Building A Precision Model of the Universe from the Biggest Color 3-D Map

Shirley Ho

Since 2000, the three Sloan Digital Sky Surveys (SDSS I, II, III) have surveyed well over a quarter of the night sky and produced the biggest color map of the universe in three dimensions ever. Now scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and their SDSS colleagues, working with DOE’s National Energy Research Scientific Computing Center (NERSC) based at Berkeley Lab, have used this visual information for the most accurate calculation yet of how matter clumps together – from a time when the universe was only half its present age until now.

“The way galaxies cluster together over vast expanses of the sky tells us how both ordinary visible matter and underlying invisible dark matter are distributed, across space and back in time,” says Shirley Ho, an astrophysicist at Berkeley Lab and Carnegie Mellon University, who led the work. “The distribution gives us cosmic rulers to measure how the universe has expanded, and a basis for calculating what’s in it: how much dark matter, how much dark energy, even the mass of the hard-to-see neutrinos it contains. What’s left over is the ordinary matter and energy we’re familiar with.”

The Sloan Digital Sky Survey III surveyed 14,000 square degrees of the sky, more than a third of its total area, and delivered over a trillion pixels of imaging data. This image shows over a million luminous galaxies at redshifts indicating times when the universe was between seven and eleven billion years old, from which the sample in the current studies was selected. Click on image for best resolution. To watch animated visualizations of the luminous galaxies in the SDSS-III dataset, click here.[Credit: David Kirkby of the University of California at Irvine and the SDSS collaboration]

For the present study Ho and her colleagues first selected 900,000 luminous galaxies from among over 1.5 million such galaxies gathered by the Baryon Oscillation Spectrographic Survey, or BOSS, the largest component of the still-ongoing SDSS III. Most of these are ancient red galaxies, which contain only red stars because all their faster-burning stars are long gone, and which are exceptionally bright and visible at great distances. The galaxies chosen for this study populate the largest volume of space ever used for galaxy clustering measurements. Their brightness was measured in five different colors, allowing the redshift of each to be estimated.

“By covering such a large area of sky and working at such large distances, these measurements are able to probe the clustering of galaxies on incredibly vast scales, giving us unprecedented constraints on the expansion history, contents, and evolution of the universe,” says Martin White of Berkeley Lab’s Physics Division, a professor of physics and astronomy at the University of California at Berkeley and chair of the BOSS science survey teams. “The clustering we’re now measuring on the largest scales also contains vital information about the origin of the structure we see in our maps, all the way back to the epoch of inflation, and it helps us to constrain – or rule out – models of the very early universe.”

After augmenting their study with information from other data sets, the team derived a number of such cosmological constraints, measurements of the universe’s contents based on different cosmological models. Among the results: in the most widely accepted model, the researchers found – to less than two percent uncertainty – that dark energy accounts for 73 percent of the density of the universe.

The team’s results are presented January 11 at the annual meeting of the American Astronomical Society in Austin, Texas, and have been submitted to the Astrophysical Journal. They are currently available online [1,2].

The power of the universe

“The way mass clusters on the largest scales is graphed in an angular power spectrum, which shows how matter statistically varies in density across the sky,” says Ho. “The power spectrum gives a wealth of information, much of which is yet to be exploited.” For example, information about inflation – how the universe rapidly expanded shortly after the big bang – can be derived from the power spectrum.

Closely related to the power spectrum are two “standard rulers,” which can be used to measure the history of the expansion of the universe. One ruler has only a single mark – the time when matter and radiation were exactly equal in density.

“In the very early universe, shortly after the big bang, the universe was hot and dominated by photons, the fundamental particles of radiation,” Ho explains. “But as it expanded, it began the transition to a universe dominated by matter. By about 50,000 years after the big bang, the density of matter and radiation were equal. Only when matter dominated could structure form.”

The other cosmic ruler is also big, but it has many more than one mark in the power spectrum; this ruler is called BAO, for baryon acoustic oscillations. (Here, baryon is shorthand for ordinary matter.) Baryon acoustic oscillations are relics of the sound waves that traveled through the early universe when it was a hot, liquid-like soup of matter and photons. After about 50,000 years the matter began to dominate, and by about 300,000 years after the big bang the soup was finally cool enough for matter and light to go their separate ways.

Differences in density that the sound waves had created in the hot soup, however, left their signatures as statistical variations in the distribution of light, detectable as temperature variations in the cosmic microwave background (CMB), and in the distribution of baryons. The CMB is a kind of snapshot that can still be read today, almost 14 billion years later. Baryon oscillations – variations in galactic density peaking every 450 million light-years or so – descend directly from these fluctuations in the density of the early universe.

BAO is the target of the Baryon Oscillation Spectroscopic Survey. By the time it’s completed, BOSS will have measured the individual spectra of 1.5 million galaxies, a highly precise way of measuring their redshifts. The first BOSS spectroscopic results are expected to be announced early in 2012.

Meanwhile the photometric study by Ho and her colleagues deliberately uses many of the same luminous galaxies but derives redshifts from their brightnesses in different colors, extending the BAO ruler back over a previously inaccessible redshift range, from z = 0.45 to z = 0.65 (z stands for redshift).

“As an oscillatory feature in the power spectrum, not many things can corrupt or confuse BAO, which is why it is considered one of the most trustworthy ways to measure dark energy,” says Hee-Jong Seo of the Berkeley Center for Cosmological Physics at Berkeley Lab and the UC Berkeley Department of Physics, who led BAO measurement for the project. “We call BAO a standard ruler for a good reason. As dark energy stretches the universe against the gravity of dark matter, more dark energy places galaxies at a larger distance from us, and the BAO imprinted in their distribution looks smaller. As a standard ruler the true size of BAO is fixed, however. Thus the apparent size of BAO gives us an estimate of the cosmological distance to our target galaxies – which in turn depends on the properties of dark energy.”

Says Ho, “Our study has produced the most precise photometric measurement of BAO. Using data from the newly accessible redshift range, we have traced these wiggles back to when the universe was about half its present age, all the way back to z = 0.54.”

Seo adds, “And that’s to an accuracy within 4.5 percent.”

Reining in the systematics

“With such a large volume of the universe forming the basis of our study, precision cosmology was only possible if we could control for large-scale systematics,” says Ho. Systematic errors are those with a physical basis, including differences in the brightness of the sky, or stars that mimic the colors of distant galaxies, or variations in weather affecting “seeing” at the SDSS’s Sloan Telescope – a dedicated 2.5 meter telescope at the Apache Point Observatory in southern New Mexico.

After applying individual corrections to these and other systematics, the team cross-correlated the effects on the data and developed a novel procedure for deriving the best angular power-spectrum of the universe with the lowest statistical and systematic errors.

With the help of 40,000 central-processing-unit (CPU) hours at NERSC and another 20,000 CPU hours on the Riemann computer cluster operated by Berkeley Lab’s High-Performance Computing Services group, these powerful computers and algorithms enabled the team to use all the information from galactic clustering in a huge volume of sky, including the full shape of the power spectrum and, independently, BAO, to get excellent cosmological constraints. The data as well as the analysis output are stored at NERSC.

“Our dataset is purely imaging data, but our results are competitive with the latest large-scale spectroscopic surveys,” Ho says. “What we lack in redshift precision, we make up in sheer volume. This is good news for future imaging surveys like the Dark Energy Survey and the Large Synoptic Survey Telescope, suggesting they can achieve significant cosmological constraints even compared to future spectroscopy surveys.”

References:
[1] “Clustering of Sloan Digital Sky Survey III photometric luminous galaxies: The measurement, systematics, and cosmological implications,” by Shirley Ho, Antonio Cuesta, Hee-Jong Seo, Roland de Putter, Ashley J. Ross, Martin White, Nikhil Padmanabhan, Shun Saito, David J. Schlegel, Eddie Schlafly, Uroŝ Seljak, Carlos Hernández-Monteagudo, Ariel G. Sánchez, Will J. Percival, Michael Blanton, Ramin Skibba, Don Schneider, Beth Reid, Olga Mena, Matteo Viel, Daniel J. Eisenstein, Francisco Prada, Benjamin Weaver, Neta Bahcall, Dimitry Bizyaev, Howard Brewinton, Jon Brinkman, Luiz Nicolaci da Costa, John R. Gott, Elena Malanushenko, Viktor Malanushenko, Bob Nichol, Daniel Oravetz, Kaike Pan, Nathalie Palanque-Delabrouille, Nicholas P. Ross, Audrey Simmons, Fernando de Simoni, Stephanie Snedden,and Christophe Yeche (submitted to Astrophysical Journal). arXiv:1201.2137.
[2] “Acoustic scale from the angular power spectra of SDSS-III DR8 photometric luminous galaxies,” by Hee-Jong Seo, Shirley Ho, Martin White, Antonio J. Cuesta, Ashley J. Ross, Shun Saito, Beth Reid, Nikhil Padmanabhan, Will J. Percival, Roland de Putter, David J. Schlegel, Daniel J. Eisenstein, Xiaoying Xu, Donald P. Schneider, Ramin Skibba, Licia Verde, Robert C. Nichol, Dmitry Bizyaev, Howard Brewington, J. Brinkmann, Luiz Alberto Nicolai da Costa, J. Richard Gott III, Elena Malanushenko, Viktor Malanushenko, Dan Oravetz, Nathalie Palanque-Delabrouille, Kaike Pan, Francisco Prada, Nicholas P. Ross, Audrey Simmons, Fernando Simoni, Alaina Shelden, Stephanie Snedden, and Idit Zehavi (submitted to Astrophysical Journal). arXiv:1201.2172 .

[This report is written by Paul Preuss of Lawrence Berkeley National Laboratory]

Quantum Complementarity Meets Gravitational Redshift

(From left to right) Magdalena Zych, Fabio Costa, Igor Pikovski, Časlav Brukner

Authors: Magdalena Zych, Fabio Costa, Igor Pikovski, Časlav Brukner

Affiliations: Faculty of Physics, University of Vienna, Austria
Link to "Quantum Foundations and Quantum Information Theory" Group >>

The unification of quantum mechanics and Einstein's general relativity is one of the most exciting and still open questions in modern physics. In general relativity, space and time are combined into a unified underlying geometry, which explains the gravitational attraction of massive bodies. Typical predictions of this theory become clearly evident on a cosmic scale of stars and galaxies. Quantum mechanics, on the other hand, was developed to describe phenomena at small scales, such as single particles and atoms. Both theories have been confirmed by many experiments independently. However, it is still very hard to test the interplay between quantum mechanics and general relativity. When considering very small systems, gravity is typically too weak to be of any significance. The most precise experiments so far have only been able to probe the non-relativistic, Newtonian limit of gravity in conjunction with quantum mechanics. Conversely, quantum effects are generally not visible in large objects.

According to general relativity, time flows differently at different positions due to the distortion of space-time by a nearby massive object. A single clock being in a superposition of two locations allows probing quantum interference effects in combination with general relativity. [Image credits: Quantum Optics, Quantum Nanophysics, Quantum Information; University of Vienna]

There is, however, a possibility to measure predictions of Einstein’s theory of general relativity without using extremely massive probe particles: one of the counterintuitive predictions of Einstein's general relativity is that gravity distorts the flow of time. The theory predicts that clocks tick slower near a massive body and tick faster the further they are away from the mass. The earth’s gravitational field produces a sufficient distortion of space-time such that the different flow of time at different altitudes can be measured with very precise clocks. This has been confirmed experimentally with classical clocks and the results were in full agreement with Einstein’s theory.

Two initially synchronized clocks placed at different gravitational potentials will eventually show different times. According to general relativity a clock near a massive body ticks slower than the clock further away from the mass. This effect is known as gravitational time dilation or gravitational redshift.

Scientists at the University of Vienna now proposed that the effect described above, which is also commonly known as the “gravitational redshift”, can also be used to probe the overlap of general relativity with quantum mechanics. In the scheme published in October in Nature Communications, the classical version of the experiment is modified such that it becomes necessary to take quantum mechanics into account. The idea is to exploit the extraordinary possibility that a single particle can be without a well-defined position, or as phrased in quantum mechanical terms: it can be in a “superposition” of two different locations. This allows single particles to produce typical wave-like detection patterns, i.e. interference.

Superpositions of particles are, however, very fragile: if the position of the particle is measured, or even if it can in principle be known, the superposition is lost. In other words, it is not possible to know the position of the particle and to simultaneously observe interference. Such a connection between information and interference is an example of quantum complementarity - a principle originally proposed by Niels Bohr. Because of the above-mentioned fragility, it is very challenging to observe and to maintain superpositions of particles. Even a very weak interaction of the particle with its surrounding leads to the demise of quantum interference. But even though the loss of superpositions is a nuisance in many quantum experiments, the newly proposed experimental scheme to probe general relativity in conjunction with quantum mechanics actually builds upon this complementarity principle.

The novel idea developed in the group of Prof. Č. Brukner is to use a single clock (which can be any particle with evolving internal degrees of freedom, such as spin) that is brought in a superposition of two locations – one closer and one further away from the surface of the Earth. Afterwards, the two parts of the superposition are brought back together, and it is observed whether or not an interference pattern is produced. According to general relativity, the clock ticks at a different rate depending on its location. But since the time measured by the clock reveals the information on where the clock was located, the interference and the wave-nature of the clock should be lost. The amount of the loss of quantum mechanical interference becomes a measure of the general relativistic redshift. To describe this effect, both general relativity and quantum mechanics are required. Such an interplay between the two theories has never been probed in experiments yet. It is therefore the first proposal for an experiment that allows testing the genuine general relativistic notion of time in conjunction with quantum complementarity.

A single clock is brought in a quantum superposition of two locations: closer and further away from the surface of the Earth. Because of the gravitational redshift, the time shown by the clock reveals the information on the clock’s location. Thus, according to the quantum complementarity principle, interference and the quantum wave-nature of the clock will be lost.

In the setup described above, the loss of quantum interference becomes a tool to measure the general relativistic time dilation. It is not even necessary to read out the clock itself: The sheer existence of the clock is sufficient to destroy the interference. But since quantum interference effects are very fragile, it is important to verify that their demise is really caused by the distortion of the flow of time. This can be done by performing the same experiment in two different ways: one where the clock is running, as described above, and one where the clock is “switched off”. In the latter case the quantum interference should become visible, as opposed to the former case.

A further application of the proposed experiment is that it can also test new physical theories. For example, in the context of theories that aim at combining general relativity and quantum mechanics into a single framework, it has been proposed that every particle carries a clock with itself, which measures time along its path. Such a possibility can be probed by the proposed experiment, without the need to directly measure such a hypothetical internal clock: if quantum interference is lost even in the case when the clock which is controlled by the experimentalist (for example, the aforementioned precession of the particle’s spin) is switched off, one can infer that there is an intrinsic mechanism which can keep track of time by itself. On the other hand, if interference is observed, the existence of an internal clock can be ruled out.

Another interesting possibility is that the quantum interference persists even with the experimentally controlled clock turned on. This would mean that quantum mechanics or general relativity breaks down when phenomena inherent to both theories become relevant. Such a scale has never been accessible for experimental tests so far.

To experimentally observe the predicted interplay of quantum interference and the gravitational redshift, three parameters are of importance: The height difference of the two locations at which the particle is held in a superposition, the time that the particle is kept in the superposition and the ticking rate of the clock. The larger any of those values, the easier it is to observe the effect. Currently, the most promising systems for such an experiment are single atoms. They can be brought into superpositions in atomic fountains and their internal states can be used as atomic clocks. There are also other systems that can be used to successfully perform the experiment: neutrons, electrons and even large molecules. There has been rapid experimental progress in the precision of clocks and in the size of the superpositions that can be created and maintained in the laboratory. It is therefore possible that within the next few years the proposed experiment with quantum clocks can be realized.

Both quantum mechanics and general relativity seem to be universal theories, though we still don’t know how to properly combine them in a universal framework. New phenomena are expected at some scale at the interplay between the two theories. Only experimentally probing this interplay may give a hint as to how to proceed in constructing a unifying description of nature.

Reference
[1] Magdalena Zych, Fabio Costa, Igor Pikovski & Časlav Brukner. "Quantum interferometric visibility as a witness of general relativistic proper time". Nature Communications, 2:505 doi: 10.1038/ncomms1498 (2011). Full Article: PDF, HTML.

Entang-bling

Ian Walmsley (left), Joshua Nunn (right) contempl -ating the universe.










Authors: Ian Walmsley and Joshua Nunn

Affiliation:
Clarendon Laboratory, Department of Physics, University of Oxford, UK.

More than 70 years ago, Erwin Schrödinger pointed out one (of many) striking features of the quantum mechanics that he’d recently invented: the possibility it allowed for stuff to do things that no one had actually seen in real life -- like cats being both dead and alive at the same time. This is one of what could be politely called the ‘interpretational difficulties’ of quantum physics. Familiar everyday objects behave in familiar everyday ways - they don’t engage in the sorts of nonsensical behaviour that Schrödinger’s equation predicts. But, as any physicist will tell you, basically quantum mechanics is not that complicated. It’s just that it takes familiar concepts, like position and direction, and makes us think about it in totally radical ways, so that in the end the results don’t make any sense at all!

Past 2Physics article by Joshua Nunn:
August 07, 2011: "Building a Quantum Internet"

Michael Sprague (top), KC Lee (left) and XianMin Jin (right), in the lab with the diamonds.

Of course, in the early 20th century, people were used to the idea that science was coming up with crazy new notions that dramatically altered our conception of things — like the notion of time in Einstein’s special theory of relativity. But Einstein’s theories deal with objects that can be seen on size and timescales that are familiar. The light from stars can be observed with a simple telescope; the timing of GPS satellites is a tangible technical problem. For this reason relativity has entered the scientific orthodoxy, which is why the recent neutrino speeding anomaly has caused such stir [1].

That’s what worried Schrödinger. In principle his theory also dealt with tangible objects — or at least there was no element in it that indicated otherwise. Yet it seemed at first as if quantum mechanics only gave good predictions for objects that are too small to be seen directly. It therefore took on the flavour of a story, in which the actors — electrons, atoms and photons — are convenient fictions we can use to explain what we see, but which are no more real than the characters in a novel.

Quantum theory tells us that in fact these characters can be in two places at once, that they are impossible to pin down exactly, and that they don’t really therefore give well defined answers to questions like ”are you red or blue”? We’re used to the idea in the ordinary world that an object, say a ball, will have a definite property like color. We may not know if a specific ball is red or blue, but we may regard it as having one of these two colors independent of whether we know which particular color it is. Quantum mechanics says, well, no, it’s not possible to be sure of that. In the quantum world, balls can be both red and blue at the same time.

But it is not the particles acting on their own that give rise to the deepest mysteries — it is when they get together that the fun really starts. For instance, it becomes possible to say that the color of one ball is well defined only in relation to a second ball. So if one is red, then the other is certainly blue -- but that neither is definite red or blue on its own. And you can actually test this proposition with microscopic particles — like photons (“particles” of light). This is the murky world of ”entanglement” in which pairs of particles are apparently connected across the universe as if by invisible filaments.

You can think about this in terms of what you know about light. Consider a beamsplitter. This is a common optical device: essentially a half-silvered mirror which passes half the incident light, and reflects the other half, as shown in Fig. 1. When a single photon encounters a beamsplitter it cannot split itself in two, so it must go one way or the other. Or does it? According to quantum physics it can go both one way and the other. In fact, the beamsplitter transforms the single photon into an entangled state[2]. If we measure if the photon is passed or reflected, we get that each option occurs 50% of the time. So this measurement alone does not help us distinguish between the entangled state and a state in which the input photon definitely goes one way or the other at random.

But consider the time reversal of this situation. Now we put a single photon into the back side of the beamsplitter. It also could be reflected or passed with 50% probability, except if the photon is in an entangled state of the two input ports. Then, by reversing argument above, we can see that it is definitely passed through the beamsplitter. Thus, by looking at how often a single input photon is passed by the beamsplitter, we can tell whether or not it was in an entangled state at the input. This kind of magic realism makes physicists (or at least, philosophers of physics) uncomfortable, but the edifice of science survives with such strangeness at its core because quantum effects are confined to the abstract domain of the microscopic, where human experience has no purchase and there can be no direct conflict with our intuitions.

Figure 1: A single photon (filled circle) cannot divide into two when it hits a beam splitter. It must either pass through, or be reflected. According to quantum mechanics, both of these possibilities occur, producing an entangled state, in which a single photon is shared between the two beams after the beam splitter. Running this process in reverse (i.e. from right to left) provides a way to detect entanglement, since only an entangled state will always produce a single photon in the same place on the left.

There is now a strong tradition of research which seeks to bring us face-to-face with our Frankenstein theory by confirming the predictions of quantum mechanics on human scales. The aim is to demonstrate quantum effects such as entanglement with increasingly large objects, containing more and more particles. Although many areas of physics have matured sufficiently that the underlying components of the theory are ‘accepted’, it is known that quantum mechanics as is cannot be the final word, since its predictions conflict with our experience of the world. Either some new physics is needed, or better arguments to explain how to reconcile quantum theory with the rest of the world.

While the philosophical debates smoulder on [3,4], experimentalists have set themselves the task of identifying the conditions under which quantum effects survive into the human realm, leading to behaviour we are not used to seeing in the familiar world of cats and elephants. Considerable progress has been made: large numbers of atoms have been entangled [5,6], and small pieces of solid material — but big enough to see with the naked eye — have been put into quantum superposition states where they were both vibrating and not vibrating at the same time [7].

These breakthroughs showed that quantum effects don’t need to be confined to small numbers of particles, or to particles without mass like photons. But so far, extremely specialised laboratory conditions have been required to observe these effects: very low temperatures just above absolute zero and high vacuum, with no air and no extraneous electric or magnetic fields. The objects were highly delicate composite devices which would not be found in nature, and careful preparation was required in order to keep them isolated from the deleterious effects of the environment.

We recognised that some materials have properties, like vibrations, that naturally lend themselves to realising these conditions in an everyday laboratory setting. These vibrations require a lot of energy to get going, so that ordinary environments at regular temperatures do not excite them. They are by nature in relatively pure quantum states, with no vibrational excitation at all. They may, however, be strongly coupled to their environment in the sense that once excited they quickly decay, so that quantum effects might be present only if we could be quick enough to observe them before they became overwhelmed.

We therefore decided to carry out an ‘easy experiment’ (though only Oxford physicists could be silly enough to think that any experiment is easy); to set one of these vibrations going using a very short duration light pulse from a laser, then to ”watch” it by means of a second short laser pulse acting as a probe of the vibrational motion. We realised that diamond was a naturally-occurring transparent material that was so hard that it could vibrate at a particular, very high-pitched frequency, which could be easily identified in a measurement. The vibrations in diamond last for just 7 picoseconds (1 ps is one thousandth of a nanosecond), so we had to use an ultrafast laser system producing laser pulses shorter than 100 femtoseconds (1 fs is one thousandth of a picosecond!).

We took an ordinary, common-or-garden diamond and set it vibrating using
a laser pulse. When the laser pulse hits the diamond, there is a small probability that just one photon from the laser pulse gives up some energy to the diamond to set it vibrating. By conservation of energy, this must mean that the photon leaves the diamond with reduced energy, and thus a longer wavelength than the original laser photon. By detecting this “red” photon, we could know that a single vibrational quantum (known technically as an optical phonon) had been created in the diamond crystal. We found that even at room temperature and pressure, in a lab with air and other vibrations and cups of tea, we could create this high-frequency vibration.

Now, we could prove this by detecting it using a second laser pulse, arriving after the first, but not so long after that the vibration had decayed away. The probe pulse detected the vibration by picking up energy from it, emerging with a shorter wavelength, so blue-shifted in color. So we detected a “red” photon to signal that a phonon has been generated, and a “blue” photon to prove that. Using this approach, we showed that we could catch a glimpse of the phonon before it vanished [8]. This type of “create-detect” experiment is precisely what has been done with cold clouds of atoms to entangle them, so we thought we would try to do that!





















Figure 2: The happy couple. We took data with the lights off but otherwise the diamonds were in a totally ordinary environment. The lenses are there to focus the laser pulses and collect the photons emitted by the crystals.

In a second experiment [9], we set up two diamonds, in ordinary little holders sitting near each other on a lab table (see Fig.2). By hitting both diamonds with a laser pulse at the same time, we created a vibration in one of the crystals, but it was impossible, even in principle, to tell which crystal was vibrating. We did this by combining the beams from the two diamonds that went to the “red” photon detector on a beamsplitter, as shown in Fig.3. When this detector fired, we knew that a single phonon has been generated, but we could not tell from which beam the red photon had arrived, and therefore in which diamond the phonon resided.

Quantum mechanics predicts that, if you don’t know this information, the right way to describe the diamonds is as an entangled quantum state, with one vibration shared between them. We then verified that the diamonds were entangled by combining the “blue” light from the diamonds at a beamsplitter (see Fig.3). We could detect first that each pulse only contained a single “blue” photon, and second that it was always passed by the beamsplitter, rather than reflected. This is only possible for a single photon if it is entering the beamsplitter in an entangled state, as argued previously, and thus was emitted from both diamonds! This means that the diamonds themselves were entangled, with a single vibration shared between both of them.

These results show, for the first time, that large, easily visible, solid objects (indeed, diamonds are naturally occurring minerals: pieces of rock), sitting in ambient conditions at room temperature and pressure, clamped to a table-top, can be put in a quantum-entangled state. Furthermore, the entanglement was created with vibrations — the motion of the crystals as a whole.

Figure 3: Generating and detecting entanglement between diamonds using ultrashort laser pulses (green lines). (a) The first set of pulses produces a single red-shifted photon from one of the diamonds. After the beams are mixed on a beam splitter, it is impossible to tell which diamond the photon came from. This means there is one vibration shared between the two diamonds — they are entangled. (b) After a small delay, we verify the entanglement by sending in a second pair of pulses, producing a blue-shifted photon in an entangled state. When this entangled state hits the beam splitter, the blue photon always emerges from just one side of the experiment (thick blue line). As shown in Fig.1, this can only happen if the diamonds are entangled.

So the positions of the atoms were entangled. This is particularly unsettling because we have an intuitive sense for position that we would not have if we had entangled magnetic fields or photons. Our measurements are, we feel, one of the most visceral demonstrations to date that the rules of quantum mechanics apply to us all: electrons and elephants alike.

References
[1] Opera Collaboraton, "Measurement of the neutrino velocity with the opera detector in the CNGS beam". arXiv:1109.4897 (2011). Link.
[2] S.J. van Enk. "Single-particle entanglement". Physical Review A, 72(6):064306 (2005). Abstract.
[3] D. Wallace. "Decoherence and Ontology: or, How I Learned to Stop Worrying and Love FAPP", in "Many Worlds? Everett, Quantum Theory, and Reality", eds. S. Saunders, J. Barrett, A. Kent and D. Wallace (OUP, 2010). Link.
[4] R. Penrose. "Wavefunction collapse as a real gravitational effect". Mathematical Physics 2000 (edited by A Fokas, A Grigoryan, T Kibble, B Zegarlinski), pages 266–282, (World Scientific eBooks, 2000). Link.
[5] K. S. Choi, H. Deng, J. Laurat, and H. J. Kimble. "Mapping photonic entanglement into and out of a quantum memory". Nature, 452:67–71 (2008). Abstract.
[6] H. Krauter, C.A. Muschik, K. Jensen, W. Wasilewski, J.M. Petersen, J.I. Cirac, and E.S. Polzik. "Entanglement generated by dissipation and steady state entanglement of two macroscopic objects". Physical Review Letters, 107(8):80503 (2011). Abstract.
[7] Adrian Cho. Faintest thrum heralds quantum machines. Science, 327(5965):516–518 (2010). Abstract.
[8] K. C. Lee, B. J. Sussman, M. R. Sprague, P. Michelberger, K. F. Reim, J. Nunn, N. K. Langford, P. J. Bustard, D. Jaksch, and I. A. Walmsley. "Macroscopic nonclassical states and terahertz quantum processing in room- temperature diamond", Nature Photonics, 6, 41-44 (2011). Abstract.
[9] KC Lee, MR Sprague, BJ Sussman, J. Nunn, NK Langford, X.M. Jin, T. Champion, P. Michelberger, KF Reim, D. England, D. Jaksch, I. A. Walmsley. "Entangling macroscopic diamonds at room temperature". Science, 334(6060):1253–1256 (2011). Abstract.

“Dressing” Atoms with Laser Allows High Angular Momentum Scattering : Could Reveal Ways to Observe Majorana Fermions

Ian Spielman (photo courtesy: Joint Quantum Institute, USA)

Scientists at the Joint Quantum Institute (JQI, a collaborative enterprise of the 'National Institute of Standards and Technology' and the University of Maryland) have for the first time engineered and detected the presence of high angular momentum collisions between atoms at temperatures close to absolute zero. Previous experiments with ultracold atoms featured essentially head-on collisions. The JQI experiment, by contrast, is able to create more complicated collisions between atoms using only lasers that dramatically influences their interactions in specific ways.

Such light-tweaked atoms can be used as proxies to study important phenomena that would be difficult or impossible to study in other contexts. Their most recent work, appearing in Science [1] demonstrates a new class of interactions thought to be important to the physics of superconductors that could be used for quantum computation.

Particle interactions are fundamental to physics, determining, for example, how magnetic materials and high temperature superconductors work. Learning more about these interactions or creating new “effective” interactions will help scientists design materials with specific magnetic or superconducting properties.Because most materials are complicated systems, it is difficult to study or engineer the interactions between the constituent electrons. Researchers at JQI build physically analogous systems using supercooled atoms to learn more about how materials with these properties work.

The key to the JQI approach is to alter the atoms’ environment with laser light. They “dress” rubidium atoms by bathing them in a pair of laser beams, which force the atoms to have one of three discrete values of momentum. In the JQI experiment, rubidium atoms comprise a Bose-Einstein condensate (BEC). BECs have been collided before. But the observation of high-angular-momentum scattering at such low energies is new.

The paper in 'Science Express' [1] includes a variety of technical issues which require some explanation:

Collisons

One of the cardinal principles of quantum science is that matter must be simultaneously thought of as both particles and waves. When the temperature of a gas of atoms is lowered, the wavelike nature of the atom emerges, and the idea of position becomes fuzzier. While an atom at room temperature might spread over a hundredth of a nm, atoms at nano-kelvin temperatures have a typical wavelength of about 100 nm. This is much larger than the range of the force between atoms, only a few nm. Atoms generally collide only when they meet face to face.

However, to study certain interesting quantum phenomena, such as searching for Majorana particles---hypothetical particles that might provide a robust means of encoding quantum information---it is desirable to engineer inter-atomic collisions beyond these low-energy, head-on type. That’s what the new JQI experiment does.

Partial Waves

Scattering experiments date back to the discovery of the atomic nucleus 100 years ago, when Ernest Rutherford shot alpha particles into a foil of gold. Since then other scattering experiments have revealed a wealth of detail about atoms and sub-atomic matter such as the quark substructure of protons.

A convenient way of picturing an interaction between two particles is to view their relative approach in terms of angular momentum. Quantized angular momentum usually refers to the motion of an electron inside an atom, but it necessarily pertains also to the scattering of the two particles, which can be thought of as parts of a single quantum object.

If the value of the relative angular momentum is zero, then the scattering is designated as “s-wave” scattering. If the pair of colliding particles has one unit of angular momentum, the scattering is called p-wave scattering. Still more higher-order scattering scenarios are referred to by more letters: d-wave, f-wave, g-wave, and so on. This model is referred to as the partial waves view.

In high energy scattering, the kind at accelerators, these higher angular-momentum scattering scenarios are important and help to reveal important structure information about the particles. In atomic scattering at low temperatures, the s-wave interactions completely swamp the higher-order scattering modes. For ultralow-temperature s-wave scattering, when two atoms collide, they glance off each other (back to back) at any and all angles equally. This isotropic scattering doesn’t reveal much about the nature of the matter undergoing collision; it’s as if the colliding particles were hard spheres.

This has changed now. The JQI experiment is the first to create conditions in which d-wave and g-wave scattering modes in an ultracold experiment could be seen in otherwise long-lived systems.

Quantum Collider

Ian Spielman and his colleagues at the National Institute for Standards and Technology (NIST) chill Rb atoms to nano-kelvin temperatures. The atoms, around half a million of them, have a density about a millionth that of air at room temperature. Radiofrequency radiation places each atom into a superposition of quantum spin states. Then two (optical light) lasers impart momentum (forward-going and backward-going motion) to the atoms.

Schematic drawing of collision between two BECs (the gray blobs) that have been “dressed” by laser light (brown arrows) and an additional magnetic field (green arrow). The fuzzy halo shows where atoms have been scattered. The non-uniform projection of the scattering halo on the graph beneath shows that some of the scattering has been d-wave and g-wave [image courtesy: JQI]

If this were a particle physics experiment, we would say that these BECs-in-motion were quantum beams, beams with energies that came in multiples of the energy kick delivered by the lasers. The NIST “collider” in Gaithersburg, Maryland is very different for the CERN collider in Geneva, Switzerland. In the NIST atom trap the particles have kinetic energies of a hundred pico-electron-volts rather than the trillion-electron-volt energies used at the Large Hadron Collider.

At JQI, atoms are installed in their special momentum states, and the collisions begin. Outward scattered atoms are detected after the BEC clouds are released by the trap. If the atoms hadn’t been dressed, the collisions would have been s-wave in nature and the observed scattered atoms would have been seen uniformly around the scattering zone.

The effect of the dressing is to screen the atoms from s-wave scattering in the way analogous to that in some solid materials, where the interaction between two electrons is modified by the presence of trillions of other electrons nearby. In other words, the laser dressing effectively increased the range of the inter-atom force such that higher partial wave scattering was possible, even at the lowest energies.

In the JQI experiment, the observed scattering patterns for atoms emerging from the collisions was proof that d-wave and g-wave scattering had taken place. “The way in which the density of scattered atoms is distributed on the shell reflects the partial waves,” said Ian Spielman. “A plot of scattered-density vs. spherical polar angles would give the sort of patterns you are used to seeing for atomic orbitals. In our case, this is a sum of s-, p-, and d- waves.”

Simulating Solids Using Gases

Ultracold atomic physics experiments performed with vapors of atoms are excellent for investigating some of the strongly-interacting quantum phenomena usually considered in the context of condensed matter physics. These subjects include superconductivity, superfluids, the quantum Hall effect, and topological insulators, and some things that haven’t yet been observed, such as the “Majorana” fermions.

Several advantages come with studying these phenomena in the controlled environment of ultracold atoms. Scientists can easily manipulate the landscape in which the atoms reside using knobs that adjust laser power and frequency. For example, impurities that can plague real solids can be controlled and even removed, and because (as in this new JQI experiment) the scattering of atoms can now (with the proper “dressing”) reveal higher-partial-wave effects. This is important because the exotic quantum effects mentioned above often manifest themselves under exactly these higher angular-momentum conditions.

“Our technique is a fundamentally new method for engineering interactions, and we expect this work will stimulate new directions of research and be of broad interest within the physics community, experimental and theoretical,” said Spielman. “We are modifying the very character of the interactions, and not just the strength, by light alone.”

On To Fermions

The JQI team, including Nobel Laureate William Phillips, is truly international, with scientists originating in the United Kingdom (lead author Ross Williams), Canada (Lindsay LeBlanc), Mexico (Karina Jiménez-García), and the US (Matthew Beeler, Abigail Perry, William Phillips and Ian Spielman).

The researchers now will switch from observing bosonic atoms (with a total spin value of 1) to fermion atoms (those with a half-integral spin). Combining the boson techniques demonstrated here with ultracold fermions offers considerable promise for creating systems which are predicted to support the mysterious Majorana fermions. “A lot of people are looking for the Majorana fermion,” says lead author and JQI postdoctoral fellow Ross Williams. “It would be great if our approach helped us to be the first.”

Reference
[1] R. A. Williams, L. J. LeBlanc, K. Jiménez-García, M. C. Beeler,A. R. Perry, W. D. Phillips, I. B. Spielman, "Synthetic partial waves in ultracold atomic collisions”, Science Express, (December 7, 2011). DOI: 10.1126/science.1212652. Abstract.