Quantum Physics

A second type of quantum computer has now performed a calculation impossible for a traditional computer. More

According to legend, Galileo dropped weights off of the Leaning Tower of Pisa, showing that gravity causes objects of different masses to fall with the same acceleration. In recent years, researchers have taken to replicating this test in a way that the Italian scientist probably never envisioned — by dropping atoms.
A new study describes the most sensitive atom-drop test so far and shows that Galileo’s gravity experiment still holds up — even for individual atoms. Two different types of atoms had the same acceleration within about a part per trillion, or 0.0000000001 percent, physicists report in a paper in press in Physical Review Letters.
Compared with a previous atom-drop test, the new research is a thousand times as sensitive. “It represents a leap forward,” says physicist Guglielmo Tino of the University of Florence, who was not involved with the new study.
Researchers compared rubidium atoms of two different isotopes, atoms that contain different numbers of neutrons in their nuclei. The team launched clouds of these atoms about 8.6 meters high in a tube under vacuum. As the atoms rose and fell, both varieties accelerated at essentially the same rate, the researchers found.

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In confirming Galileo’s gravity experiment yet again, the result upholds the equivalence principle, a foundation of Albert Einstein’s theory of gravity, general relativity. That principle states that an object’s inertial mass, which determines how much it accelerates when force is applied, is equivalent to its gravitational mass, which determines how strong a gravitational force it feels. The upshot: An object’s acceleration under gravity doesn’t depend on its mass or composition.
So far, the equivalence principle has withstood all tests. But atoms, which are subject to the strange laws of quantum mechanics, could reveal its weak points. “When you do the test with atoms … you’re testing the equivalence principle and stressing it in new ways,” says physicist Mark Kasevich of Stanford University.
Kasevich and colleagues studied the tiny particles using atom interferometry, which takes advantage of quantum mechanics to make extremely precise measurements. During the atoms’ flight, the scientists put the atoms in a state called a quantum superposition, in which particles don’t have one definite location. Instead, each atom existed in a superposition of two locations, separated by up to seven centimeters. When the atoms’ two locations were brought back together, the atoms interfered with themselves in a way that precisely revealed their relative acceleration.
Many scientists think that the equivalence principle will eventually falter. “We have reasonable expectations that our current theories … are not the end of the story,” says physicist Magdalena Zych of the University of Queensland in Brisbane, Australia, who was not involved with the research. That’s because quantum mechanics — the branch of physics that describes the counterintuitive physics of the very small — doesn’t mesh well with general relativity, leading scientists on a hunt for a theory of quantum gravity that could unite these ideas. Many scientists suspect that the new theory will violate the equivalence principle by an amount too small to have been detected with tests performed thus far.
But physicists hope to improve such atom-based tests in the future, for example by performing them in space, where objects can free-fall for extended periods of time. An equivalence principle test in space has already been performed with metal cylinders, but not yet with atoms (SN: 12/4/17).
So there’s still a chance to prove Galileo wrong. More

Positronium is positively puzzling.
A new measurement of the exotic “atom” — consisting of an electron and its antiparticle, a positron — disagrees with theoretical calculations, scientists report in the Aug. 14 Physical Review Letters. And physicists are at a loss to explain it.
A flaw in either the calculations or the experiment seems unlikely, researchers say. And new phenomena, such as undiscovered particles, also don’t provide an easy answer, adds theoretical physicist Jesús Pérez Ríos of the Fritz Haber Institute of the Max Planck Society in Berlin. “Right now, the best I can tell you is, we don’t know,” says Pérez Ríos, who was not involved with the new research.
Positronium is composed of an electron, with a negative charge, circling in orbit with a positron, with a positive charge — making what’s effectively an atom without a nucleus (SN: 9/12/07). With just two particles and free from the complexities of a nucleus, positronium is appealingly simple. Its simplicity means it can be used to precisely test the theory of quantum electrodynamics, which explains how electrically charged particles interact.

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A team of physicists from University College London measured the separation between two specific energy levels of positronium, what’s known as its fine structure. The researchers formed positronium by colliding a beam of positrons with a target, where they met up with electrons. After manipulating the positronium atoms with a laser to put them in the appropriate energy level, the team hit them with microwave radiation to induce some of them to jump to another energy level.
The researchers pinpointed the frequency of radiation needed to make the atoms take the leap, which is equivalent to finding the size of the gap between the energy levels. While the frequency predicted from calculations was about 18,498 megahertz, the researchers measured about 18,501 megahertz, a difference of about 0.02 percent. Given that the estimated experimental error was only about 0.003 percent, that’s a wide gap.
The team searched for experimental issues that could explain the result, but came up empty. Additional experiments are now needed to help investigate the mismatch, says physicist Akira Ishida of the University of Tokyo, who was not involved with the study. “If there is still significant discrepancy after further precise measurements, the situation becomes much more exciting.”
The theoretical prediction also seems solid. In quantum electrodynamics, making predictions involves calculating to a certain level of precision, leaving out terms that are less significant and more difficult to calculate. Those additional terms are expected to be too small to account for the discrepancy. But, “it’s conceivable that you could be surprised,” says theoretical physicist Greg Adkins of Franklin & Marshall College in Lancaster, Pa., also not involved with the research.
If the experiments and the theoretical calculations check out, the discrepancy might be due to a new particle, but that explanation also seems unlikely. A new particle’s effects probably would have shown up in earlier experiments. For example, says Pérez Ríos, positronium’s energy levels could be affected by a hypothetical axion-like particle. That’s a lightweight particle that has the potential to explain dark matter, an invisible type of matter thought to permeate the universe. But if that type of particle was causing this mismatch, researchers would also have seen its effects in measurements of the magnetic properties of the electron and its heavier cousin, the muon.
That leaves scientists still searching for an answer, says physicist David Cassidy, a coauthor of the study. “It’s going to be something surprising. I just don’t know what.­” More

Astronaut John Glenn was wary about trusting a computer. It was 1962, early in the computer age, and a room-sized machine had calculated the flight path for his upcoming orbit of Earth — the first for an American. But Glenn wasn’t willing to entrust his life to a newfangled machine that might make a mistake. […] More

On the International Space Station, astronauts are weightless. Atoms are, too. That weightlessness makes it easier to study a weird quantum state of matter known as a Bose-Einstein condensate. Now, the first Bose-Einstein condensates made on the space station are reported in the June 11 Nature. The ability to study the strange state of matter […] More

Physicists have harnessed the aloofness of quantum particles to create a new type of crystal. Some particles shun one another because they are forbidden to take on the same quantum state as their neighbors. Atoms can be so reluctant to overlap that they form a crystal-like arrangement even when they aren’t exerting any forces on […] More

Quantum computing’s deep freeze is beginning to thaw. Computers that harness quantum physics could trump standard computers on certain types of calculations. But the machines typically work only at temperatures tiny fractions of a degree above absolute zero. Now, two teams of physicists report that they’ve created silicon-based quantum computers that work under warmer conditions. […] More

Even if you could measure three black holes’ locations as precisely as physically possible, you still might not know where the black holes would go. Such a trio’s complex dance can be so chaotic that the motions are fundamentally unpredictable, new computer simulations show. The paths of three black holes orbiting each other can be […] More