Physics Tip Sheet #63, June 20, 2006
American Physical Society
Highlights in this Issue: 36 hour news cycles, the secret lives of pebbles, a theory of cloudbursts, and entangled quantum entanglement.
Quantum Experiment Breakthrough: Interference of Independent Photons
R. Kaltenbaek et al.
Austrian physicists have managed the first demonstration of interference among independent photons - a phenomenon predicted decades ago, but experimentally unproven until now. The achievement is vital for future quantum computer designs as well as long distance versions of secure communication schemes that rely on quantum mechanics. Specifically, the experiment confirms that it will be possible to build quantum repeaters that transfer quantum information from one portion of a system to a remote portion, which is important if quantum computers and communications are ever to be realized.
At least one group has previously made similar claims (see PRL, 26 March 2006, http://link.aps.org/abstract/PRL/v96/e110501). In the earlier experiment, however, independent photons passed through some shared optical components, which would not be practical for physically separated portions of a real world quantum device and may lead to interactions that compromise the photons’ independence. The latest experiment demonstrates interference between photons that pass through completely separate sets of optical components. In addition to computational and communication applications, the experiment is an important demonstration of photon interactions that cannot be explained with classical physics, and therefore may lead to further experiments testing the foundations of quantum mechanics.
Linking quantum components together is a hot topic at the moment. At least two more PRL papers, one published this week and the other in the works, describe new quantum repeater proposals. One suggests quantum mechanically linking atoms through a mediating electron (A. T. Costa et al., http://link.aps.org/abstract/PRL/v96/e230501), another describes a quantum repeater design that relies on bright, coherent light rather than independent photons (P. van Loock et al., forthcoming PRL). Both papers are available to journalists on request.
Bidding Last Minute is Best on eBay
I. Yang and B. Kahng
If you’ve ever tried to bid in an online auction, the chances are you’ve been sniped. That is, someone came along and placed a high bid just moments before the bidding deadline - eliminating your chances of outbidding them. Many people consider sniping unethical, robbing other bidders of a chance to buy an item and taking money out of sellers’ pockets by stifling fair competition.
Ethical or not, it turns out that sniping is the best way to win an auction. Researchers at Seoul National University have produced a model that mimics bidding behavior on eBay and a Korean auction site (auction.co.kr). The model confirms previous statistical studies of winning bidders that show that people who refrain from bidding at all until the very last seconds are much more likely to win than people who take part in earlier incremental bidding. Although snipers miss out on occasion (if their late bids are not registered in time for the auction close) they are usually successful. Unless online auction companies adjust their rules to extend bidding deadlines when large, last second bids come in (as live auctioneers do), you’re going to be better off sniping if you really must have that rare Pokemon card or Chia Pet planter.
K. Jensen et al.
Researchers at the University of California at Berkeley (UCB) have developed a tunable, nanotube resonator that could lead to exquisitely sensitive and versatile sensors.
Nanoresonators are tiny vibrating beams, bridges or other structures. Because their resonant frequencies are highly dependent on various factors, such as their mass, length, and the stresses they are experiencing, nanoresonators make extremely sensitive measurement devices. (Recently, a nanoresonator-based scale managed to detect mass small enough to register the presence of a single bacterium.) Most nanoresonators operate at a single frequency or a very narrow band of frequencies. If a different frequency is required, you have to build a different resonator.
The UCB nanoresonators, however, are tunable because they are made of telescoping nanotubes that can extend like a trombone slide. By securing the telescoping nanotubes between two surfaces that can be moved relative to each other, the researchers were able to vary the nanoresonator frequencies over ranges of 50 to 75 megahertz. Each nanometer change in length leads to roughly a 1 megahertz shift in frequency, making the nanoresonators highly sensitive position and force sensors as well as tunable mass and frequency measurement devices.
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