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jmlloren

Scientists discover how to turn light into matter after 80-year quest - 5 views

phys.org/...-05-scientists-year-quest.html

PHY MAD

shared by jmlloren on 19 May 14 - No Cached
  • Thijs Versloot on 19 May 14
     
    Theoretized 80 years ago was Breit-Wheeler pair production in which two photons result in an electron-positron pair (via a virtual electron). It is a relatively simple Feynmann diagram, but the problem is/was how to produce in practice a high energy photon-photon collider... The collider experiment that the scientists have proposed involves two key steps. First, the scientists would use an extremely powerful high-intensity laser to speed up electrons to just below the speed of light. They would then fire these electrons into a slab of gold to create a beam of photons a billion times more energetic than visible light. The next stage of the experiment involves a tiny gold can called a hohlraum (German for 'empty room'). Scientists would fire a high-energy laser at the inner surface of this gold can, to create a thermal radiation field, generating light similar to the light emitted by stars. They would then direct the photon beam from the first stage of the experiment through the centre of the can, causing the photons from the two sources to collide and form electrons and positrons. It would then be possible to detect the formation of the electrons and positrons when they exited the can. Now this is a good experiment... :)
  • ...6 more comments...
  • jmlloren on 19 May 14
     
    The solution of thrusting in space.
  • dejanpetkow on 20 May 14
     
    Thrusting in space is solved already. Maybe you wanted to say something different?
  • Thijs Versloot on 21 May 14
     
    Thrusting until your fuel runs out is solved, in this way one can produce mass from, among others, solar/star energy directly. What I like about this experiment is that we have the technology already to do it, many parts have been designed for inertial confinement fusion.
  • johannessimon81 on 29 May 14
     
    I am quite certain that it would be more efficient to use the photons directly for thrust instead of converting them into matter. Also, I am a bit puzzled at the asymmetric layout for photon creation. Typically, colliders use two beam of particle with equal but opposite momentum. Because the total momentum for two colliding particles is zero the reaction products are produced more efficiently as a minimum of collision energy is waisted on accelerating the products. I guess in this case the thermal radiation in the cavity is chosen instead of an opposing gamma ray beam to increase the photon density and increase the number of collisions (even if the efficiency decreases because of the asymmetry). However, a danger from using a high temperature cavity might be that a lot of thermionic emission creates lots of free electrons with the cavity. This could reduce the positron yield through recombination and would allow the high energetic photons to loose energy through Compton scattering instead of the Breit-Wheeler pair production.
  • Thijs Versloot on 30 May 14
     
    Well, the main benefit from e-p pair creation might be that one can accelerate these subsequently to higher energies again. I think the photon-photon cross-section is extremely low, such that direct beam-beam interactions are basically not happening (below 1/20.. so basically 0 according to quantum probability :P), in this way, the central line of the hohlraum actually has a very high photon density and if timed correctly maximizes the reaction yield such that it could be measured.
  • johannessimon81 on 30 May 14
     
    I agree about the reason for the hohlraum - but I also keep my reservations about the drawbacks. About the pair production as fuel: I pretty sure that your energy would be used smarter in using photon (not necessarily high energy photons) for thrust directly instead of putting tons of energy in creating a rest-mass and then accelerating that. If you look at E² = (p c)²+(m0 c)² then putting energy into the mass term will always reduce your maximum value of p.
  • Thijs Versloot on 31 May 14
     
    True, but isnt it E2=(pc)^2 + (m0c^2)^2 such that for photons E\propto{pc} and for mass E\propto{mc^2}. I agree it will take a lot of energy, but this assumes that that wont be the problem at least. The question therefore is whether the mass flow of the photon rocket (fuel consumed to create photons, eg fission/fusion) is higher/lower than the mass flow for e-p creation. You are probably right that the low e-p cross-section will favour direct use of photons to create low thrust for long periods of time, but with significant power available the ISP might be higher for e-p pair creation.
  • johannessimon81 on 03 Jun 14
     
    In essence the equation tells you that for photons with zero rest mass m0 all the energy will be converted to momentum of the particles. If you want to accelerate e-p then you first spend part of the energy on creating them (~511 keV each) and you can only use the remaining energy to accelerate them. In this case the equation gives you a lower particle momentum which leads to lower thrust (even when assuming 100% acceleration efficiency). ISP is a tricky concept in this case because there are different definitions which clash in the relativistic context (due to the concept of mass flow). R. Tinder gets to a I_SP = c (speed of light) for a photon rocket (using the relativistic mass of the photons) which is the maximum possible relativistic I_SP: http://goo.gl/Zz5gyC .
jmlloren

Experimental verification of the feasibility of a quantum channel between space and Earth - 0 views

www.iop.org/...njp8_3_033038.html

NAN MAD technology

shared by jmlloren on 14 Jul 09 - Cached
  • jmlloren on 14 Jul 09
     
    Extending quantum communication to space environments would enable us to perform fundamental experiments on quantum physics as well as applications of quantum information at planetary and interplanetary scales. Here, we report on the first experimental study of the conditions for the implementation of the single-photon exchange between a satellite and an Earth-based station. We built an experiment that mimics a single photon source on a satellite, exploiting the telescope at the Matera Laser Ranging Observatory of the Italian Space Agency to detect the transmitted photons. Weak laser pulses, emitted by the ground-based station, are directed toward a satellite equipped with cube-corner retroreflectors. These reflect a small portion of the pulse, with an average of less-than-one photon per pulse directed to our receiver, as required for faint-pulse quantum communication. We were able to detect returns from satellite Ajisai, a low-Earth orbit geodetic satellite, whose orbit has a perigee height of 1485 km.
  • pacome delva on 16 Jul 09
     
    hello Jose! Interesting it was proposed to do the same with the ISS as part of the ACES experiment. I don't remember the paper but i can look if you're interested
annaheffernan

Physicists create 'molecules' of light - 0 views

physicsworld.com/...ists-create-molecules-of-light

PHY science

shared by annaheffernan on 27 Sep 13 - No Cached
LeopoldS liked it
  • annaheffernan on 27 Sep 13
     
    The first "molecules" made from two photons have been created by physicists in the US. The breakthrough could allow both conventional and quantum computers to encode and process information using photons.
  • johannessimon81 on 30 Sep 13
     
    I wonder if the effect also works with much larger numbers of photons involved. By slowing down any initial (Rydberg) photons and letting the later photons catch up it might be possible to create a very sharp "shock" front of photons. This could produce in itself be an ultra-short pulse of light or maybe it could be used as the trigger for massive stimulated emission in a laser.
santecarloni

[1101.6015] Radio beam vorticity and orbital angular momentum - 1 views

arxiv.org/1101.6015

radio light antennas technology

shared by santecarloni on 20 Oct 11 - No Cached
  • It has been known for a century that electromagnetic fields can transport not only energy and linear momentum but also angular momentum. However, it was not until twenty years ago, with the discovery in laser optics of experimental techniques for the generation, detection and manipulation of photons in well-defined, pure orbital angular momentum (OAM) states, that twisted light and its pertinent optical vorticity and phase singularities began to come into widespread use in science and technology. We have now shown experimentally how OAM and vorticity can be readily imparted onto radio beams. Our results extend those of earlier experiments on angular momentum and vorticity in radio in that we used a single antenna and reflector to directly generate twisted radio beams and verified that their topological properties agree with theoretical predictions. This opens the possibility to work with photon OAM at frequencies low enough to allow the use of antennas and digital signal processing, thus enabling software controlled experimentation also with first-order quantities, and not only second (and higher) order quantities as in optics-type experiments. Since the OAM state space is infinite, our findings provide new tools for achieving high efficiency in radio communications and radar technology.
  • santecarloni on 20 Oct 11
     
    It has been known for a century that electromagnetic fields can transport not only energy and linear momentum but also angular momentum. However, it was not until twenty years ago, with the discovery in laser optics of experimental techniques for the generation, detection and manipulation of photons in well-defined, pure orbital angular momentum (OAM) states, that twisted light and its pertinent optical vorticity and phase singularities began to come into widespread use in science and technology. We have now shown experimentally how OAM and vorticity can be readily imparted onto radio beams. Our results extend those of earlier experiments on angular momentum and vorticity in radio in that we used a single antenna and reflector to directly generate twisted radio beams and verified that their topological properties agree with theoretical predictions. This opens the possibility to work with photon OAM at frequencies low enough to allow the use of antennas and digital signal processing, thus enabling software controlled experimentation also with first-order quantities, and not only second (and higher) order quantities as in optics-type experiments. Since the OAM state space is infinite, our findings provide new tools for achieving high efficiency in radio communications and radar technology.
  • LeopoldS on 20 Oct 11
     
    and how can we use this?
santecarloni

Physicists entangle 100,000 photons - physicsworld.com - 2 views

physicsworld.com/...icists-entangle-100000-photons

phy

shared by santecarloni on 29 Oct 12 - No Cached
LeopoldS liked it
  • santecarloni on 29 Oct 12
     
    Pulses of light comprising around 100,000 entangled photons have been created by physicists in Germany and Russia. 
Tobias Seidl

Evolutionary Photonics with a Twist -- Vukusic 325 (5939): 398 -- Science - 0 views

www.sciencemag.org/...398

BIO optics photonics PHY

shared by Tobias Seidl on 13 Aug 09 - Cached
pacome delva liked it
  • Tobias Seidl on 13 Aug 09
     
    Again bio-photonics. Cool subject, should have a look into.
  • pacome delva on 31 Aug 09
     
    really interesting and fascinating what evolution can do...!
santecarloni

Three electrons for the price of one - physicsworld.com - 0 views

physicsworld.com/...47533

technology space phy

shared by santecarloni on 21 Oct 11 - No Cached
  • santecarloni on 21 Oct 11
     
    Researchers have created a new material that can produce three or more free electrons every time it absorbs a single photon. This is unlike conventional semiconductors, which produce just one free electron per photon. Based on tiny semiconductor structures called quantum dots, the new material - developed by researchers at Delft University of Technology in the Netherlands and Toyota Europe in Belgium - could someday be used to make more efficient solar cells.
Thijs Versloot

Relativistic rocket: Dream and reality - 3 views

www.sciencedirect.com/...S009457651400040X

propulsion antimatter relativistic

shared by Thijs Versloot on 27 Mar 14 - No Cached
  • Thijs Versloot on 27 Mar 14
     
    An exhaustive overview of all possible advanced rocket concepts, eg.. "As an example, consider a photon rocket with its launching mass, say, 1000 ton moving with a constant acceleration a =0.1 g=0.98 m/s2. The flux of photons with E γ=0.5 MeV needed to produce this acceleration is ~1027/s, which corresponds to the efflux power of 1014 W and the rate of annihilation events N'a~5×1026 s−1 [47]. This annihilation rate in ambiplasma l -l ann corresponds to the value of current ~108 A and linear density N ~2×1018 m−1 thus any hope for non-relativistic relative velocity of electrons and positrons in ambiplasma is groundless." And also, even if it would work, then one of the major issues is going to be heat dispersal: "For example, if the temperature of radiator is chosen T=1500 K, the emitting area should be not less than 1000 m2 for Pb=1 GW, not less than 1 km2 for Pb=1 TW, and ~100 km2 for Pb=100 TW, assuming ε=0.5 and δ=0.2. Lower temperature would require even larger radiator area to maintain the outer temperature of the engine section stable for a given thermal power of the reactor."
  • ...2 more comments...
  • johannessimon81 on 27 Mar 14
     
    We were also discussing a while ago a propulsion system using the relativistic fragments from nuclear fission. That would also produce an extremely high ISP (>100000) with a fairly high thrust. Never really got any traction though.
  • dejanpetkow on 29 Mar 14
     
    I absolutely do not see the point in a photon rocket. Certainly, the high energy releasing nulcear processes (annihilation, fusion, ...) should rather be used to heat up some fluid to plasma state and accelerate it via magnetic nozzle. This would surely work as door-opener to our solar system...and by the way minimize the heat disposal problem if regenerative cooling is used.
  • Thijs Versloot on 31 Mar 14
     
    The problem is not achieving a high energy density, that we can already do with nuclear fission, the question however is how to confine or harness this power with relatively high efficiency, low waste heat and at not too crazy specific mass. I see magnetic confinement as a possibility, yet still decades away and also an all-or-nothing method as we cannot easily scale this up from a test experiment to a full-scale system. It might be possible to extract power from such a plasma, but definitely well below breakeven so an additional power supply is needed. The fission fragments circumvent these issues by a more brute force approach, thereby wasting a lot of energy for sure but at the end probably providing more ISP and thrust.
  • dejanpetkow on 03 Apr 14
     
    Sure. However, the annihilation based photon rocket concept unifies almost all relevant drawbacks if we speak about solar system scales, making itself obsolete...it is just an academic testcase.
annaheffernan

High-gain optical transistors flipped by just one photon - 0 views

physicsworld.com/...ors-flipped-by-just-one-photon

technology PHY science

shared by annaheffernan on 05 Aug 14 - No Cached
  • annaheffernan on 05 Aug 14
     
    Two research teams have designed an optical gate that can 'switch off' a stream of photons as well as store them ... good news for optical communications as well as future photon-based quantum information systems.
jcunha

Synthetic Landau levels for photons - 1 views

www.nature.com/...nature17943.html

PHY quantum hall effect topology experiment photons

shared by jcunha on 09 Jun 16 - No Cached
  • jcunha on 09 Jun 16
     
    Very nice experiment on the verge of Condensed matter Physics! The presence of Landau levels is a necessary condition to obtain a Quantum Hall state. Quantum Hall states have first appeared in 2D electronic gases when applied a perpendicular magnetic field that induces a new topological state of the "electronic gas". This new topological state is believed to "protect" some parameters of the system, such as conductance making it possible to measure fundamental constants with very high precision even in imperfect experimental conditions. In this fundamental experiment, a synthetic magnetic field was created that acts in continuum photons, producing "an integer quantum Hall system in curved space, a long-standing challenge in condensed matter physics".
johannessimon81

Entangled photons make a picture from a paradox - 3 views

www.nature.com/...picture-from-a-paradox-1.15781

science PHY

shared by johannessimon81 on 28 Aug 14 - No Cached
  • johannessimon81 on 28 Aug 14
     
    Physicists have devised a way to take pictures using light that has not interacted with the object being photographed. This form of imaging uses pairs of photons, twins that are 'entangled' in such a way that the quantum state of one is inextricably linked to the other.
  • Ma Ru on 28 Aug 14
     
    The actual article: http://www.nature.com/nature/journal/v512/n7515/full/nature13586.html ScienceDaily version: "Scientists photograph the Schrödinger's cat for the first time!"
ESA ACT

Glow in the Dark - 0 views

www.nature.com/...nphoton.2007.275.html

BIO Photonics

shared by ESA ACT on 24 Apr 09 - Cached
  • ESA ACT on 24 Apr 09
     
    The ability of living organisms to generate light by bioluminescence is a fascinating phenomenon. Nature Photonics spoke to Yoriko Ando from the University of Tokyo about her recent quantitative analysis of the effect in fireflies.
ESA ACT

nature01941.pdf (application/pdf Object) - 0 views

Photonics structures in nature

BIO NAN optics

shared by ESA ACT on 24 Apr 09 - No Cached
  • ESA ACT on 24 Apr 09
     
    A number of biological models with interesting photonic properties are reviewed, such as butterfly wings or moth eyes.
ESA ACT

A solid-state light-matter interface at the single-photon level - 0 views

www.nature.com/...nature07607.html

PHY NAN technology communications

shared by ESA ACT on 24 Apr 09 - Cached
  • ESA ACT on 24 Apr 09
     
    First demostration of coherent and reversible mapping of a light field with less than one photon per pulse onto an ensemble of approx10^7 atoms naturally trapped in a solid.
Marcus Maertens

A scalable photonic computer solving the subset sum problem | Science Advances - 0 views

advances.sciencemag.org/...eaay5853

INF

shared by Marcus Maertens on 03 Feb 20 - No Cached
  • Marcus Maertens on 03 Feb 20
     
    +1 for team photon!
johannessimon81

Nuclear isomer - 2 views

en.wikipedia.org/...Nuclear_isomer

technology science PHY space

shared by johannessimon81 on 23 Nov 12 - No Cached
LeopoldS liked it
  • johannessimon81 on 23 Nov 12
     
    We had a short discussion yesterday about using nuclear isomers as batteries for spacecraft. The principle is that energy is stored as an excitation of the nucleus which can then release the energy as a gamma-photon. However angular momentum has to be conserved an this suppresses the decay strongly - making these states stable up to 10^35 longer than a typical decay.
  • dejanpetkow on 27 Nov 12
     
    The key is the triggering of the dacay wheras the triggering comsumes less energy than the decay provides. The x-ray based triggering of the gamma photon decay turned out to be quite controversial and needs significantly more scientific attention.
  • LeopoldS on 28 Nov 12
     
    I think that there should be some references and traces of the discussions we had on this on the shared drive or wiki ... One other aspect: converting the omnidirectional gamma bursts into useful energy ....
jcunha

Superfast light source made from artificial atom - 0 views

phys.org/...st-source-artificial-atom.html

technology quantum single photon light dot

shared by jcunha on 02 May 16 - No Cached
  • jcunha on 02 May 16
     
    A new more efficient type of single photon light source consisting of a quantum dot reproduces 1954 Robert Dicke theoretical proposal. Applications in quantum communications directly on the target. "All light sources work by absorbing energy - for example, from an electric current - and emit energy as light. But the energy can also be lost as heat and it is therefore important that the light sources emit the light as quickly as possible, before the energy is lost as heat."
santecarloni

Chinese Physicists Smash Distance Record For Teleportation  - Technology Review - 1 views

www.technologyreview.com/...27843

technology phy teleport

shared by santecarloni on 11 May 12 - No Cached
  • santecarloni on 11 May 12
     
    The ability to teleport photons through 100 kilometres of free space opens the way for satellite-based quantum communications, say researchers
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