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Recent Successes Boost Hopes for Applications “Squeeze lii t” is the sobriquet for a Dhenomenon hat is causing quite a stir these days in the field of optics. Recently, squeezed-light experimenters stunned the scientific community when they achieved a significant goal: the surpassing of a seemingly inviolable limit set by the quantum nature of optical noise. This miler’--reached after more tl theoretical work. J
Work on squeezed light began in the mid-19708, hut only in the past few years has noise reduction heen realized-thereby paving the way for technological applications. To successfully implement this technology in an analytical situation, three things must happen: Squeezed states must be generated, analytical information must bihedded in the reduced-noise compc n t of the electromagnetic field. am
the 1980s was followed by a generation of experimenters who devised a variety of methods for generating and detecting squeezed states. In searching for optimal methods for eliminating noise in optical communications and measurements, researchers summoned an old idea of quantum theory: the phase-dependent redistribution of quantum fluctuations. Essenise (fluctuations) in the
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G IS made by cogeneratJeams of particular phasings and mixing them in a nonlinear atomic or molecular medium. A “squeezed” beam emerging from the medium exhibits a startling macroscopic quantum effect: The value of one optical noise component is found to he lower than the value that results in a vacuum. At the same time, the value of a conjugate noise component is found to he higher, so that the uncertainty principle is satisfied. (There is in fact no violation of quantum mechanics, just a redistrihution of noise on the quantum level.) Experiments have now produced increases in the signal-to-noise (S/N) ratio with respect to the reduced component of more than 4 dB (relative to the vacuum limit). The likelihood that the noise reduction property of squeezed light will lead to profound application in the future is inspiring today’s researchers. Some of them go so far as to suggest that squeezed light might lead to a revolution in optics comparable to that caused hv the laser. .16Yu
the neio IW mat seneratresearchr.. Ilmvr DIL’.crsaLul.J ed and detected squeezed states, they can proceed with investigations of specific applications. They often cite the delicate experiments now underway to measure gravity waves as an example of an area that could benefit from the low noise levels of squeezed light. However, squeezed light could he a boon to many analytical methods-such as various spectroscopic methods-in which precision measurements a t the quantum level are required. Avarktydmethods The theoretical foundation for the “squeezing” of the electromagnetic field was laid down more than a decade ago and was based on quantum ideas stretching hack 50 years. It took some time for the work of Hidetosi Takahasi at the University of Tokyo, David Stoler, formerly a t Brooklyn Polytech, Horace Yuen and Jeffrey Shapiro at MIT, and Carlton Caves a t Cal Tech, to he noticed. This generation of theorists working in the 19608, 19709, and into
electromagneti two separate q-u.ar..vn For coherent or vacuum states,the Values of these components are equal and their product is minimized. However, theory indicated that by properly choosing and mixing fields with different phases, the fluctuations (actually the mean-square fluctuation) in one quadrature could he reduced to a value below that minimum state, while fluctuations in the other quadrature would he raised (see Figures 1and 2). Ingenious methods for properly mixing the fields and detecting the separate quadrature components have heen devised. In 1985,a group at AT&TBell Laboratories, led by Richard Slusher, produced the first successful results of light squeezing. The group employed four-wave mixing in an atomic sodium beam in an optical cavity and created squeezed states with a 7% noise reduction in one quadrature ( I ) . In 1986, a t the University of Texas, a team led by Jeffrey Kimhle improved upon the results of Slusher, using a slightly different method. The Texas
ANALYTICAL CHEMISTRY, VOL. BO, NO, 5, MARCH 1, 1988
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group produced 30%noise reduction in optical bistahility in a sodium atomic beam. Of greater importance, however, were the results obtained by Kimhle’s group when they switched to another method the squeezing of light in nonlinear crystals. Into a cavity containing the crystal MgOLiNbOs, the team fed two beams of photons-with one beam having half the frequency of the other. Resulting noise in one quadrature plummeted to 37%relative to the vacuum noise level (Z),which represented a 4.3-dB reduction below the shot-noise limit. (The shot-noise limit refers to the ordinary noise associated with quantum vacuum fluctuations.) Separately, Kimhle’s group applied squeezed light in an interferometer and achieved a 3-dB increase in the S/N ratio relative to operation at the shotnoise limit (3). The squeezed beam was supplied through the ordinary vacuum port of the interferometer. Kimhle’s group and others have repeatedly stressed that further improvements in noise reduction are being hampered “by losses in propagation and detection, and not by the degree of available squeezing.” In 1987, Slusher’s group used KTiOPOa crystals and pulsed light to produce squeezed light that exhibited noise reduction of 0.6 dB below the shot-noise limit (4). The pulsed laser light led to high peak powers. In addition to significant noise reduction, the team’s method afforded squeezed light over a wide range of wavelengths in the visible and infrared and minimized loss associated with random absorption of photons. The Bell team also applied squeezed IR light in a polarization interferometer and improved the response time (5).
Figure 1. Detectable phase changes
(6@) are limited by quantum fluctuations (shaded regions). Squeezed states (b) make possible liner measurement than do Vacuum states (a). (Figures adapted with permission hom John Wiley 8 Sons. F r m me lonhcoming Lases, Molecules and M e m . by HirsChlelder. Wyatt, and Coalson.)
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They generated the squeezed states in an optical cavity containing KTiOPOl crystals. The light was not pulsed. At the IBM Almaden Research Center (San Jose, Calif.), a team led by Robert Shelby and Marc Levenson has been producing squeezed states in optical fibers. Generation in optical fibers is complicated by disrupting factors such as light scattering. Nevertheless, the IBM group has achieved noise reductions of about 20% helow the shotnoise limit (6). Their technique again involvespassage of laser light through a nonlinear material-in this case, the fiher-however, they apply a four-mode squeezing procedure. Because of the inherent difficulties of generation in fihers, the IBM group stresses quantum nondemolition (QND) detection, or, in their words, “the measurement of a quantum observable without adding uncertainty to that variable.” This is accomplished by coupling an ohservable of interest to a second variable, which, upon reading, provides direct evidence for‘the value of the ohservable-hence the use of four light modes rather than the normal two. Before applying the QND methods, the group achieved 12%squeezing in fibers (7). Jeffrey Shapiro of MIT has been instrumental in advancing the theory of squeezed light, and, along with his coworkers, has done important experimental work. He and his group have continued to study squeezed states formed in a nonlinear vapor. They have used sodium vapor and have achieved noise reduction of 4% helow the shotnoise limit (8).Realizing that the sodium medium presents many interfering factors, such as resonance effects, the group is now attempting to switch to an ytterbium vapor medium in the hope of lessening those effects. Interest in squeezed light is international. Some examples of groups outside the United States who have made important contributions to the ongoing work (and people often mentioned by those with whom I spoke) are Claude Fabre and eo-workers at the University of Pierre and Marie Curie (France), D. F. Walls and co-workers at the University of Waikato (New Zealand), and Yoshihisa Yamamoto a t Nippon Telegraph and Telephone (Japan). There are many more, both in the United States and abroad. Areas Or appllcatlon In 1981, Carlton Caves, then at Cal Tech, proposed that squeezed light be used to improve the sensitivities of large interferometers being employed in the search for gravity waves. The effectiveness of using squeezed light to improve the sensitivities of smaller in-
ANALYTICAL CHEMISTRY, VOL. 60. NO. 5. MARCH 1. 1988
Figure 2. A detectable amplitude change (6A) is also limited by Quantum fluctuations. Snadea regions represent n~nualo m lor (ai YBC. .Jm 6 1 1 e S end IO) Squet)LM stales NOte lhsl bA improves lor b e s q ~ e z e a6Lates. smo.~nme a r m 01 the shaded regions remain vln same iuncsrtainh pr nc~piei.
terferumeters has nuw been demonstrated (by the Texas and Bell Labs groups). Caves’s proposal can now be carried out and very likely will he by a group at the Max Planck Institute for Quantum Optics (Munich, Germany), which is building such a device. Detecting gravity waves is not easy, and to this day no indisputable results have been obtained. The sensitivities required are daunting; indeed, the contribution made by squeezed light to the improvement in sensitivity of a gravity-wave interferometer would he only a small fraction of what is needed. Caves has remarked that by the time the technology of gravity.wave detectors approaches the levels of sensitivity needed for success, squeezed-light generation likely will have become routine and presumably employed in the efforts. More to the point for analytical chemists are the potential uses of squeezed light in molecular studies. David Neshitt of the University of Colorado suggests that squeezed light could be used as a direct-absorption, analytical probe. He and his co-workers pass IR laser light through weak molecular complexes and measure ahsorption of the light. So far, using ordinary laser light, they have been able to detect species down to orders of 10’108molecules(ferntogram range). They have achieved these results while operating essentially at theshot-noise limit. Neshitt believes that if even greater sensitivities could he achieved by using squeezed light, then nonintrusive prob-
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ing methods such as this could be extended to unique applications such as measurement in hostile environments. He adds that these techniques could he applied in all areas of the spectrum, not just the infrared. Furthermore, the technologies envisioned could, in Nesbitt's words, "translate into any absorption technique that is limited by shot noise." Fred Lytle of Purdue University, an expert in the field of spectroscopy, suggests that because solution applications would experience overtone problems at the sensitivities described, initial efforts in applying squeezed light should be in nonsolution areas. In particular, he cites methods for gettiilg absorption spectra of ion beams in mass spectrometers. Other examples of possible future uses include two-dimensional techniques such as gas chromatography and laser absorption (suggested by Nesbitt). More unusual applications are also possible. At Stanford University, David Bloom is using picosecond laser pulses to test circuit pathways in microchips and is operating at the shot-noise limit. Because Slusher has already demonstrated the feasibility of pulsed squeezed light, work such as Bloom's could conceivably benefit. The lack of progress in generating squeezed states for many years kept scientists from envisioning possible uses of this phenomenon. Now that a working technology is closer to reality, that should change. In the near future, we can expect scientists from many fields to propose novel squeezed-light applications. That process will feed back to optics researchers, and great things will undoubtedly ensue. Stay Don Cunningham tuned. Reterences (1) Slusher, R. E.; Hollberg, L. W.; Yurke, B.; Mertz, J. C.; Valley, J. F. Phys. Reu. Lett. 1985,55,2409.
Wu, L.-A,; Kirnble, H. J.; Hall, J. L.; Wu, H. Phys. Re". Lett. 1986,57,2520-3. (3) Xiao, M.; Wu,L.-A.;Kirnble,H. J.Phys.
(2)
Rev. Lett. 1987,59,27&81. (4) Slusher,R.E.;Grangier,P.;LaPorta,A.;
cbvy my 0 VISANuicrCud 0 American Expms
0 Diners ClubiCartc Blanche 0 B m b y m d 0 Accm
Yurke, B.; Potasek, M. J. Phys. Re". Lett.
1987,59,25669. (5) Grangier, P.; Slusher, R. E.;
Yurke, B.; LaPorta, A. Phys. Reu. Lett. 1987, 59, 21534
1
(6)Schurnaker, B. L.; Perlrnutter, S. H.; Shelby,R.M.; Levenson,M. D. Phys. Reu. Lett. 1987,58,357. ( 7 ) Levenson, M. D.; Shelby, R. M.; Reid, M.;Walls, D. F. Phys. Re". Lett. 1986.57,
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(8) Maeda, M. W.; Kumar, P.; Shapiro, J. H. Opt. Lett. 1987.Z2, 161. city. SlltC, zip
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svggestd reolml J . Opt. SOC.Am. B., October 1987.
