domenica 16 maggio 2010

Quantum Dynamics of Matter Waves Reveal Exotic Multibody Collisions.

Source: ScienceDaily
ScienceDaily (May 16, 2010) — At extremely low temperatures atoms can aggregate into so-called Bose Einstein condensates forming coherent laser-like matter waves. Due to interactions between the atoms fundamental quantum dynamics emerge and give rise to periodic collapses and revivals of the matter wave field.
A group of scientists led by Professor Immanuel Bloch (Chair of Experimental Physics at the Ludwig-Maximilians-Universität München (LMU) and Director of the Quantum Many Body Systems Division at the Max Planck Institute of Quantum Optics in Garching) has now succeeded to take a glance 'behind the scenes' of atomic interactions revealing the complex structure of these quantum dynamics. By generating thousands of miniature BECs ordered in an optical lattice the researchers were able to observe a large number of collapse and revival cycles over long periods of time.
The research is published in the journal Nature.
The experimental results imply that the atoms do not only interact pairwise -- as typically assumed -- but also perform exotic collisions involving three, four or more atoms at the same time. On the one hand, these results have fundamental importance for the understanding of quantum many-body systems. On the other hand, they pave the way for the generation of new exotic states of matter, based on such multi-body interactions.
The experiment starts by cooling a dilute cloud of hundreds of thousands of atoms to temperatures close to absolute zero, approximately -273 degrees Celsius. At these temperatures the atoms form a so-called Bose-Einstein condensate (BEC), a quantum phase in which all particles occupy the same quantum state. Now an optical lattice is superimposed on the BEC: This is a kind of artificial crystal made of light with periodically arranged bright and dark areas, generated by the superposition of standing laser light waves from different directions. This lattice can be viewed as an 'egg carton' on which the atoms are distributed. Whereas in a real egg carton each site is either occupied by a single egg or no egg, the number of atoms sitting at each lattice site is determined by the laws of quantum mechanics: Depending on the lattice height (i.e. the intensity of the laser beam) the single lattice sites can be occupied by zero, one, two, three and more atoms at the same time.
The use of those "atom number superposition states" is the key to the novel measurement principle developed by the researchers. The dynamics of an atom number state can be compared to the dynamics of a swinging pendulum. As pendulums of different lengths are characterized by different oscillation frequencies, the same applies to the states of different atom numbers. "However, these frequencies are modified by inter-atomic collisions. If only pairwise interactions between atoms were present, the pendulums representing the individual atom number states would swing synchronously and their oscillation frequencies would be exact multiples of the pendulum frequency for two interacting atoms," Sebastian Will, graduate student at the experiment, explains.
Using a tricky experimental set-up the physicists were able to track the evolution of the different superimposed oscillations over time. Periodically interference patterns became visible and disappeared, again and again. From their intensity and periodicity the physicists found unambiguous evidence that the frequencies are actually not simple multiples of the two-body case. "This really caught us by surprise. We became aware that a more complex mechanism must be at work," Sebastian Will recalls. "Due to their ultralow temperature the atoms occupy the energetically lowest possible quantum state at each lattice site. Nevertheless, Heisenberg's uncertainty principle allows them to make -- so to speak -- a virtual detour via energetically higher lying quantum states during their collision. Practically, this mechanism gives rise to exotic collisions, which involve three, four or more atoms at the same time."
The results reported in this work provide an improved understanding of interactions between microscopic particles. This may not only be of fundamental scientific interest, but find a direct application in the context of ultracold atoms in optical lattices. Owing to exceptional experimental controllability, ultracold atoms in optical lattices can form a "quantum simulator" to model condensed matter systems. Such a quantum simulator is expected to help understand the physics behind superconductivity or quantum magnetism. Furthermore, as each lattice site represents a miniature laboratory for the generation of exotic quantum states, experimental set-ups using optical lattices may turn out to be the most sensitive probes for observing atomic collisions.
Story Source:
Adapted from materials provided by
Ludwig-Maximilians-Universität München.
Journal Reference:
Sebastian Will, Thorsten Best, Ulrich Schneider, Lucia Hackermüller, Dirk-Sören Lühmann, Immanuel Bloch. Time-resolved observation of coherent multi-body interactions in quantum phase revivals. Nature, 2010; 465 (7295): 197 DOI:

domenica 9 maggio 2010

Anton Zeilinger vs. Daniel Salart about "Spooky action at a distance".

Comment on: Testing the speed of ‘spooky action at a distance’.
(Johannes Kofler, Rupert Ursin, Časlav Brukner, Anton Zeilinger)
In a recent experiment, Salart et al. addressed the important issues of the speed of hypothetical communication and of reference frames in Bell-type experiments. The authors report that they "performed a Bell experiment using entangled photons" and conclude from their experimental results that "to maintain an explanation based on spooky action at a distance we would have to assume that the spooky action propagates at speeds even greater than the bounds obtained in our experiment", exceeding the speed of light by orders of magnitude. Here we show that, analyzing the experimental procedure, explanations with subluminal or even no communication at all exist for the experiment.
In order to explain the violation of Bell inequalities within the view where, to use the author‟s wording, "correlated events have some common causes in their shared history", one needs to assume hypothetical communication between the observer stations. This communication must be faster than light if the outcome at one station is space-like separated from all relevant events at the other station.
In the experiment pairs of time-bin entangled photons were sent over 17.5 km optical fibers to two receiving stations, located in Jussy and Satigny, both equipped with a Franson-type interferometer and detectors. The out-comes were observed space-like separated from each other. The phase in the interferometer, i.e. the setting, in Jussy was continuously scanned, while the setting at Satigny was kept stable.
However, if the setting at one side remains unchanged, the results at both observer stations can be described by a "common-cause" without having to invoke any communication at all, let alone superluminal spooky action at a distance. This is signified, e.g., by the fact that no formulation of a bipartite Bell type inequality exists which does not use at least two settings at each side. Therefore, contrary to the claim in the paper, no Bell test was performed.
Furthermore, had the experiment been repeated with a second stable setting at Satigny, a "common-cause" explanation would still be possible. This is because in order to exclude subluminal communication, it is crucial that the outcome event on each side is space-like separated from the setting choice on the other side – which was not done in Ref. [1]. Thus, such experimental data – even if they were taken with two measurement settings at Satigny and even granting the fair-sampling assumption – could be explained by a "common-cause" model. In other words, the experiment tests the superluminal speed of hypothetical influences between outcome events under the assumption of no, not even subluminal, hypothetical influences between setting choices and outcome events.
We also remark that in a Franson-type experiment like the one reported in Ref. [1] the considered Clauser-Horne-Shimony-Holt Bell inequality is not applicable even with perfect detectors because of the inherent postse-lection.2 One would (i) have to use a chained Bell inequality2, (ii) achieve fast switching with a rate depending on the geometry of the interferometer, and (iii) reach a better visibility than the one reported in Ref. [1]. None of these three issues is covered by the experiment.
We would like to stress that this comment should not be seen as a defence of local realism. And neither do we demand that Ref. [1] must present a loophole-free Bell test. However, it is the purpose of our comment to point out "common-cause" explanations of an experiment which aims at putting "stringent experimental bounds on the speed of all such hypothetical influences".
Reply to the: "Comment on: Testing the speed of `spooky action at a distance' "
(D. Salart, A. Baas, C. Branciard, N. Gisin, and H. Zbinden)
Quantum correlations cannot be described by local common causes. This prediction of quantum theory, surprising as it might appear, has been widely con rmed by numerous experiments. In our Nature Letter [1] we considered this point as established and addressed another issue: the alternative assumption that quantum correlations are due to supra-luminal influences of a first event onto a second event. For this purpose we believe that it suffices to observe 2-photon interferences with a visibility high enough to potentially violate Bell's inequality, as we reported (over 2 x 17.5 km). Simultaneously closing other loopholes, like the locality loophole as desired by Koer and colleagues, would certainly be an interesting addition, as would be any Bell tests that simultaneously address several of the loopholes.
Indeed, to rigorously exclude any common cause explanation of the observed quantum correlation one should, ideally, simultaneously close the locality and the detection loophole (and assume the existence of independent randomness and that quantum measurements are nished when detectors re or at least when a mesoscopic mass has sufficiently moved as insured in our experiment, see our recent article [2]). This is a formidable task and any progress towards achieving it is most welcome. So far, however, all experiments have addressed at most one of these loopholes; ours is no exception.
Concerning the comment on the use of a Franson interferometer for testing quantum nonlocality, we stress that this is not a fundamental issue. In principle it suffices to replace the entrance beam splitters of each interferometer by a fast switch. In this way the non-interfering lateral peaks observed in the 2-photon interferogram would disappear. However, in practice such switches suffer due to losses of around 3 dB. Hence, with today's technology it is much more
convenient to replace the ideal switch by a passive coupler, as we did in our experiment in a way very similar to [3].