The Quantum Times
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TThhee QQuuaannttuumm TTiimmeess APS Topical Group on Quantum Information, Concepts, and Computation Spring 2007 Volume 2, Number 1 Quantum Information and Quantum Foundations I sense a new interest in quantum foundations among people Changes afoot… working in quantum information. There are at least two motives There are a few subtle (and for this. First is the recognition that the ideas one finds in the perhaps a few not-so-subtle) standard textbooks are not quite what we need for quantum changes that mark their appearance information. Understanding the time development of systems that with this issue. The first and are constantly interacting with experimental probes and the hopefully most aesthetically environment requires more than cross sections and perturbation obvious is that I have endeavored theory. Second is the desire to use quantum information theory to to make some changes to fonts for tackle and, hopefully, resolve the well-known conceptual headings and in the masthead. My difficulties of Copenhagen – I use it as a convenient albeit hope is that it not only looks more inaccurate abbreviation for what one finds in current textbooks – professional but is a bit easier on quantum mechanics. the eyes. I think both motives are valid. But I also worry that instead Another change that should be of cleaning up the conceptual mess of quantum foundations, we fairly obvious from the start is that may simply end up by importing it into quantum information, we have essentially settled into a building a new structure on top of old, flawed ideas, and quarterly schedule here at The confusing not only ourselves but also the computer scientists, Times, and thus have opted to drop electrical engineers, mathematicians, and others now interested in the specific month in favor of the our discipline. I already see signs that this is going on, and I am ‘season.’ As such, this is the concerned. Spring 2007 issue (my apologies to As someone who has worked for many years in quantum readers from the Southern foundations, let me admit that the field deserves some of the bad Hemisphere – I had to draw the reputation it has acquired in the broader physics community. line somewhere and the equator There has been lots of work but not all that much progress in seemed a good place to do it). resolving the central issues, most of which date back to the good Very roughly we expect to put out old days of the 1920s or 30s. Disagreement is more common than an issue toward the end of each agreement among the practitioners. Sometimes it seems more ‘season’ which is really no like academic philosophy – where some critics think we belong – different than we’ve been doing: than like physics. Despite this I believe there are useful lessons to May, July or August, November, be learned from previous work in quantum foundations. Even the and February. failures can teach us something, and there have been some You will notice that once again interesting ideas that are worth further development. Perhaps the we have a section for letters. I am first and most obvious lesson is that the foundational problems hoping that each issue contains a are not trivial. Do not let the failures of Einstein, Feynman, robust letters section so please Schrödinger and Wigner discourage you from attempting your write to me (e-mail is preferred). own attack on these matters, but do expect to have to give them Specific information on sustained, serious thought. submissions can be found on the One of the few points of widespread agreement in quantum last page of the newsletter. In foundations is that measurement, despite its presence in every addition, a section of short news textbook, is a most unsatisfactory foundation for interpreting items makes a permanent return quantum mechanics. Everyone thinks the Copenhagen approach (well, at least until we change our results in a nasty unsolved “measurement problem,'' even if there minds). is little agreement on what to do about it. No doubt the textbook approach assigns correct probabilities to measurement outcomes -Ian T. Durham, Editor (the “pointer positions'' in the archaic terminology of a discipline Department of Physics that predates the computer age). But what does the pointer Saint Anselm College (continued on next page) 1 position tell us about the microscopic system that nonlocal influences are needed. It is simply a was (supposedly) being measured? I call this the matter of statistical correlation, and obviously does first measurement problem, and will return to it not conflict with relativity. later. The second measurement problem has to do The quantum case can be worked out in a with finding a fully quantum mechanical similar way. Alice and Bob share an entangled description of a real measurement process carried singlet state of two spin- "0 = ( 00 # 11 ) 2 out in the laboratory using equipment made of half particles (qubits). Alice measures S for her atoms that (presumably) follow quantum laws. z particle, and if the pointer points up, concludes that There are compelling arguments that a consistent Sz was +1/2, corresponding to the state 0 just quantum description of real measurements is ! impossible within the Copenhagen framework of before the measurement took place. From this and wavefunction collapse [1]. Yes, POVMs have the information provided by "0 she can infer that been considered, and no, they do not help. To my S = "1/2 for Bob's particle. This is a correct z ! mind the failure to solve the second problem is inference whether or not Bob carries out a particularly serious, as it indicates a basic measurement on his particle, as long as he does inconsistency in the textbook approach. The hope nothing to perturb !it s spin. If we understand the that things can somehow be salvaged usi!ng a wavefunction as somehow representing genuinely classical apparatus fades with each information, there is no danger of falling into the advance in creating more and more exotic nonlocality trap and supposing that Alice's entangled states in the laboratory. So I ask – and it measurement has an instantaneous influence on is a serious question – do we really want to Bob's particle. Instead, her measurement provides construct the edifice of quantum information her with information about the state of her particle theory with measurement as one of the axioms? before the measurement, and hence with And if not, what are the alternatives? information about the correlated state of Bob's There is at least one idea coming out of particle. In this respect the quantum case is no quantum information theory that I think could be different from its classical analog. But in what helpful in quantum foundations: the notion that a sense does " represent information? I use the wavefunction can represent information, rather 0 term pre-probability in Sec. 9.4 of my book [3] than physical reality [2]. While this does not by (hereafter referred to as CQT). That is, , while itself solve the measurement problem, it could be "0 not itself a probability, can be used to calculate helpful in disposing of a somewhat different ! conceptual mess: the nonlocality ghost. It is probabilities, in particular the joint probability widely believed that when Alice makes a distribution of Sz values for Alice's and for Bob's measurement over here on her half of an entangled particles, respectively. This !is the counterpart of state there is an instantaneous, superluminal, the joint probability for colors of the two slips of nonlocal influence that somehow changes Bob's paper used in the classical analog discussed above. half, no matter how far away. There are theorems Thus information, in the broad sense of statistical showing that the nonlocality ghost cannot transmit correlation, is playing a similar role in both cases. information, which is to say that it can never In the background I hear someone shouting, manifest itself in experiments. The only thing it is “How dare you claim that Alice's measurement capable of doing is causing confusion, and for this revealed the value Sz had before the measurement reason it needs to be permanently laid to rest, was made. That just isn't good quantum especially as it has given rise to the notion that mechanics!'' Well, it certainly is not good quantum mechanics is incompatible with special Copenhagen (i.e., textbook) quantum mechanics, relativity. which is indeed very difficult to merge with a Can we deal with this problem using the notion of the wavefunction as information. But it notion of wavefunction-as-information? I think so. is good physics if we assume that Alice is a To see how, consider the classical analog in which competent experimentalist who knows how to Charlie places a red slip of paper in one opaque construct a piece of apparatus which will measure envelope, a green slip in another, and after mixing the z component of angular momentum of a them up mails one envelope to Alice in Atlanta and particle. Do you think those folks are crazy who the other to Bob in Boston. If Alice opens her take results of their particle detectors and envelope and sees a red slip she can instantly extrapolate the tracks backwards to a point where – conclude, as she knows the protocol, that the slip in so they claim – a collision occurred between an Bob's envelope is green, independent of whether he electron and a positron? Let me suggest, at the risk has opened or ever will open his envelope. No of losing some friends, that the problem is not with 2 the way experimentalists think about real reasoning. But what constitutes sound quantum measurements.