In quantum physics, the divisions between object and observer—the systems and environment—become blurred. Because any measuring device is governed by the laws of quantum mechanics, the act of measurement involves an interaction between two quantum systems. The exact mechanisms by which this works are still unclear in many instances, but much of the quasi-mystical language once used to describe quantum mechanics has given way to precise scientific descriptions.
One remaining frontier is comprehension of how systems gradually lose coherence via interactions with their environment, which prevents their usefulness in quantum computing. A new set of experiments by Yinnon Glickman, Shlomi Kotler, Nitzan Akerman, and Roee Ozeri revealed part of the mechanism by which environment disrupts quantum systems: photons. They found that photons that interacted with a quantum system can end up correlated with the system's state, the hallmark of entanglement. By careful preparation of the atom's state, it may be possible to reduce the loss of quantum information to the environment, and thus extend the life of these systems.
Measurement in quantum physics transforms an indeterminate system—one that behaves as if it's in a number of different states simultaneously—into one with a definite set of physical attributes. For example, the spin of an electron cannot typically be known in the absence of a measurement. However, sending the atom through a (nonuniform) magnetic field will deflect it either up or down relative to the field, showing that the electron is either aligned with or aligned against the magnet.
Since that outcome will happen whichever way the magnetic field is oriented, the electron's state is indeterminate before the measurement, but "collapses" into one of two alternatives after the fact. Those two alternatives comprise the "measurement basis," relative to the magnetic field; a different orientation will yield a different measurement basis.
Even if a quantum system is in a definite state (say after a measurement), interactions with its environment will tend to nudge it back toward indeterminacy. This process is known as decoherence, and it stands as one of the biggest obstacles to quantum computing. In essence, the environment itself—which includes ambient photons that make up the electromagnetic field—is performing "measurements" on the system. If these (or any other) measurements involve light, then the interaction creates entanglement between the quantum states of the photons and the matter.
In the current experiment, the researchers trapped a single strontium (Sr) atom in crossed electromagnetic fields. Strontium has an unpaired electron in its outermost shell, which dictates the spin of the atom. The experimenters subjected this spin to an additional weak magnetic field, coaxing it into a preferred quantum state. They then shone a laser on the atom in a direction perpendicular to the magnetic field, and collected the photons that scattered.
The scattering process involves an atom absorbing a photon, which induces a transition between two quantum states inside the atom. Subsequently, the atom undergoes a second transition, emitting a new photon. So, one photon entered, one photon left, but the second takes some information with it: the polarization of the photon depends on the spin state of the electron in the strontium atom. By measuring the polarization of these scattered photons, the researchers could reconstruct the quantum state transitions of the atom.
To reconstruct the full physical system, the researchers prepared the atoms so that the electron spin started in one of several different directions, then compared the photon polarizations after scattering. The outcomes were very different: if the electron spin was aligned with or against the direction of photon scatter, then the light came out with one of two simple polarization orientations, corresponding to the measurement basis. However, for any other electron spin direction, the photons came out entangled with the state of the atom.
That meant that the result of the polarization measurement was correlated with the state of the atom. The outcome was a mixture of possible polarizations and, in most cases, they results didn't correspond to one or the other state in the measurement basis.
The finding is significant because the quantum state of a trapped atom has been used as a qubit in a quantum computer. Unfortunately, according to quantum physics, the environment contains ambient photons, which interact with any atom, and this can lead to decoherence. In effect, the environment is erasing the state needed for computations.
This experiment showed that orienting the detection devices in a clever way could avoid this form of decoherence, by exploiting scattering directions in which entanglement between environment photons and the atom's spin state does not take place. While this may sound simple, in practice it would be somewhat more challenging. Nevertheless, these new results indicate how future experiments could compensate for some aspects of decoherence.
Science, 2013. DOI: 10.1126/science.1229650 (About DOIs).
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