Biodegradable mems. Vocabbox. A lot of machines made by physicists are used in medicine now, страница 7

PHOTONS  SINGLED  OUT

Vocabbox

noun collocations

§  quantum cryptography

§  single-photon source

§  solid-state device

§  efficiency

§  fluorescent dye molecule

§  impurity

§  carbon atom

§  nitrogen atom

§  beam splitter

verb collocations

§  generate photons

§  to be encoded on

§  emit

§  to be miniaturized

§  degrade rapidly

§  excite colour centres

Pre-reading task

Do you agree that the ability to generate single photons on demand is one of the most crucial tasks in quantum optics? Why?

Reading

Read the text. Find some information concerning the single-photon sources for practical application.

The ability to generate single photons on demand is one of the most crucial tasks in quantum optics. Such a photon source could be used in quantum cryptography, where each bit of information is encoded on a single photon, or to make quantum logic gates. Now two teams of researchers from Germany and France have shown that certain defects within synthetic diamond can emit single photons at room temperature. Moreover, these solid-state devices have high quantum efficiency and could potentially be miniaturized – making them ideal candidates as single-photon sources for practical applications.

Single photons have previously been generated in experiments using single trapped atoms, or by regulating the recombination of electrons and holes in a solid-state light-emitting device. These experiments, however, are technically difficult and operate at temperatures lower than 1 kelvin. And although single photons can be created at room temperature using fluorescent dye molecules, the molecules degrade rapidly after emitting around 10⁹ photons.

In the latest experiments, a team of researchers used so-called colour centres in diamond as single-photon emitters. These impurities form naturally when a carbon atom is replaced by a nitrogen atom. As these nitrogen atoms are bound tightly within the crystal, colour centres are very stable.

Both teams used a laser beam focused to a tiny spot inside the diamond to excite individual colour centres and cause them to emit light over a range of wavelengths from 640-750 nm. This fluorescence was filtered and then analysed by passing it through a beam splitter. Two photodiodes were used to measure the light reflected and transmitted by the beam splitter, and the correlation between the signals was measured as a function of time. The lack of a correlation between the signals from the two detectors gave a clear indication that only single photons were generated.

Moreover, the German team found the characteristics of the light remained unchanged even after some 10¹³ photons had been emitted, and conclude that the superior stability of the solid-state devices makes them “very attractive” for use as practical single-photon sources.

Comprehension check

Answer the following questions:

1.  What are the experiments carried out by the researchers?

2.  How are single photons created?

3.  What are two photodiodes used for?

Discussion

Comment on the text using the following prompts: It goes without saying…;

As far as I know…; I mean to say…; I must confess, I don’t know…

Give a brief summary of the text.

UNIT 9

NEUTRONS  CAUGHT  IN  A  TRAP

Vocabbox

noun collocations

§  radioactive beta decay

§  this level of uncertainty

§  fluctuations in the initial number of neutrons

§  intrinsic magnetic moments

§  a trapping vessel

§  collision with the helium

§  an important yardstick in cosmology

verb collocations

§  turn attention to

§  succeed in storing neutrons

§  be measured with an accuracy

§  confine neutral atomic gases

§  slow down atomic gases

§  pass a beam from… into

§  be absorbed by

Pre-reading task