NEWS for November 2008 www.jqi.umd.edu J Q I Joint Quantum Institute Postdoc Positions Available Page 7 I N S I D E 1 Rolston Is Named New JQI Co-Director Quantum Dots in Photonic Crystals Entangled States: Awards, Grants, Meetings & More Page 8 JQI Fellow Steven Rolston of the University of Maryland (UMD) Physics Department has been named the new Co-Director of the Joint Quantum Institute, effective Novem- ber 1. Rolston succeeds JQI Fellow Christo- pher Lobb, who has held the position since the Institute's creation in September 2006. JQI Fellow Carl Williams of the National In- stitute of Standards and Technology (NIST) will continue as the other Co-Director. Rolston, a Fellow of the American Physical Society, and pioneer in the field of ultra- cold plasmas, said "I am excited to be able to help the Joint Quantum Institute in our exploration of quantum phenomena. continued, page 2 Every day, a growing amount of the world’s information moves at the speed of light in the form of photons that fly through optical fibers. And increasingly, society is depending on quantum information science to ensure that critical communications traveling over those lines can be made impregnably secure. There are many possible ways of reaching that goal, and JQI Fellow Edo Waks is exploring one of the most promising: experimental arrangements that combine the blazing speed of photonic crystals with continued, page 3 Christopher Lobb (l) turns over the position of Co-Direc- tor to Steven Rolston (r) after a two-year term. The photonic crystal (left, in red dot) lies in the center of a cryogenic enclosure (white). An infrared beam coming from the right travels through a beamsplitter (glass cube, center) to the crystal.
Transcript
NEWS for November 2008www.jqi.umd.edu
JQ I Joint Quantum Institute
Postdoc PositionsAvailable
Page 7
INSIDE
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Rolston Is Named New JQI Co-Director
Quantum Dots in Photonic Crystals
Entangled States: Awards, Grants, Meetings & More
Page 8
JQI Fellow Steven Rolston of the University of Maryland (UMD) Physics Department has been named the new Co-Director of the Joint Quantum Institute, effective Novem-ber 1. Rolston succeeds JQI Fellow Christo-pher Lobb, who has held the position since the Institute's creation in September 2006. JQI Fellow Carl Williams of the National In-stitute of Standards and Technology (NIST) will continue as the other Co-Director.
Rolston, a Fellow of the American Physical Society, and pioneer in the field of ultra-cold plasmas, said "I am excited to be able to help the Joint Quantum Institute in our exploration of quantum phenomena. continued, page 2
Every day, a growing amount of the world’s information moves at the speed of light in the form of photons that fly through optical fibers. And increasingly, society is depending on quantum information science to ensure that critical communications traveling over those lines can be made impregnably secure.
There are many possible ways of reaching that goal, and JQI Fellow Edo Waks is exploring one of the most promising: experimental arrangements that combine the blazing speed of photonic crystals with
continued, page 3
Christopher Lobb (l) turns over the position of Co-Direc-tor to Steven Rolston (r) after a two-year term.
The photonic crystal (left, in red dot) lies in the center of a cryogenic enclosure (white). An infrared beam coming from the right travels through a beamsplitter (glass cube, center) to the crystal.
Rolston Named JQI Co-Director, from page 1
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We may be at the brink of a second quantumrevolution, where the special properties of quantum mechanics are exploited to break the bonds of classical physics and develop new technologies previously unthinkable."
Lobb resigned in October to devote more time to research. In a letter to the Fellows, Lobb said "I want to thank all of you for the hard work you've done to make the JQI what it is today. The best part of being Co-Director has been the willingness of everyone to take the initiative to improve the infrastructure for research at the JQI. . . . The JQI is in a very strong position because of all of your work, and I anticipate that it will con-tinue to go from strength to strength."
Steve Halperin, Dean of UMD's College of Computer, Mathematical and Physical Sciences, said that "Chris was instrumental in helping to resolve numerous critical issues in making the JQI a reality, a situation involving meetings with accountants and lawyers, something that few scientists ever learn to excel at. He did this with no complaints and with a vision of creating a world-class institute. As a result, Chris helped to create an environment among the Fellows of the JQI that has led to numer-ous collaborations and discussions, helping to create the scientific foundations and inter-actions that will make the JQI truly success-ful. We all thank Chris for his service and his sacrifice.
"With Chris Lobb stepping down, Steve Rol-ston has been appointed the next UMD Co-Director. Steve has a long history with NIST and that deep understanding and knowledge should help take JQI to the next step where we can further work out the administrative kinks as we work over the next few years to
take the JQI from an existence as two parties in two locations to that of a unified institute on the UMD campus. Together with the wonderful scientific collaborations that have been born over the last two years, including the establishment of a Physics Frontier Center within the JQI, we are looking forward to a grand future. "
Co-Director Carl Williams of NIST noted that "Chris has been extraordinarily valuable to the NIST Fellows in both making us feel welcome
on the UMD campus and in helping us to learn the numer-ous administrative and other ropes required for us to success-fully interact on the campus. He has eased the process of making us Adjunct Professors at UMD, has helped to teach us about various administrative procedures for our students and postdocs, and has made us feel welcome in this new environ-ment. All of the JQI Fellows, but especially the NIST Fellows of the JQI , wish to thank Chris for his service and support.
"We also wish to welcome aboard Steve Rolston as the new
UMD Co-Director and look forward to working with him. Steve is well known by many of us, having spent more than a decade as part of Bill Phillips' research group."
Drew Baden, Chair of the UMD Physics Depart-ment, said that "Chris was one of the first fac-ulty experimentalists interested in quantum information, and how condensed matter phys-ics can evolve to have an impact in this new field. He played an early leadership role in making JQI possible. Chris is a natural experi-mentalist, and it is good to see him back in his natural habitat. In addition, he is one of our best teachers, and the department and stu-dents will benefit enormously by having him back in the classroom. The department, Col-lege and University are indebted to him."
Rolston, a Fellow of the Optical Society of America, worked at NIST from 1988-2003.
Quantum Dots in Photonic Crystals, from Page 1
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the remarkable, newly discovered “switching” abilities of tiny imbedded structures called quantum dots.
A primary long-term objective is to create a system for generating “entanglement” – the
quantum process whereby the properties of two objects become inextricably linked, no matter how far they are separated – between two quantum dots. The dots interact with light beams channeled through the crystal, thus making the whole assembly well-suited to quantum encryption schemes or to fabrication of “quantum repeaters” that can move encrypted messages over long distances.
Getting to that point, however, will first require
understanding how to precision-tune the interactions among crystal features, dots and photons with high fidelity using nothing but light to manipulate the system.
“It’s a complete quantum operation,” Waks says. “That’s really important to quantum networking and quantum computation. In principle, all quantum computations can be performed using this operation alone. And you can take this idea and extend it to entangle two quantum dots.” That feat has not yet been accomplished, but Waks believes it may soon be possible in photonic crystals (PCs).
A Quantum Light Switch
PCs are extremely small structures, typically no more than a few micrometers on a side, which are made of alternating regions of insulating material and air. One way this can be achieved is by drilling or etching holes in the material at regular intervals in a grid pattern. [Figure above left] A beam of photons passing through a PC thus experiences periodic changes in refractive index – high in the insulator, low in the air holes.
A common configuration for photonic crystal fabrication with embedded dots.
TWO KINDS OF TRANSISTORS Top: In a field-effect transistor, charges cannot move from the source to the drain if no voltage is applied to the gate. When a voltage is applied, the semiconductor region under the gate is affected by the applied field, and current flows. Bottom: In a photonic crystal, when a light beam (yellow) moving down a waveguide encounters a cavity without an embedded dot -- or when the dot is not coupled to the cavity -- the beam is reflected. When the dot is coupled, the cavity effect is "switched off" and the beam is transmitted as if there were no cavity present.
GATE GATE
CAVITY AREA CAVITY AREA
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That effect is strikingly analogous to what electrons experience as they move through the geometry of a semiconductor lattice, and it allows researchers to manipulate the passage of light through PCs
in much the same way that electrons are controlled in a transistor. For example, creating a defect in the crystal lattice can strongly confine photons much the same way that electrons are confined in lattice defects of a crystal.
This localization can enhance the interaction of light inside the cavity with an atomic system, such as a quantum dot, by both holding the light in place, which increases the interaction time, and tightly localizing the light, which leads to an increase of electric field intensity.
Three years ago, Waks and Jelena Vuckovic at Stanford University discovered that the enhanced atom-photon interactions in a photonic crystal cavity can lead to transistor action at the quantum level. A
quantum transistor could be made using the device shown below. The transistor is composed of a photonic crystal waveguide (formed by removing a row of holes) that is closely coupled to a cavity. If the cavity is empty, light injected into the input port of the waveguide would be completely reflected due to the coupled cavity. By placing a quantum dot inside the cavity, one can disable the coupling between the cavity and waveguide and the light is instead transmitted.
This outcome – called “dipole induced transparency” (DIT) – has obvious utility for controlling the movement of light in a PC. Of course, it is physically impossible to implant or remove a dot every time it is desirable to switch the cavity from transmitting to reflecting or vice versa. Instead, the QD can be effectively removed by exciting it to an optically inactive quantum state, which can be achieved by exciting it with a laser pulse of appropriate wavelength and duration. In this way, the QD behaves as a switch which turns the transparency of the waveguide on or off.
Entangling Dots?
The switching action of the quantum dot in DIT is reminiscent of transitor action where the logical value of a bit register (in this case the QD state) modifies the flow of current (in this case photons). There is one important distinction, however. Unlike classical transistors the QD is a quantum object, and therefore behaves like a quantum bit.
Quantum bits, or “qubits,” are the quantum-mechanical analogue of the minimum information unit in a classical computer. A conventional electronic binary digit, abbreviated “bit,” can have only one of two values: on or off, 0 or 1, as represented by electrical charges, voltages or magnetization. A quantum bit, however, can be 0, 1, or a “superposition” of both at once, owing to the inherently
Dots of indium arsenide in a gallium arsenide matrix.
Orange: Initial light path on PC waveguide. Blue: Reflected path. Red: Transmitted path. Small triangle marks the dot location. The dotted oval marks the cavity area. Yellow arrow indicates interaction with light beam. When the dot couples to the cavity, the beam is transmitted. When the dot is "switched off," and does not couple to the cavity, then the beam is reflected as if no dot were present.
Quantum Dots in Photonic Crystals, continued
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indeterminate nature of unmeasured quantum states.Certain kinds of quantum operations produce superposition outcomes in which the states of two objects are inseparably entangled, no matter how far apart they are moved. Until some measurement is taken, both objects exist in a superposition of possible states. But once one object is measured, the state of the other is instantly known. That’s the hallmark of entanglement, and it is an essential component of systems that can send and transfer quantum-encrypted messages.
Waks and UMD graduate student Deepak Sridharan have devised a protocol, based on the earlier Stanford DIT work, to generate entanglement between two dots, and are now testing parts of it in various ways, using nanometer dots of indium arsenide (InAs) embedded in cavities of photonic crystals made of gallium arsenide. Photonic crystal cavities are formed around the quantum dots by patterning the GaAs host substrate using electron beam lithography.
Each dot-and-cavity unit controls the path of a light beam, as in conventional DIT. But instead of switching the dot definitively to the “on” or “off” state, the researchers use a laser pulse adjusted to have an equal probability of putting the dot in either the coupled (“on,” transparent cavity) state or the decoupled (“off,” reflecting cavity) state.
For a simple single-dot system comprising one dot/cavity and one light beam, the uncertainty of the 50-50 chance adds no ambiguity to the outcome: If the beam is transmitted, the researcher knows for certain that the dot was coupled to the cavity; if the beam is reflected,
the dot was decoupled. But a much more interesting situation arises in a system of two dot/cavity pairs. (Call them left and right to match the diagram below.)
In the two-dot entanglement protocol, a coherent light beam is split into two parts, here labeled |α> and |β>. One travels to the left dot, one to the right. Each dot is exposed to an identical laser pulse producing the 50-50 condition, and the traveling light beam is either reflected or transmitted to the beam stop.
If it is transmitted, it is discarded for purposes of the experiment. If it is reflected, it is directed to an optical device called a beamsplitter, which divides the incoming beam and sends it to one or the other of two separate detectors labeled Det. d1 and Det. d2 in the diagram.
There are four possible outcomes for this arrangement: Both dots reflect; both dots transmit; the left dot reflects while the right transmits; and the left transmits while the right reflects.
Only two of those outcomes can produce light that arrives at the beamsplitter: Either both dots reflected, or only one did. If both of them reflected, then the combined beam will constructively interfere at detector 1. If only one dot reflected the light, there is a probability that the beam will register on detector 2.
At this point, the peculiar nature of quantum mechanics comes into play. If detector 2 registers a hit, it is certain that only one dot reflected. But it is impossible to tell which of the two dots did so. So the states of the two dots are entangled – each is in a superposition of the reflecting and transmitting states.
Quantum Dots in Photonic Crystals, continued
Entanglement Protocol for Two Quantum Dots
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Quantum Dots in Photonic Crystals
Above left: One of the lasers used in sending beams to the embedded dots. Above right: The lab table. Below: Apparatus for directing the beam to the quantum dot and detecting the output. Inset: Red laser beam strikes the photonic crystal in its cryogenic housing.For a complete description, see "Gener-ating entanglement between quantumdots with different resonant frequen-cies based on Dipole Induced Trans-parency," Deepak Sridharan and Edo Waks. arXiv:quant-ph/0703089v2.
Around the Waks Lab at the Institute for Re-search in Electronics and Applied Physics
Multiple Postdoctoral Openings
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UMD Condensed Matter Theory Center/JQI
We invite applications for new theoretical post-doctoral positions, available immediately. We are looking for technically strong, highly motivated candidates to work in interdisciplinary topics of to-pological phases of matter, topological quantum
computation, strongly correlated phases in cold atoms, non-Abelian quasiparticle statistics in condensed matter sys-tems (solid state and cold atomic gases), many-body and effective field theory, quantum criti-cal phenomena, p-wave superconductivity, frac-
tional quantum Hall effects, Kitaev model, quan-tum coherence and entanglement in solids and atoms, and related topics.
Candidates must have PhDs in theoretical physics preferably with some background in one or more of these areas. Although our interest is mostly in analytical theories, computational experts in DMRG, DMFT, QMC, etc. will also be considered. Some of these positions are associated with the newly created Physics Frontier Center at JQI and others are positions at the Condensed Matter Theory Center (www.physics.umd.edu/cmtc). We may be able to offer prize fellowships to truly ex-ceptional candidates.
There is considerable opportunity to interact with UMD and JQI scientists well as with a dozen other postdoctoral fellows in a highly stimulat-ing environment. Prospective candidates should send brief email messages (with a short CV and a publication list) to [email protected] for consideration. Please do not send reference let-ters unless asked. The relevant faculty members are Sankar Das Sarma and Victor Galitski.
Superconducting Quantum Computing
JQI is looking for an experimental postdoc with experience in SQUIDs, dilution refrigerators, mi-crowave techniques and the physics of quantum circuits. The postdoc will work on qubits based on Josephson junctions in collaboration with Fred Wellstood, Chris Lobb, and Bob Anderson of the Superconducting Quantum Computing Group.
We are looking for a scientist who works well with students, other postdocs and faculty, and who is creative at meeting experimental challenges. Ap-
plicants should send a CV, a statement of research inter-ests, and three letters of reference to [email protected].
IREAP/JQI
The nanophotonics group is seeking to hire a Postdoctoral Fellow. The main interests for our research group are: experimental and theoretical quantum optics, quantum information process-ing, photonic crystal design and fabrication, quantum dots, physics of optoelectronic devices, and plasmonics. We are particularly interested in candidates with optics lab and nanofabrication experience.
The nanophotonics group, led by Edo Waks, is part of the Department of Electrical and Com-puter Engineering. Our group is part of JQI d the Institute for Research in Electronics and Applied Physics (IREAP). Postdocs will have the oppor-tunity to establish close ties with researchers at UMD, NIST, and IREAP, and will also have access to state-of-the-art nanofabrication facilities at the UMD Fablab and NIST.
The initial appointment is 1 year with the pos-sibility of renewal for an additional 1-2 years
subject to satisfactory progress and availability of funds. Qualified candidates must have attained a Ph.D. prior to the beginning of their appoint-ment. Applicants should send a detailed CV and 3-4 references to [email protected]. Put “Nano-photonics Research Position” in the subject line. For additional information, see http://www.ireap.umd.edu/NanoPhotonics/. Email applications are preferred. Hard copy can be mailed to: Edo WaksIREAP/PhysicsUniversity of Maryland College Park, MD 20742-3511
NOTICE for all positions: The University of Maryland is an equal opportunity employer. Women and minorities are strongly encouraged to apply.
Entangled States
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On Oct. 16, Charles Clark, along with col-leagues from NIST and UMD's Institute for Physical Science & Technology (IPST), was honored at the 2008 R&D 100 awards dinner in Chicago for development of the Lyman Alpha Neutron Detector (LAND). See photo at right of the group with its award.
On Nov. 7, Clark will give the Stanford Pho-tonics Research Center Photonics Pioneers Seminar, "Relativity at a billionth the speed of light."
Paul Lett gave a talk at the ITAMP/Harvard-Smithsonian Center for Astrophysics en-titled "Entangled images from four-wave mixing in an atomic vapor" on Oct. 22.
Ian Spielman will deliver an invited seminar on Nov. 14 at the University of Illinois at Urbana-Champaign's "QI/AMO and Condensed Matter Seminar."
Sankar Das Sarma was one of 48 invitees to the 24th Solvay Conference in Physics entitled
"Quantum Theory of Condensed Matter." The Solvay Conferences, held in Brussels since 1911, have a long and distinguished history (see photo below of the 1928 invitees). Das Sarma gave talks on Spin Quantum Computation and Topological Matter in Cold Atomic Systems.
Left to Right: Michael Coplan (IPST), Clark, Alan Thompson (NIST) and Muhammad Arif (NIST)
Joint Quantum InstituteDepartment of Physics, Univ. of MarylandCollege Park, MD 20742E-mail: [email protected]: (301) 405-6129
Grants and Awards
Sankar Das Sarma is the PI on a DARPA QuEST (Quantum Entanglement Science and Technolo-gy) award that will total about $500,000 per year for five years. "Topologi-cal Quantum Entangle-ment" is a collaboration among UMD, UC Santa Barbara, UC Riverside, and Microsoft. The award will likely fund several grad students and/or postdocst at UMD.
Chris Monroe is the PI on “Trapped Ion Quan-tum Networks," a $480,000 award from the Army Research Office, and "Ultrafast Robust Quantum Gates for Trapped Ion Qubits," a $227,424 award from the National Geospatial Intelligence Agency.
Publications
Victor Yakovenko and coauthor Krishnendu Sengupta published "Spontaneous Spin Accumu-lation in Singlet-Triplet Josephson Junctions " in
Physical Review Letters 101, 187003 (30 Oct 2008).
Roman M. Lutchyn, Victor Galitski, Sankar Das Sarma and coauthor Gil Refael published "Dissipation-Driven Quantum Phase Transition in Super-conductor-Graphene Systems" in Phys-
ical Review Letters 101, 106402 (5 Sept 2008).
PRL has accepted "Adiabaticity and localization in one-dimensional incommensurate lattices" by Steve Rolston and UMD researchers.
Paul Julienne, Eite Tiesinga and Jeremy Hutson of the Unversity of Durham, UK, pub-lished "Avoided crossings between bound states of ultracold cesium dimers" in Physics Review A.
Entangled States, continued
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JQI is a joint venture of the University of Maryland and the National Institute of Stan-dards and Technology, with support from the Laboratory for Physical Sciences.
From BECs to the Whole Shebang
Kris Helmerson wrote a News & Views essay for the 16 October 2008 issue of Nature (comment-
ing on a paper by Weiler et al in the same issue) that discusses the role of non-equilibrium phase transi-tions of condensed matter systems in spontaneous formation of topological defects. In Bose-Einstein condensation of an atomic gas, this can take the form of multiple localized mini-
BECs. No one knows if that process obeys scaling laws. But something similar may have occurred in the first 10-35 seconds after the Big Bang, creating seeds of structure in the expanding universe.
