Study analyzes the interaction of quantum systems with the environment

A paper in Nature Publishing Group's Scientific Reports describes the enhanced understanding of qubit control.

By José Tadeu Arantes

Agência FAPESP – When open quantum systems are studied, two components should be considered: the physical system itself and the environment. However, in almost all studies, the environment is not taken into account because of a lack of information about it. Only the intrinsic dynamics of the system, such as entanglement and the correlations between its parts, are considered.

A paper published in December in Scientific Reports, which belongs to Nature Publishing Group, shows that even without access to information about the environment it is possible to infer how the system and its parts entangle with it.

The article “Extracting information from qubit-environment correlations” was written by John H. Reina and Cristian E. Susa, who are both affiliated with the Universidad del Valle and Center for Bioinformatics & Photonics, Cali, Colombia, and Felipe F. Fanchini, a professor at the Department of Physics, School of Sciences, São Paulo State University (UNESP), Bauru Campus, Brazil.

The article resulted from a project entitled A study of quantum correlations in open quantum systems, supported by a research grant from FAPESP. “We set out to stress that the environment can provide important information for the understanding of a system’s dissipative dynamics,” Fanchini told Agência FAPESP. The article considers a situation comprising two atoms coupled to a control laser. The atoms are the qubits of the system and are subject to external disturbances originating in the environment.

“Our aim was to understand how these atoms are influenced by the environment to the point at which their quantum correlations are lost. We did this by looking not only at the system comprising the two atoms but also at the entanglements and quantum correlations between the system and the environment,” Fanchini said.

For this purpose, the authors used a well-established mathematical tool: the monogamous relationship proposed by Masato Koashi of the University of Bristol in the UK and the Graduate University for Advanced Studies in Japan and Andreas Winter of the University of Bristol in the paper “Monogamy of quantum entanglement and other correlations,” published in Physical Review A in 2004.

“It was believed that the entanglement between two atoms diminished because they were also entangling with the environment. However, we highlighted that the correlation is not as proportional as everyone thought,” Fanchini said.

“Sometimes atoms lose entanglement with each other but entangle more slowly with the environment because part of what is lost may be converted into other correlations that characterize quantum discord.”

The article by Fanchini and colleagues aims to show that the dissipative process can be more complex than has been supposed and that new studies are required to understand it. The goal is to achieve a theory regarding the control of quantum systems in their relationship with the environment that addresses the challenge of shielding these systems to prevent the loss of information.

One obvious application of shielding is a way to overcome the main obstacle to quantum computing: the effect of the environment on the system, which causes system components to lose quantum correlations and to behave according to classical physics.

Another possible application would be quantum simulation in such situations as understanding the behavior of a molecule that cannot be accessed by creating another quantum system capable of mimetizing it.

“Why can’t we simulate large molecules or achieve effective quantum computing? Because we can’t block the effects of the reservoir [environment] on the system. So, we need to understand how the environment works. The more we know about that, the better we’ll be able to protect quantum systems from the influence of the environment,” Fanchini said.

“Our paper doesn’t say how to protect quantum systems, but it does present a new frontier for understanding dissipative processes, focusing not only on the system but also on the interactions between the system and the environment.”

Quantum computing

Fanchini is optimistic about the prospects for quantum computing. “Until 2002, I believed that quantum computing was entirely feasible. In 2005, I started to think it was unlikely. Now, I’ve changed my mind again; I believe that we’re almost there,” he said.

“There are experimental groups that are developing new, extremely efficient semiconductor technologies. We’re controlling the physical system, protecting it, accessing it, and reading the quantum information. What’s left is to put all of this together. Judging from the success achieved in 2014, we may have a quantum computer within a decade.”

According to Fanchini, this possibility is derived from the fact that a quantum computer does not need to be 100% accurate. “Approximately 15 years ago, it was shown that 100% fidelity isn’t necessary. There is a fidelity threshold, above which you can concatenate protection methods and make an efficient quantum computer,” he said.

Moreover, classical computers will not have to be replaced by quantum computers for all tasks.

“Initially, this substitution will occur only for some tasks rather than for everything. There are people who are studying this. There are proposals relating to power generation using more efficient voltaic cells. There is also the possibility of new search algorithms. These will be highly specific applications,” Fanchini said.

Quantum simulation, according to Fanchini, is even more promising than quantum computing. An example of its use is an advance in chemistry to offer the possibility of intervening in the material world atom by atom, molecule by molecule.