Molecular dynamics simulated by photons

The microscopic behaviour of molecules can be difficult to model using ordinary computers because it is governed by quantum physics. A photonic chip provides a versatile platform for simulating such behaviour.
Fabien Gatti is at the Institut des Sciences Moléculaires d’Orsay, CNRS, Université de Paris-Sud, F-91405 Orsay, France.

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Quantum-computing devices could one day outperform ordinary computers, particularly in the simulation of quantum systems. Such devices share their quantum nature with the system to be simulated and are therefore inherently suited to describing quantum phenomena1. In a paper in Nature, Sparrow et al.2 report a device based on a single photonic chip that can simulate a range of quantum dynamics associated with different molecules. The results are in excellent agreement with simulations carried out by ordinary computers, reaffirming the potential of quantum technology in this area.

In conventional industrial chemistry, the yields of chemical processes are optimized by controlling macroscopic variables, such as temperature and pressure. But the use of high temperatures and pressures wastes a substantial amount of energy and generates unwanted by-products, leading to high energy consumption and pollution. To overcome these issues, a promising optimization approach exploits the quantum nature of the reacting molecules.

A central tenet of quantum physics is the superposition principle, which asserts that possible quantum states of a system can be added together and the result will be another possible state. The non-classical aspect of this principle is demonstrated, for example, by quantum bits. These objects can exist in both an on state and an off state at the same time. Such states exhibit quantum coherence, which means that they are correlated in a non-classical way.

The ability to systematically control quantum coherence is considered one of the main challenges in energy science. Such control might enable the synthesis of highly desirable materials and devices, including superfluids (fluids that flow without resistance) and quantum computers. It could also give rise to more-efficient chemical processes than are currently possible.

In conventional chemistry, the quantum states involved in chemical processes are incoherent. However, coherent superpositions of molecular states can be produced using the light emitted by a laser. The ability to consistently generate these superpositions could improve the efficiency of the corresponding chemical processes and reduce the energy required to control such processes. It might even open up chemical-reaction mechanisms that are otherwise inaccessible3,4.

Laser pulses are the main tool for manipulating molecules in this field. Improvements in the design of these pulses, such as increases in power and tunability, as well as the ability to reduce the duration of the pulses to attosecond (10–18 s) timescales, have enabled greater control of light-induced processes in molecules5. Since the pioneering work of Ahmed Zewail, who was awarded the 1999 Nobel Prize in Chemistry, laser pulses have been used to study quantum coherence in chemistry6.

For example, quantum coherence has been used to enhance the rates of chemical reactions in biological systems at room temperature7. Such studies conclusively showed that quantum coherence can be partially preserved even in molecular systems open to the external environment8.

These experimental advances call for accurate models of the quantum evolution of molecular systems. This is a challenging task for quantum-computing devices, although much progress has been made, thanks to the development of improved algorithms for simulating quantum dynamics9,10.

Sparrow and colleagues engineered a quantum-computing device that is based on a single photonic chip. They used the quantum superposition of photons in the chip to carry quantum information and to model molecular systems. By adjusting the optical circuitry of the chip, the authors simulated a range of quantum dynamics associated with different molecules.

The authors began by simulating vibrational excitations in a variety of four-atom molecules. They then modelled energy transport in the chemical bond of a protein and the transfer of vibrational energy in liquid water. Finally, they tested an algorithm designed to identify quantum states that can lead to the break-up of ammonia. The results of these simulations were in almost perfect agreement with those obtained using ordinary computers.

The first quantum revolution occurred at the turn of the twentieth century, and provided us with the physical laws that govern reality. Sparrow and colleagues have now simulated the time evolution of a quantum superposition of molecular states with the aid of an experimental device that uses the quantum superposition of photons. Such a feat suggests that we could be entering a second quantum revolution, in which the physical laws of nature are used to develop innovative technologies.

Despite these promising prospects, it is not difficult to envisage the problems that follow-up studies will encounter. In this seminal work, the authors used rather simple molecular models, involving a limited number of mathematical terms. However, this number will increase exponentially when aiming to closely reproduce experimental conditions. Such an increase might dramatically enhance what the authors refer to as the “fundamental errors” in photonics, which include the loss of photons and the loss of quantum coherence.

Nevertheless, Sparrow and colleagues have demonstrated that simulations carried out by quantum-computing devices can be both reliable and efficient, by tackling problems that can be solved using well-established standard techniques. As the authors point out, slight improvements in their method could yield simulations that cannot be achieved using ordinary computers.

Nature 557, 641-642 (2018)


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