EXACT FUNDAMENTAL LIMIT TO THE SPEED OF PHYSICAL COMPUTATION IS DETERMINED
We have established the exact fundamental limit on the rate (number per
operations per second) of any computing system. No system can exceeed this
limit. However, a system with suitably chosen parameters and states can approach
the limit as closely as desired.
The limit is set by the minimum time required for a physical computing system
to perform an elementary operation, namely, the transition from a given state
to another state distinguishable from it with certainty. This minimum time
follows from the laws of quantum mechanics and depends on the average energy
and the energy spread (uncertainty) of the system's state. Since the maximum
rate is determined by the smallest of those two parameters, the optimum is
achieved when they are equal. Then, adding one joule of energy to the system
allows its rate to increase by 6*10^32 elementary operations per second.
Today's computers are slower by many orders of magnitude. Our research reveals
the prospects for future development---what is possible to achieve and what is
required for that.
Note that our results amount to a general "law of nature": the limit does not
depend on a specific model of computation or a specific technology.
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CH10233
Radioactive half-lives do not depend on temperature after all
If the half-lives of radioactive isotopes could be altered by changes in the temperature of their surroundings, then it might be possible to speed up the disposal of radioactive waste. Hopes for this simple solution to one of society’s problems were raised several years ago by published reports of appreciable half-life changes having been observed at low temperatures. These claims have now been scotched.
The original reports encompassed the three most-common forms of radioactivity, referred to as alpha, beta and electron-capture decays. Since then, however, well controlled and more precise measurements have discredited the claims one by one. First, it was demonstrated that beta-decay half-lives are, in fact, stable to within less than a tenth of a percent at low temperature; then the claimed changes in alpha-decay half-lives were shown to be wrong. Now, the third class of radioactivity, electron-capture decay, has also been proven stable to within a tenth of a percent (CH10233).
For nearly a century, physicists have considered the half-lives of radioactive isotopes to be independent of temperature, but without modern proofs. Now, it turns out that they were right all along.
***
LG12785
Magnetic Monopoles in the Mirror
A novel class of materials with topological order, called topological
insulators, has recently been proposed theoretically and has been
observed experimentally. One striking aspect of topological insulators
is that when an electric charge is brought close to their surface it
behaves as if it had a magnetic monopole partner on the other side of
the interface: when seen in the mirror of a topological insulator, an
electron looks like a monopole. In this work I re-obtain expressions for
this mirror charge in a formalism that treats electric and magnetic
charges on an equal footing. This “duality covariant formalism” helps to
present compact expressions for the final result and hopefully paves the
way for a deeper understanding of these fascinating materials.
***
LE12655
Creating superpositions of quantum operations to directly prove the
commutation relation
The commutation relation is one of the pillars of quantum mechanics and
brings about a fundamental quantum paradox, the uncertainty principle,
according to which precise simultaneous measurements of two
non-commutative observables is impossible.
Although fundamental and ubiquitous, the commutation relation is often
learnt as a mere mathematical notion and little physical insight is ever
gained. For a light field, it really means that annihilating a photon
after its creation is different from the opposite sequence of
operations, and the difference is equal to the identity operator.
Here we provide the first direct experimental verification of the
commutation relation by means of a general scheme for superposing
distinct quantum operations. In the language of the Schrödinger's cat
paradox, it corresponds to having implemented a new versatile and
powerful tool, allowing one to superpose the two operations "to kill"
and "not to kill", so that they may be eventually applied to any
"animal" (i.e., to any quantum state, including macroscopic classical
ones), not just unfortunate "cats".
Besides clearly illustrating one of the main textbook concepts of
quantum mechanics, the possibility of implementing general operator
superpositions is an alternative promising tool to control and engineer
quantum information for future technologies.
***
CJ10179
Addressing Nuclear Data Needs for Thorium-Based Nuclear Reactors
The energy crisis is in the forefront of the international arena and thorium-based nuclear energy systems provide a channel for massive energy generation without the overproduction of ozone-depleting greenhouse gases. The thorium-uranium fuel cycle has several advantages, with respect to nuclear nonproliferation and radioactive waste management, when compared with the conventional uranium-plutonium fuel cycle. However, much of the experimental nuclear data required for design calculations for thorium-based reactor systems are not precise or even absent. To achieve improved design calculations for thorium-based reactors, the determination or reevaluation of neutron-induced fission cross sections for short-lived radioactive isotopes of thorium is required. In this paper, we present the first measurement of the 231Th(n,f) cross section, formerly a major source of uncertainty in thorium-based reactor design calculations, to an unprecedented level of accuracy. This work fosters responsible environmental practices through sustainable nuclear energy.
***
LC12048
Experimental indication of the Majorana fermions on a surface of superfluid 3He.
Although particles are usually different from their antiparticles, Majorana fermions have a unique property: they are their own antiparticles.
They were originally posed in the field of the elementally particle physics but have not been identified yet. Theoretical developments, however,
have shown that Majorana fermions should also appear on a surface of condensed materials if their bulk properties are topologically nontrivial.
One of such material is superfluid 3He at ultra-low temperatures which has been a model system of quantum condensates with internal degrees of
freedom. In this paper, we made a systematic study of the surface states of superfluid 3He by the transverse acoustic impedance measurement with
controlling a roughness of a wall. Indication of gapless excitations with a linear dispersion, which are referred to as the Majorana cone, was
experimentally obtained if the roughness is small enough.
***
EGJ1041
Cell Adhesion
Adhesion is fundamental in cell functioning, and in various inter-cell activities of biological tissues. When an external pulling force is applied onto a cell stuck to its substrate, a reacting "suction-cup” force, due to the slow penetration of the surrounding fluid between the cell and the substrate, opposes to the separation. It can overcome other known adhesive forces when the process is sufficiently violent (typically 105N/m2). The physical origin of this effect may be compared with that leaning a suction-cup against a bathroom wall. Indeed, when the cell begins to move, the pressure under the cell diminishes, which leads to penetration of the surrounding liquid between the cell and its substrate. Then, the induced pressure difference between the top and the underside of the cell generates the suction-cup force. In contrast to similar hydrodynamic forces caused, for instance, by shear flow, the suction-cup force is purely attractive. The suction-cup force can answer to problems and questions raised
by experimentalists.
***
LZ11637BR
Graphene quantum dot
Quantum dots defined by electrostatic potential were realized in a
graphene nano-ribbon device sandwiched between a narrow top gate and a
back gate. The dot consists of a small local island filled with holes,
separated by pn junctions from the neighboring regions filled with
electrons. This is one of the first experiments where pn junctions are
intentionally used for confining electrons and holes. The quantum dot
devices demonstrated here would enable one to measure single particle
energies of Dirac particles in graphene, study their spin and valley
degrees of freedom, and explore their potential for quantum coherent
control. Our work is an important starting point for further quantum
dots applications where great tunability is desired.
On the other hand, we also found that without the intentional barriers,
in the high density limit, electrons become localized in one large
island, in contrast to the multiple localized islands found in previous
work. The reason for this localization is not yet understood.
