Physics Tip Sheet #76, November 29, 2007

Quantum Computer Breakthrough

Chao-Yang Lu, Daniel E. Browne, Tao Yang, and Jian-Wei Pan
Physical Review Letters
and
B. P. Lanyon, T. J. Weinhold, N. K. Langford, M. Barbieri, D. F. V. James, A. Gilchrist, and A. G. White
Physical Review Letters

Two research groups have independently managed to experimentally solve a mathematical problem with light-based quantum computers. The simultaneous achievements appear to be the first experimental demonstrations of true (though rudimentary) quantum mechanical computations. Both groups manipulated quantum mechanically entangled photons to calculate the prime factors of the number 15.

Although the physicists could have gotten the answer to the problem much more easily by querying an average elementary school child, the method both groups used involved a quantum mechanical approach commonly known as Shor's algorithm. Previous theoretical work has shown that the algorithm could potentially crack cryptographic codes that are practically unbreakable with non-quantum mechanical (classical) computers.

While there's no great need to factor numbers as small as 15, the research demonstrates that quantum computation is feasible with existing technology and could in principle be scaled up to tackle problems that would take longer than the age of the universe to solve with any classical computer, but would require only minutes on a quantum computer.

In addition to factoring large numbers and solving other challenging mathematical problems, quantum computers based on the work of these two groups could help model quantum mechanical problems in physics and chemistry (see example of a quantum simulator experiment by C.-Y. Lu, et al.), and lead to ultra high speed searching algorithms.

Chao-Yang Lu and his group are currently expanding on their work by trying to manipulate larger numbers of quantum bits. In the long run, they plan to add quantum memory to their quantum computers, which could further increase the number of photons they can control. In addition, because the loss of photons is a huge problem for light-based quantum computation, they are working on some basic quantum codes that can protect the quantum information from photon loss error. These sorts of issues are crucial in the effort to scale up photonic quantum computation. - JR

Dark Matter Stars

Douglas Spolyar, Katherine Freese, and Paolo Gondolo
Physical Review Letters

Before stars were fueled by nuclear fusion, they may have been fueled by dark matter. Researchers have theorized that "Dark Stars" may have been supported by the huge release of energy from dark matter annihilation (i.e. the release of energy that comes when matter and antimatter encounter each other) in the early universe. The physicists from UC Santa Cruz, UM Ann Arbor, and the University of Utah believe that despite many theories stating otherwise, dark matter did have an effect on the first stars in the universe.

The release of energy from dark matter/anti-dark matter annihilation may have prevented the first proto-stars from collapsing and beginning fusion, but in turn could have heated a star’s core enough to support it. This would change the time scale of the formation of second generation stars, the appearance of elements like nitrogen, carbon, and oxygen in our universe, and other aspects of stellar evolution.

Products of the annihilation, such as neutrinos, gamma-rays, or antimatter may make these dark stars or their remnants detectable. Although stars composed of dark matter are likely to be much dimmer than normal stars, they may produce some light. The next step for researchers will be to determine how much visible light the dark stars give off, and how long they survive. Dark stars may have died out millions of years ago, or they may still exist today.

The idea of dark stars relies on the Lightest Super symmetric Particle (LSP), a highly favored candidate for particles that make up dark matter. The properties of the LSPs are consistent with current information about dark matter in the universe. Many physicists are hopeful that new experiments in particle colliders will soon yield more discoveries on the nature of dark matter, and perhaps offer insight into the possibility of dark stars in the early universe. - CC

Calla Cofield and James Riordon contributed to this Tip Sheet.

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