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An Upper Bound on Computation for the Anharmonic Oscillator

Reza Pirmoradian, Mohammad Reza Tanhayi

Published Mar 4, 2021• 0 citations
OpenAlex

Abstract

For a quantum system with energy E, there is a limitation in quantum computation which is identified by the minimum time needed for the state to evolve to an orthogonal state. In this paper, we will compute the minimum time of orthogonalization (i.e. quantum speed limit) for a simple anharmonic oscillator and find an upper bound on the rate of computations. We will also investigate the growth rate of complexity for the anharmonic oscillator by treating the anharmonic terms perturbatively. More precisely, we will compute the maximum rate of change of complexity and show that for even order perturbations, the rate of complexity increases while for the odd order terms it has a decreasing behavior.

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For a quantum system with energy E, there is a limitation in quantum computation which is identified by the minimum time needed for the state to evolve to an orthogonal state. In this paper, we will compute the minimum time of orthogonalization (i.e. quantum speed limit) for a simple anharmonic oscillator and find an upper bound on the rate of computations. We will also investigate the growth rate of complexity for the anharmonic oscillator by treating the anharmonic terms perturbatively. More precisely, we will compute the maximum rate of change of complexity and show that for even order perturbations, the rate of complexity increases while for the odd order terms it has a decreasing behavior.
The laws of quantum physics place a limit on the speed of computation, in particular the evolution time of a system cannot be arbitrarily fast. Bounds on the speed of evolution for unitary dynamics have been long studied. A few bounds on the speed of evolution for noisy dynamics have also been obtained recently, these bounds, however, are in general not tight. In this article we present a new framework for quantum speed limit of noisy dynamics. With this framework we obtain the exact maximal angle that a noisy dynamics can achieve at any given time, this then provides tight bounds on the evolution time for noisy dynamics. The obtained bound reveals that noisy dynamics are generically different from unitary dynamics, in particular we show that the 'orthogonalization' time, which is the minimum time needed to evolve any state to its orthogonal states, is in general not applicable to noisy dynamics.
In this article the anharmonic oscillator in quantum mechanics is discussed. First propagators of less complex systems are found by using the path integral method. With perturbation theory an expression, up to any order, has been found for the vacuum propagator of the anharmonic oscillator. In this article only the first and second order were calculated and by looking at the n-pointfunctions the Feynman diagrams of these orders were found. It appears that the anharmonic term up to first order causes a change in the frequency of the system. Looking at second order perturbation theory gives five diagrams. The first four of these diagrams are simple diagrams and again these diagrams alter the frequency of the system. The change in the frequency looks a lot like a series and it is possible to ’guess’ a general expression up to any order for all simple diagrams. This expression seems to fit qualitatively for higher orders however the prefactors were not checked nor is there a proof formulated. The fifth diagrams is a complex diagram and this diagram has not been solved in a satisfying way. Finally some recommendations for additional research are given.
One version of the energy-time uncertainty principle states that the minimum time ${T}_{\ensuremath{\perp}}$ for a quantum system to evolve from a given state to any orthogonal state is $h/(4\mathrm{\ensuremath{\Delta}}E)$, where $\mathrm{\ensuremath{\Delta}}E$ is the energy uncertainty. A related bound called the Margolus-Levitin theorem states that ${T}_{\ensuremath{\perp}}\ensuremath{\ge}h/(2\ensuremath{\langle}E\ensuremath{\rangle})$, where $\ensuremath{\langle}E\ensuremath{\rangle}$ is the expectation value of energy and the ground energy is taken to be zero. Many subsequent works have interpreted ${T}_{\ensuremath{\perp}}$ as defining a minimal time for an elementary computational operation and correspondingly a fundamental limit on clock speed determined by a system's energy. Here we present local time-independent Hamiltonians in which computational clock speed becomes arbitrarily large relative to $\ensuremath{\langle}E\ensuremath{\rangle}$ and $\mathrm{\ensuremath{\Delta}}E$ as the number of computational steps goes to infinity. We argue that energy considerations alone are not sufficient to obtain an upper bound on computational speed, and that additional physical assumptions such as limits to information density and information transmission speed are necessary to obtain such a bound.
A bstract We show that quantum circuit complexity for the unitary time evolution operator of any time-independent Hamiltonian is bounded by linear growth at early times, independent of any choices of the fundamental gates or cost metric. Deviations from linear early-time growth arise from the commutation algebra of the gates and are manifestly negative for any circuit, decreasing the linear growth rate and leading to a bound on the growth rate of complexity of a circuit at early times. We illustrate this general result by applying it to qubit and harmonic oscillator systems, including the coupled and anharmonic oscillator. By discretizing free and interacting scalar field theories on a lattice, we are also able to extract the early-time behavior and dependence on the lattice spacing of complexity of these field theories in the continuum limit, demonstrating how this approach applies to systems that have been previously difficult to study using existing techniques for quantum circuit complexity.
The traditional quantum speed limits are not attainable for many physical processes, as they tend to be loose and fail to determine the exact time taken by quantum systems to evolve. To address this we derive exact quantum speed limits for the unitary dynamics of pure-state quantum systems that outperform the existing quantum speed limits. Using these exact quantum speed limits, we can precisely estimate the evolution time for two- and higher-dimensional quantum systems. Additionally, for both finite- and infinite-dimensional quantum systems, we derive an improved Mandelstam-Tamm bound for pure states and show that this bound always saturates for any unitary generated by self-inverse Hamiltonians. Furthermore, we show that our speed limits establish an upper bound on the quantum computational circuit complexity and yield a tighter quantum Cramér-Rao bound. These results will have a significant impact on our understanding of quantum physics as well as rapidly developing quantum technologies, such as quantum computing, quantum control, and quantum batteries.
The time needed by a quantum system to reach a state fully distinguishable from the original one provides a natural way of determining how fast the corresponding dynamical evolution is. This orthogonality time admits a lower bound, expressible in terms of the energy's expectation value and the energy's standard deviation, that yields a 'quantum speed limit'. Composite quantum systems need entanglement in order to achieve this limit. So far, most studies on the connection between entanglement and the quantum speed limit have focused on the case of non-interacting systems. The connection between quantum speed and entanglement is systematically investigated here for the case of a system of two interacting qubits, taking into consideration all possible initial states that evolve into an orthogonal one.