量子傅里叶变换:Quantum Phase Estimation

Posted 安徽思远

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Overview

如果仅仅想使用QPE,那我们只需要知道QPE干了什么就好。

通过Inverse Fourier Transform,我们可以得到:
1 2 t / 2 ∑ j = 0 2 t − 1 e 2 π i φ j ∣ j ⟩ ∣ u ⟩ → ∣ φ ~ ⟩ ∣ u ⟩ \\frac12^t / 2 \\sum_j=0^2^t-1 e^2 \\pi i \\varphi j|j\\rangle|u\\rangle \\rightarrow|\\tilde\\varphi\\rangle|u\\rangle 2t/21j=02t1e2πiφjjuφ~u

Derivation

As mentioned above, this circuit estimates the phase of a unitary operator U. It estimates $\\theta $in U ∣ ψ ⟩ = e 2 π i θ ∣ ψ ⟩ U\\vert\\psi \\rangle =e^\\boldsymbol2\\pi i \\theta |\\psi \\rangle Uψ=e2πiθψ, where ∣ ψ ⟩ |\\psi\\rangle ψ is an eigenvector and e 2 π i θ e^\\boldsymbol2\\pi i\\theta e2πiθ is the corresponding eigenvalue.

矩阵特征值是对特征向量进行伸缩和旋转程度的度量。

“特征”的含义就是“不变”。

i. Setup: ∣ ψ ⟩ \\vert\\psi\\rangle ψ is in one set of qubit registers. An additional set of n qubits form the counting register on which we will store the value 2 n θ 2^n\\theta 2nθ:

ψ 0 = ∣ 0 ⟩ ⊗ n ∣ ψ ⟩ \\psi_0 = \\lvert 0 \\rangle^\\otimes n \\lvert \\psi \\rangle ψ0=0nψ

ii. Superposition: Apply a n-bit Hadamard gate operation H ⊗ n H^\\otimes n Hn on the counting register:

ψ 1 = 1 2 n 2 ( ∣ 0 ⟩ + ∣ 1 ⟩ ) ⊗ n ∣ ψ ⟩ \\psi_1 = \\frac 12^\\frac n2\\left(|0\\rangle +|1\\rangle \\right)^\\otimes n \\lvert \\psi \\rangle ψ1=22n1(0+1)nψ

iii. Controlled Unitary Operations: We need to introduce the controlled unitary C-U that applies the unitary operator U on the target register only if its corresponding control bit is ∣ 1 ⟩ |1\\rangle 1. Since U is a unitary operator with eigenvector |\\psi\\rangle such that U ∣ ψ ⟩ = e 2 π i θ ∣ ψ ⟩ U|\\psi \\rangle =e^\\boldsymbol2\\pi i \\theta |\\psi \\rangle Uψ=e2πiθψ, this means:

U 2 j ∣ ψ ⟩ = U 2 j − 1 U ∣ ψ ⟩ = U 2 j − 1 e 2 π i θ ∣ ψ ⟩ = ⋯ = e 2 π i 2 j θ ∣ ψ ⟩ U^2^j|\\psi \\rangle =U^2^j-1U|\\psi \\rangle =U^2^j-1e^2\\pi i\\theta |\\psi \\rangle =\\cdots =e^2\\pi i2^j\\theta |\\psi \\rangle U2jψ=U2j1Uψ=U2j1e2πiθψ==e2πi2jθψ
Applying all the n controlled operations C − U 2 j U^2^j U2j with 0 ≤ j ≤ n − 1 0\\leq j\\leq n-1 0jn1, and using the relation ∣ 0 ⟩ ⊗ ∣ ψ ⟩ + ∣ 1 ⟩ ⊗ e 2 π i θ ∣ ψ ⟩ = ( ∣ 0 ⟩ + e 2 π i θ ∣ 1 ⟩ ) ⊗ ∣ ψ ⟩ |0\\rangle \\otimes |\\psi \\rangle +|1\\rangle \\otimes e^2\\pi i\\theta |\\psi \\rangle =\\left(|0\\rangle +e^2\\pi i\\theta |1\\rangle \\right)\\otimes |\\psi \\rangle 0ψ+1e2πiθψ=(0+e2πiθ1)ψ:

ψ 2 = 1 2 n 2 ( ∣ 0 ⟩ + e 2 π i θ 2 n − 1 ∣ 1 ⟩ ) ⊗ ⋯ ⊗ ( ∣ 0 ⟩ + e 2 π i θ 2 1 ∣ 1 ⟩ ) ⊗ ( ∣ 0 ⟩ + e 2 π i θ 2 0 ∣ 1 ⟩ ) ⊗ ∣ ψ ⟩ = 1 2 n 2 ∑ k = 0 2 n − 1 e 2 π i θ k ∣ k ⟩ ⊗ ∣ ψ ⟩ \\beginaligned \\psi_2 & =\\frac 12^\\frac n2 \\left(|0\\rangle+e^\\boldsymbol2\\pi i \\theta 2^n-1|1\\rangle \\right) \\otimes \\cdots \\otimes \\left(|0\\rangle+e^\\boldsymbol2\\pi i \\theta 2^1\\vert1\\rangle \\right) \\otimes \\left(|0\\rangle+e^\\boldsymbol2\\pi i \\theta 2^0\\vert1\\rangle \\right) \\otimes |\\psi\\rangle\\\\\\\\ & = \\frac12^\\frac n2\\sum _k=0^2^n-1e^\\boldsymbol2\\pi i \\theta k|k\\rangle \\otimes \\vert\\psi\\rangle \\endaligned ψ2=22n1(0+e2πiθ2n11)(0+e2πiθ211)(0+e2πiθ201)ψ=22n1k=02n1e2πiθkkψ

where k denotes the integer representation of n-bit binary numbers.

iv. Inverse Fourier Transform: Notice that the above expression is exactly the result of applying a quantum Fourier transform as we derived in the notebook on Quantum Fourier Transform and its Qiskit Implementation. Recall that QFT maps an n-qubit input state ∣ x ⟩ \\vert x\\rangle x into an output as

Q F T ∣ x ⟩ = 1

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