Quantum Algorithms Database
Quantum Phase Estimation (QPE)
Estimates the eigenvalue (phase) associated with a quantum operation, a critical building block for many other quantum algorithms.
Year
1995
Inventor(s)
Alexei Kitaev
Speedup Type
Exponential Speedup
Difficulty
★★★★☆
The Problem
Determining the precise phase associated with an eigenstate of a unitary quantum operation.
How It Works
Combines controlled application of the unitary operation with the inverse Quantum Fourier Transform to extract the phase to high precision.
Real-World Impact
Used inside Shor's Algorithm, quantum chemistry simulations, and the HHL algorithm for solving linear systems — a true workhorse subroutine.