Physics Today

Demolishing quantum nondemolition Christopher Monroe

Citation: Physics Today 64(1), 8 (2011); doi: 10.1063/1.3541926 View online: http://dx.doi.org/10.1063/1.3541926 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/64/1?ver=pdfcov Published by the AIP Publishing

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[[[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to ]]] IP: 129.2.8.82 On: Mon, 14 Oct 2013 20:45:55 letters Demolishing quantum nondemolition

Most physicists are taught that almost exactly by the probabilities of pulse of photocurrent, the photon is lost quantum physics has two parts: coher- the underlying superposition, as pre- forever. Or is it? The photocurrent may ent wave mechanics, in which quantum scribed by the conventional Copen- interact with a macroscopic system of a states evolve according to the reversible hagen rules that we are taught. bulk conductor, and we may measure Schrödinger wave equation, and the However, a new term, the quantum the resulting voltage across the conduc- measurement process, in which quan- nondemolition, or QND, has crept tor. Sure, the photon has disappeared, tum states irreversibly collapse into a into the literature these past decades. but if our detector indicates that we had particular state given by the details of Also known as “quantum back-action- one photon, we can always create an- the measurement. So when a quantum evading measurement,” it’s a needlessly other and get the same answer again superposition state such as a|0〉 + b|1〉 is confusing term that seems to highlight and again, exactly like a QND measure- measured with a device that discrimi- the mundane aspect of a perfect quan- ment. Even if we begin with a coherent nates perfectly between |0〉 and |1〉, the tum measurement: If one already superposition of zero and one photon, system evolves to the definite state |0〉 knows that the system is in a particular which is not an eigenstate of our meas- or |1〉 with respective probabilities |a|2 eigenstate of the measuring device, urement device, that approach works. and |b|2. Of course, when such an ideal then, obviously, a measurement on the After the first measurement collapses measurement is repeated on the surviv- system will produce that eigenstate and the state into |0〉 or |1〉, we know which ing state, the same definite state will ap- leave the system intact. Zero informa- one came out and we either prepare pear again and again. Many of us, start- tion is gained from the repeated meas- zero or one photon forever after. A real ing with Albert Einstein, do not like the urement. On the other hand, when the photodetector is not 100% efficient, of apparent disconnect between those two system is not in an eigenstate of the course, meaning that a single photon rules and may not even subscribe to measuring device, the quantum state striking the detector could register that conventional “Copenhagen inter- can be thought to collapse to one of its nothing, or that even in the absence of pretation” of quantum mechanics. It is eigenstates. In that case, information is photons the noisy detector fires any- nevertheless useful because it correctly gained from the system, and the QND way. Again we can think of this as a predicts the results of all experiments measurement most certainly demol- QND measurement in which we have performed to date, even though it gives ishes the system. The concept of QND lost information to the environment. In the appearance that the observer neces- measurement adds nothing to the usual practice, we trace over the environment sarily affects the system. rules of quantum measurement, regard- and then must describe the system in In the past two decades, the advent less of interpretation. But worse, the terms of a density matrix containing the of quantum information science and the term confounds students and newcom- probabilities of various outcomes. ability of experiments to control quan- ers who may even be comfortable with In every case, the concept of QND tum systems such as single atoms have the prescriptions of quantum physics. measurement is confusing and unnec- provided the happy opportunity to Not all measurements are perfect, of essary. It wasn’t needed in the formative once again ponder the disconnect be- course, and some people may wish to years of quantum mechanics; and now tween the fundamental quantum con- use the QND nomenclature to distin- with quantum information theory, en- cepts of coherent evolution and meas- guish the efficiency of a measuring de- tanglement, and decoherence theory, urement. Quantum systems can now be vice. However, in the spirit of quantum we still do not need it. Marvelous ad- measured with nearly perfect effi- decoherence theory (see, for example, vances in many experimental quantum ciency, meaning that the observed dis- Wojciech Zurek’s article in PHYSICS systems, such as individual atoms and tribution of a measurement is given TODAY, October 1991, page 36), we can superconducting circuits, are allowing still think of any measurement as a quantum measurements to be per- formed almost exactly like in textbooks, Letters are encouraged and should be QND measurement plus a possible in- teraction with a reservoir that can either as most of us have learned. Why not de- sent by e-mail to [email protected] molish the term “QND”? (using your surname as “Subject”), or by mask the information obtained from the system or even uncontrollably change Christopher Monroe standard mail to Letters, PHYSICS TODAY, ([email protected]) American Center for Physics, One Physics the quantum state. The interaction with Joint Quantum Institute Ellipse, College Park, MD 20740-3842. the reservoir is what can make quan- University of Maryland, College Park Please include your name, affiliation, tum problems difficult, and it is there mailing address, e-mail address, and that we confront the conceptual diffi- daytime phone number on your attach- culties with quantum mechanics and the quantum–classical divide. ment or letter. You can also contact us A building moves As a common example of an imper- online at http://www.physicstoday.org/ fect measurement, consider photo - with the music pt/contactus.jsp. We reserve the right to detection. When a single photon strikes In my capacity as a consulting acousti- edit submissions. a photomultiplier tube and generates a cal engineer for 39 years, I’ve been in-

[[[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to ]]] IP: 8 January 2011 Physics Today © 2011 American Institute of Physics, S-0031-9228-1101-220-8 129.2.8.82 On: Mon, 14 Oct 2013 20:45:55