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QPE output law and conditional state for a spectral input
Statement
Let be a finite-dimensional complex inner-product space, let be Hermitian with distinct eigenvalues and spectral projections , and let be a unit vector. Run the ideal phase-estimation sampler on , with unitary whose declared phase class on is . Write for its unique representative, so for . Write for the phase-register amplitude at output when the input is in that eigenspace, so as in Phase-estimation finite outcome law. Then the output law is For every outcome with positive probability, the normalized conditional system vector is Moreover, is a probability distribution and, for every , The conditional system is generally a coherent sum across eigenspaces, not an exact eigenspace projection.
Facts & Assumptions
Given: A finite-dimensional Hermitian , its spectral resolution, a unit vector , the ideal sampler, and its phase encoding on every eigenspace.
A normal endomorphism is a sum of its eigenvalues times pairwise orthogonal projections, and each spectral projection is a polynomial in the endomorphism: , for distinct eigenvalues, and the projections are self-adjoint.
Quantum phase-estimation spectral sampler and Phase-estimation finite outcome law: on an eigenvector of phase representative , the phase register has amplitudes with squared magnitudes .
Finite projective measurements and Projective measurement outcome weights are probabilities give the computational-basis outcome probability as the squared norm of the projected vector. The readout in Quantum phase estimation specifies the Lüders branch ; on a pure input, its positive-probability branch is therefore the projected vector normalized.
Dirac Notation Conversion: for an orthogonal projection.
Proof
The orthogonality in [F1] gives and . Thus the weights in [F2] are nonnegative and sum to one. By [F5]–[F6], .
By linearity, the QPE unitary sends the initial phase state and to , using [F3] on each eigenspace. Projecting the phase register onto gives the system vector . Since the summands for distinct are orthogonal by [F1], [F4] yields . Summing these probabilities over the finite output set gives one, as each eigenphase kernel is a measurement law.
When , [F4] normalizes the projected branch to , which is the displayed formula. Its coefficients need not vanish outside one eigenvalue, so no exact eigenspace-collapse claim follows; the formula itself gives the precise conditional state.
Depends on
- State-dependent spectral distribution
- Quantum phase-estimation spectral sampler
- Quantum phase estimation
- Phase-estimation finite outcome law
- Finite projective measurements
- Projective measurement outcome weights are probabilities
- Dirac Notation Conversion
- The spectral functional calculus f(T) for a normal endomorphism
- A normal endomorphism is a sum of its eigenvalues times pairwise orthogonal projections, and each spectral projection is a polynomial in the endomorphism
- Complex spectral theorem: a normal endomorphism of a finite-dimensional complex inner product space has an orthonormal eigenbasis, and conversely
Used by
- Spectral distribution and phase resolution for a two-level matrix Example
- Coherent spectral minimum and a frozen Gibbs energy shift Lemma
- Additive spectral-estimation accuracy and cost Theorem
Cited to discharge well-definedness by State-dependent spectral distribution.
Dependency tree · two levels
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Sources
- Andrew M. Childs, Lecture Notes on Quantum Algorithms, §4.3 (standard reference, not scraped)
- Ronald de Wolf, Quantum Computing: Lecture Notes, version 6, §4.6 (standard reference, not scraped)