Hot Buckyballs Lose Quantum Coherence

Small things are quantum and big things are classical. But where is the boundary between the two regimes and what happens there? Once the preserve of gedanken experiments, such questions are increasingly tackled in the lab. Markus Arndt, Anton Zeilinger, and their colleagues at the University of Vienna explore the quantum-classical boundary by subjecting ever larger molecules to matter interferometry. Their latest experiment, on hot C₇₀ buckyballs, supports the emerging consensus that an object’s interaction with the environment, rather than its mere size or even its complexity, dictates its classicality. ¹

The Vienna group’s experiment is conceptually simple: Heat buckyballs, send them through an interferometer, then watch how the fringes depend on the buckyballs’ temperature. The higher the temperature, runs the theory, the more a molecule radiates as it cools and the more it should lose its fringe-forming coherence to the environment.

Freshly baked buckyballs emerge from the lab’s oven at about 900 K. At that temperature, they still form fringes. To induce decoherence, Arndt and company heat the buckyballs further with lasers. The lasers excite electronic transitions that swiftly dump about 2.5 eV of energy per photon into the molecule’s many vibrational states.

Each absorbed photon causes a 140-K rise in temperature, but the molecules start cooling radiatively even as they’re heated. Determining their temperature is difficult. Fortunately, what might have been an experimental nuisance—laser-induced ionization—provides an accurate thermometer. The hotter the molecules become, the more likely some of them are to shed an electron. Pulling ions out of the beam and counting them yields the temperature of the molecules as they enter the interferometer.

As an interferometer, the Vienna group uses three parallel transmission gratings spaced 38 cm apart. The first grating acts as a collimator, the second plays the role of Young’s slits, while the third serves as a movable mask. When fringes form, moving the mask across the beam modulates the intensity I, which is measured by ionizing and counting the molecules as they exit the interferometer.

The figure below shows the results. At zero laser power, the fringe visibility V, defined as (I _max−I _min)/(I _max + I _min), is 0.47. At 3 W, the overall intensity rises (because hotter ions are easier to ionize at the detector), but V drops to 0.29. At 6 W, the overall intensity stops rising (because more molecules are lost at the heating stage) and V drops further to 0.07. At 10.5 W, visibility vanishes.

The results, which the group modeled with standard quantum mechanics, don’t challenge standard decoherence theory. However, they exemplify a growing appreciation for the role of the environment in quantum mechanics. If the photons emitted by a hot buckyball are numerous enough and have short enough wavelengths, they carry information that can localize its path. Whether detected or not, the information entangles the environment in a state that picks out one path. The buckyball becomes classical.

Other experiments have revealed environment-induced decoherence, but not in objects large enough to radiate thermally. “The Vienna group,” says Wojciech Zurek of Los Alamos National Laboratory, “has produced the biggest, cleanest Schrödinger’s cat so far.”