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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

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Solution

This problem involves quantum mechanics and the theory of quantum information. Let's break it down step by step.

  1. 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.

  2. The expected success probability of the POVM is given by ∑xpX (x) Tr {Λxρx}.

  3. We are given that there exists an 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.

  4. We need to show that Tr {τ } is an upper bound on the expected success probability of the POVM. This can be done by using the properties of the trace and the positive semi-definiteness of the operator τ - pX (x)ρx.

  5. The trace of a positive semi-definite operator is non-negative, and the trace of a sum of operators is the sum of the traces of the operators. Therefore, Tr {τ } - ∑xpX (x) Tr {ρx} ≥ 0, which implies that Tr {τ } ≥ ∑xpX (x) Tr {ρx}. This shows that Tr {τ } is an upper bound on the expected success probability of the POVM.

  6. Now, consider the case of encoding n bits into a d-dimensional subspace. If we choose states uniformly at random (in the case of the ensemble {2−n, ρi}i∈{0,1}n ), the expected success probability is bounded above by d 2−n.

  7. This result implies that it is not possible to store more than n classical bits in n qubits and have a perfect success probability of retrieval. This is because the expected success probability decreases exponentially with the number of bits, while the dimension of the subspace increases linearly. Therefore, the expected success probability cannot exceed 1, which is the maximum possible success probability.

This problem has been solved

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