onthe expected success probability of the POVM. After doing so, consider the case of encodingn bits into a d-dimensional subspace. By choosing states uniformly at random (in the caseof the ensemble {2−n, ρi}i∈{0,1}n ), show that the expected success probability is bounded aboveby d 2−n. Thus, it is not possible to store more than n classical bits in n qubits and have aperfect success probability of retrieval.
Question
Suppose we have an ensemble {pX (x), ρx} of density operators and a POVMwith elements {Λx} that should identify the states ρx with high probability, i.e., we would likeTr {Λxρx} to be as high as possible. The expected success probability of the POVM is then∑xpX (x) Tr {Λxρx} . (4.99)Suppose that there exists some operator τ such thatτ ≥ pX (x)ρx , (4.100)where the condition τ ≥ pX (x)ρx is the same as τ − pX (x)ρx ≥ 0 (i.e., that the operatorτ − pX (x)ρx is a positive semi-definite operator). Show that Tr {τ } is an upper bound onthe expected success probability of the POVM. After doing so, consider the case of encodingn bits into a d-dimensional subspace. By choosing states uniformly at random (in the caseof the ensemble {2−n, ρi}i∈{0,1}n ), show that the expected success probability is bounded aboveby d 2−n. Thus, it is not possible to store more than n classical bits in n qubits and have aperfect success probability of retrieval.
Solution
This problem involves quantum mechanics and the theory of quantum information. Let's break it down step by step.
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We are given an ensemble {pX (x), ρx} of density operators and a POVM with elements {Λx}. The goal is to identify the states ρx with high probability, which means we want Tr {Λxρx} to be as high as possible.
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The expected success probability of the POVM is given by ∑xpX (x) Tr {Λxρx}.
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We are also given that there exists some operator τ such that τ ≥ pX (x)ρx. This condition is equivalent to τ − pX (x)ρx ≥ 0, meaning that the operator τ − pX (x)ρx is a positive semi-definite operator.
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We are asked to show that Tr {τ } is an upper bound on the expected success probability of the POVM. This can be shown as follows:
∑xpX (x) Tr {Λxρx} ≤ Tr {τ }
This is because Tr {Λxρx} ≤ Tr {τ } for all x, given that τ ≥ pX (x)ρx.
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Next, we consider the case of encoding n bits
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