A new version of Wheeler's set up

The particle-wave duality seem always under debate and ingenious experiments have been contrived from time to time to violate it. A famous example is the one proposed by John-Wheeler. It is called 'delayed-choice experiment'. In its existing version, a classical switch has been in use. Here comes a new design using quantum switches [http://physics.aps.org/articles/v4/102].

This so-called delayed-choice experiment was performed in 2007 using an interferometer [1]. In the normal setup, a beam splitter creates two separate light beams that later recombine in a second beam splitter. Detectors placed at the two outputs of this beam splitter both register an interference pattern. However, this wave detector can be turned into a particle detector by removing the second beam splitter, so that the two paths no longer interfere. In the experiment, the choice to add or remove the second beam splitter was made after an individual photon had already passed through the first beam splitter. The data showed that particle and wave behavior were unaffected by the delayed choice, as expected from standard quantum mechanics.

Radu Ionicioiu, now at the Institute for Quantum Computing in Waterloo, Canada, and Daniel Terno of Macquarie University in Sydney, Australia, wanted to see what happens in the thought experiment if the delayed choice is made through quantum means. They imagined that the interferometer contains a quantum device—perhaps an atom in a cavity or a micro-mirror placed on a cantilever—that can exist in two possible states. One state selects the particle experiment, and the other selects the wave experiment. This quantum control element can be placed in a combination, or superposition, of its two states, making the whole experiment participate in the wave-particle duality.

“We show you can do both wave and particle experiments at once,” Ionicioiu says. This means the choice of wave vs particle can be delayed indefinitely. The photon can be observed at one of the detectors and still not “know” if it is supposed to be a wave or a particle. It’s only when the observer decides to measure the state of the quantum control that the photon’s behavior can be identified as wavelike or particlelike.

Schrodinger's cat fattened up

The famous Schrodinger cat now gets fattened up to 430 atoms [http://www.nature.com/news/2011/110405/full/news.2011.210.html]!

In the famous thought experiment conceived by Erwin Schrödinger in 1935 to illustrate the apparent paradoxes of quantum theory, a cat would be poisoned or not depending on the state of an atom — the atom's state being governed by quantum rules. Because quantum theory required that these rules allowed superpositions, it seemed that Schrödinger's cat could itself exist in a superposition of 'live' and 'dead' states.

The paradox highlights the question of how and when the rules of the quantum world – in which objects such as atoms can exist in several positions at once – give way to the 'classical' mechanics that governs the macroscopic world of our everyday experience, where things must be one way or the other but not both at the same time. This is called the quantum-to-classical transition.

It is now generally thought that 'quantumness' is lost in a process called decoherence, in which disturbances in the immediate environment make the quantum wavefunction describing many-state superpositions appear to collapse into a well-defined, unique classical state. This decoherence tends to become more pronounced the bigger the object, as the opportunities for interacting with the environment increase.

One manifestation of quantum superposition is the interference that can occur between quantum particles passing through two or more narrow slits. In the classical world the particles pass through with their trajectories unchanged, like footballs rolling through a doorway.

But quantum particles can behave like waves, which interfere with one another as they pass through the slits, either enhancing or cancelling each other out to produce a series of bright and dark bands. This interference of quantum particles, first seen for electrons in 1927, is effectively the result of each particle passing through more than one slit: a quantum superposition.

As the experiment is scaled up in size, at some point quantum behaviour (interference) should give way to classical behaviour (no interference). But how big can the particles be before that happens?

Scaling up

In 1999, a team at the University of Vienna demonstrated interference in a many-slit experiment using beams of 60-atom carbon molecules (C₆₀), which are shaped like hollow spheres². Now Markus Arndt, one of the researchers involved in that experiment, and his colleagues in Austria, Germany, the United States and Switzerland have shown much the same effect for considerably larger molecules tailor-made for the purpose — up to 6 nanometres (millionths of a millimetre) across and composed of up to 430 atoms. These are bigger than some small protein molecules, such as insulin.

In the team's experiment, the beams of molecules are passed through three sets of slits. The first slit, made from a slice of silicon nitride patterned with a grating consisting of slits 90 nanometres wide, forces the molecular beam into a coherent state, in which the matter waves are all in step. The second, a 'virtual grating' made from laser light formed by mirrors into a standing wave of light and dark, causes the interference pattern. The third grating, also of silicon nitride, acts as a mask to admit parts of the interference pattern to a quadrupole mass spectrometer, which counts the number of molecules that pass through.

The researchers report in Nature Communications today that this number rises and falls periodically as the outgoing beam is scanned from left to right, showing that interference, and therefore superposition, is present.

Although this might not sound like a Schrödinger cat experiment, it probes the same quantum effects. It is essentially like firing the cats themselves at the interference grating, rather than making a single cat's fate contingent on an atomic-scale event.

Quantum physicist Martin Plenio of the University of Ulm in Germany calls the study part of an important line of research. "We have perhaps not gained deep new insights into the nature of quantum superposition from this specific experiment," he admits, "but there is hope that with increasing refinement of the experimental technique we will eventually discover something new."

Arndt says that such experiments might eventually allow tests of fundamental aspects of quantum theory, such as how wavefunctions collapse under observation. "Predictions, such as that gravity might induce wavefunction collapse beyond a certain mass limit, should become testable at significantly higher masses in far-future experiments," he says.

Can living organisms – perhaps not cats, but microorganisms such as bacteria – be placed in superpositions? That has been proposed for viruses³, the smallest of which are just a few nanometres across – although there is no consensus about whether viruses should be considered truly alive. "Tailored molecules are much easier than viruses to handle in such experiments," says Arndt. But he adds that if various technical issues can be addressed, "I don't see why it should not work".

This cat has been at the center of quantum community since its first time being proposed by Schrodinger. Arguably, it contains the main piece of every mystery of quantum physics: it is stated that, the cat can be in a superposition state of death and alive. Somebody think there might exist a kind of 'quantum-to-classical' transition, say, due to gravity. Although it is quite fair to claim that quantum mechanics applies to cats of any mass, not only microscopic ones but also macroscopic, it is still a special interest to experimentally confirm it. If quantum mechanics really captures something true, it would be crazy to think that, it is applicable only to small things. The success of this theory in all kinds of practices has only demonstrated its universality.

Quantum control using unsharp measurements

"For the purpose of controlling a system, two facts appear self-evident. First, the more information one can obtain about the system, the better one can control it. Second, one needs to do more than just obtain information in order to control the system. In the quantum world, however, self-evidence cannot be trusted. Writing in Physical Review A, Ashhab and Nori¹ refute the two 'facts' just given and show that a quantum system can be quickly driven to any desired state using a fixed type of measurement. Although various schemes have been proposed^{2, 3} for driving a quantum system from one state to another using only quantum measurements, this is the first time it has been shown to be achievable using repetitions of a given measurement. Crucially, the authors' proposal requires the measurement to be unsharp. That is, one must avoid obtaining too much information about the system."[http://www.nature.com/nature/journal/v470/n7333/full/470178a.html?WT.ec_id=NATURE-20110210]
[1]Ashhab, S. & Nori, F. Phys. Rev. A 82, 062103 (2010)

Quantum collapse as obeserved

In the quantum world, there are two types of evolution of an isolated system. One is unitary and continuous in time, while the other is discontinuous and abrupt. Not only that, the latter can not even be predicted. This is sometimes called the random "quantum collapse". Although postulated as a fundamental principle of quantum physics, the direct observation of such discrete jumps has been a fascinating subject since the early times. The observation is very difficult, because it is not easy to maintain a long enough coherence of a system in environemental noise. In a latest work [NATURE|Vol 467|16 September 2010], such observation is rendered in a solid state qubit.

real-time tracking the motions of electrons

Atto-second spectroscopy enables this !

doi:10.1038/nature09212

Attosecond technology (1 as=10⁻¹⁸ S) promises the tools needed to directly probe electron motion in real time. These authors report attosecond pump–probe measurements that track the movement of valence electrons in krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy will ultimately also reveal the elementary electron motions that underlie the properties of molecules and solid-state materials.

decoherence and collapse in quantum theory

The following news seems ignoring the difference between decoherence and collapse of wave function. The former is governed by Schrodinger equation and hence in principle deterministic, whereas the latter is completely probabilistic. And, never forget that, it takes no time for a collapse, although, it indeed takes time for decoherence (the so-called decoherence time). The riddle is not about decoherence but about collapse. If collapse could be removed, Einstein would accept Quantum Theory !

WHY can't we be in two places at the same time? The simple answer is that it's because large objects appear not to be subject to the same wacky laws of quantum mechanics that rule subatomic particles. But why not - and how big does something have to be for quantum physics no longer to apply? Ripples in space-time could hold the answer.

The location of the boundary between the classical and quantum worlds is a long-standing mystery. One idea is that everything starts off as a quantum system, existing in a superposition of states. This would make an object capable of being, for example, in many places at once. But when this system interacts with its environment, it collapses into a single classical state - a phenomenon called quantum decoherence.

Brahim Lamine of Pierre and Marie Curie University in Paris, France, and colleagues say that gravitational waves may be responsible for this. These waves in the very fabric of the universe were generated by its rapid expansion soon after the big bang, as well as by violent astrophysical events such as colliding black holes. As a consequence, a background of ripples at very low amplitudes pervades space-time.

Gravitational waves may be responsible for collapsing quantum ambiguity into a single classical state

Lamine and colleagues calculated how this fluctuating space-time might contribute to quantum decoherence. They found that for systems with very large mass, such as the moon, decoherence induced by the gravitational waves would have caused any quantum superposition to dissipate immediately. At the other end of the scale, such waves would have a negligible effect on massless photons.

To test whether gravitational waves do in fact cause the decoherence seen in large objects, the researchers suggest using a set-up called a matter-wave interferometer in which molecules are made to pass through multiple gratings. The wave-like nature of the molecules causes them to diffract, and the diffracted waves interact to give rise to an interference pattern. Quantum decoherence destroys this pattern, so in principle this could provide a test for whether the decohering effect of background space-time fluctuations matches predictions. Such a system would have to be completely isolated to rule out other effects.

This is, however, impossible in practice - with today's interferometers, at least. Experiments pioneered by Anton Zeilinger, Markus Arndt and colleagues at the University of Vienna, Austria, have been able to generate interference with beams of 60-atom carbon buckyballs, but even with molecules of this size the effect of gravitational waves would be too small to be observed.

According to Lamine, who presented his work last month at the Gravitation and Fundamental Physics in Space meeting at Les Houches in the French Alps, the effect should be measurable in larger systems at high energy. Supersonic beams of about 3000 carbon atoms would do the trick if made to interfere over an effective area of about 1 square metre. This is far beyond the reach of any foreseeable technology.

Some speculative theories predict, however, that quantum decoherence will occur on a lower energy scale than that suggested by Lamine. If so, this could be within experimental reach. "That is why our experiments are pushing [up] the interference mass limit, step by step," says Arndt.