Quantum Square Dance

William Phillips [photo courtesy: NIST]

A team of physicists led by 1997 Nobel Laureate William Phillips at the National Institute of Standards and Technology (NIST)could induce thousands of atoms trapped by laser beams to swap their internal spin states with partners simultaneously. Such repeated exchanges, like a quantum version of swinging your partner in a square dance but lasting a total of just 10 milliseconds, might someday carry out logic operations in quantum computers.

In the binary language of computers, the atoms swap values from 1 (“spin up”) to 0 (“spin down”), or vice versa. Unlike classical bits, which would either swap or not, quantum bits can be simultaneously in an unusual state of having swapped and not swapped at the same time. Under these conditions, spin swapping has the effect of “entangling” the pairs, a quantum phenomenon that links the atoms' properties even when they are physically separated. Entanglement is one of the features that make quantum computers potentially so powerful.

[image credit: Trey Porto/NIST]

The NIST experiment was performed with about 60,000 rubidium atoms in a Bose-Einstein condensate (BEC), a special state of matter in which all atoms are in the same quantum state. They were trapped within a three-dimensional grid of light formed by three pairs of infrared laser beams. The lasers were arranged to create two horizontal lattices overlapping like two mesh screens, one twice as fine as the other in one dimension. This created many pairs of energy “wells” for trapping atoms.

The swapping process is a way of creating logical connections among data, crucial in any computer. A logic operation is the equivalent of an “if/then” statement, such as: If two qubits have opposite states, then they should exchange values. The logical connections in quantum computers are created using entanglement, which in effect allows for multiple simultaneous, correlated possibilities.

The scientists attempted to place a single atom in each well, with one atom spin up (or 1) and the other down (or 0). Then, they merged all double wells to force each pair of atoms into the same well, where they could interact with each other. When two such identical atoms are forced into the same physical location, quantum mechanics imposes a specific type of symmetry (only two of four seemingly possible combinations of quantum states are allowed). Due to this restriction, the merged atoms oscillate between the condition in which one atom is 1 and the other is 0, to the opposite condition. This behavior is unique to identical particles.

As they swap spins, the atoms pass in and out of entanglement. At the “half-swap” points the spin of each atom is uncertain and, if measured, might turn out to be either up or down. But whatever the result, a measurement on the other atom, equally uncertain before the measurement, would be sure to be the opposite. This entanglement is the key feature that enables quantum computation.

"This is the first time these spin-entangling interactions have been demonstrated between pairs of atoms in an optical lattice,” says Trey Porto, one of the authors. “Other research groups have entangled atoms in lattices as extended clusters. By isolating pairs, we can focus on the simplest units for quantum logic.” The current set-up is not directly scalable to an arbitrary computer architecture, Porto says, since it performs the same spin-swap in parallel for all pairs of atoms.

Researchers are developing ways to address and manipulate any pair of atoms in the lattice, which should allow for scalable architectures. Furthermore, not all atoms participated in the swap process, primarily because of imperfect initial loading of the atoms in the lattice. (Some double-wells contained only one atom and had no partner to exchange with). The scientists estimate that the swap worked for at least 65% of the double wells. The NIST group is continuing to work on improving the reliability of each step and on completing the logic operation by separating atoms after they interact.

Reference:
"Controlled exchange interaction between pairs of neutral atoms in an optical lattice"
M. Anderlini, P.J. Lee, B.L. Brown, J. Sebby-Strabley, W.D. Phillips, and J.V. Porto.
Nature, 448, p452-456 (2007). Link to Article

[We thank Media Relations, NIST Boulder for materials used in this posting]

High Energy Physics: 5 Needed Breakthroughs -- Pierre Ramond

Pierre Ramond [photo courtesy: University of Florida, Gainesville]

[ Prof. Pierre Ramond, Distinguished Professor of Physics at University of Florida in Gainesville, is today's guest in our ongoing feature '5-Breakthroughs'.

During his long career starting with the PhD work at Syracuse University in 1969, Prof. Ramond contributed in some significant developments in the study of elementary particles and fields. Notable among those is the crucial role he played in the early development of superstring theory.

Early string theory proposed by Yoichiro Nambu and others in 1970 was based on bosonic string. At that point, Pierre Ramond took the crucial step of generalizing the Virasoro algebra, the symmetry algebra of the bosonic string, to a superconformal algebra including anticommuting operators. The inclusion of a fermionic string to accompany the bosonic ones completed the theory of strings. In 1971, he generalized Dirac's equation for point-like particles to string-like ones, which laid a solid foundation for the superstring theory. A comprehensive list of the variety of work he did can be found in Google Scholar link.

Prof. Ramond is a Fellow of American Physical Society and American Academy of Arts & Sciences. In August 2004, he was awarded Oskar Klein Medal by Swedish Royal Academy of Sciences and Stockholm University.

Many of us grew up with his celebrated book "Field Theory: A Modern Primer" (Addison / Wesley, 1981) and also experienced the pleasure of "Journeys Beyond the Standard Model"(Perseus, 1999), his other book. It's thus our pleasure to present the 5 most important breakthroughs that Prof. Ramond would like to see in High Energy Physics.
-- 2Physics.com Team]

Here is my list of five:

Finding Supersymmetry with the Large Hadronic Collider, and if found, understanding Supersymmetry breaking.

Understanding why there are three chiral families of Elementary Particles (closely related to finding the organizing principle behind chiral symmetry breaking, e.g. Yukawa interactions).

Observation of Proton Decay in the Laboratory.

Determining the Character (Majorana or Dirac) of Neutrino masses.

Identifying Dark Matter.