LOS ALAMOS, N.M. The behavior of elementary particles in the quantum realm is markedly different from particles in the large-scale aggregate world because of a phenomenon called "einselection".
Coined by physicist Wojciech Zurek at Los Alamos National Laboratory here, "einselection" stipulates that any disturbance to matter in the environment, however subtle, records information about the quantum state of a system.
Einselection is the same physical operation as an "observation," which need not directly involve an observer. For example, a photon counter could remotely record the arrival of photons, without a human observer participating. Likewise, the Saturn moon Hyperion is continually "recording" its position by disturbing photons and other small particles in its path. The continual record of its movement would prevent the moon from adopting an undetermined superposition of states.
"These interactions with the environment can be so slight that they don't affect the object, but the object nevertheless leaves an imprint on the environment the environment is in some sense constantly monitoring objects," Zurek said.
"There are many macroscopic phenomena that should show quantum effects, but do not. I believe the reason is that the environment itself observes quantum phenomena, forcing it to select between what were previously thought to be indeterminate states what I call einselection," Zurek said.
Physicists readily acknowledge that the underlying world of quantum mechanics clashes with the indisputable appearance of everyday reality, but a full explanation of this dichotomy has been lacking, Zurek said.
Phenomena like chaotic systems "should get into all sorts of bizarre trouble from a quantum-mechanical perspective," he said. Chaotic systems make rapid transitions to a very large number of states. Zurek uses Saturn's Hyperion as an example of a large body in a chaotic state. Because of the strong influence of Saturn's gravitational field, the moon tumbles, and its axis is continually redirected in a non-repeating chaotic sequence.
According to quantum theory, the rapid state change should lead to a "superposition of states" an indeterminate situation that is common at the quantum level. But large objects never enter indeterminate quantum states. The standard explanation involves the mechanism of "de-coherence," in which the numerous quantum particles making up the object are out of step with one another.
In special circumstances, groups of elementary particles can be forced into a coherent state, where they produce strange behavior that can be observed. Superconductivity and laser light are two technologically important examples, and physicists can create other coherent systems in the lab. But even coherent-particle systems do not fall into indeterminate states.
While einselection ensures that the everyday macroscopic world is free from quantum paradox, it does not always resolve indeterminacy at the microscopic level, according to Zurek. Fortunately, many quantum phenomena involve more moving parts than a single photon, and thus do resolve most of their indeterminacy even when no one is watching.
Though einselection brings assurance that quantum weirdness won't spring into existence in the midst of everyday life, the principle could also dash researchers' hopes to harness indeterminate quantum states to perform computations in future quantum computers. Scientists have been speculating about the possibility of processing many different computations in parallel, by operating on a collection of superposed states, and then decoding the results of those parallel computations. Although einselection does not put an end to hopes for quantum computers, it does put a damper on those hopes by greatly reducing the number of possible parallel computations that could be harnessed.
The idea of "einselection does not completely resolve indeterminacy, but it does reduce the number of possibilities to a small number of possible selections. It also poses the biggest known obstacle to an experimental implementation of a quantum computer," said Zurek.
Experimental confirmation of Zurek's middle ground between the classical and quantum realms is just beginning, but many scientists worldwide are already at work on it. Most are driven to resolve the paradoxes that quantum mechanics poses, but some are planning to use einselection as a blueprint for building quantum computers.
"Quantum computers are still possible there is some very promising work that uses quantum entanglement to perform some of the parallel-processing tasks that were speculated on by others before my concept of einselection [was announced]," Zurek said.