From Quantum to Classical

Researchers show that classical observations of quantum systems are a fundamental part of quantum mechanics – no assumptions necessary.

Understanding how the macroscopic classical world emerges from the microscopic quantum world is a fascinating topic that has been puzzling scientists since the early days of quantum mechanics.  This “quantum-to-classical” transition is important both conceptually and for the future of quantum computing. We need to know, for example, if a one-thousand qubit quantum computer can maintain the quantum features necessary to process quantum information.

New research from Institute for Quantum Computing (IQC) research assistant professor Marco Piani and collaborators shows for the first time that the emergence of classical objectivity, as described by the quantum Darwinism framework, is actually a direct consequence of the laws of quantum mechanics itself.

Quantum Darwinism, a theoretical framework championed by quantum theorist Wojciech Zurek, tries to explain the quantum-to-classical transition and the emergence of classical objectivity, meaning that all observers get the same classical information when interacting with a quantum system. Quantum Darwinism predicts that a quantum system environment is actually a means for information about the system to reach the observer. Even more interestingly, in this process the environment selects some special information about the system, which multiplies in the environment and is accessible to many observers at the same time. Each observer can access this information by independently collecting some part of the environment. This makes the selected information objective.

Two elements of quantum Darwinism include the objectivity of observables, which states that the environment selects the same specific classical information to be made potentially available to all observers, and the objectivity of outcomes, which states that the observers will have access to the outcome of the observation and agree on it.

The paper Generic emergence of classical features in quantum Darwinism proves that the objectivity of observables is completely general and always present, independent of the interaction between the system and its environment. Piani and collaborators found that the objectivity of outcomes, however, is model-dependent and does depend on the type of interaction between the system and its environment. Piani, now at the University of Strathclyde, says, “Future work will focus on the assumptions required to ensure the objectivity of outcomes.” 

In addition to the significant research progress in quantum Darwinism, the paper also derived a clear-cut operational meaning for quantum discord, which is a measure of non-classical correlations between two subsystems of a quantum system. The researchers found that when one share of the correlations between two parties is redistributed to many parties, quantum discord is equal to the minimum average loss of correlations.

The paper, co-authored by Fernando G.S.L. Brandão (Microsoft Research, University College London) and Pawel Horodecki (National Quantum Information Center of Gdańsk, Technical University of Gdańsk), appeared in Nature Communications August 12. It exemplifies how modern techniques of quantum information can find applications in many other fields of physics, including the foundations of physics. Understanding the quantum-to-classical transition will also lead to improved technological control over quantum features, which may be helpful for future implementation of quantum technologies.

August 2015