Scientists at the U.S. Naval Research Laboratory (NRL) have developed a new quantum algorithm, the Cascaded Variational Quantum Eigensolver (CVQE), which could revolutionize the study of physical properties in electronic systems. The CVQE algorithm, a variant of the Variational Quantum Eigensolver (VQE), increases computational throughput by only requiring the execution of quantum circuits once.
The algorithm uses a quantum computer to probe probability mass functions and a classical computer for remaining calculations. Researchers John Stenger, Steve Hellberg, and Dan Gunlycke believe this technology could significantly speed up calculations and aid in developing new materials and chemistry.
Quantum Algorithm Development by U.S. Naval Research Laboratory Scientists
U.S. Naval Research Laboratory (NRL) scientists have recently published their work on the Cascaded Variational Quantum Eigensolver (CVQE) algorithm in a Physical Review Research article. This algorithm is anticipated to be a significant tool for investigating physical properties in electronic systems. The CVQE algorithm is a variant of the Variational Quantum Eigensolver (VQE) algorithm, but it only requires the execution of a set of quantum circuits once, rather than at every iteration during the parameter optimization process. This feature increases the computational throughput.
John Stenger, Ph.D., a Theoretical Chemistry Section research physicist, explains that both algorithms produce a quantum state close to the ground state of a system, which is used to determine many of the system’s physical properties. The CVQE algorithm can perform calculations that previously took months in just hours.
The Role of Quantum and Classical Computers in the CVQE Algorithm
The CVQE algorithm employs a quantum computer to probe the needed probability mass functions and a classical computer to perform the remaining calculations, including the energy minimization. Steve Hellberg, Ph.D., a Theory of Advanced Functional Materials Section research physicist, explains that finding the minimum energy is computationally challenging as the size of the state space grows exponentially with the system size. Even the world’s most powerful supercomputers are unable to find the exact ground state for anything but very small systems.
To overcome this challenge, scientists use a quantum computer with a qubit register, whose state space also increases exponentially, in this case with qubits. By representing the states of a physical system on the state space of the register, a quantum computer can be used to simulate the states in the exponentially large representation space of the system.
The Sampling and Optimization Processes in the CVQE Algorithm
Data can subsequently be extracted by quantum measurements. As quantum measurements are not deterministic, the quantum circuit executions must be repeated multiple times to estimate probability distributions describing the states, a process known as sampling. Variational quantum algorithms, including the CVQE algorithm, identify trial states by a set of parameters that are optimized to minimize the energy.
Dan Gunlycke, D.Phil., Theoretical Chemistry Section Head, who also leads the NRL quantum computing effort, explains that the key difference between the original VQE method and the new CVQE method is that the sampling and optimization processes have been decoupled in the latter. This allows the sampling to be performed exclusively on the quantum computer and the parameters processed exclusively on a classical computer.
The Impact of the CVQE Algorithm on Material and Chemistry Development
Quantum computing is a component of quantum science, which has been designated as a Critical Technology Area within the USD(R&E) Technology Vision for an Era of Competition by the Under Secretary of Defense for Research and Engineering Heidi Shyu.
Gunlycke explains that understanding the properties of quantum-mechanical systems is essential in the development of new materials and chemistry for the Navy and Marine Corps. For instance, the CVQE algorithm can be used to study the chemical reactions causing corrosion, providing critical information to existing anticorrosion teams in their quest to develop better coatings and additives.
The U.S. Naval Research Laboratory’s Role in Quantum Science Research
For decades, NRL has been conducting fundamental research in quantum science, which has the potential to yield disruptive Defense technologies for precision, navigation, and timing; quantum sensing; quantum computing; and quantum networking. NRL is a scientific and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seafloor to space and in the information domain. NRL is located in Washington, D.C. with major field sites in Stennis Space Center, Mississippi; Key West, Florida; Monterey, California, and employs approximately 3,000 civilian scientists, engineers and support personnel.
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