| Notable Physics Abstracts | (39/2007) |
| collected by Rainer W. Scharf |
|
| Coherent quantum state
storage and transfer between two phase qubits via a resonant cavity
Mika A. Sillanpää, Jae I. Park & Raymond W. Simmonds As with classical information processing, a quantum information processor requires bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states, and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits. Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large-scale quantum information processor. Although these systems have successfully passed tests of coherent coupling for up to four qubits, communication of individual quantum states between superconducting qubits via a quantum bus has not yet been realized. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a quantum bus. This quantum bus is a resonant cavity formed by an open-ended superconducting transmission line of length 7 mm. After preparing an initial quantum state with the first qubit, this quantum information is transferred and stored as a nonclassical photon state of the resonant cavity, then retrieved later by the second qubit connected to the opposite end of the cavity. Beyond simple state transfer, these results suggest that a high-quality-factor superconducting cavity could also function as a useful short-term memory element. The basic architecture presented here can be expanded, offering the possibility for the coherent interaction of a large number of superconducting qubits. Nature 449, 438 (27.9.2007)
Coupling superconducting
qubits via a cavity bus
Superconducting circuits are promising candidates for constructing quantum bits (qubits) in a quantum computer; single-qubit operations are now routine, and several examples of two-qubit interactions and gates have been demonstrated. These experiments show that two nearby qubits can be readily coupled with local interactions. Performing gate operations between an arbitrary pair of distant qubits is highly desirable for any quantum computer architecture, but has not yet been demonstrated. An efficient way to achieve this goal is to couple the qubits to a 'quantum bus', which distributes quantum information among the qubits. Here we show the implementation of such a quantum bus, using microwave photons confined in a transmission line cavity, to couple two superconducting qubits on opposite sides of a chip. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. Using fast control of the qubits to switch the coupling effectively on and off, we demonstrate coherent transfer of quantum states between the qubits. The cavity is also used to perform multiplexed control and measurement of the qubit states. This approach can be expanded to more than two qubits, and is an attractive architecture for quantum information processing on a chip. Nature 449, 443 (27.9.2007)
Antti O. Niskanen & Yasunobu
Nakamura: Qubits ride the photon bus. Nature
449, 415 (27.9.2007)
Jon Cartwright: Microchip
“bus” links up quantum bits (27.9.2007)
 Symmetrized Characterization
of Noisy Quantum Processes
A major goal of developing high-precision control of many-body quantum systems is to realize their potential as quantum computers. A substantial obstacle to this is the extreme fragility of quantum systems to "decoherence" from environmental noise and other control limitations. Although quantum computation is possible if the noise affecting the quantum system satisfies certain conditions, existing methods for noise characterization are intractable for present multibody systems. We introduce a technique based on symmetrization that enables direct experimental measurement of some key properties of the decoherence affecting a quantum system. Our method reduces the number of experiments required from exponential to polynomial in the number of subsystems. The technique is demonstrated for the optimization of control over nuclear spins in the solid state. Science 317, 1893
(28.9.2007)
Dave Bacon: Does Our Universe
Allow for Robust Quantum Computation? Science 317, 1876 (28.9.2007)
Hamish Johnston: A faster
way to better quantum computers (28.9.2007)
Nuclei-Induced Frequency
Focusing of Electron Spin Coherence
The hyperfine interaction of an electron with the nuclei is considered as the primary obstacle to coherent control of the electron spin in semiconductor quantum dots. We show, however, that the nuclei in singly charged quantum dots act constructively by focusing the electron spin precession about a magnetic field into well-defined modes synchronized with a laser pulse protocol. In a dot with a synchronized electron, the light-stimulated fluctuations of the hyperfine nuclear field acting on the electron are suppressed. The information about electron spin precession is imprinted in the nuclei and thereby can be stored for tens of minutes in darkness. The frequency focusing drives an electron spin ensemble into dephasing-free subspaces with the potential to realize single frequency precession of the entire ensemble. Science 317, 1896
(28.9.2007)
Spin-Polarisation mit Langzeitgedächtnis
(27.9.2007)
Emission from Electronically
Excited Metal Atoms during Single-Bubble Sonoluminescence
Variations in sonoluminescence (SL) from an acoustically driven but rapidly translating bubble in solutions of sulfuric acid with alkali-metal salts coincide with variations in translational bubble dynamics. At low acoustic pressures, emission from Ar excited states is observed and the bubble motion is smooth and elliptical. At elevated acoustic pressures, SL intensity decreases, emission from excited alkali-metal atoms is observed, and the bubble motion becomes increasingly erratic with frequent and abrupt changes in direction. These results provide a direct experimental link between single and multibubble SL and point toward the origins of sonochemical reactivity of nonvolatile species. Phys. Rev. Lett. 99,
134301 (27.9.2007)
Nonclassical Rotational
Inertia in Helium Crystals
It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large 4He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon. Phys. Rev. Lett. 99,
135302 (26.9.2007)
Passive Cooling of a Micromechanical
Oscillator with a Resonant Electric Circuit
We cool the fundamental mode of a miniature cantilever by capacitively coupling it to a driven rf resonant circuit. Cooling results from the rf capacitive force, which is phase shifted relative to the cantilever motion. We demonstrate the technique by cooling a 7 kHz cantilever from room temperature to 45 K, obtaining reasonable agreement with a model for the cooling, damping, and frequency shift. Extending the method to higher frequencies in a cryogenic system could enable ground state cooling and may prove simpler than related optical experiments in a low temperature apparatus. Phys. Rev. Lett. 99,
137205 (2007)
Phil Schewe: Radio-Cooled
Macroscopic Object (17.9.2007)
Why macroscopic quantum
tunnelling in Josephson junctions differs from tunnelling of a quantum
particle
We show that the macroscopic quantum tunnelling of a fluxon in a Josephson junction cannot be described, even qualitatively, as the tunnelling of a quantum particle in a potential U(phi), where the phase difference phi plays the role of particle position, if the length of the junction d exceeds a fluxon length. We calculate the probability per unit time of tunnelling (or escape rate), Gamma, which has a form Gamma=A exp(-B). In contrast to particles, where the B is proportional to d, our field-theory predicts a different behavior of B for either usual, 0–pi, or stacks of Josephson junctions, giving rise to a renormalization of Gamma by many orders of magnitude. EPL 80, 17009 (17.9.2007)
Nonergodicity and central-limit
behavior for long-range Hamiltonians
We present a molecular dynamics test of the Central-Limit Theorem (CLT) in a paradigmatic long-range-interacting many-body classical Hamiltonian system, the HMF model. We calculate sums of velocities at equidistant times along deterministic trajectories for different sizes and energy densities. We show that, when the system is in a chaotic regime (specifically, at thermal equilibrium), ergodicity is essentially verified, and the Pdfs of the sums appear to be Gaussians, consistently with the standard CLT. When the system is, instead, only weakly chaotic (specifically, along longstanding metastable Quasi-Stationary States), nonergodicity (i.e., discrepant ensemble and time averages) is observed, and robust q-Gaussian attractors emerge, consistently with recently proved generalizations of the CLT. EPL 80, 26002 (21.9.2007)
Fabrication of doped nano-electromechanical
systems
We present a new generation of nano-electromechanical systems (NEMS), which are realized by doping the semiconductor base material. In contrast to the traditional approach these doped NEMS (D-NEMS) do not require a metallization layer. This makes them far lighter and hence increases resonance frequency and quality factor. Additionally, D-NEMS can be tuned from the conductive state into an insulating one. This will enable a host of new device designs, like mechanically tunable pin-junctions and nanomechanical single electron switches. We demonstrate D-NEMS fabrication and operation from the intrinsic, to the light, and to the heavy regime of doping. phys. stat. sol. (RRL) 1,
205 (30.8.2007)
Loss of entanglement in
quantum mechanics due to the use of realistic measuring rods
We show that the use of real measuring rods in quantum mechanics places a fundamental gravitational limit to the level of entanglement that one can ultimately achieve in quantum systems. The result can be seen as a direct consequence of the fundamental gravitational limitations in the measurements of length and time in realistic physical systems. The effect may have implications for long distance teleportation and the measurement problem in quantum mechanics. Phys. Lett. A: Accepted (20.9.2007)
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