Heisenberg-limited quantum phase estimation of multiple eigenvalues with few control qubits

Alicja Dutkiewicz; Barbara M. Terhal; Thomas E. O'Brien

doi:10.22331/q-2022-10-06-830

Heisenberg-limited quantum phase estimation of multiple eigenvalues with few control qubits

Alicja Dutkiewicz¹, Barbara M. Terhal², and Thomas E. O'Brien^1,3

¹Instituut-Lorentz, Universiteit Leiden, 2300 RA Leiden, The Netherlands
²QuTech, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands and JARA Institute for Quantum Information, Forschungszentrum Juelich, D-52425 Juelich, Germany
³Google Quantum AI, 80636 Munich, Germany

Published:	2022-10-06, volume 6, page 830
Eprint:	arXiv:2107.04605v3
Doi:	https://doi.org/10.22331/q-2022-10-06-830
Citation:	Quantum 6, 830 (2022).

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Abstract

Quantum phase estimation is a cornerstone in quantum algorithm design, allowing for the inference of eigenvalues of exponentially-large sparse matrices.The maximum rate at which these eigenvalues may be learned, –known as the Heisenberg limit–, is constrained by bounds on the circuit complexity required to simulate an arbitrary Hamiltonian. Single-control qubit variants of quantum phase estimation that do not require coherence between experiments have garnered interest in recent years due to lower circuit depth and minimal qubit overhead. In this work we show that these methods can achieve the Heisenberg limit, $also$ when one is unable to prepare eigenstates of the system. Given a quantum subroutine which provides samples of a `phase function' $g(k)=\sum_j A_j e^{i \phi_j k}$ with unknown eigenphases $\phi_j$ and overlaps $A_j$ at quantum cost $O(k)$, we show how to estimate the phases $\{\phi_j\}$ with (root-mean-square) error $\delta$ for total quantum cost $T=O(\delta^{-1})$. Our scheme combines the idea of Heisenberg-limited multi-order quantum phase estimation for a single eigenvalue phase [Higgins et al (2009) and Kimmel et al (2015)] with subroutines with so-called dense quantum phase estimation which uses classical processing via time-series analysis for the QEEP problem [Somma (2019)] or the matrix pencil method. For our algorithm which adaptively fixes the choice for $k$ in $g(k)$ we prove Heisenberg-limited scaling when we use the time-series/QEEP subroutine. We present numerical evidence that using the matrix pencil technique the algorithm can achieve Heisenberg-limited scaling as well.

Featured image: A Heisenberg-limited scheme for detecting multiple phases $\phi_j$. First, an initial estimate of $\phi_j\in[0,2\pi]$ is made from a time series $\{\langle U\rangle, \langle U^2\rangle \ldots\}$. Then, an estimate is made of $k\phi_j$ for some $k>1$, using $\{\langle U^{k}\rangle, \langle U^{2k}\rangle,\ldots \}$. This generates a more-accurate estimate of $\phi_j$ (smaller error bars), but modulo $2\pi/k$. Data from the first estimation is used to stabilize this estimate and remove unwanted aliases (dashed lines). This is repeated for ever-larger values of $k$ to achieve the Heisenberg limit. The multiplier $k$ is chosen based on the previous estimates to enable unambiguous estimation.

Popular summary

A common task for a quantum computer is the estimation of the eigenphases of a unitary operator U, so-called quantum phase estimation or QPE. One may reduce the quantum overhead for QPE by turning it into a problem of classically processing the expectation values of $U^k$ as a time-series in $k$. However, it was not clear whether such a method could achieve known bounds on the cost of QPE — the so-called Heisenberg limit — when estimating multiple eigenphases. This work gives an algorithm with provable performance bounds that achieve the Heisenberg limit.

► BibTeX data

@article{Dutkiewicz2022heisenberglimited,
  doi = {10.22331/q-2022-10-06-830},
  url = {https://doi.org/10.22331/q-2022-10-06-830},
  title = {Heisenberg-limited quantum phase estimation of multiple eigenvalues with few control qubits},
  author = {Dutkiewicz, Alicja and Terhal, Barbara M. and O'Brien, Thomas E.},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {6},
  pages = {830},
  month = oct,
  year = {2022}
}

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