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arXiv:2009.05580 · Physical Review Letters

Autoregressive Neural Network for Simulating Open Quantum Systems via a Probabilistic Formulation

Di Luo, Zhuo Chen, Juan Carrasquilla, Bryan K. Clark

Published Feb 28, 2022• 77 citations
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Abstract

The theory of open quantum systems lays the foundation for a substantial part of modern research in quantum science and engineering. Rooted in the dimensionality of their extended Hilbert spaces, the high computational complexity of simulating open quantum systems calls for the development of strategies to approximate their dynamics. In this Letter, we present an approach for tackling open quantum system dynamics. Using an exact probabilistic formulation of quantum physics based on positive operator-valued measure, we compactly represent quantum states with autoregressive neural networks; such networks bring significant algorithmic flexibility due to efficient exact sampling and tractable density. We further introduce the concept of string states to partially restore the symmetry of the autoregressive neural network and improve the description of local correlations. Efficient algorithms have been developed to simulate the dynamics of the Liouvillian superoperator using a forward-backward trapezoid method and find the steady state via a variational formulation. Our approach is benchmarked on prototypical one-dimensional and two-dimensional systems, finding results which closely track the exact solution and achieve higher accuracy than alternative approaches based on using Markov chain Monte Carlo method to sample restricted Boltzmann machines. Our Letter provides general methods for understanding quantum dynamics in various contexts, as well as techniques for solving high-dimensional probabilistic differential equations in classical setups.

DOI
10.1103/physrevlett.128.090501

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The theory of open quantum systems lays the foundation for a substantial part of modern research in quantum science and engineering. Rooted in the dimensionality of their extended Hilbert spaces, the high computational complexity of simulating open quantum systems calls for the development of strategies to approximate their dynamics. In this Letter, we present an approach for tackling open quantum system dynamics. Using an exact probabilistic formulation of quantum physics based on positive operator-valued measure, we compactly represent quantum states with autoregressive neural networks; such networks bring significant algorithmic flexibility due to efficient exact sampling and tractable density. We further introduce the concept of string states to partially restore the symmetry of the autoregressive neural network and improve the description of local correlations. Efficient algorithms have been developed to simulate the dynamics of the Liouvillian superoperator using a forward-backward trapezoid method and find the steady state via a variational formulation. Our approach is benchmarked on prototypical one-dimensional and two-dimensional systems, finding results which closely track the exact solution and achieve higher accuracy than alternative approaches based on using Markov chain Monte Carlo method to sample restricted Boltzmann machines. Our Letter provides general methods for understanding quantum dynamics in various contexts, as well as techniques for solving high-dimensional probabilistic differential equations in classical setups.
The theory of open quantum systems lays the foundations of a substantial part of modern research in quantum science and engineering. Rooted in the dimensionality of their extended Hilbert spaces, the high computational complexity of simulating open quantum systems calls for the development of strategies to approximate their dynamics. In this paper, we present an approach for tackling open quantum system dynamics. We simulate the dynamics of the Liouvillian superoperator using a forward-backward trapezoid method and find the steady-state via a variational formulation. We make use of a probabilistic formulation of quantum physics based on a positive operator-valued measure (POVM) in combination with neural autoregressive models, which bring significant algorithmic flexibility due to their efficient sampling and tractable density. We introduce improved ansatzs, String States, which partially restore the symmetry of the neural auto-regressive model and improve the description of local correlations. We benchmark our approaches on prototypical one and two-dimensional systems, finding results which closely track the exact solution and achieve higher accuracy in comparison to a recently proposed method based on restricted Boltzmann machines. We anticipate this approach will be widely applicable to evolving density matrices in various contexts.
The theory of quantum dynamics is crucial for quantum science and engineering. The computation cost for exact quantum simulation is expensive due to the exponential growth of the dimension of the Hilbert space. There are recent attempts that utilize neural networks to simulate long-time quantum dynamics. We conduct a comparative study on different approaches that simulate dynamics based on parameterizing the quantum states with state-of-the-art autoregressive neural networks. Pixel Convolution Neural Networks, Recurrent Neural Networks, and Transformer models are examined for their representability and accuracy. We identify that Recurrent Neural Networks and Transformers are superior to Pixel Convolution Neural Networks. In addition, we compare the performance of different algorithms to simulate quantum dynamics, which include the Euler’s method, the forward-backward trapezoid method, the stochastic reconfiguration method, and the spacetime method. We found that the spacetime method performs better than the other algorithms, the Euler’s method and the forward-backward method are comparable, while the SR method exhibits stability issues. Our study is performed on the transverse-field Ising model on one-dimensional systems
Artificial neural networks were recently shown to be an efficient representation of highly entangled many-body quantum states. In practical applications, neural-network states inherit numerical schemes used in variational Monte Carlo method, most notably the use of Markov-chain Monte Carlo (MCMC) sampling to estimate quantum expectations. The local stochastic sampling in MCMC caps the potential advantages of neural networks in two ways: (i) Its intrinsic computational cost sets stringent practical limits on the width and depth of the networks, and therefore limits their expressive capacity; (ii) its difficulty in generating precise and uncorrelated samples can result in estimations of observables that are very far from their true value. Inspired by the state-of-the-art generative models used in machine learning, we propose a specialized neural-network architecture that supports efficient and exact sampling, completely circumventing the need for Markov-chain sampling. We demonstrate our approach for two-dimensional interacting spin models, showcasing the ability to obtain accurate results on larger system sizes than those currently accessible to neural-network quantum states.
In experimentally realistic situations, quantum systems are never perfectly isolated and the coupling to their environment needs to be taken into account. Often, the effect of the environment can be well approximated by a Markovian master equation. However, solving this master equation for quantum many-body systems becomes exceedingly hard due to the high dimension of the Hilbert space. Here we present an approach to the effective simulation of the dynamics of open quantum many-body systems based on machine-learning techniques. We represent the mixed many-body quantum states with neural networks in the form of restricted Boltzmann machines and derive a variational Monte Carlo algorithm for their time evolution and stationary states. We document the accuracy of the approach with numerical examples for a dissipative spin lattice system.
We propose a neural-network variational quantum algorithm to simulate the time evolution of quantum many-body systems.Based on a modified restricted Boltzmann machine (RBM) wave function ansatz, the proposed algorithm can be efficiently implemented in near-term quantum computers with low measurement cost.Using a qubit recycling strategy, only one ancilla qubit is required to represent all the hidden spins in an RBM architecture.The variational algorithm is extended to open quantum systems by employing a stochastic Schrödinger equation approach.Numerical simulations of spin-lattice models demonstrate that our algorithm is capable of capturing the dynamics of closed and open quantum many-body systems with high accuracy without suffering from the vanishing gradient (or "barren plateau") issue for the considered system sizes.
The simulation of quantum circuits presents a significant bottleneck in the development and understanding of quantum algorithms. Traditional methods relying on the full wave function description quickly become computationally intractable as the number of qubits and circuit depth increases. This work proposes a novel approach utilizing neural networks to approximate the probabilities of measurement outcomes within a quantum circuit. The core idea is to train a neural network to learn the complex dynamics of quantum evolution, thereby enabling efficient simulation with a significantly reduced parameter space. We introduce a framework that maps the quantum circuit to a neural network architecture, allowing for approximate but tractable simulation. The effectiveness of this approach is demonstrated through theoretical analysis and a conceptual implementation strategy, highlighting its potential to unlock the exploration of complex quantum algorithms and accelerate the development of quantum computing. The primary objective is to provide a computationally feasible alternative to traditional simulation techniques, bridging the gap between theoretical quantum algorithm design and practical implementation. The key innovation lies in leveraging the representational power of neural networks to capture the essential correlations inherent in quantum systems.