Embedding classical dynamics in a quantum computer
We develop a framework for simulating measure-preserving, ergodic dynamical
systems on a quantum computer. Our approach provides a new operator-theoretic
representation of classical dynamics by combining ergodic theory with quantum
information science. The resulting quantum embedding of classical dynamics
(QECD) enables efficient simulation of spaces of classical observables with
exponentially large dimension using a quadratic number of quantum gates. The
QECD framework is based on a quantum feature map for representing classical
states by density operators on a reproducing kernel Hilbert space, $\mathcal H
$, and an embedding of classical observables into self-adjoint operators on
$\mathcal H$. In this scheme, quantum states and observables evolve unitarily
under the lifted action of Koopman evolution operators of the classical system.
Moreover, by virtue of the reproducing property of $\mathcal H$, the quantum
system is pointwise-consistent with the underlying classical dynamics. To
achieve an exponential quantum computational advantage, we project the state of
the quantum system to a density matrix on a $2^n$-dimensional tensor product
Hilbert space associated with $n$ qubits. By employing discrete Fourier-Walsh
transforms, the evolution operator of the finite-dimensional quantum system is
factorized into tensor product form, enabling implementation through a quantum
circuit of size $O(n)$. Furthermore, the circuit features a state preparation
stage, also of size $O(n)$, and a quantum Fourier transform stage of size
$O(n^2)$, which makes predictions of observables possible by measurement in the
standard computational basis. We prove theoretical convergence results for
these predictions as $n\to\infty$. We present simulated quantum circuit
experiments in Qiskit Aer, as well as actual experiments on the IBM Quantum
System One.
Dimitrios Giannakis Abbas Ourmazd Philipp Pfeffer Joerg Schumacher Joanna Slawinska