4,992 research outputs found
Coherent control of macroscopic quantum states in a single-Cooper-pair box
A small superconducting electrode (a single-Cooper-pair box) connected to a
reservoir via a Josephson junction constitutes an artificial two-level system,
in which two charge states that differ by 2e are coupled by tunneling of Cooper
pairs. Despite its macroscopic nature involving a large number of electrons,
the two-level system shows coherent superposition of the two charge states, and
has been suggested as a candidate for a qubit, i.e. a basic component of a
quantum computer. Here we report on time-domain observation of the coherent
quantum-state evolution in the two-level system by applying a short voltage
pulse that modifies the energies of the two levels nonadiabatically to control
the coherent evolution. The resulting state was probed by a tunneling current
through an additional probe junction. Our results demonstrate coherent
operation and measurement of a quantum state of a single two-level system, i.e.
a qubit, in a solid-state electronic device.Comment: 4 pages, 4 figures; to be published in Natur
Coulomb implosion mechanism of negative ion acceleration in laser plasmas
Coulomb implosion mechanism of the negatively charged ion acceleration in
laser plasmas is proposed. When a cluster target is irradiated by an intense
laser pulse and the Coulomb explosion of positively charged ions occurs, the
negative ions are accelerated inward. The maximum energy of negative ions is
several times lower than that of positive ions. The theoretical description and
Particle-in-Cell simulation of the Coulomb implosion mechanism and the evidence
of the negative ion acceleration in the experiments on the high intensity laser
pulse interaction with the cluster targets are presented.Comment: 4 page
Demonstration of conditional gate operation using superconducting charge qubits
Since the first demonstration of coherent control of a quantum state of a
superconducting charge qubit a variety of Josephson-junction-based qubits have
been implemented with remarkable progress in coherence time and read-out
schemes. Although the current level of this solid-state device is still not as
advanced as that of the most advanced microscopic-system-based qubits, these
developments, together with the potential scalability, have renewed its
position as a strong candidate as a building block for the quantum computer.
Recently, coherent oscillation and microwave spectroscopy in
capacitively-coupled superconducting qubits have been reported. The next
challenging step toward quantum computation is a realization of logic gates.
Here we demonstrate a conditional gate operation using a pair of coupled
superconducting charge qubits. Using a pulse technique, we prepare different
input states and show that they can be transformed by controlled-NOT (C-NOT)
gate operation in the amplitude of the states. Although the phase evolution
during the gate operation is still to be clarified, the present results are a
major step toward the realization of a universal solid-state quantum gate
Resonant Cooper-Pair Tunneling: Counting Statistics and Frequency-Dependent Current Noise
We discuss the counting statistics and current noise associated with the
double Josephson quasiparticle resonance point in a superconducting single
electron transistor. The counting statistics are in general phase-dependent,
despite the fact that the average current has no dependence on phase. Focusing
on parameter regimes where the counting statistics have no phase-dependence, we
use a general relation first derived by MacDonald in 1948 to obtain the full
frequency-dependent shot noise directly from the counting statistics, without
any further approximations. We comment on problems posed by the
phase-dependence of the counting statistics for the finite-frequency noise.Comment: 13 pages, 2 figures; to appear in the proceedings of the NATO ASI
"New Directions in Mesoscopic Physics", Erice, 200
Quantum oscillations in two coupled charge qubits
Despite an apparent progress in implementing individual solid-state qubits,
there have been no experimental reports so far on multi-bit gates required for
building a real quantum computer. Here we report a new circuit comprising two
coupled charge qubits. Using a pulse technique, we coherently mix quantum
states and observe quantum oscillations whose spectrum reflects interaction
between the qubits. Our results demonstrate the feasibility of coupling of
multiple solid-state qubits and indicate the existence of entangled two-qubit
states.Comment: 4 pages, 4 figures, submitted to Natur
Constraints for the nuclear parton distributions from Z and W production at the LHC
The LHC is foreseen to finally bring also the nuclear collisions to the TeV
scale thereby providing new possibilities for physics studies, in particular
related to the electro-weak sector of the Standard Model. We study here the Z
and W production in proton-lead and lead-lead collisions at the LHC,
concentrating on the prospects of testing the factorization and constraining
the nuclear modifications of the parton distribution functions (PDFs).
Especially, we find that the rapidity asymmetries in proton-nucleus collisions,
arising from the differences in the PDFs between the colliding objects, provide
a decisive advantage in comparison to the rapidity-symmetric nucleus-nucleus
case. We comment on how such studies will help to improve our knowledge of the
nuclear PDFs.Comment: The version accepted for publication in JHEP. New figures has been
added, and we also discuss the single charged lepton productio
Voluntary exercise can strengthen the circadian system in aged mice
Consistent daily rhythms are important to healthy aging according to studies linking disrupted circadian rhythms with negative health impacts. We studied the effects of age and exercise on baseline circadian rhythms and on the circadian system's ability to respond to the perturbation induced by an 8 h advance of the light:dark (LD) cycle as a test of the system's robustness. Mice (male, mPer2luc/C57BL/6) were studied at one of two ages: 3.5 months (nβ=β39) and >18 months (nβ=β72). We examined activity records of these mice under entrained and shifted conditions as well as mPER2::LUC measures ex vivo to assess circadian function in the suprachiasmatic nuclei (SCN) and important target organs. Age was associated with reduced running wheel use, fragmentation of activity, and slowed resetting in both behavioral and molecular measures. Furthermore, we observed that for aged mice, the presence of a running wheel altered the amplitude of the spontaneous firing rate rhythm in the SCN in vitro. Following a shift of the LD cycle, both young and aged mice showed a change in rhythmicity properties of the mPER2::LUC oscillation of the SCN in vitro, and aged mice exhibited longer lasting internal desynchrony. Access to a running wheel alleviated some age-related changes in the circadian system. In an additional experiment, we replicated the effect of the running wheel, comparing behavioral and in vitro results from aged mice housed with or without a running wheel (>21 months, nβ=β8 per group, all examined 4 days after the shift). The impact of voluntary exercise on circadian rhythm properties in an aged animal is a novel finding and has implications for the health of older people living with environmentally induced circadian disruption
Cooling a nanomechanical resonator with quantum back-action
Quantum mechanics demands that the act of measurement must affect the
measured object. When a linear amplifier is used to continuously monitor the
position of an object, the Heisenberg uncertainty relationship requires that
the object be driven by force impulses, called back-action. Here we measure the
back-action of a superconducting single-electron transistor (SSET) on a
radiofrequency nanomechanical resonator. The conductance of the SSET, which is
capacitively coupled to the resonator, provides a sensitive probe of the
latter's position;back-action effects manifest themselves as an effective
thermal bath, the properties of which depend sensitively on SSET bias
conditions. Surprisingly, when the SSET is biased near a transport resonance,
we observe cooling of the nanomechanical mode from 550mK to 300mK-- an effect
that is analogous to laser cooling in atomic physics. Our measurements have
implications for nanomechanical readout of quantum information devices and the
limits of ultrasensitive force microscopy (such as single-nuclear-spin magnetic
resonance force microscopy). Furthermore, we anticipate the use of these
backaction effects to prepare ultracold and quantum states of mechanical
structures, which would not be accessible with existing technology.Comment: 28 pages, 7 figures; accepted for publication in Natur
Rare Codons Cluster
Most amino acids are encoded by more than one codon. These synonymous codons are not used with equal frequency: in every organism, some codons are used more commonly, while others are more rare. Though the encoded protein sequence is identical, selective pressures favor more common codons for enhanced translation speed and fidelity. However, rare codons persist, presumably due to neutral drift. Here, we determine whether other, unknown factors, beyond neutral drift, affect the selection and/or distribution of rare codons. We have developed a novel algorithm that evaluates the relative rareness of a nucleotide sequence used to produce a given protein sequence. We show that rare codons, rather than being randomly scattered across genes, often occur in large clusters. These clusters occur in numerous eukaryotic and prokaryotic genomes, and are not confined to unusual or rarely expressed genes: many highly expressed genes, including genes for ribosomal proteins, contain rare codon clusters. A rare codon cluster can impede ribosome translation of the rare codon sequence. These results indicate additional selective pressures govern the use of synonymous codons, and specifically that local pauses in translation can be beneficial for protein biogenesis
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