第1期第4期.docx

1、第1期第4期目 录2006第1期第4期纳米前沿New look for nanodetection4DNA pyramids make their debut5Nanotubes beam out bright light9Nanotubes break superconducting record11Nanotube foams are strong and flexible12通信快讯Search engines are not unfair12DCS Announces OptiWatch Intelligent Fiber System 28Ricoh Launches Encrypt

2、Ease CD-R Hybrid Security Disc28物理新知Breakthrough for quantum measurement1 Faster Plastic Circuits6Doing physics with bacteria9Quantum Memory Advancement15Quantum chaos demonstrated during atom ionisation for the first time21Photons create primitive quantum network23Quantum gravity for real24科界动态Calc

3、ulating with Bose condensates2 Fluids mix in reverse3Ions trapped on a chip8Optical devices get fishy10Electrons lose their mass in carbon sheets17Cosmic magnetism revealed18Pseudogap puzzle for superconductors19Entanglement reaches new levels 20Theorists claim dark energy does not exist25The import

4、ance of staying clean26Silicon chip puts the brakes on light26Fluid lenses feel the pressure27主办单位：电子科技大学图书馆 编员会：张晓东 黄思芬 汪育健 侯壮 曹学艳 李世兰 刘静 张宇娥 张毓晗 黄崇成 陈茂兴本期责编：张毓晗 编辑部电话：(028)83202340 2006年3月10日出版 Breakthrough for quantum measurementTwo teams of physicists have measured the capacitance of a Josephson

5、 junction for the first time. The methods developed by the two teams could be used to measure the state of quantum bits in a quantum computer without disturbing the state.A Josephson junction consists of two superconducting layers separated by a thin insulating layer. Brian Josephson of Cambridge Un

6、iversity won the Nobel prize in 1973 for predicting, while he was still a PhD student, that the Cooper pairs in the superconducting layers would be able to tunnel through the insulating layer without losing their superconducting properties. Josephson junctions are widely used in many electronic devi

7、ces, including logic circuits, memory cells and amplifiers. Superconducting quantum interference devices (SQUIDs), also rely on the junctions to measure extremely small magnetic fields.Figure 1In the classical regime, the junction behaves like an inductance. In the 1980s, however, theorists predicte

8、d that a Josephson junction would behave like a capacitor if it was small enough. Now, Per Delsing and colleagues at Chalmers University of Technology in Sweden, and independently, Pertti Hakonen and co-workers at Helsinki University of Technology and the Landau Institute of Theoretical Physics in M

9、oscow have observed this effect in experiments for the first time. Figure 2The Sweden team measured the effect in a Cooper-pair transistor, a device that contains two Josephson junctions in series (Phys. Rev. Lett. 95 206806). The Helsinki-Moscow group saw the effect in a Cooper-pair box, which cont

10、ains one junction (Phys. Rev. Lett. 95 206807). Delsing and colleagues at Chalmers University began by embedding their Cooper-pair transistor in a resonant circuit. Next, they cooled the device down to millikelvin temperatures and measured how the phase of a radio-frequency signal changed when it wa

11、s reflected from the circuit. Based on these measurements, the team was able to show that the device behaved like a quantum capacitor. Hakonen and co-workers in Helsinki and Moscow group employed a similar technique. Both teams found that the devices behaved as predicted by theory. The effect could

12、be used to read out quantum bits (qubits) in a reliable way because the quantum capacitance of the excited state of the qubit has the opposite sign to the ground state. These states could be used as the 1s and 0s in a quantum computer. Indeed Hakonen and colleagues have already used this approach to

13、 read the value of a qubit without changing its value - which is almost always a problem when measuring the quantum state of any system. In the future, the Josephson capacitance could be used for operations in a large-scale quantum computer, says Mika Sillanpaa of Helsinki University. The Josephson

14、inductance and Josephson capacitance together would also allow us to build new types of quantum band engineered electronic devices, such as low-noise parametric amplifiers.http:/physicsweb.org/articles/news/9/11/13/1Calculating with Bose condensatesCould Bose Einstein condensates be used as calculat

15、ors? The answer to this question could be yes according to physicists in France and Italy. Yvan Castin and colleagues of the Kastler Brossel Laboratory in Paris and Sandro Stringari at the University of Trento say that condensates could be used to experimentally observe the roots of mathematical exp

16、ressions called random polynomials for the first time (Phys. Rev. Lett. 96 040405). The results might help improve our understanding in many areas of physics - such as chaos, for example - in which random polynomials or matrices are often appliedPolynomials are mathematical expressions involving a s

17、um of powers in one or more variables multiplied by coefficients. Random polynomials are simply polynomials with random coefficients that have a Gaussian or bell-shaped probability distribution. Solving the roots of random polynomials is an important field in theoretical physics and although such po

18、lynomials have been extensively studied, no one has ever seen what these expressions might actually look like. Now, Castin and colleagues have shown that the location of vortices in a rotating 2D Bose-Einstein gas could be used to physically represent the roots of a polynomial. A Bose-Einstein conde

19、nsate (BEC) is an ultra-cold cloud of gas atoms that are all in the same quantum state, and can therefore be described by the same wavefunction. Quantum vortices can form in these condensates if they are rapidly rotated. According to Castin and co-workers, the wavefunction of a rotating condensate f

20、ormed with non-interacting atoms can be described by a random polynomial. Figure 1A vortex sits at each location where the wavefunction vanishes, and it can be associated with a root of the polynomial. Each root is a complex number with a real and imaginary part, which can be viewed as the two spati

21、al coordinates of the vortex. The roots interact with each other and this is represented by the vortices repelling each other. Figure 2The team says that the fictitious BEC formed by the roots is interesting in this context because it is a very rare example of an exactly solvable many-body problem i

22、n physics. The mathematical theory of random polynomials and random matrices has already found many applications in physics, says team member Jean Dalibard. Indeed the Hamiltonian of a complex or chaotic system can often be viewed as such a matrix. So how did the France-Italy team find the unlikely

23、connection between random polynomial theory and BECs? We were interested in the case of interacting atoms in which the roots of the polynomials - that is the location of the quantized vortices - form a regular array, explains Dalibard. To our surprise we noticed that even for strictly non-interactin

24、g atoms, a local order of the vortex distribution remained. This is exactly what happens in mathematical random polynomials - although the coefficients of the polynomial are independent, the root distribution still exhibits some correlations. Fluids mix in reverseWhen you stir cream into your cup of

25、 coffee, you would amazed to see the two fluids return to where they started simply by reversing the direction of stirring. However, a team of physicists in the US and Israel has now discovered that such mixing can indeed be reversible under certain conditions. The work could be important for mixing

26、 processes in industry and biology (Nature 438 997).David Pine of New York University and colleagues at the Haverford College, the California Institute of Technology and the Israel Institute of Technology studied the motion of tiny polymer beads suspended in a viscous fluid trapped between two conce

27、ntric cylinders held 2.5 millimetres apart. When the team rotated the inner cylinder in one direction and then back again, they found that the beads returned to their starting positions. But the behaviour is only seen if the solution is relatively dilute and the beads are stirred for a short time. A

28、t higher concentration and longer times, mixing becomes irreversible. According to the researchers, the observed behaviour can be explained by collisions between individual beads. Mixing can be reversed if the particles do not collide with each other, which is the case at low concentrations. But as

29、the solution becomes more concentrated - and more collisions occur - the process becomes irreversible. The irreversibility of these particles may be explained by the extreme sensitivity of their trajectories to imperceptibly small changes of the particle positions, explains Pine. Such perturbations

30、might arise from almost anything from small imperfections in the particles or by small external forces - and are magnified exponentially because of the motion of other particles suspended in the liquid, he says. Physical systems that exhibit such extreme sensitivity to small perturbations are said t

31、o be chaotic, which means that their behaviour cannot be determined in advance. The US-Israel team says that an irreversible flow could be transformed into a reversible one at a predictable point by reducing the number of particles since this makes collisions between the particles less likely. This

32、could be important for scaling up laboratory experiments to industrial levels, which is difficult simply because of the unpredictable behaviour of the particles involved. Possible applications include mixing of pharmaceutical suspensions and the catalysis of petrochemicals in fluid beds. The work could also help in understanding particle migration during ceramic processing and in the culture of blood-making cells.http:/physicsweb.org/arti