FIGURE 1. End view of a superconducting quadrupole magnet used for focusing of high-energy particlesat the Brookhaven National Laboratory. The diameter of the bore is slightly larger than 2.5 cm.

In the broad sense the term "quantum electronics" said, in effect, was: "My brain child has a functionrelates to the motion of electrons as governed by similar to that of a newborn British subject; you mayquantum mechanical laws, but used more specifically tax it later." The history of science has shown that it isit refers to electronic processes involving transitions much safer to say that some technological applicationsbetween discrete energy levels. The latter category will follow scientific advances than to deny futureincludes the laser, and it is to this development, which applications. The economic scale and impact of theis responsible for most of the new horizons in the field, applications are, however, very difficult to predict andthat this article is primarily devoted. Two specific I shall not venture into a realm that is beyond myexamples are described: a tunable light oscillator and competence.an intense-light-pulse generator. It is pointed out that,despite their promise, the ultimate impact of the laser Defining our termsdevices on industrial technology cannot be predicted. Presumably the term "quantum electronics" relates to

the motion of electrons, as governed by quantum mechan-This article is not an attempt to predict the future ical laws. Although the term is relatively new, the field

of quantum electronics, but rather to sketch some new defined in this manner is four decades old, starting in thevistas that have been opened by recent developments. late 1920s, or perhaps earlier if we include the photoelec-One might ask: "What are these scientific developments tric effect and photocells as part of quantum electronics.good for?" In answer to that question, I would like to Looking back to past horizons to get our bearings, weremind you of Faraday's reply to the British prime find that the quantum electronic effect with the largestminister who wondered what purpose the discovery of technological impact is probably the existence of severalelectromagnetic induction could have. Faraday did not types of carriers of electricity in solids-electrons andpredict the rise of an electric power industry; what he holes with different effective masses. This fact, which was

82 Bloembergen-New horizons in quantum electronics

New horizons inquantum electronicsThe development of the laser has opened new horizons inquantum electronics, but although the future promises even greaterpossibilities for its use, experience teaches us that we cannot safelypredict the economic scale or impact of such applications

N. Bloembergen Harvard University

first clearly stated by Peierls and Wilson in the late power generation and to containment of plasmas fortwenties, is at the basis of semiconductor electronics and controlled fusion. I shall not venture into propheciestransistors. about these fields, which still harbor many uncertainties,The tunneling of an electron through a potential bar- but superconducting magnets are definitely useful in the

rier is another typical quantum mechanical effect; the laboratory. Figure 1 shows a quadrupole superconductingEsaki and Zener diodes are successful applications. The magnet for focusing high-energy particles; its compactfield-emission microscope, with which individual mole- size is a distinct advantage. Superconducting phenomena,cules adsorbed on a metal point may be made visible, is such as tunneling of electrons between superconductingalso based on this effect but has not had a large-scale junctions, have also opened new horizons beyond whichtechnological impact. These examples should remind us there may lie new applications. Superconducting filmsthat there is no one-to-one correspondence between may be used in switching elements and junctionsexciting discoveries in electron physics and important may be developed as sensitive detectors in the mostdevelopments in electronic technology, although there is, inaccessible region of the far infrared. Very recentlyof course, a large amount of correlation, and the economic the effect was used for a new precision determination ofscale of applications is difficult to predict. the fundamental constant h/e. This may be said to be aThe phenomenon of superconductivity is another metrological application.

manifestation of the laws of quantum mechanics relating The term "quantum electronics" is often used in ato the motion of electrons. Although the effect has been narrower sense also. It then refers more specifically toknown since 1908, only in the last 20 years has it been electronic processes, which involve transitions betweenunderstood, and it has become technologically interesting discrete energy levels-as opposed to the continuum ofonly in the past five years or so. Type II superconductors energy levels, which are involved in semiconductors,allow the construction of superconducting magnets with superconductors, and plasmas. Discrete energy levelfield strengths of about 150 kilogauss and zero power devices are numerous and many were well establishedconsumption. These magnets do, however, consume before the term came into vogue, so perhaps "discreteliquid helium. Nevertheless, because of advances in electronics" would bemore accurate. In this field familiarcryogenic technology, liquid helium has become accept- horizons of the recent past are formed by the skyline ofable in technological operations, and so superconducting applications of magnetic resonance and microwavemnagnets may be the answer to magnetohydrodynamic spectroscopy; the gyrator, including isolators and circu-

IEEE spectrum JULY 1967 83

lators, magnetoacoustic delay lines, and microwave 1:1012. The hyperfine splitting of the atomic groundmasers, are well-established landmarks. The cesium-beam state of hydrogen is known to 11 significant decimalatomic clock has been adopted as the new standard of places.time and man finally has decoupled the measurement of The three-level solid-state maser is still the ultimatetime from the motion of the earth around the sun; he in low-noise microwave reception. This device, conceivedhas turned from an astronomical precession to the preces- in 1956, quickly became operational in the groundsion of electrons inside the atom. And the cesium clock stations of satellite communication systems, in a fewmay well be replaced as a frequency standard by the radiotelescopes, and in some radar installations. Itsatomic hydrogen maser if this instrument becomes more technological use, however, is not widespread and inuniversally available. It is capable of a short-time stability most applications it is not competitive with simple cooledof 1:1013 and a long-term stability and resettability of parametric devices, which were developed shortly after-

wards. Here we have another example of how hard it isto predict technological application. Although the devicewas soon perfected to the point that very little furtherdevelopment is now necessary, alternative solutions forlow-noise reception had advantages of economy and

2 cm- Cr3+ in ruby simplicity, and thus the technological impact of the2_

solid-state maser was severely limited.The pumping principle employed in the maser to create

4///l/4F2 a medium with an electromagnetic gain, as shown in Fig.2, has endured in many forms of lasers. And lasers have,

Damping of course, opened up most of the new horizons in quantumelectronics to which the remainder of this article will

1.5 be devoted.

The new devicesVery succinctly, but not too inaccurately, one might

say that the quantum electronics of lasers consists ofdoing at light frequencies what is already done at radioand microwave frequencies. Although the difference in

Laser time and spatial scale (the frequency 10 000 timeshigher, the wavelength 10 000 times smaller than for

R1 microwaves) makes the physical embodiment andE design of such items as coherent tunable light oscillators,

cm-, light amplifiers, light modulators and demodulators,harmonic generators, light flip-flop and logical circuits,

0.5 parametric converters, and light waveguides radicallydifferent from their radio and microwave counterparts,the underlying principles of Maxwell's theory and quan-

,Damping tum mechanics are the same at all frequencies. I havePump l chosen two recent examples to convey an idea of what

ties ahead.My first example is the tunable light oscillator, which is

0 L - 4A2 Maser shown schematically in Fig. 3. An intense green lightbeam, obtained by harmonic doubling of a neodymium-

FIGURE 2. Energy levels of the chromium ion in glass laser beam in an oriented crystal of lithium niobate,ruby. The four levels of the spin quartet ground is sent into another crystal, which has reflective coatingsstate 4A2 are shown on an enlarged scale at right. for infrared light. In this second crystal the green quantaThe pumping and damping mechanism essential split up into pairs of smaller quanta. The exact frequenciesfor maser operation was first proposed and dem- of these smaller quanta depend sensitively on the geom-onstrated at microwave frequencies. The sameprinciple is used in the ruby laser. The different etry and optical properties of the crystal. Data obtainedenergy levels at light frequencies are shown at left. for signal and idler by variation of the temperature

of a niobate crystal are shown in Fig. 4. A tunable

FIGURE 3. Schematic of optical parametric down- coherent-light oscillator results. To date, operationconverter. Mirrors M, and M2 transmit the pump

between 7800 angstroms and two micrometers has beenlaser beam at cp. Depending on the orientation and achieved. With a crystal of potassium dihydrogen phos-temperature of the crystal, frequencies wi and co. are phate (KDP) and a pump beam in the ultraviolet, tunablegenerated, such that Wj + w5 = con. laser-like beams in the visible region of the spectrum have

been obtained. If such an oscillator were perfected andcould be obtained commercially as easily as a radio signal

Xj ,;~~~~~~~~~generator, for example, the field of optical spectroscopyXp > { ] Zp : I > ~~~~~~wouldbe revolutionized. One would dial the wavelength

> ." \ \ ~~~~~~~~~~~~ofthe laser-like beam.KrLNOP G1The second example is an inltense light pulse generator

orystlNQ Filter with pulse durations as short as a few picoseconds. The

84 IEEE spectrum JULY 1967

0.96di 12

0.98

1.00

1.02 A Dye

1.042.~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~d

W106Rg. B 2d __ ____

4.2 nsEM 1.0s

1.12 ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~~ - ~~~~~~2(d, + d2) ____.ss

FIGURE 5. The operation of a Q-switched mode-locked1.14 neodymium-glass laser. (A) Experimental arrangement.

(B) Oscillogram of the output at a sweep speed of 2 X10-S seconds per division.

46 50 54 58 62 66Temperature. 'C

FIGURE 4. Wavelengths of parametrically down-convertedlight as a function of the temperature of an LiNbO3 crystal.

microwave radar techniques is hard to beat.Continuous-wave power levels of a few kilowatts have

been attained with CO2 lasers at a 10.6-micrometer wave-length. An infrared beam of this type of only 100 wattscan, when focused, easily cut through a standard two- by

principle of operation is based on mode locking of a Q- four-inch board or vaporize the most refractory materials.switched laser. A saturable dye solution is placed in the Although it is unlikely that a carpenter would use a laser inlaser cavity, as shown in Fig. 5. After one round trip his daily work, the cutting of emery paper in a factory thatthrough the laser rod the light is so amplified that it at present rapidly wears out its cutting tools is a possiblebleaches the filter. The population in the excited state of application. Laser beams have also been used for drillingthe dye becomes equal to the population in the ground holes in diamond dyes for wire drawing and for precisionstate. After the intense amplified light pulse has passed tooling operations, but electron-beam machining is athrough, the dye returns rapidly to its normal absorbing powerful competitor. Medical applications, such as retinastate. The process repeats after another round-trip time of welding and cutting of tissues with a laser beam, also arelight in the cavity. The duration of the extremely short being tested.pulse is measured by the scheme shown in Fig. 6. The light When hot CO.2 gas is suddenly expanded, the excitedpulse is split into two pulses of orthogonal polarization, vibrational state has a long enough lifetime that popula-which are recombined in a piezoelectric crystal of such tion inversion results with respect to rapidly depletedorientation that second-harmonic light is formed only lower-lying rotational levels. This is the principle of gaswhen both polarizations are present. The second-har- dynamic lasers, which promiseCW oscillation levels con-monic generator of light acts as an extremely fast coinci- siderably above the kilowatt level.dence counter. If the light path in one arm is increased by a Lasers are definitely well established as laboratoryfew millimeters, no second harmonic is generated. The tools; they are in wide use for aligning and testing opticalpulse duration can then be measured, and is found to be instruments, for demonstration and teaching, and for pre-shorter than 10-11 second. This clearly opens new vistas cision metrology. A laser slaved to a reference crystal,for time measurement and ultrafast switching. The power kept permanently in ultrahigh vacuum at liquid-heliumflux density in such pulses can be staggering, reaching 1012 temperature, may well provide the length standard of thewatts/cm2, corresponding to light field amplitudes of 101 future. The laser will transform Raman spectroscopy fromvolts/cm. If such fields persisted for longer times, which a time-consuming tool of limited usefulness to an impor-still means only 10-9 or 10-8 second, the material would tant analytical technique; for example, the hour-long ex-break down mechanically and electrically. Somewhat less posures of Raman spectra on photographic plates areintense pulses of about 10-8-second duration from power- eliminated. Raman spectroscopy with gas-laser beamsful solid-state or pulsed gas lasers are used for ranging. should have widespread application in analytical chemis-Light radars with extremely high Doppler resolution and try and solid-state physics.fast response at short-distance ranging appear promising, It also appears fairly certain that the far-infrared regionbut again the competition from extremely sophisticated will be investigated more intensively and will be con-

Blocmbergen-New horizons in quantum electronics 85

50 ohms To dual-beam

l scope

RCA photomultiplier7326

4-64 filter Fixed prism0.53-gmmh3rmonim CuSo4 attenuatormonitor

Z-cut quartz

2-64 filter Z-quartz900 opticalLens rotation

Glass-slide at 1.06 urm GaAsbeam crystalSplitters harmonic

1.06 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~generatornpu6-bum Polarize|

0.D.5 .CuS04 attenuator

Coaxial filtoell CntrS04nc attenuato

T 7 ~~~~~~~Polarizer r125 ohms To ,l519

scope \/ Photomultiplier RCA- ~~~~~~~~ ~~~~~~~~Movable 6810-A

1.06-pm trigger and monitor prismchannel To dual- 50

beam ohms

FIGURE 6. Diagram to measure the duration of light pulses shorter than 10-1" second.

quered by new techniques. The gap between the milli- as a carrier frequency in communications needed? Formeter-wave region and wavelengths shorter than 100 transmission in space the acquisition and aiming of themicrometers is rapidly disappearing. Gas-laser sources light beams pose formidable problems. In the atmosphere,and difference-frequency generation techniques are useful rain, smog, fog, haze, snow, etc., make a light a poorboth as sources and as detectors. competitor of microwaves. Can a system ofenclosed tubesHolography is another field for which the laser has with controlled atmosphere and light repeater stations be

opened many possibilities. Perhaps it will find useful ap- built on a technologically sound and economically com-plication in pattern recognition and in storage of three- petitive basis?dimensional information as a Fourier transform. A bad The application of light in the computer area is also thespot in a photographic image will not spoil all bits of in- subject of speculation. Superficially, it appears attractiveformation completely; the Fourier transform of such a to have fast switching, high storage density, direct visualplate will still give a good image. It is too early, however, display. Such developments would depend heavily on theto tell how much impact holography will have on our so- availability of cheap, small, high-quality semiconductorciety. Perhaps it will only be used as a tempting three-di- lasers. If these were available, the entire organization ofmensional display for advertising purposes. Three-dimen- computers using them would probably be different. Couldsional displays of airfield approaches in the cockpit ofa jet such a system compete with an existing and rapidly de-liner with the correct viewing angle from the position of veloping computer technology that thrives without lasers?the aircraft would be a more interesting application. The The burden of proof is on the laser.ultimate dream of making visible three-dimensional X-ray One should not expect the near future to bring suddenpictures of crystals and molecules seems remote. How can dramatic technological change. Rather, a gradual widen-one control the dimension to within a quarter wavelength ing of the perspective and an increasing number of variedfor X ray during the photographic processing? applications on a smaller scale in many different fields of

endeavor appear a more likely course for the historicalWhat of the future? development. Even if full development is realized, theThese are some of the new horizons that have come into strength of the technological impact behind our new honi-

view because of recent research, but they are horizons not zons is difficult to estimate. However, experience tells usyet attained. Even if the devices that seem promising are that it would be remarkable if so many new scientific pos-fully developed, the size of their impact on industrial tech- sibilities did not lead to some technological change.nology cannot be prejudged, as past experience in otherfields has shown. Essentially full text of a paper presented at the Symposium on* ~~~~~~~~~~NewHorizons in Science and Technology at the 1967 IEEE

Is the enormous increase in bandwidth offered by light International Convention, New York, N.Y., March 20-23.

86 Bloembergen-New horizons in quantum electronics