Tag Archives: quarks

Simulating elementary physics in a quantum simulation (particle zoo in a quantum computer?)

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Whoever wrote the news release used a very catchy title “Particle zoo in a quantum computer”; I just wish they’d explained it. Looking up the definition for a ‘particle zoo’ didn’t help as much as I’d hoped. From the particle zoo entry on Wikipedia (Note: Links have been removed),

In particle physics, the term particle zoo[1][2] is used colloquially to describe a relatively extensive list of the then known “elementary particles” that almost look like hundreds of species in the zoo.

In the history of particle physics, the situation was particularly confusing in the late 1960s. Before the discovery of quarks, hundreds of strongly interacting particles (hadrons) were known, and believed to be distinct elementary particles in their own right. It was later discovered that they were not elementary particles, but rather composites of the quarks. The set of particles believed today to be elementary is known as the Standard Model, and includes quarks, bosons and leptons.

I believe the writer used the term to indicate that the simulation undertaken involved elementary particles. If you have a better explanation, please feel free to add it to the comments for this post.

Here’s the news from a June 22, 2016 news item on ScienceDaily,

Elementary particles are the fundamental buildings blocks of matter, and their properties are described by the Standard Model of particle physics. The discovery of the Higgs boson at the CERN in 2012 constitutes a further step towards the confirmation of the Standard Model. However, many aspects of this theory are still not understood because their complexity makes it hard to investigate them with classical computers. Quantum computers may provide a way to overcome this obstacle as they can simulate certain aspects of elementary particle physics in a well-controlled quantum system. Physicists from the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences have now done exactly that: In an international first, Rainer Blatt’s and Peter Zoller’s research teams have simulated lattice gauge theories in a quantum computer. …

A June 23, 2016 University of Innsbruck (Universität Innsbruck) press release, which seems  to have originated the news item, provides more detail,

Gauge theories describe the interaction between elementary particles, such as quarks and gluons, and they are the basis for our understanding of fundamental processes. “Dynamical processes, for example, the collision of elementary particles or the spontaneous creation of particle-antiparticle pairs, are extremely difficult to investigate,” explains Christine Muschik, theoretical physicist at the IQOQI. “However, scientists quickly reach a limit when processing numerical calculations on classical computers. For this reason, it has been proposed to simulate these processes by using a programmable quantum system.” In recent years, many interesting concepts have been proposed, but until now it was impossible to realize them. “We have now developed a new concept that allows us to simulate the spontaneous creation of electron-positron pairs out of the vacuum by using a quantum computer,” says Muschik. The quantum system consists of four electromagnetically trapped calcium ions that are controlled by laser pulses. “Each pair of ions represent a pair of a particle and an antiparticle,” explains experimental physicist Esteban A. Martinez. “We use laser pulses to simulate the electromagnetic field in a vacuum. Then we are able to observe how particle pairs are created by quantum fluctuations from the energy of this field. By looking at the ion’s fluorescence, we see whether particles and antiparticles were created. We are able to modify the parameters of the quantum system, which allows us to observe and study the dynamic process of pair creation.”

Combining different fields of physics

With this experiment, the physicists in Innsbruck have built a bridge between two different fields in physics: They have used atomic physics experiments to study questions in high-energy physics. While hundreds of theoretical physicists work on the highly complex theories of the Standard Model and experiments are carried out at extremely expensive facilities, such as the Large Hadron Collider at CERN, quantum simulations may be carried out by small groups in tabletop experiments. “These two approaches complement one another perfectly,” says theoretical physicist Peter Zoller. “We cannot replace the experiments that are done with particle colliders. However, by developing quantum simulators, we may be able to understand these experiments better one day.” Experimental physicist Rainer Blatt adds: “Moreover, we can study new processes by using quantum simulation. For example, in our experiment we also investigated particle entanglement produced during pair creation, which is not possible in a particle collider.” The physicists are convinced that future quantum simulators will potentially be able to solve important questions in high-energy physics that cannot be tackled by conventional methods.

Foundation for a new research field

It was only a few years ago that the idea to combine high-energy and atomic physics was proposed. With this work it has been implemented experimentally for the first time. “This approach is conceptually very different from previous quantum simulation experiments studying many-body physics or quantum chemistry. The simulation of elementary particle processes is theoretically very complex and, therefore, has to satisfy very specific requirements. For this reason it is difficult to develop a suitable protocol,” underlines Zoller. The conditions for the experimental physicists were equally demanding: “This is one of the most complex experiments that has ever been carried out in a trapped-ion quantum computer,” says Blatt. “We are still figuring out how these quantum simulations work and will only gradually be able to apply them to more challenging phenomena.” The great theoretical as well as experimental expertise of the physicists in Innsbruck was crucial for the breakthrough. Both Blatt and Zoller emphasize that they have been doing research on quantum computers for many years now and have gained a lot of experience in their implementation. Innsbruck has become one of the leading centers for research in quantum physics; here, the theoretical and experimental branches work together at an extremely high level, which enables them to gain novel insights into fundamental phenomena.

Here’s a link to and a citation for the paper,

Real-time dynamics of lattice gauge theories with a few-qubit quantum computer by Esteban A. Martinez, Christine A. Muschik, Philipp Schindler, Daniel Nigg, Alexander Erhard, Markus Heyl, Philipp Hauke, Marcello Dalmonte, Thomas Monz, Peter Zoller, & Rainer Blatt.  Nature 534, 516–519 (23 June 2016)  doi:10.1038/nature18318 Published online 22 June 2016

This paper is behind a paywall.

There is a soundcloud audio file featuring an explanation of the work from the lead author, Esteban A. Martinez,

Fermionic atoms and the microscopes that can see them

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The new fermionic microscope built at the Massachusetts Institute of Technology (MIT) allows you to image 1000 or more fermionic atoms according to a May 13, 2015 news item on ScienceDaily,

Fermions are the building blocks of matter, interacting in a multitude of permutations to give rise to the elements of the periodic table. Without fermions, the physical world would not exist.

Examples of fermions are electrons, protons, neutrons, quarks, and atoms consisting of an odd number of these elementary particles. Because of their fermionic nature, electrons and nuclear matter are difficult to understand theoretically, so researchers are trying to use ultracold gases of fermionic atoms as stand-ins for other fermions.

But atoms are extremely sensitive to light: When a single photon hits an atom, it can knock the particle out of place — an effect that has made imaging individual fermionic atoms devilishly hard.

Now a team of MIT physicists has built a microscope that is able to see up to 1,000 individual fermionic atoms. The researchers devised a laser-based technique to trap and freeze fermions in place, and image the particles simultaneously.

A May 13, 2015 MIT news release, which originated the news item, provides intriguing detail about the microscope and fascinating insight into fermions (for those who are interested but not expert and sufficiently brave to endure certain failure to understand everything in this piece),

The new imaging technique uses two laser beams trained on a cloud of fermionic atoms in an optical lattice. The two beams, each of a different wavelength, cool the cloud, causing individual fermions to drop down an energy level, eventually bringing them to their lowest energy states — cool and stable enough to stay in place. At the same time, each fermion releases light, which is captured by the microscope and used to image the fermion’s exact position in the lattice — to an accuracy better than the wavelength of light.

With the new technique, the researchers are able to cool and image over 95 percent of the fermionic atoms making up a cloud of potassium gas. Martin Zwierlein, a professor of physics at MIT, says an intriguing result from the technique appears to be that it can keep fermions cold even after imaging.

“That means I know where they are, and I can maybe move them around with a little tweezer to any location, and arrange them in any pattern I’d like,” Zwierlein says.

Zwierlein and his colleagues, including first author and graduate student Lawrence Cheuk, have published their results today in the journal Physical Review Letters.

Seeing fermions from bosons

For the past two decades, experimental physicists have studied ultracold atomic gases of the two classes of particles: fermions and bosons — particles such as photons that, unlike fermions, can occupy the same quantum state in limitless numbers. In 2009, physicist Markus Greiner at Harvard University devised a microscope that successfully imaged individual bosons in a tightly spaced optical lattice. This milestone was followed, in 2010, by a second boson microscope, developed by Immanuel Bloch’s group at the Max Planck Institute of Quantum Optics.

These microscopes revealed, in unprecedented detail, the behavior of bosons under strong interactions. However, no one had yet developed a comparable microscope for fermionic atoms.

“We wanted to do what these groups had done for bosons, but for fermions,” Zwierlein says. “And it turned out it was much harder for fermions, because the atoms we use are not so easily cooled. So we had to find a new way to cool them while looking at them.”

Techniques to cool atoms ever closer to absolute zero have been devised in recent decades. Carl Wieman, Eric Cornell, and MIT’s Wolfgang Ketterle were able to achieve Bose-Einstein condensation in 1995, a milestone for which they were awarded the 2001 Nobel Prize in physics. Other techniques include a process using lasers to cool atoms from 300 degrees Celsius to a few ten-thousandths of a degree above absolute zero.

A clever cooling technique

And yet, to see individual fermionic atoms, the particles need to be cooled further still. To do this, Zwierlein’s group created an optical lattice using laser beams, forming a structure resembling an egg carton, each well of which could potentially trap a single fermion. Through various stages of laser cooling, magnetic trapping, and further evaporative cooling of the gas, the atoms were prepared at temperatures just above absolute zero — cold enough for individual fermions to settle onto the underlying optical lattice. The team placed the lattice a mere 7 microns from an imaging lens, through which they hoped to see individual fermions.

However, seeing fermions requires shining light on them, causing a photon to essentially knock a fermionic atom out of its well, and potentially out of the system entirely.

“We needed a clever technique to keep the atoms cool while looking at them,” Zwierlein says.

His team decided to use a two-laser approach to further cool the atoms; the technique manipulates an atom’s particular energy level, or vibrational energy. Each atom occupies a certain energy state — the higher that state, the more active the particle is. The team shone two laser beams of differing frequencies at the lattice. The difference in frequencies corresponded to the energy between a fermion’s energy levels. As a result, when both beams were directed at a fermion, the particle would absorb the smaller frequency, and emit a photon from the larger-frequency beam, in turn dropping one energy level to a cooler, more inert state. The lens above the lattice collects the emitted photon, recording its precise position, and that of the fermion.

Zwierlein says such high-resolution imaging of more than 1,000 fermionic atoms simultaneously would enhance our understanding of the behavior of other fermions in nature — particularly the behavior of electrons. This knowledge may one day advance our understanding of high-temperature superconductors, which enable lossless energy transport, as well as quantum systems such as solid-state systems or nuclear matter.

“The Fermi gas microscope, together with the ability to position atoms at will, might be an important step toward the realization of a quantum computer based on fermions,” Zwierlein says. “One would thus harness the power of the very same intricate quantum rules that so far hamper our understanding of electronic systems.”

Zwierlein says it is a good time for Fermi gas microscopists: Around the same time his group first reported its results, teams from Harvard and the University of Strathclyde in Glasgow also reported imaging individual fermionic atoms in optical lattices, indicating a promising future for such microscopes.

Zoran Hadzibabic, a professor of physics at Trinity College [University of Cambridge, UK], says the group’s microscope is able to detect individual atoms “with almost perfect fidelity.”

“They detect them reliably, and do so without affecting their positions — that’s all you want,” says Hadzibabic, who did not contribute to the research. “So far they demonstrated the technique, but we know from the experience with bosons that that’s the hardest step, and I expect the scientific results to start pouring out.”

Here’s a link to and a citation for the published paper,

Quantum-Gas Microscope for Fermionic Atoms by Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, and Martin W. Zwierlein. Phys. Rev. Lett. 114, 193001 – Published 13 May 2015 (print: Vol. 114, Iss. 19 — 15 May 2015) DOI: http://dx.doi.org/10.1103/PhysRevLett.114.193001

I believe this paper is behind a paywall.

There is an earlier version available on arXiv.org,

A Quantum Gas Microscope for Fermionic Atoms by Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, Martin W. Zwierlein. (Submitted on 9 Mar 2015 (v1), last revised 10 Mar 2015 (this version, v2))

This an open access website.