"Spooky Action-at-a-Distance" with Individual Atoms

By: Christopher Monroe

Albert Einstein never liked Quantum Mechanics, with its fuzzy superpositions and confused states of reality. In 1935, he and colleagues Boris Podolsky and Nathan Rosen proposed a thought experiment that they believed would finally show cracks in the new quantum theory. The essentials of their famed proposal can be seen by cconsidering two “quantum coins,” that are prepared in a strange superposition of being both heads-up and both tails-up at the same time. When such coins are brought far apart from each other and then measured, quantum mechanics predicts that the only possible results can be HH and TT – the orientation of the coins always matches in perfect correlation. But when either individual coin is observed, its value is expected to be totally random (H or T). What’s interesting here is that while an individual coin is in an indeterminate state until observed, the observer immediately knows that orientation of the other coin, and this knowledge happens faster than the speed of light can traverse the distance between the coins.

Einstein called this quantum behavior “spooky action-at-a-distance,” and concluded that either quantum mechanics is incomplete, or it is just very weird. We now know, thanks to John Bell in 1964, that if quantum mechanics is indeed incomplete, than any more complete theory must be just as weird, so we might as well stick with quantum mechanics. Bell devised a measure of this weirdness: an inequality involving measured pair-correlations that is violated for situations like the one considered by Einstein, Podolsky and Rosen.

This weird type of quantum state the Einstein introduced is now known as an “entangled state,” and the spooky action-at-a-distance that he bemoaned is now the central resource in the field of Quantum Information Science. Replace the coins by quantum bits that can be in the state 0 and 1 simultaneously, and these qubits can be used for superfast computing applications, or fundamentally secure communication. Qubits are now being investigated in a variety of physical systems, from individual atoms and photons, to superconducting circuits and semiconductor quantum dots.

Recently, a team of researchers from the University of Maryland Department of Physics and Joint Quantum Institute have observed for the first time, quantum entanglement of individual atoms separated by a large distance [Moehring, et al., Nature 449, 68, (2007)]. Two atoms, held in electromagnetic traps one meter apart, were synchronized with a laser pulse, and the resulting emitted light was interfered on a beamsplitter and detected. This detection produced an entangled state of the two atoms, where qubits were stored in the magnetic orientation of each atom. This entanglement (the correlations of the atomic-scale magnets and the randomness of each one individually) was directly verified by measuring the magnetic orientation with a separate laser, resulting in a clear violation of Bell inequalities. This type of quantum linking between atomic qubits may ultimately lead to the fabrication of a large-scale quantum computer, where atomic memories will be able to store exponentially-rich amounts of data and be connected through optical interconnects as demonstrated here. In the nearer term, this is among the most promising roads to a “quantum repeater,” where qubits can be propagated over very large (or even geographic) distances with the use of optical fibers.

First Measurement of ηb

Collaborators on the BaBar experiment, at the U.S. Department of Energy (DOE)supported Stanford Linear Accelerator Center (SLAC), have detected and measured the lowest energy bound state of the "bottomonium" family: ηb. This is the first observation of the ground state of a b and anti-b quark pair.

The significance of this observation has to do with the fact that in the absence of spin-spin interactions the ηb would be degenerate with the lowest s-wave state with the quark spins aligned: the b/anti-b bound state called the "upsilon 1S", denoted Y(1S). The spin-spin interaction generates a very small hyperfine splitting between the Y(1S) and the ηb. For this data, the PEP-II center-of-mass energy was tuned to the 3rd radially excited state Y(3S) resonance, and the analysis consisted of looking for the photon transistions from the Y(3S) to the ηb, a magnetic dipole transition, and thus just as in atomic physics, highly suppressed. (In fact, our very own Joe Sucher wrote a wonderful article on M1 transitions in atomic and particle physics in 1978 right after the charmonium J/ψ was discovered, Rep. Prog. Phys. 41 1781-1838, 1978, available here).

Given the suppressed transition, this state has been anticipated but has gone unobserved for quite a long time. Failure to observe it has led many particle physicists to speculate that it may be interfering with a light CP-odd Higgs state, which masks its signature. The ηb observation at BaBar provides a key element of our understanding of the physics of the b/anti-b system, and allows probing the role of spin in the strong interactions. Lattice QCD predictions for these hyperfine splittings will now be able to be tested and calibrated. This is only the first step – to be able to dig so deep in the role of spin in quark anti-quark interactions. The BaBar group expects to be able to map out a number of states and transitions that are all sensitive to hyperfine splitting effects. This will open up an entirely new area in the field of precision physics of the strong interaction, since the higher mass of the b quarks (relative to the up, down, charm, and strange quarks) allows strong interaction calculations that are more reliable than in the lighter quark sectors, and testing the lattice QCD calculations in these systems.

This is a tremendous achievement for both the PEP-II accelerator and the BaBar Collaboration

"This is a tremendous achievement for both the PEP-II accelerator and the BaBar Collaboration", said SLAC Director Persis Drell. SLAC is the home of the PEP-II accelerator complex, which consists of two independent storage rings bringing a 9-GeV electron beam in collision with a 3.1-GeV positron beam at the center of the BaBar detector. The asymmetric energies result in a collision center-of-mass that is moving in the laboratory frame of BaBar, and this motion is crucial for study of CP violation in bottom meson decays.

The BaBar collaboration consists of 459 physicists and 74 institutions in 10 countries, is supported by the Department of Energy as well as by international funding, and is led by UMD Professor Hassan Jawahery, their current "Spokesperson" since 2006. Professor Jawahery has been involved with BaBar since it's inception in 1993, and served as physics coordinator in 2001-2002 during initial running that began in 2000 and subsequent measurements of CP violation in the bottom-quark sector. "This very significant observation was made possible by the tremendous luminosity of the PEP-II accelerator and the great precision of the BaBar detector, which was extremely well calibrated over the BaBar experiment's 8-plus years of operation", said Professor Jawahery. "These results were highly sought after for over 30 years and will have an important impact on our understanding of the strong interactions. It's amazing, we are doing atomic physics at 10 GeV, and that's the beauty of this."