Tag Archives: physics

Shape from sound — new methods to probe the universe (Physical Review Letters)

By Morgan Kelly, Office of Communications

As the universe expands, it is continually subjected to energy shifts, or “quantum fluctuations,” that send out little pulses of “sound” into the fabric of spacetime. In fact, the universe is thought to have sprung from just such an energy shift.

A recent paper in the journal Physical Review Letters reports a new mathematical tool that should allow one to use these sounds to help reveal the shape of the universe. The authors reconsider an old question in spectral geometry that asks, roughly, to what extent can the shape of a thing be known from the sound of its acoustic vibrations? The researchers approached this problem by breaking it down into small workable pieces, according to author Tejal Bhamre, a Princeton University graduate student in the Department of Physics.

To understand the authors’ method, consider a vase. If one taps a vase with a spoon, it will make a sound that is characteristic of its shape. Similarly, the technique Bhamre and her coauthors developed could, in principle, determine the shape of spacetime from the perpetual ringing caused by quantum fluctuations.

The researchers’ technique also provides a unique connection between the two pillars of modern physics — quantum theory and general relativity — by using vibrational wavelengths to define the geometric property that is spacetime.

Bhamre worked with coauthors David Aasen, a physics graduate student at Caltech, and Achim Kempf, a Waterloo University professor of physics of information.

Read the abstract.

David Aasen, Tejal Bhamre and Achim Kempf. 2013. Shape from Sound: Toward New Tools for Quantum Gravity. Physical Review Letters. Article first published online: March 18, 2013. DOI: 10.1103/PhysRevLett.110.121301.

This research was supported by the Natural Sciences and Engineering Research Council of Canada.

Quantum computing moves forward (Science)

By Catherine Zandonella, Office of the Dean for Research

New technologies that exploit quantum behavior for computing and other applications are closer than ever to being realized due to recent advances, according to a review article published this week in the journal Science.

A silicon chip levitates individual atoms used in quantum information processing. Photo: Curt Suplee and Emily Edwards, Joint Quantum Institute and University of Maryland. Credit: Science.

These advances could enable the creation of immensely powerful computers as well as other applications, such as highly sensitive detectors capable of probing biological systems. “We are really excited about the possibilities of new semiconductor materials and new experimental systems that have become available in the last decade,” said Jason Petta, one of the authors of the report and an associate professor of physics at Princeton University.

Petta co-authored the article with David Awschalom of the University of Chicago, Lee Basset of the University of California-Santa Barbara, Andrew Dzurak of the University of New South Wales and Evelyn Hu of Harvard University.

Two significant breakthroughs are enabling this forward progress, Petta said in an interview. The first is the ability to control quantum units of information, known as quantum bits, at room temperature. Until recently, temperatures near absolute zero were required, but new diamond-based materials allow spin qubits to be operated on a table top, at room temperature. Diamond-based sensors could be used to image single molecules, as demonstrated earlier this year by Awschalom and researchers at Stanford University and IBM Research (Science, 2013).

The second big development is the ability to control these quantum bits, or qubits, for several seconds before they lapse into classical behavior, a feat achieved by Dzurak’s team (Nature, 2010) as well as Princeton researchers led by Stephen Lyon, professor of electrical engineering (Nature Materials, 2012). The development of highly pure forms of silicon, the same material used in today’s classical computers, has enabled researchers to control a quantum mechanical property known as “spin”. At Princeton, Lyon and his team demonstrated the control of spin in billions of electrons, a state known as coherence, for several seconds by using highly pure silicon-28.

Quantum-based technologies exploit the physical rules that govern very small particles — such as atoms and electrons — rather than the classical physics evident in everyday life. New technologies based on “spintronics” rather than electron charge, as is currently used, would be much more powerful than current technologies.

In quantum-based systems, the direction of the spin (either up or down) serves as the basic unit of information, which is analogous to the 0 or 1 bit in a classical computing system. Unlike our classical world, an electron spin can assume both a 0 and 1 at the same time, a feat called entanglement, which greatly enhances the ability to do computations.

A remaining challenge is to find ways to transmit quantum information over long distances. Petta is exploring how to do this with collaborator Andrew Houck, associate professor of electrical engineering at Princeton. Last fall in the journal Nature, the team published a study demonstrating the coupling of a spin qubit to a particle of light, known as a photon, which acts as a shuttle for the quantum information.

Yet another remaining hurdle is to scale up the number of qubits from a handful to hundreds, according to the researchers. Single quantum bits have been made using a variety of materials, including electronic and nuclear spins, as well as superconductors.

Some of the most exciting applications are in new sensing and imaging technologies rather than in computing, said Petta. “Most people agree that building a real quantum computer that can factor large numbers is still a long ways out,” he said. “However, there has been a change in the way we think about quantum mechanics – now we are thinking about quantum-enabled technologies, such as using a spin qubit as a sensitive magnetic field detector to probe biological systems.”

Read the abstract.

Awschalom D.D., Bassett L.C., Dzurak A.S., Hu E.L. & Petta J.R. (2013). Quantum spintronics: engineering and manipulating atom-like spins in semiconductors. Science 339 (6124) 1174–1179. PMID: 23471400

The research at Princeton University was supported by the Alfred P. Sloan Foundation, the David and Lucile Packard Foundation, US Army Research Office grant W911NF-08–1-0189, DARPA QuEST award HR0011-09–1-0007 and the US National Science Foundation through the Princeton Center for Complex Materials (DMR-0819860) and CAREER award DMR-0846341.

Cancer cells exchange leaders during invasion (PNAS)

By Catherine Zandonella, Office of the Dean for Research

A new study has found that cancer cells appear to exchange leading roles as they migrate out of a tumor in the early stages of invasion, or metastasis, of other sites in the body. Metastatic cancer accounts for more than 90% of cancer-related deaths.

A team led by Robert Austin, professor of physics at Princeton University, found that individual cancer cells take turns as trailblazers when they carve their way through the dense wall — known as the extracellular matrix — that stands between a tumor and the blood vessels which can carry the cells to other parts of the body.

The researchers also found that the cells leave the tumor in search of food, since cells that had plenty of available nutrients did not migrate. The finding reinforces the hypothesis that metastasis occurs when tumors become so densely packed that blood vessels cannot penetrate the interior and cancer cells must migrate to survive.

The researchers included first author Liyu Liu of the Chinese Academy of Sciences; Guillaume Duclos of the National Center for Scientific Research in Paris; Bo Sun, Jeongseog Lee, Amy Wu, Howard Stone and James Sturm of Princeton University; Yoonseok Kam and Robert Gatenby of H. Lee Moffitt Cancer Center in Tampa; and Eduardo Sontag of Rutgers University. The article appeared in the Proceedings of the National Academy of Sciences.

To study cancer cell behavior, the researchers constructed a small chamber with three compartments arranged like floors in an apartment building. On the bottom floor was a well of glucose, the preferred food for metastatic cells. The middle floor contained a dense layer of collagen, a protein that makes up the extracellular matrix that surrounds tumors. On the top floor they placed metastatic cancer cells, which were labeled with fluorescent dye for visibility. They trained a microscope and camera on the chamber.

Through the microscope, the researchers filmed the cancer cells as they moved down through the chamber toward the glucose. The researchers found that a single cell would become the leader for some time, then drop back as another cell took the lead in what the authors term a “collective invasion strategy.” They also found that the collagen was pushed aside, leaving a wake in which cells behind the leader could travel.

Because the collagen is very dense, the cells must expend a lot of energy to reach the glucose, and indeed the researchers found that cells without a need for glucose did not bother to burrow down into the collagen. The researchers used collagen with a density similar to that of human breast tissue.

The study adds to the growing understanding of metastasis and could serve to assist researchers in developing strategies for its prevention.

Liyu Liu, Guillaume Duclos, Bo Sun, Jeongseog Lee, Amy Wu, Yoonseok Kam, Eduardo D. Sontag, Howard A. Stone, James C. Sturm, Robert A. Gatenby, and Robert H. Austin. Minimization of thermodynamic costs in cancer cell invasion. PNAS January 14, 2013 201221147.

Read the paper (open access).

This work was supported by the National Science Foundation and the National Cancer Institute.

Pump-Induced Exceptional Points in Lasers (Physics Review Letters)

Scientists Predict Paradoxical Laser Effect

New laser-effect, discovered by scientists from the Vienna University of Technology, Princeton, Yale and ETH Zurich: If coupled, lasers can switch each other off, leading to a “laser blackout.”

Princeton’s Hakan Tureci and Li Ge collaborated with researchers at the Institute for Theoretical Physics at Vienna University of Technology.
Read the press release.

Read the abstract.

M. Liertzer^1,*, Li Ge², A. Cerjan³, A. D. Stone³, H. E. Türeci^2,4, and S. Rotter^1,†
¹Institute for Theoretical Physics, Vienna University of Technology, A-1040 Vienna, Austria, EU; ²Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA;³Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA; ⁴Institute for Quantum Electronics, ETH-Zürich, CH-8093 Zürich, Switzerland
Received 2 September 2011; revised 20 January 2012; published 24 April 2012

We demonstrate that the above-threshold behavior of a laser can be strongly affected by exceptional points which are induced by pumping the laser nonuniformly. At these singularities, the eigenstates of the non-Hermitian operator which describes the lasing modes coalesce. In their vicinity, the laser may turn off even when the overall pump power deposited in the system is increased. Such signatures of a pump-induced exceptional point can be experimentally probed with coupled ridge or microdisk lasers. © 2012 American Physical Society

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Sci­en­tists Pre­dict Para­dox­i­cal Laser Effect

New laser-effect, dis­cov­ered by sci­en­tists from the Vienna Uni­ver­sity of Tech­nol­ogy, Prince­ton, Yale and ETH Zurich: If cou­pled, lasers can switch each other off, lead­ing to a “laser blackout.”

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Scientists Predict Paradoxical Laser Effect

New laser-effect, discovered by scientists from the Vienna University of Technology, Princeton, Yale and ETH Zurich: If coupled, lasers can switch each other off, leading to a “laser blackout.”