T a r g e t F a c t o r y

1. How many targets a second can be manufactured?

2. What techniques are used for making the high – precision capsules?

3. What material is required for target manufacturing.

4. How much could the inventory be?

 

           

   OPTICAL FREQUENCY MEASUREMENT

                                                         IS GETTING A LOT MORE PRECISE

I. Translate the following words and phrases from the text.

 

1. … the difficulty of measuring with a precision adequate for the scientific and technological task at hand.

2. … frequencies in the infrared not to mention the visible and ultraviolet are already much too fast for electronic counting.

3. … and the optical frequency one is trying to measure.

4. … because the 2S state in hydrogen is very long-lived.

5. … the problem was now to determine the absolute frequency of resonance with high precision.

7. Where ▲ is the mismatch one seeks to measure between it and the standard’s 7^th harmonic.

8. That’s less likely to be a problem with the theory than with the measured charge radius of the proton.

9. One would like to be able to compare it with more precise measurements of the proton charge radius.

10.High-precision measurements seem to have stimulated a renaissance of quantum electrodynamics calculations.

 

Ultraviolet transition, frequency, optical frequencies, the best previous accuracy, two orders of magnitude, verification, a hot topic, opposite directions, tunable dye laser, a reference standard, mismatch, a powerful phase-locked colour centre laser, ragged edge, laser illumination, a time delay, to convince, frequency divider chain.

 

II. Translate the passages from English into Russian.

 

1. Page 19. from … at the Max Planck Institute to any astronomical data we have.

2. Page 20. from … the general problem to … too fast for electronic counting.

3.           from … very accurate optical reference … to … is trying to measure.

4.       …the essence of the new technique … to … the original frequency gap by a factor by a factor 2 ⁿ .

5.     … the two opposing beams … of the resonance with high precision.

6.     … the calibrated frequency of the He-Ne standard to … for further comparison.

7.     … the new Garching result improves … to … a single optical cycle.

8.     … the paragraph “Spreading the technique.”

 

III. Give English equivalents to the following words and expressions from the text.

 

Точный, постоянная, высокоточный метод, делитель частоты, практическое применение, портативный, атомные часы, полупроводниковый лазер, сжимать, совпадение, лазер на красителе, частота, инфракрасный, ультрафиолетовый, видимый, луч лазера, резонанс, внешнее электрическое поле, техническая задача, улучшать, сдвиг, измерение, радиус, исходная частота, измерять.

 

IV. Translate the following sentences from Russian into English.

 

1.Самое точное измерение любой частоты возможно при помощи нового высокоточного метода в видимом или ультрафиолетовом диапазоне.

2.Хэнси и его коллеги разработали этот высокоточный метод.

3.Трудность состоит в том, чтобы измерить оптическую частоту с точностью, адекватной современным научным и техническим задачам.

4.Каждый каскад делителя принимает две входные частоты лазера.

5.Источник пучка – лазер на красителе плюс кристалл.

6.Вы можете думать о нашем результате как о новом измерении радиуса заряда.

V. Answer the following questions.

 

1. Where can the frequency be measured the most accurately nowadays?

2. Where does Theodore Hänsch work?

3. What was Dirac’s opinion about varying the fundamental constants?

4. What is the essence of the new technique?

5. In what way were the two opposing beams produced?

6. What advantage did Hänsch and his colleagues of coincidence?

7. What is the calibrated frequency f of the He-Ne standard?

8. What was taken as a first divider stage by the Garching group?

9. Does the new Garching result improve considerably the previous determination of the Rydberg constant?

10.What has the Garching group succeeded inin recent months?

 

Optical Frequency Measurement
Is Getting a Lot More Precise

Abridged

A new trick for the repeated halving of optical frequency intervals now permits the measurement of optical atomic transitions with unprecedented accuracy.

Bertram Schwarzschild

At the Max Planck Institute for Quantum Optics in the Munich suburb of Garching, Theodor Hänsch and colleagues have measured the ultraviolet

transition frequency between the IS and 2S states of atomic hydrogen to be 2.466 061 413 187 34(84) x 10¹⁵ Hz.

With an uncertainty of only 3 parts in   10¹³, this result exceeds the accuracy
of the best previous measurement of the 1S-2S transition by two orders of magnitude. It is, in fact, the most accurate measurement to date of any frequency in the visible or ultraviolet. It’s so accurate that simply repeating the measurement a year from now would provide a better and more direct verification (or falsification) of the constancy of the fine-structure constant over cosmological time than any astro-physical data we have.

Dirac, among others, conjectured that the fundamental constants might be varying very slowly. But that's certainly not a hot topic at present. "Of course, it's not why we developed this high-pre­cision technique," Hansch told us. "But if it lets us do the best test ever, we should."

 

 

 

FREQUENCY MINUS 2 466 061 102 400 (kHz)

 

 

The problem

The general problem addressed by the Garching group is the difficulty of measuring optical frequencies with a precision adequate for the scientific and technological tasks at hand. Tra­ditional laser wavelength measure­ment, which is essentially counting in­terference fringes in space and split­ting the last wave, runs out of steam at about a part in 10¹⁰.

One can measure gigahertz micro­wave frequencies to a part in 10¹⁴ by electronically counting oscillations in time. That's how atomic clocks work, exploiting microwave hyperfine tran­sitions. In fact, the standard second is now defined in terms of the 9 GHz hyperfine transition frequency of a ce­sium atomic clock. But terahertz fre­quencies in the infrared, not to mention the visible and ultraviolet, are already much too fast for electronic counting. "Nowadays we can make incredibly stable lasers with very narrow reso­nances," says MIT atomic physicist Daniel Kleppner. "But we can't do much with them if we don't have an adequate method for measuring the laser frequency."

Very accurate optical reference fre­quencies can be produced by construct­ing elaborate chains that start with an atomic clock and gener­ate higher and higher harmonics in crystals and other nonlinear de­vices. But typically there's a terahertz mis­match between the nearest such reference and the optical freqency one is trying to measure. The tour de force accom­plished by Hansch and company is their dem­onstration of a general technique for bridging such large frequency in­tervals with great accu­racy.

The essence of the new technique is the re­peated application of a trick for coherently gen­erating the mean be­tween any two optical frequencies. Each so-called divider stage re­ceives two input laser frequencies, fi, and f2, and forces a third laser to oscillate at the precise midpoint, fз = (f1 + f2) /2, by electronically phase locking its second harmonic, 2f3, to the sum f1 + f2 by means of a low-frequency beat signal. A cascaded chain of n such stages shrinks the original frequency gap by a factor 2ⁿ.

 

The experiment

 

Because the 2S state in hydrogen is very long-lived, the natural line width of the 1S-2S transition is exceedingly narrow—only 1.3 Hz. It's actually a two-photon transition, because the se­lection rules prohibit a single-photon jump from one S-wave state to another. Therefore the Garching group bom­barded their cold atomic hydrogen beam with 243 nm photons from two opposite directions at once. This con­figuration has the merit of canceling out Doppler broadening to first order.

The two opposing beams were pro­duced by injecting a single laser beam into an optical resonator. The beam source was an ultrastable 486 nm dye laser plus a crystal that generated its first (243 nm) harmonic. As the fre­quency of the tunable dye laser was swept through the 1S-2S resonance, the rate at which the 2S state was being excited was monitored by count­ing 121.6 nm Lyman-α (2P→1S) de­cays in an external electric field that mixed the 2S and 2P substates. The figure above shows the resulting 1S-2S resonance curve, only a few kilohertz wide. The problem was now to deter­mine the absolute frequency of the resonance with high precision.

That's where the new optical fre­quency divider technique comes in.

 

Hansch and company took advantage of the coincidence that the 1S-2S ex­citation frequency differs by only about 2 THz from the 28th harmonic of a methane-stabilized 3.39 µm heliumneon laser that can serve as a portable reference standard when calibrated (to 3 parts in 10¹³) against a cesium atomic clock at the Physikalisch-Technische Bundesanstalt, the national standards laboratory in Braunschweig. To put it another way, the resonant dye-laser frequency, being one fourth of the full IS—2S transition frequency, is in near coincidence with the portable stand­ard's 7th harmonic.

The calibrated frequency f of the He-Ne standard is about 88 THz in the near-infrared. It's instructive to write the resonant dye laser frequency as 7 f -∆, where ∆ is the mismatch one seeks to measure between it and the standard's 7th harmonic. As a first divider stage, the Garching group split the difference between f and the reso­nant dye laser frequency, producing the midpoint frequency 4 f -∆/2.

For further comparison they then generated 4f, the fourth harmonic of the He-Ne reference, through the in­tervention of an AgGaSe crystal and a powerful phase-locked color center la­ser. With four additional cascaded di­vider stages they then shrank the dif­ference between this 4th harmonic and 4f-∆/2 by a factor of 16, producing a final beat frequency of about 66 GHz, low enough for standard microwave frequency counting techniques.

 

Testing QED

 

"Our high-precision measurements in the last few years seem to have stimu­lated a renaissance of quantum elec­trodynamics calculations," Hänsch told us. "Calculating small higher-order QED effects can yield surprises. But the calculations are extremely in­volved, and theorists don't like to un­dertake them unless there's suffi­ciently precise data to compare them with."

The new Garching result improves somewhat on the best previous deter­mination of the Rydberg constant. It also yields a precise new measurement of the IS state's Lamb shift, whose agreement with the theoretical value is only on the ragged edge of consis­tency. "That's less likely to be a prob­lem with the theory," suggests Hansch, "than with the measured charge radius of the proton, which enters the QED calculation as an external parameter known only to within 10%. You can think of our result as a new measure­ment of the charge radius." One would like to be able to compare it with more precise measurements of the proton charge radius by new electron scattering experiments or by spectroscopy with muonic atoms.

In recent months the Garching group has succeeded in narrowing the 4 kHz of the published resonance curve on page 20 down to only 1 kHz. This they accomplished by chopping the la­ser illumination into pulses and then imposing a time delay that admits only the slowest hydrogen atoms in the beam. That selection minimizes both second-order Doppler broadening and the "transit broadening" dictated by the uncertainty principle. Hänsch and coworkers have also employed an elec­tro-optic "comb generator" to convince themselves that the divider stages don't lose even a single optical cycle.