We implement a quantum optimal control algorithm based on automatic differentiation and harness the acceleration afforded by graphics processing units (GPUs). Automatic differentiation allows us to specify advanced optimization criteria and incorporate them in the optimization process with ease. We show that the use of GPUs can speedup calculations by more than an order of magnitude. Our strategy facilitates efficient numerical simulations on affordable desktop computers and exploration of a host of optimization constraints and system parameters relevant to real-life experiments. We demonstrate optimization of quantum evolution based on fine-grained evaluation of performance at each intermediate time step, thus enabling more intricate control on the evolution path, suppression of departures from the truncated model subspace, as well as minimization of the physical time needed to perform high-fidelity state preparation and unitary gates.
We implement a quantum optimal control algorithm based on automatic differentiation and harness the acceleration afforded by graphics processing units (GPUs). Automatic differentiation allows us to specify advanced optimization criteria and incorporate them in the optimization process with ease. We show that the use of GPUs can speedup calculations by more than an order of magnitude. Our strategy facilitates efficient numerical simulations on affordable desktop computers and exploration of a host of optimization constraints and system parameters relevant to real-life experiments. We demonstrate optimization of quantum evolution based on fine-grained evaluation of performance at each intermediate time step, thus enabling more intricate control on the evolution path, suppression of departures from the truncated model subspace, as well as minimization of the physical time needed to perform high-fidelity state preparation and unitary gates.
We present a gradient-based optimal-control technique for open quantum systems that utilizes quantum trajectories to simulate the quantum dynamics during optimization. Using trajectories allows for optimizing open systems with less computational cost than the regular density matrix approaches in most realistic optimization problems. We introduce an improved-sampling algorithm which minimizes the number of trajectories needed per optimization iteration. Together with employing stochastic gradient descent techniques, this reduces the complexity of optimizing many realistic open quantum systems to the complexity encountered with closed systems. Our optimizer harnesses automatic differentiation to provide flexibility in optimization and to suit the different constraints and diverse parameter regimes of real-life experiments. We utilize the optimizer in a variety of applications to demonstrate how the use of quantum trajectories significantly reduces the computation complexity while achieving a multitude of simultaneous optimization targets. Demonstrated targets include high state-transfer fidelities despite dissipation, faster gate times, and maximization of qubit-readout fidelity while maintaining the quantum nondemolition nature of the measurement and allowing for subsequent fast resonator reset.
This thesis describes our work on optimizing quantum control and scaling up quantum computers. We demonstrate the application of automatic differentiation in quantum optimal control, which allows us to specify advanced optimization criteria and incorporate them in the optimization process of quantum control with ease. These criteria enable more intricate control on the evolution path, suppression of departures from the truncated model subspace, as well as minimization of the physical time needed to perform high-fidelity state preparation and unitary gates. We subsequently present our experimental efforts in efficiently scaling up a quantum computer. The innovation of quantum random access memory (qRAM) architecture with multimode circuit QED establishes a promising path towards expanding the information processing power of a quantum device, without introducing control-resources overhead. Using a single Josephson junction transmon circuit serving as the central processor, we demonstrate universal operations on a nine-qubit random access memory. We further present our experimental results in establishing bidirectional and deterministic photonic communication between two remote superconducting multimode processors, connected through a one-meter long coaxial cable. This device demonstrates a prototype of setting up a scalable and distributed quantum computing cluster that could solve a harder computational problem.
We develop a framework of "semi-automatic differentiation" that combines existing gradient-based methods of quantum optimal control with automatic differentiation. The approach allows to optimize practically any computable functional and is implemented in two open source Julia packages, GRAPE.jl and Krotov.jl , part of the QuantumControl.jl framework. Our method is based on formally rewriting the optimization functional in terms of propagated states, overlaps with target states, or quantum gates. An analytical application of the chain rule then allows to separate the time propagation and the evaluation of the functional when calculating the gradient. The former can be evaluated with great efficiency via a modified GRAPE scheme. The latter is evaluated with automatic differentiation, but with a profoundly reduced complexity compared to the time propagation. Thus, our approach eliminates the prohibitive memory and runtime overhead normally associated with automatic differentiation and facilitates further advancement in quantum control by enabling the direct optimization of non-analytic functionals for quantum information and quantum metrology, especially in open quantum systems. We illustrate and benchmark the use of semi-automatic differentiation for the optimization of perfectly entangling quantum gates on superconducting qubits coupled via a shared transmission line. This includes the first direct optimization of the non-analytic gate concurrence.
This paper presents a novel approach to simulating quantum circuits with differentiability, enabling gradient-based optimization for quantum algorithms. The core of the method lies in combining the accuracy of quantum circuit Monte Carlo simulation with the efficiency of automatic differentiation. We introduce a framework where the quantum circuit's evolution is simulated using Monte Carlo techniques, and the resulting expectation values are computed using automatic differentiation. This allows us to calculate gradients with respect to the circuit's parameters, which can then be used with standard optimization algorithms to refine the circuit's design. The key innovation is the seamless integration of these two techniques, offering a pathway to directly optimize quantum circuits for improved performance. The presented methodology provides a foundation for exploring advanced quantum algorithms and improving their efficiency through gradient-based approaches.
ipie is a Python-based auxiliary-field quantum Monte Carlo (AFQMC) package that has undergone substantial improvements since its initial release [Malone et al., J. Chem. Theory Comput. 19(1), 109-121 (2023)]. This paper outlines the improved modularity and new capabilities implemented in ipie. We highlight the ease of incorporating different trial and walker types and the seamless integration of ipie with external libraries. We enable distributed Hamiltonian simulations of large systems that otherwise would not fit on a single central processing unit node or graphics processing unit (GPU) card. This development enabled us to compute the interaction energy of a benzene dimer with 84 electrons and 1512 orbitals with multi-GPUs. Using CUDA and cupy for NVIDIA GPUs, ipie supports GPU-accelerated multi-slater determinant trial wavefunctions [Huang et al. arXiv:2406.08314 (2024)] to enable efficient and highly accurate simulations of large-scale systems. This allows for near-exact ground state energies of multi-reference clusters, [Cu2O2]2+ and [Fe2S2(SCH3)4]2-. We also describe implementations of free projection AFQMC, finite temperature AFQMC, AFQMC for electron-phonon systems, and automatic differentiation in AFQMC for calculating physical properties. These advancements position ipie as a leading platform for AFQMC research in quantum chemistry, facilitating more complex and ambitious computational method development and their applications.
This work presents a portable, modular quantumcircuit emulator designed to accelerate algorithm prototyping by leveraging Central Processing Unit (CPU) and Compute Unified Device Architecture (CUDA)-enabled Graphic Processing Unit (GPU) backends for dense linearalgebra workloads.The emulator provides a lightweight backend abstraction that unifies NumPy and CuPy, supports both statevector and density-matrix representations, and implements efficient multi-target unitary embedding and Kraus-map noise channels.These capabilities enable rapid exploration of quantum algorithms that require full 2 n × 2 n operators, such as quantum phase estimation, full Quantum Fourier Transform (QFT), and noise-aware variational circuits, while minimizing host/device transfer and preserving numerical parity between backends.Implementation strategies are described for embedding single-and multi-qubit gates, parameterized rotations, and controlled unitaries, along with an "apply_unitary_on_targets" routine that employs reshape/matrix-multiply/reshape patterns to map arbitrary target sets to dense GPU kernels for high throughput.Using a curated benchmark suite of dense quantum workloads and microbenchmarks, including large matrix multiplies and Fast Fourier Transforms (FFT), performance is quantified across a range of qubit counts and precisions.The results demonstrate substantial speedups on modern NVIDIA GPUs for workloads dominated by dense linear algebra, identify cross-over points where GPU acceleration becomes beneficial, and analyse memory/precision trade-offs for complex64 versus complex128.The presented emulator lowers the barrier for noise-aware algorithm prototyping by combining practical software ergonomics with GPU performance, enabling faster iteration on algorithm design and noise-mitigation strategies on commodity single-node hardware.While the abstraction layer ensures portability, achieving peak performance currently requires hardwarespecific tuning-a limitation we address through future automation strategies.