PERSPECTIVES increasing the accuracy of the measurement quantum motion of a collective vibrational The experiment of Purdy et al.repre- by increasing the intensity.As individual degree of freedom of a mechanical element. sents one of the few approaches that dem- photons are reflected from a movable mir- In their experiment,the mechanical element onstrate the physical mechanism respon- ror,it will recoil,and the shot noise of indi- is a 7-ng square dielectric membrane placed sible for enforcing the Heisenberg uncer- vidual photon arrivals will lead to a fluctu- inside a Fabry-Perot optical interferometer. tainty limit.Can we beat this limit by creat- ating force on the mirror:it will begin to The bulk mechanical resonance frequency is ing light sources with no intensity fluctua- shake ever more violently as the intensity above I MHz with a linewidth less than 1 Hz tions at all?Such squeezed light sources are is increased,limiting the gain in accuracy (see the figure). indeed under development,yet they will be due to increasing intensity.This is the radi- As the membrane moves,it modulates the of no use in trying to beat the Heisenberg ation pressure shot noise,and it is the phys- frequency of the optical resonance and con- limit in optomechanical systems.The rea- ical mechanism enforcing the Heisenberg sequently modulates the amplitude and phase son is that if the intensity of a light source is uncertainty principle in an optomechanical of the transmitted light.This motion can be well defined,its phase becomes increasingly position transducer. detected with high efficiency by means of randomized-a quantum optical Heisen- Balancing the effects of shot noise and optical detection.Fluctuations of the mem- berg principle-and an interferometer can- radiation pressure noise leads to the stan- brane due to radiation pressure noise appear not be operated with a rapidly fluctuating dard quantum limit for an optomechanical in the noise power spectrum of the optical phase.A natural extension of the Purdy et position transducer.Large interferometers signal.In the experiment,two optical modes al.experiment would demonstrate precisely designed to detect gravitation waves are not were used:one with high power,the signal. this trade-off. yet at this limit,which would require more is the source of the radiation pressure shot 9 laser power than the optical components noise,while the other,weaker probe beam References could stand,yet it will need to be taken into monitors the displacement of the membrane 1.T.P.Purdy,R.W.Peterson,C.A.Regal,Science 339,801 (2013). account in the next generation of detectors. to detect the radiation pressure noise.The 2.P.Meystre,Atom Optics (Springer Verlag,New York,2001). Purdy et al.report the first direct demon- experiment can also be thought of as a quan- 3.C.M.Caves,K.S.Thorne,R.W.P.Drever,V.D.Sand- stration of the radiation pressure shot noise. tum nondemolition measurement of the pho- berg.M.Zimmermann,Rev.Mod.Phys.52.341(1980). Their study is a landmark in the emerging 4.M.Aspelmeyer,P.Meystre,K.Schwab,Phys.Today 65,29 ton number in the strong signal beam using a 5 (2012). field of quantum optomechanics(4),where coupling to the signal beam mediated by the the objective is to coherently control the mechanical element. 10.1126/science.1234109 PLANETARY SCIENCE A meteorite with a chemical makeup A Unique Piece of Mars resembling that of rocks from Mars'Gusev Crater has been identified. W Munir Humayun ollowing the pioneering Mars Explo- known martian meteorites with respect to Ga,older than SNC meteorites but younger ration Rovers,NASA's Curiosity age,oxygen isotopes,and petrology. than ALH 84001 (6).What makes NWA rover is actively exploring the crustal One might wonder whether such a rock 7034 so exciting is that its major element rocks of Mars.Despite the exciting results should even be construed to be of mar- composition is a close fit with the chemi- returned by the rovers,there is no substitute tian origin.Oxygen isotopes are the mete- cal data returned by the Mars rovers,as well for a hand sample of crustal rock.Because oritic equivalent of DNA fingerprinting, as with the global chemical composition of such samples will not be returned to Earth each unique signature implying a different Mars'crust returned by the orbiting gamma- anytime soon,geochemists who want a piece planetary reservoir (5).One of the big sur- ray spectrometer onboard Mars Odyssey(4). of Mars in their labs must satisfy themselves prises in NWA 7034 is that its oxygen iso- NWA 7034 appears to be a chemical analog with martian meteorites (/)These comprise topes are shifted to greater 60 depletion, of rocks from Gusev Crater (3). a group of igneous rocks with telltale signs and to heavier isotopic compositions,than Given that the other known martian mete- of martian alteration products (2)and have those of the SNC(shergottite,nakhlite,and orites are a poor match for crust exposed at provided ground truth for the information chassignite)martian meteorites.Had it not the martian surface,what can we learn from returned by the rovers.Oddly,however,the been for the alertness of Agee et al.,NWA a meteorite that resembles just about any hundred or so known martian meteorites are 7034 would likely have been classified as rock from Mars?The first important dis- chemically unrepresentative of the martian a unique achondrite (an asteroidal sample) covery is that the new meteorite has a dis- crust determined by missions(3).On page on the basis of its distinct oxygen isotope tinctly greater deficiency of the 0 isotope 780 of this issue,Agee et al.(4)put an end composition.However,they showed that the among its oxygen isotopes than the other to this conundrum with the finding of a new mineral chemistry of pyroxenes from NWA martian meteorites,which are bunched so martian meteorite,Northwest Africa(NWA) 7034 plot on the distinct FeO versus MnO close together on an oxygen isotope plot that 7034.a basaltic breccia unique among trend defined by other martian meteorites. they can be classified as a group on the basis Another distinguishing characteristic of of a single measurement of their A70(5). Department of Earth,Ocean and Atmospheric Science and martian meteorites over achondrites is their That this should be the case was anticipated National High Magnetic Field Laboratory,Florida State Uni- versity,Tallahassee,FL 32310,USA.E-mail:humayun@ young radiometric ages[<4 billion years ago by pioneering studies that found the mar- magnet.fsu.edu (Ga)];NWA 7034 has been dated about 2 tian hydrosphere and atmosphere to have www.sciencemag.org SCIENCE VOL 339 15 FEBRUARY 2013 771 Published by AAASwww.sciencemag.org SCIENCE VOL 339 15 FEBRUARY 2013 771 PERSPECTIVES A Unique Piece of Mars PLANETARY SCIENCE Munir Humayun A meteorite with a chemical makeup resembling that of rocks from Mars’ Gusev Crater has been identifi ed. increasing the accuracy of the measurement by increasing the intensity. As individual photons are reflected from a movable mir￾ror, it will recoil, and the shot noise of indi￾vidual photon arrivals will lead to a fluctu￾ating force on the mirror; it will begin to shake ever more violently as the intensity is increased, limiting the gain in accuracy due to increasing intensity. This is the radi￾ation pressure shot noise, and it is the phys￾ical mechanism enforcing the Heisenberg uncertainty principle in an optomechanical position transducer. Balancing the effects of shot noise and radiation pressure noise leads to the stan￾dard quantum limit for an optomechanical position transducer. Large interferometers designed to detect gravitation waves are not yet at this limit, which would require more laser power than the optical components could stand, yet it will need to be taken into account in the next generation of detectors. Purdy et al. report the first direct demon￾stration of the radiation pressure shot noise. Their study is a landmark in the emerging field of quantum optomechanics (4), where the objective is to coherently control the quantum motion of a collective vibrational degree of freedom of a mechanical element. In their experiment, the mechanical element is a 7-ng square dielectric membrane placed inside a Fabry-Perot optical interferometer. The bulk mechanical resonance frequency is above 1 MHz with a linewidth less than 1 Hz (see the figure). As the membrane moves, it modulates the frequency of the optical resonance and con￾sequently modulates the amplitude and phase of the transmitted light. This motion can be detected with high efficiency by means of optical detection. Fluctuations of the mem￾brane due to radiation pressure noise appear in the noise power spectrum of the optical signal. In the experiment, two optical modes were used; one with high power, the signal, is the source of the radiation pressure shot noise, while the other, weaker probe beam monitors the displacement of the membrane to detect the radiation pressure noise. The experiment can also be thought of as a quan￾tum nondemolition measurement of the pho￾ton number in the strong signal beam using a coupling to the signal beam mediated by the mechanical element. The experiment of Purdy et al. repre￾sents one of the few approaches that dem￾onstrate the physical mechanism respon￾sible for enforcing the Heisenberg uncer￾tainty limit. Can we beat this limit by creat￾ing light sources with no intensity fluctua￾tions at all? Such squeezed light sources are indeed under development, yet they will be of no use in trying to beat the Heisenberg limit in optomechanical systems. The rea￾son is that if the intensity of a light source is well defined, its phase becomes increasingly randomized—a quantum optical Heisen￾berg principle—and an interferometer can￾not be operated with a rapidly fluctuating phase. A natural extension of the Purdy et al. experiment would demonstrate precisely this trade-off. References 1. T. P. Purdy, R. W. Peterson, C. A. Regal, Science 339, 801 (2013). 2. P. Meystre, Atom Optics (Springer Verlag, New York, 2001). 3. C. M. Caves, K. S. Thorne, R. W. P. Drever, V. D. Sand￾berg, M. Zimmermann, Rev. Mod. Phys. 52, 341 (1980). 4. M. Aspelmeyer, P. Meystre, K. Schwab, Phys. Today 65, 29 (2012). 10.1126/science.1234109 Following the pioneering Mars Explo￾ration Rovers, NASA’s Curiosity rover is actively exploring the crustal rocks of Mars. Despite the exciting results returned by the rovers, there is no substitute for a hand sample of crustal rock. Because such samples will not be returned to Earth anytime soon, geochemists who want a piece of Mars in their labs must satisfy themselves with martian meteorites (1). These comprise a group of igneous rocks with telltale signs of martian alteration products (2) and have provided ground truth for the information returned by the rovers. Oddly, however, the hundred or so known martian meteorites are chemically unrepresentative of the martian crust determined by missions (3). On page 780 of this issue, Agee et al. ( 4) put an end to this conundrum with the finding of a new martian meteorite, Northwest Africa (NWA) 7034, a basaltic breccia unique among known martian meteorites with respect to age, oxygen isotopes, and petrology. One might wonder whether such a rock should even be construed to be of mar￾tian origin. Oxygen isotopes are the mete￾oritic equivalent of DNA fingerprinting, each unique signature implying a different planetary reservoir (5). One of the big sur￾prises in NWA 7034 is that its oxygen iso￾topes are shifted to greater 16O depletion, and to heavier isotopic compositions, than those of the SNC (shergottite, nakhlite, and chassignite) martian meteorites. Had it not been for the alertness of Agee et al., NWA 7034 would likely have been classified as a unique achondrite (an asteroidal sample) on the basis of its distinct oxygen isotope composition. However, they showed that the mineral chemistry of pyroxenes from NWA 7034 plot on the distinct FeO versus MnO trend defined by other martian meteorites. Another distinguishing characteristic of martian meteorites over achondrites is their young radiometric ages [<4 billion years ago (Ga)]; NWA 7034 has been dated about 2 Ga, older than SNC meteorites but younger than ALH 84001 (6). What makes NWA 7034 so exciting is that its major element composition is a close fit with the chemi￾cal data returned by the Mars rovers, as well as with the global chemical composition of Mars’ crust returned by the orbiting gamma￾ray spectrometer onboard Mars Odyssey (4). NWA 7034 appears to be a chemical analog of rocks from Gusev Crater (3). Given that the other known martian mete￾orites are a poor match for crust exposed at the martian surface, what can we learn from a meteorite that resembles just about any rock from Mars? The first important dis￾covery is that the new meteorite has a dis￾tinctly greater deficiency of the 16O isotope among its oxygen isotopes than the other martian meteorites, which are bunched so close together on an oxygen isotope plot that they can be classified as a group on the basis of a single measurement of their ∆ 17O ( 5). That this should be the case was anticipated by pioneering studies that found the mar￾tian hydrosphere and atmosphere to have Department of Earth, Ocean and Atmospheric Science and National High Magnetic Field Laboratory, Florida State Uni￾versity, Tallahassee, FL 32310, USA. E-mail: humayun@ magnet.fsu.edu Published byAAAS on February 15, 2013 www.sciencemag.org Downloaded from