The Carleman linearization algorithm the Zoo credits with T² scaling, together with its convergence theorem, its R < 1 condition, and the accompanying intractability bound for R ≥ √2; it also reports that the condition can be met in realistic epidemiological models.
arxiv.org/abs/2011.03185 ↗Quantum algorithms for nonlinear differential equations
Given a system of nonlinear ordinary differential equations, in the primary algorithm's case a dissipative quadratic n-dimensional system, produce the solution at a chosen evolution time T to error ε, encoded in the amplitudes of a quantum state.
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Given a system of nonlinear ordinary differential equations, in the primary algorithm's case a dissipative quadratic n-dimensional system, produce the solution at a chosen evolution time T to error ε, encoded in the amplitudes of a quantum state. Carleman linearization maps the nonlinear system to an infinite-dimensional system of linear differential equations, which is then truncated, discretized with the forward Euler method, and solved with the quantum linear system algorithm; the paper supplies a new convergence theorem for the Carleman linearization. Efficiency is conditional on the parameter R, which compares the nonlinearity and the forcing against the linear dissipation, staying below 1, and the same work proves a worst-case lower bound making general quadratic differential equations intractable once R ≥ √2. Exponential decay of the solution precludes efficiency, but driven equations can avoid that issue despite the presence of dissipation. The earlier route of Leyton and Osborne is structurally different: it combines a subroutine that applies a nonlinear transformation to the probability amplitudes of an unknown state with a quantum implementation of Euler's method, and is nondeterministic.
Circuit & simulation
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- Solve a nonlinear ODE dy/dt = F(y) Slot
Takes Access oracles for the components of (for example a linear part , a quadratic part , a forcing term ), a preparation unitary for , the evolution time , and an error tolerance . Returns A normalized state -close to , a history state over , or an estimate of an observable of the solution.
How it works
Carleman linearization maps the nonlinear system to an infinite-dimensional system of linear differential equations, which is then truncated, discretized with the forward Euler method, and solved with the quantum linear system algorithm; the paper supplies a new convergence theorem for the Carleman linearization. Efficiency is conditional on the parameter R, which compares the nonlinearity and the forcing against the linear dissipation, staying below 1, and the same work proves a worst-case lower bound making general quadratic differential equations intractable once R ≥ √2. Exponential decay of the solution precludes efficiency, but driven equations can avoid that issue despite the presence of dissipation. The earlier route of Leyton and Osborne is structurally different: it combines a subroutine that applies a nonlinear transformation to the probability amplitudes of an unknown state with a quantum implementation of Euler's method, and is nondeterministic. This record's speedup class, "Superpolynomial", is a secondary source's classification of the optimization, numerics, and machine learning it files this under — not a claim its primary paper makes. Not checked against the primary source yet. Reported cost: For dissipative quadratic n-dimensional ODEs with R < 1, complexity T² q poly(log T, log n, log 1/ε)/ε, where T is the evolution time, ε the allowed error and q measures decay of the solution; a worst-case lower bound in the same paper shows the general quadratic problem is intractable for R ≥ √2. The earlier Leyton and Osborne algorithm has expected resource requirements polylogarithmic in the number of variables but exponential in the integration time, against a best classical algorithm scaling linearly in the number of variables..
Implementation
ALGORITHM: Quantum algorithms for nonlinear differential equations
PROBLEM: Given a system of nonlinear ordinary differential equations, in the primary algorithm's case a dissipative quadratic n-dimensional system, produce the solution at a chosen evolution time T to error ε, encoded in the amplitudes of a quantum state.
IDEA: Carleman linearization maps the nonlinear system to an infinite-dimensional system of linear differential equations, which is then truncated, discretized with the forward Euler method, and solved with the quantum linear system algorithm; the paper supplies a new convergence theorem for the Carleman linearization. Efficiency is conditional on the parameter R, which compares the nonlinearity and the forcing against the linear dissipation, staying below 1, and the same work proves a worst-case lower bound making general quadratic differential equations intractable once R ≥ √2. Exponential decay of the solution precludes efficiency, but driven equations can avoid that issue despite the presence of dissipation. The earlier route of Leyton and Osborne is structurally different: it combines a subroutine that applies a nonlinear transformation to the probability amplitudes of an unknown state with a quantum implementation of Euler's method, and is nondeterministic.
REPORTED COST: For dissipative quadratic n-dimensional ODEs with R < 1, complexity T² q poly(log T, log n, log 1/ε)/ε, where T is the evolution time, ε the allowed error and q measures decay of the solution; a worst-case lower bound in the same paper shows the general quadratic problem is intractable for R ≥ √2. The earlier Leyton and Osborne algorithm has expected resource requirements polylogarithmic in the number of variables but exponential in the integration time, against a best classical algorithm scaling linearly in the number of variables.
BASIS: abstract of arXiv:2011.03185: "Assuming R < 1, where R is a parameter characterizing the ratio of the nonlinearity and forcing to the linear dissipation, this algorithm has complexity T^2 q poly(log T, log n, log 1/epsilon)/epsilon, where T is the evolution time, epsilon is the allowed error, and q measures decay of the solution", and "a lower bound on the worst-case complexity of quantum algorithms for general quadratic differential equations, showing that the problem is intractable for R >= sqrt(2)"; second bound from the abstract of arXiv:0812.4423: "its expected resource requirements are polylogarithmic in the number of variables and exponential in the integration time. The best classical algorithm runs in a time scaling linearly with the number of variables."
PRIMARY SOURCE: Jin-Peng Liu, Herman Øie Kolden, Hari K. Krovi, Nuno F. Loureiro, Konstantina Trivisa, Andrew M. Childs (2020), Efficient quantum algorithm for dissipative nonlinear differential equations — https://arxiv.org/abs/2011.03185
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Quantum vs classical
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Literature & references
The earlier approach the Zoo describes as exponential in t: a nondeterministic algorithm for sparse nonlinear systems with polynomial nonlinear terms, built from a nonlinear transformation of the amplitudes of an unknown state and a quantum implementation of Euler's method.
arxiv.org/abs/0812.4423 ↗