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| Issue 12, 2024 |
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Extracts from the Internet | ||
Testing of Bell's inequalities in a system of Josephson qubits1 November 2009Researchers at the University of California at Santa Barbara experimentally confirmed that Bell's inequalities are violated in a macroscopic system composed of two Josephson qubits (quantum bits) implemented using superconducting Josephson contacts. Violation of Bell's inequalities has already been tested in a number of quantum processes which excluded the possibility of the “hidden-variables” interpretation. In the new experiment two Josephson qubits were transferred to entangled quantum state (Phys. Usp. 49 1111 (2006)) using an electromagnetic resonator after which quantum correlations between the states of qubits were measured. For this particular experiment, Bell's inequality can be written in the form S<2 where S depends on the state of qubits. Measurements showed that S = 2.0732 ± 0.0003 i.e. Bell's inequality is violated at the level of 244 standard deviations and therefore the state of the qubits in this experiment cannot be classically described. Source: Nature 461 504 (2009) Persistent current in a ring1 November 2009J. Harris (Yale University, USA) and his co-workers measured for the first time the persistent electric current in metallic (non-superconducting) rings. The persistent ring current predicted theoretically by Ì. Buttiker, Y. Imry and R.B. Landauer in 1983 is an element of the equilibrium quantum state of electrons in the ring. It was predicted that the persistent current in micron-size rings at temperatures T<1 Ê may reach 1 nÀ. The external magnetic field breaks the time reversal symmetry which forces one of the possible current directions to be selected, plus this current is a periodic function I(Φ) of the magnetic flux Φ across the ring (due to the Aharonov – Bohm effect). Attempts were made earlier to use superconducting quantum interferometers (SQUIDs) to measure the magnetic field generated by the persistent current but the sensitivity of this method proved insufficient owing to, among other factors, the feedback to the ring current from oscillations of the superconducting current in SQUIDs. J. Harris and his team measured the effect induced by the magnetic field of the current in the micromechanical silicon cantilever probe. Aluminum rings were attached to the end of the probe and the system measured the shift of the resonance frequency of mechanical vibrations of the cantilever due to the interaction between the magnetic moments of ring currents and the external magnetic field. The oscillations of the cantilever were caused by a piezo-mechanical vibrator and were observed using a laser interferometer. This technique has the sensitivity (≈20 pÀ Hz-1/2) approximately an order of magnitude better than the approach based on SQUIDs. The results of measurements both with a single aluminum ring and with a large set of rings are in good agreement with theoretical predictions. It was found that current oscillations I(Φ) in different rings of the set had uncorrelated phases both in the first and the second harmonics. Measuring persistent currents in microscopic rings can help in studying quantum phase transitions and quantum coherence at low temperatures. Source: Science 326 272 (2009) Splitting of Cooper pairs1 November 2009Two independent groups of researchers created efficient sources of electrons entangled in spin states (EPR-pairs); they act by splitting Cooper pairs of electrons tunneling through a superconductor. The difficulty that faced earlier attempts to generate EPR pairs stemmed from the fact that electrons in metals sit below the Fermi surface so that releasing them immediately destroys entanglement. The ground state in superconductors is the condensate of Cooper pairs in spin-singlet state so these pairs can be separated from the superconductor by way of tunneling. L. Hofstetter and his colleagues in Switzerland, Hungary and Denmark solved the remaining problem of splitting Cooper pairs into individual electrons by using the Coulomb repulsion of the electrons that went through tunneling into two quantum dots. Quantum dots would appear in the region where the nanowire made of indium arsenide intersected with the central superconductor and two metal contacts in normal (non-superconducting) state. Control electrodes make it possible to vary the depth of the potential well of quantum dots in such a way that only one electron passed through each quantum dot at any one moment of time. The electrons of a Cooper pair are inherently quantum-correlated (entangled) along the direction of spin and this entanglement of electrons survived after the pair was split. The experiment of L.G. Herrmann and colleagues in France, Spain and Germany generally resembles the one described above but it used carbon nanotubes instead of nanowire. New sources of EPR pairs of electrons may find important applications in fundamental research, such as the study of the Einstein – Podolsky – Rosen paradox. Sources: Nature 461 960 (2009) ; arXiv:0909.3243v1 [cond-mat.mes-hall] Bose – Einstein condensate of calcium atoms1 November 2009S. Kraft (Federal Physico-Technical Institute, Germany) and his co-workers were able to produce for the first time the Bose – Einstein condensate of atoms of the alkali earth metal, 40Ca. At the initial stage of the experiment atoms were laser-cooled in a magneto-optical trap using the transitions 1S0-1P1, 1S0-3P1 etc. The main process that restricted the effectiveness of cooling was losses via three-particle atomic interactions. At the last stage the atomic cloud was loaded into an optical dipole trap and cooled evaporatively. The transition of about 2×104 atoms to the Bose – Einstein condensate state at 170 nK was identified using the characteristic Gaussian density profile. The formation of the condensate was also confirmed by the large chemical potential of the gas, which was calculated on the basis of the measured velocities of anisotropic expansion of the cloud. Quantum intercombination transitions with very narrow spectral lines 1S0-3P1 (only 370 Hz) are possible in atoms of 40Ca. Consequently, Bose – Einstein condensates of 40Ca atoms are very promising for highest-precision measurements of the gravitational field. Furthermore, 40Ca atoms in the non-degenerate ground state have no magnetic moment, which additionally increases the accuracy of measurements in view of the absence of interaction with external magnetic fields. Source: Phys. Rev. Lett. 103 130401 (2009) Magnetism of carbon1 November 2009Researchers in Czechia and the Netherlands J. Cervenka, M.I. Katsnelson and C.F.J. Flipse have clarified the mechanism of formation of ferromagnetic properties of specimens of polycrystalline graphite at room temperatures. A number of experiments established that various forms of carbon manifest ferromagnetism (see e.g. Phys. Usp. 47 102 (2004)) but its nature remained unclear. Hypotheses were advanced that carbon owes the observed magnetism either to metal impurities or to defects of crystal structure. The new experiment used a magnetic force microscope, a SQUID magnetometer and an atomic force microscope for measurements; this made it possible to study simultaneously and at high spatial resolution both magnetic and electronic properties of samples. The obtained microscopic images provide direct evidence supporting the second hypothesis: the graphite magnetism arises owing to defects in the structure of atomic layers and that impurities do not play a significant role. A two-dimensional network of defects (only 2 nm thick each) is formed along atomic planes of carbon in graphite; these networks delimit homogeneous regions, i.e. grains of the polycrystal. Ferromagnetism originates with unpaired electrons localized on defects at grain boundaries. Magnetic carbon may find applications in spintronic devices and in medical fields for designing biological sensors. Source: Nature Physics 5 840 (2009) ; arXiv:0910.2130v1 [cond-mat.mtrl-sci] |
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources. | |
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