In the 1870s, when Max Planck was still a young German university student, his professor Philipp von Jolly discouraged him from continuing to pursue physics, reportedly saying that nothing was left to discover in the field except for a few minor details.
Undaunted, Planck became a professor of physics at the University of Berlin, and by 1900 had developed a theory that would turn physics upside-down: Electromagnetic energy could only be emitted in discrete packets, or “quanta.” The field of quantum mechanics was born, and its ramifications continue to echo through physics today. Indeed, modern quantum researchers aren’t just filling in minor details; they’re still adding in leaps and bounds to our knowledge of how the world fundamentally works.
Planck’s breakthrough came out of his studies of “black bodies,” idealized objects that perfectly absorb and then re-emit electromagnetic radiation. In reality, nothing can absorb light so perfectly, but many real-world objects, like a hunk of iron, absorb and emit electromagnetic radiation similarly to a black body. As an iron ingot is heated, it begins to emit electromagnetic radiation, energy that travels on a spectrum of frequencies. When it’s quite hot, the ingot turns red—and as its temperature rises further, the ingot will progressively turn orange, then yellow, then white. These are only the frequencies we can see—the ingot, of course, is emitting invisible electromagnetic radiation too, in frequencies like infrared. Planck studied this “black-body spectrum,” and precisely measured how changing temperature affected the radiation a black body emitted. In his work, he came to realize that the emitted radiation didn’t smoothly increase with temperature, but in fact changed in sudden steps. Planck never quite understood the implications of his discovery, but Einstein and other physicists soon began to see its reach. Their conclusion: Everything in the universe—energy, light, particles, and all the macroscopic objects they form and influence—is somehow quantized, and subject to strange probabilistic behavior that defies classical explanations. In the quantum world, objects can be in multiple places at the same time, can simultaneously harbor mutually exclusive states, and can pop in and out of existence spontaneously. Even Richard Feynman, the Nobel-Prize-winning physicist who arguably had a better grasp of quantum mechanics than anyone else in the 20th century, quipped that no one really understood it.
Quantum phenomena are most dramatic in extremes that humans can’t tolerate or perceive, like near-absolute-zero temperatures, or in hard vacuum, or at the scale of atomic nuclei. But this doesn’t mean quantum principles don’t apply to larger objects. Chad Orzel, a physics professor at Union College, blogs about a March study that may document the first observation of a certain type of quantum behavior in an object visible to the naked eye.
“Visible” may be a stretch. The object in question is a fork-shaped device fabricated from aluminum nitride and sandwiched between sheets of aluminum. It’s about 40 microns long, or roughly the width of a human hair. You wouldn’t be able to see it at all if you looked at it from the side, because it’s just one micron thick. Still, since quantum behavior is typically observed at the scale of a single atom or subatomic particle, this research represents an astonishing leap: The device is composed of about 10 trillion atoms.
Orzel says the researchers, led by Aaron O’Connell, cooled the object to its “quantum ground state,” at 0.025 degrees above absolute zero. At warmer temperatures, the device has some resonance, meaning it mechanically oscillates back and forth a bit like a tuning fork. At this experiment’s chilly temperatures, the device still oscillates, but only due to unavoidable quantum effects that cannot be subtracted. Hence, it resides in its quantum ground state. Now, classical physics would say that as the device’s temperature gradually increases, its resonance should smoothly increase, too. O’Connell’s team was able to show instead that the object resonated in discrete intervals. Its resonance was, in other words, quantized.
The resonance itself is not visible; instead this had to be measured indirectly by coupling the resonator to another device, a loop of wire the team fabricated to act as a “qubit,” a register for a single unit of quantum information. This arrangement allowed the researchers to induce resonance in two different ways. They could energize the qubit, which in turn caused the resonator to oscillate, or they could apply microwave radiation to the resonator, and observe the oscillatory response in the qubit. In each case, the resonance occurred only at frequencies predicted exactly by quantum theory.
Greg Fish, a science writer and computer science graduate student, says that this research could also lead to innovations in quantum computing. Because the state of a quantum resonator can’t be directly detected, it leads to a classic conundrum: If an object can be in one of two states, and if we don’t know its state, then it can be said to be in both possible states at the same time. For a computer, this ambiguity can lead to tremendous number-crunching power, because it means that instead of a binary 1 or 0 as in a classical computing “bit,” a qubit can simultaneously embody multiple probabilistic states. And since there are many more than just two possible probabilities, a quantum computer could theoretically process much more information in a given interval of time than a classical computer could. The O’Connell team’s project, therefore, can be seen as a test-run for a quantum computer integrating many resonators like the one in their device. In theory, Fish says, a quantum computer can be as much as 50,000 times faster than a modern supercomputer at solving some types of mathematical problems.
In just over a century, quantum theory has moved from being an abstract curiosity to a powerful driver of technological development, with implications not just for theoretical physicists, but nearly every branch of science. As new developments are unveiled, watch for discussion and analysis on ResearchBlogging.org.
Front page image courtesy of Matt Brown.
Originally published April 7, 2010