In a new paper published recently by the scholarly journal Physical Review Letters, Associate Professor of Physics Jonathan Friedman continues to grapple with the big questions about a very small object.
Friedman is particularly interested in that fuzzy line between the seemingly abstract world of quantum mechanics and the everyday world.
“Nobody has ever made a definitive observation that contradicts quantum mechanics, but we do seemingly have this everyday cognitive dissonance between the way we perceive the world and quantum mechanics,” he says. “Is there a boundary between the classical and the quantum worlds? Or is it that the everyday is somewhat illusory? It seems like quantum effects don't happen in the everyday world most of the time.”
The new paper, “Geometric-Phase Interference in a Mn12 Single-Molecule Magnet with Fourfold Rotational Symmetry,” is one of many building upon Friedman’s groundbreaking experimental research in quantum mechanics. While a graduate student at CUNY in the 1990s, Friedman and collaborators found the first unambiguous evidence of what’s known as magnetization tunneling, a phenomenon exhibited in molecule-sized magnets. The behavior of these molecules can shed light on fundamental questions in quantum mechanics. The molecules may also one day find application in quantum computers.
Tunneling, according to Friedman, is a uniquely quantum phenomenon in which a system—a particle, for example—can cross an energy barrier without having enough energy to overcome it. The particle is said to have “tunneled through” the barrier. Friedman’s work was the first to show definitively that a magnet could reverse direction from pointing up to down by tunneling. For that work, Friedman shared the 2002 Agilent Technologies Europhysics Prize. His original paper on the discovery has been cited more than 1,200 times by other researchers. In 2008, the journal Nature proclaimed the discovery to be one of the 23 most important milestones in over a century of spin physics.
The new work combines two telltale quantum effects: tunneling and interference. “In quantum mechanics, every object is also a wave,” says Friedman, “and waves can show interference.” Interference, he explains, is what happens when waves overlap. “Think about water waves in the ocean. In some parts of the wave, the water is peaked – above sea level – and in other parts, it’s below sea level – that’s called a trough. When two waves moving in different directions cross paths, the wave heights add up: two peaks can add up to create a bigger peak; when a peak and a trough cross, they can cancel each other out.”
The waviness of quantum mechanics in Friedman’s molecules determines the probability that they can tunnel from pointing up to down. Friedman and his students used magnetic fields to coax the interference to be the sort in which the waves cancel, which suppresses tunneling. “Like most things in quantum mechanics,” says Friedman, “it’s counterintuitive. Each of the individual ways the magnet can tunnel has a given probability of happening. But add them all up and the total probability ends up being close to zero.” You can read Friedman’s recent paper here, or through the journal here.
The work spanned several years, as attested by the number of names at the head of the paper, researchers that include Spencer Adams ’13, Edwardo H. da Silva Neto ’08 (now a graduate student at Princeton), John Ware ’11 (now a graduate student at University of Michigan), and S. Datta, a recent postdoc in Friedman’s lab.