## Posts Tagged ‘**photons**’

## The LHC as a photon collider

Yes, that’s correct: photon collider.

The Large Hadron Collider is known for smashing together protons. The energy from these collisions gets converted into matter, producing new particles that allow us to explore the nature of our Universe. The protons are not fired at one another individually; instead, they are circulated in bunches inside the LHC, each bunch containing some 100 billion (100,000,000,000) particles. When two bunches cross each other in the centre of CMS, a few of the protons — around 25 or so — will collide with one another. The rest of the protons continue flying through the LHC unimpeded until the next time two bunches cross.

Sometimes, something very different happens. As they fly through the LHC, the accelerating protons radiate photons, the quanta of light. If two protons going in opposite directions fly very close to one another within CMS, photons radiated from each can collide together and produce new particles, just as in proton collisions. The two parent protons remain completely intact but recoil as a result of this photon-photon interaction: they get slightly deflected from their original paths but continue circulating in the LHC. We can determine whether the photon interactions took place by identifying these deflected protons, thus effectively treating the LHC as a photon collider and adding a new probe to our toolkit for exploring fundamental physics. Read the rest of this entry »

## Photon in a cavity — a Gedankenexperiment

**Klaus Wilhelm, Bhola N. Dwivedi**

The inertial and gravitational mass of electromagnetic radiation (i.e., a photon distribution) in a cavity with reflecting walls has been treated by many authors for over a century.

After many contending discussions, a consensus has emerged that the mass of such a photon distribution is equal to its total energy divided by the square of the speed of light.

Nevertheless, questions remain unsettled on the interaction of the photons with the walls of the box.

In order to understand some of the details of this interaction, a simple case of a single photon with an energy Ε_{ν} = h ν bouncing up and down in a static cavity with perfectly reflecting walls in a constant gravitational field **g**, constant in space and time, is studied and its contribution to the weight of the box is determined as a temporal average….

Read more at http://arxiv.org/pdf/1307.2830v1.pdf

## How stable is the photon?

**Julian Heeck**

Yes, the photon. While a nonzero photon mass has been under experimental and theoretical study for years, the possible implication of a finite photon lifetime lacks discussion.

The tight experimental upper bound of the photon mass restricts the kinematically allowed final states of photon decay to the lightest neutrino and/or particles beyond the Standard Model.

We discuss the modifications of the well-measured cosmic microwave background spectrum of free streaming photons due to photon mass and lifetime and obtain model-independent constraints on both parameters—most importantly a lower direct bound of 3 yr on the photon lifetime, should the photon mass be at its conservative upper limit.

In that case, the lifetime of microwave photons will be time-dilated by a factor order 10^{15}.

Read more at http://arxiv.org/pdf/1304.2821v2.pdf

## Observation of eight-photon entanglement

The creation of increasingly large multipartite entangled states is not only a fundamental scientific endeavour in itself, but is also the enabling technology for quantum information.

Tremendous experimental effort has been devoted to generating multiparticle entanglement with a growing number of qubits.

So far, up to six spatially separated single photons have been entangled based on parametric downconversion. Multiple degrees of freedom of a single photon have been exploited to generate forms of hyper-entangled states.

Here, using new ultra-bright sources of entangled photon pairs, an eight-photon interferometer and post-selection detection, we demonstrate for the first time the creation of an eight-photon Schrödinger cat state1 with genuine multipartite entanglement.

The ability to control eight individual photons represents a step towards optical quantum computation, and will enable new experiments on, for example, quantum simulation21, 22, topological error correction and testing entanglement dynamics under decoherence….

Read more: nature.com and arstechnica.com

## The secret lives of photons revealed

An international team of researchers has, for the first time, mapped complete trajectories of single photons in Young’s famous double slit experiment. The finding takes an important first step towards measuring complimentary variables of a quantum system – which until now has been considered impossible as a consequence of the Heisenberg uncertainty principle.

In the double slit experiment, a beam of light is shone onto a screen through two slits, which results in an interference pattern on the screen. The paradox was that one could not tell which slit single photons had passed through, as measuring this would directly distort the interference pattern on the screen. “In most science, it is possible to look at what a system is doing presently and so, determine its past or future. But in quantum mechanics, it is considered inconceivable to consider the past at all.” says physicist Aephraim Steinberg of the Centre for Quantum Information and Quantum Control at the University of Toronto, Canada who has led this new research.

Now, using a technique known as “weak measurement” Steinberg and his team say they have managed to accurately measure both position and momentum of single photons in a two-slit interferometer experiment. The work was inspired by one of Steinberg’s colleagues, Howard Wiseman of Griffith University, Australia, who proposed in 2007 that it may be possible to use weak measurements to determine momenta and positions in the double slit experiment. Steinberg was immediately fascinated and began to see how this would become experimentally viable.

### Catching a glimpse

The theory of “weak measurement” first proposed in 1988 and developed by physicist Yakir Aharonov and his group at Tel Aviv University, Israel, has seen a fair amount of interest in recent years…. Read the rest of this entry »