Physics News Update No. 618

Tuning Carbon Nanotube Resonance Frequencies can be achieved by varying a static voltage applied between the nanotubes and a counter electrode. This was demonstrated for several nanotubes (NT) grown on a nickel support tip. The natural resonant NT frequencies are those where the NT oscillates with a large amplitude, a motion that can be monitored directly by watching the pattern of electrons spraying out the end of the tubes (like water spraying out of a wiggling garden hose). The vibration is excited by applying an additional sinusoidal voltage of the correct frequency to one of the electrodes (see figure at www.aip.org/mgr/png/2002/173.htm). This technique gives researchers yet another handle for manipulating the versatile NT's for what promises to be wide variety of applications. According to Stephen Purcell of the University of Lyon (stephen.purcell@dpm.univ-lyon1.fr), one of the co-authors on a new paper reporting the results, carefully excited and tuned NT's may act as the core for future nanometric oscillator circuitry, nano-balances or nano-force sensors. (Purcell et al., Physical Review Letters, 30 December 2002; for a past summary of NT thermal, electrical, and optical emissions, see Update 580, http://www.aip.org/enews/physnews/2002/split/580-2.html.)

Quantum Simulations With Continuous Variables. Furthering efforts to answer hard-to-test questions about the quantum world, a NIST ion-trap computer can now simulate how the unique rules of quantum mechanics can affect a microscopic particle's "continuous variables," quantities such as position and momentum which can have a smooth continuum of values. Acting as a form of quantum computer, the NIST ion trap might only need a couple of seconds to simulate a quantum physics experiment that can take days to carry out. Moreover, the ion trap can simulate experiments that require rare commodities, like entangled photons, which are created relatively infrequently. Since quantum computers embrace the unusual logic of the microscopic world, they can perform powerful simulations of its often counterintuitive phenomena.

First envisioned by Richard Feynman, quantum simulators are perhaps the earliest practical application of quantum computing--in fact, they have been around for several years now. However, previous versions (Update 438, http://www.aip.org/enews/physnews/1999/split/pnu438-2.htm) have only re-created quantum phenomena involving "discrete variables," such as an electron's energy in an atom, which can only have certain prescribed values. The new version recreates quantum processes involving both discrete and continuous variables. To construct their simulator, NIST researchers in Colorado trap a single beryllium-9 ion with electric fields. As the ion vibrates in the trap, its position and momentum are continuous. This allows the researchers to easily simulate any other complementary pair of continuous variables-such as an electric field's amplitude and phase-which have the exact same mathematical interrelationship. To perform simulations, the researchers shine a series of carefully engineered light pulses on the ion. The pulses cause the ion to act like something it's not, such as an electron bound by an atom, or even a photon as it hits a beamsplitter. Under the influence of the pulses, the ion's quantum states evolve in a way identical to the situation the researchers want to study.

For now, the researchers have performed simple, proof-of-principle demonstrations. As an example, they have investigated how a photon would behave if entangled with other photons by sending it through a beamsplitter. Shining light pulses on the ion to simulate the effects of a beamsplitter on a photon, the researchers have demonstrated that interferometry with up to three other entangled photons would be three times as precise as interferometers using single photons, in line with the recent experimental results on bi-photon interferometry (Update 613, http://www.aip.org/enews/physnews/2002/split/613-1.html). (Leibfried et al, Physical Review Letters, 9 December 2002; Dietrich Leibfried, 303-497-7880, dil@boulder.nist.gov)

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