PERSPECTIVES a means for further optimizing the editing more,the recombination should be fast,effi- References performance of Cas9. cient,and scalable.Compared to the most 1.L.Cong et al.,Science 339.819 (2013): Efficient strategies for directed editing 10.1126/science..1231143 promising currently available genome edit- 2.P.Mali et al.,Science 339,823 (2013): of mammalian genomes will enable sophis- ing systems(zinc finger domains and TAL- 10.1126/science.1232033. ticated genetic engineering for both funda- ENs),the RNA-guided Cas9 nuclease prob- 3.B.L Stoddard,Structure 19,7 (2011). 4.F.D.Urnov,E.]Rebar,M.C.Holmes,H.S.Zhang. mental and applied purposes.Especially in ably is closest to meeting these requirements P.D.Gregory,Nat.Rev.Genet.11,636 (2010). medical applications,high-fidelity target rec- However,efficiency and specificity still can 5.A J.Bogdanove,D.F.Voytas,Science 333,1843 (2011) ognition is critical,as off-site nuclease activ- be improved-for instance,by laboratory 6.K.Eisenschmidt et ol.,Nucleic Acids Res.33,7039 (2005). ity will jeopardize the safety of the engineer- evolution.Applying these genome surgery 7.B.Wiedenheft,S.H.Sternberg,]A.Doudna,Nature ing operation;thus,long stretches of nucle- techniques to correct human disease-asso- 482,331(2012) otides should be specifically recognized.In ciated genetic mutations,resulting in func- 8.E.R.Westra et al..Annu.Rev.Genet.46.311(2012). 9.E.Deltcheva et al,Nature 471,602(2011). addition,adjusting the system's specificity tional gene therapy and in curing genetic dis- 10.M.Jinek et al.,Science 337,816 (2012). toward new target sequences should be easy orders,will therefore take time.The spectac- 11.G.Gasiunas,R.Barrangou,P.Horvath,V.Siksnys,Proc. and affordable;this is a major advantage of ular recent development of dedicated nucle- Natl.Acad.Sci.U.S.A 109,E2579 (2012). 12.M.Jinek et ol.,ele2013;2:300471(2013). the Cas9 system,as it merely requires chang- ases suggests,however,that we are entering ing the sequence of the guide RNA.Further- the final stage of this quest. 10.1126 science.1234726 PHYSICS Demonstrating Uncertainty Heisenberg's uncertainty principle is demonstrated with a vibrating macroscopic mirror. Gerard J.Milburn s nyone using a modern camera is cles.This observation eventually led to laser changed by the tidal forces exerted by gravi- implementing an optical position cooling and the field of atom optics. tational waves.This length difference leads measurement.In an active autofo- The history of using optical transduc- to changes in the interference of light at the cus camera,a pulse of infrared light is emit- ers to monitor the quantized position of an output mirror.Monitoring the output inten- ted from the camera,and the time taken for object at the Heisenberg limit goes back to sity can thus be used to measure the relative it to be reflected back to the camera is used the early proposals for the optical detec- position of the end mirrors. to compute the distance between the object tion of gravitational radiation (3).The rel- The effect is small,however,and very and the image plane.Imagine how difficult ative length of the two orthogonal arms small changes in the intensity need to be it would be to operate such a system if the in a Michelson-Morley interferometer is detected.This eventually runs into a problem object recoiled every time the caused by the essential granular infrared pulse was reflected from nature of light (light pulses are it.Heisenberg suggested that this made up of individual photons). is precisely what would happen if Even the most carefully stabi- light were used to determine the lized laser produces light with eo] position of a quantum object as intensity fluctuations due to the accurately as his famous uncer- random arrival of individual pho- tainty principle would allow.On Inaident tons,called shot noise.We rarely page 801 of this issue,Purdy et need to account for this as the rel- Transmitted al.(/demonstrate this quantum Reflected ative size of the intensity fluctua- back-action effect in an optical tion falls off as the inverse square measurement of the position of a root of the intensity,so we can macroscopic mirror. always increase the intensity to The mechanical action of improve the signal-to-noise ratio. light has long been known(2). But there is another problem. Kepler suggested that the rea- If we take into account the son comet tails point away from mechanical action of light,we the Sun is due to the mechani- see that a price must be paid for cal action of light.In the early 1970s,Arthur Ashkin of Bell An optical cavity with a vibrat- Laboratories showed that optical ing mirror.As this mirror moves,the intensity of the transmitted light can intensity gradients could exert a Probe be used to monitor its position.(Inset) force on micrometer-size parti- When the quantum nature of light is included,the random reflection of Centre for Engineered Quantum Systems,The University of Queensland,StLucia,Brisbane, individual photons shakes the mirror, QLD 4072,Australia.E-mail:milburn@ adding radiation pressure noise to the physics.uq.edu.au position measurement. 770 15 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org Published by AAAS770 15 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org PERSPECTIVES Demonstrating Uncertainty PHYSICS Gerard J. Milburn Heisenberg’s uncertainty principle is demonstrated with a vibrating macroscopic mirror. a means for further optimizing the editing performance of Cas9. Efficient strategies for directed editing of mammalian genomes will enable sophis￾ticated genetic engineering for both funda￾mental and applied purposes. Especially in medical applications, high-fidelity target rec￾ognition is critical, as off-site nuclease activ￾ity will jeopardize the safety of the engineer￾ing operation; thus, long stretches of nucle￾otides should be specifically recognized. In addition, adjusting the system’s specificity toward new target sequences should be easy and affordable; this is a major advantage of the Cas9 system, as it merely requires chang￾ing the sequence of the guide RNA. Further￾more, the recombination should be fast, effi- cient, and scalable. Compared to the most promising currently available genome edit￾ing systems (zinc finger domains and TAL￾ENs), the RNA-guided Cas9 nuclease prob￾ably is closest to meeting these requirements. However, efficiency and specificity still can be improved—for instance, by laboratory evolution. Applying these genome surgery techniques to correct human disease-asso￾ciated genetic mutations, resulting in func￾tional gene therapy and in curing genetic dis￾orders, will therefore take time. The spectac￾ular recent development of dedicated nucle￾ases suggests, however, that we are entering the final stage of this quest. References 1. L. Cong et al., Science 339, 819 (2013); 10.1126/science.1231143. 2. P. Mali et al., Science 339, 823 (2013); 10.1126/science.1232033. 3. B. L. Stoddard, Structure 19, 7 (2011). 4. F. D. Urnov, E. J. Rebar, M. C. Holmes, H. S. Zhang, P. D. Gregory, Nat. Rev. Genet. 11, 636 (2010). 5. A. J. Bogdanove, D. F. Voytas, Science 333, 1843 (2011). 6. K. Eisenschmidt et al., Nucleic Acids Res. 33, 7039 (2005). 7. B. Wiedenheft, S. H. Sternberg, J. A. Doudna, Nature 482, 331 (2012). 8. E. R. Westra et al., Annu. Rev. Genet. 46, 311 (2012). 9. E. Deltcheva et al., Nature 471, 602 (2011). 10. M. Jinek et al., Science 337, 816 (2012). 11. G. Gasiunas, R. Barrangou, P. Horvath, V. Siksnys, Proc. Natl. Acad. Sci. U.S.A. 109, E2579 (2012). 12. M. Jinek et al., elife 2013; 2:300471 (2013). 10.1126/science.1234726 Anyone using a modern camera is implementing an optical position measurement. In an active autofo￾cus camera, a pulse of infrared light is emit￾ted from the camera, and the time taken for it to be reflected back to the camera is used to compute the distance between the object and the image plane. Imagine how difficult it would be to operate such a system if the object recoiled every time the infrared pulse was reflected from it. Heisenberg suggested that this is precisely what would happen if light were used to determine the position of a quantum object as accurately as his famous uncer￾tainty principle would allow. On page 801 of this issue, Purdy et al. ( 1) demonstrate this quantum back-action effect in an optical measurement of the position of a macroscopic mirror. The mechanical action of light has long been known (2). Kepler suggested that the rea￾son comet tails point away from the Sun is due to the mechani￾cal action of light. In the early 1970s, Arthur Ashkin of Bell Laboratories showed that optical intensity gradients could exert a force on micrometer-size parti￾cles. This observation eventually led to laser cooling and the field of atom optics. The history of using optical transduc￾ers to monitor the quantized position of an object at the Heisenberg limit goes back to the early proposals for the optical detec￾tion of gravitational radiation (3). The rel￾ative length of the two orthogonal arms in a Michelson-Morley interferometer is changed by the tidal forces exerted by gravi￾tational waves. This length difference leads to changes in the interference of light at the output mirror. Monitoring the output inten￾sity can thus be used to measure the relative position of the end mirrors. The effect is small, however, and very small changes in the intensity need to be detected.This eventually runs into a problem caused by the essential granular nature of light (light pulses are made up of individual photons). Even the most carefully stabi￾lized laser produces light with intensity fluctuations due to the random arrival of individual pho￾tons, called shot noise. We rarely need to account for this asthe rel￾ative size of the intensity fluctua￾tion falls off as the inverse square root of the intensity, so we can always increase the intensity to improve the signal-to-noise ratio. But there is another problem. If we take into account the mechanical action of light, we see that a price must be paid for Centre for Engineered QuantumSystems,The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia. E-mail: milburn@ physics.uq.edu.au Incident Reflected Transmitted Signal Probe Signal Probe An optical cavity with a vibrat￾ing mirror. As this mirror moves, the intensity of the transmitted light can be used to monitor its position. (Inset) When the quantum nature of light is included, the random reflection of individual photons shakes the mirror, adding radiation pressure noise to the position measurement. Published byAAAS on February 15, 2013 www.sciencemag.org Downloaded from