NEWS for October 2008www.jqi.umd.edu

JQ I Joint Quantum Institute

Das Sarma on Screening in Graphene Page 2

INSIDE

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Ultracold Polar Molecules -- at Last

Quantum Dots in a Whole New Light

Entangled States: JQI on 2 Coasts and 2 Continents Page 9

Scientists at JILA, a joint institute of the Nation-al Institute of Standards and Technology (NIST) and the University of Colorado at Boulder (CU-Boulder), and the Joint Quantum Institute have applied their expertise in ultracold atoms and lasers to produce the first high-density gas of ultracold molecules—two different atoms bonded together—that are both stable and capable of strong interactions.

The long-sought milestone in physics has potential applications in quantum computing, precision measurement and designer chemis-try. Described in the Sept. 18 issue of Science Express,* JILA’s creation of ultracold “polar” molecules— featuring a positive electric charge at one end and a negative charge at the other—paves the way for controlled

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A team headed by JQI Fellow Glenn Solomon of NIST has set a new experimental standard for understanding and ma-nipulating the exotic creations called “quantum dots,” or QDs for short.

QDs are artificial three-di-mensional structures, made of semiconductor material, that are only a few tens of nanome-ters at their widest. That’s small: About 1,000 dots placed side by side would barely equal the

width of a human hair. But it’s exactly the right size to per-form certain kinds of tricks that are much in demand in nano-electronics and information processing, as well as quantum optics and encryption.

In particular, researchers are interested in QDs’ ability to act like individual atoms, each of which has a strictly limited, clearly demarked set of permit-ted energy states -- and hence

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By using laser beams to manipulate a gas mixture of very cold potassium and rubidium atoms, the research team created a population of bound mol-ecules in their lowest and most stable energy level.

Glenn Solomon and Andreas Muller. The quantum dot is in the liquid helium cryostat at 4.2K.

Graphene Screening Tops the Charts

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Science Watch, a service of Thomson Reuters, identifies highly cited research. In September, the organization flagged a paper* by JQI Fel-low Sankar Das Sarma and colleague Euyheon Hwang as “the highest cited paper in the re-search area of two-dimensional graphene.” Science Watch asked Das Sarma to comment on the paper, and published the following interview on-line at http://sciencewatch.com/dr/fmf/2008/08sepfmf/08sepfmfSarma/.

Why do you think your paper is highly cited?

This paper is highly cited because it solves the outstanding quantum mechanical problem of theoretically and quanti-tatively understanding the dielectric screening properties of graphene.

Graphene has two-dimension-al electrons and holes. These electrons and holes respond to an external electric field by screening the external field, i.e., modifying or changing the external field in a complex manner.

It is extremely important to know how this screening behavior manifests itself in the actual two-dimensional graphene system since all graphene properties will, in the end, depend on how these two-dimen-sional carriers respond to external perturba-tions. Our work solves this important problem. Since this screening behavior manifests itself in many graphene properties, other research-ers need to use our theoretical results in their work, leading to a high rate of citations for this paper.

* “Dielectric function, screening, and plas-mons in two-dimensional graphene.” Hwang, E.H and Das Sarma, S. Physical Review B, 75 (20): 205418 (May 2007)Does it describe a new discovery, method-

ology, or synthesis of knowledge?

It describes a new theoretical discovery, i.e., the screening properties of graphene were simply unknown before our work, and be-came quite well-known afterwards.

Would you summarize the significance of your paper in layman’s terms?

I touch upon this issue in my response to the first question. But let me elaborate. An im-portant reason for the high level of interest in two-dimensional (2D) graphene is the pos-sibility of its eventual use in micro- or nano-electronic applications such as transistors or

other electronic compo-nents.

Our work in this paper and the closely related work, (EH Hwang, S Adam, and S Das Sarma, “Carrier Trans-port in Two-Dimensional Graphene Layers,” Phys. Rev. Lett. 98[18]: 186806, 2007), establish theoretically how the movement of electrons carrying electrical current in 2D graphene will be affect-ed by unintentional charged

impurities and defects invariably present in the graphene environment—these impurities cause electrical circuits to have finite resistiv-ity.

Since the response of the electrons in 2D graphene to the impurities and defects must include the effect of screening, i.e., how the graphene electrons themselves respond to external electric fields, our paper on screening takes on a special significance.

In summary, our paper is significant (and highly cited) because it enables research-ers to estimate the electrical resistance of a graphene-based electronic device. Our work has been fully verified by subsequent experi-

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Screening in Graphene, continued

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ments carried out all around the world, and therefore, future gra-phene-based electronic devices will use our theory in figuring out the device’s performance.

How did you become involved in this research? Were there particu-lar problems encountered along the way?

I became involved in this work because the United States Office of Naval Research (USONR) ap-proached me, asking if I would be interested in looking into the prospects for 2D graphene-based electronic applications.

Since I have extensive experience and background in theoretical research on semiconductor-based 2D transistors (e.g., Si MOSFETs and GaAs HEMTs and HIGFETs), it was quite natural for me and my long-term collaborator Dr. Euyheon Hwang (who did his Ph.D. under my supervision 10 years ago and has con-tinued a very fruitful research collaboration with me for the last 15 years) to become involved in this research.

The surprising thing is that we had very great dif-ficulties publishing this highly cited paper.

It took eight months for the paper to be pub-lished—submission in October of 2006 and publication in May of 2007—and in fact, the first journal (which shall remain un-named) that we submitted our paper to decided not to publish it!

This is perhaps not as strange as it sounds. First, the field of 2D graphene is highly competitive, and second, original theoretical discoveries are often not appreciated when they are put forward for the first time.

Where do you see your research leading in the future?

I continue to remain very active in the 2D gra-phene field, and have several other highly cited publications. I believe that our screening paper which we are discussing right now will continue to be cited in the literature for a while since the prospective use of graphene in microelectron-ics remains an active area of research all around the world. I believe that our screening work will remain an important theoretical ingredient in graphene research for the reasons I’ve discussed above.

Do you foresee any social or political implica-tions for your research?

Our work is mathematical and theoretical physics research. It can at best have some technological implications, and I believe that it does. Physics research typically does not have any direct social or political significance.

Ultra-cold Polar Molecules, continued from page 1

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interactions of molecules separated by relatively long distances, offer-ing a richer selection of features than is possible with individual atoms and potentially leading to new states of matter.

“Ultracold polar mole-cules really represent now one of the hottest fron-tiers in physics,” says NIST/JILA Fellow Jun Ye, an au-thor of the paper. “They are potentially a new form of matter, a quantum gas with strong interactions that vary by direction and that you can control us-ing external tools such as electric fields.”

The authors say atoms are like basketballs, round and somewhat feature-less, whereas molecules are more like footballs, with angles, and char-acteristics that vary by direction.

“This is really a big deal,” says NIST/JILA Fellow Deborah Jin, another author of the new paper. “This is something people have been trying to do for a long time, using all kinds of different approaches.”

Jin and Ye are adjoint professors of physics at CU-Boulder and both teach undergraduate and grad-uate students. Other authors of the paper include JQI Fellow Paul Julienne, and S. Kotochigova, a theorist at Temple University in Philadelphia.

Two types of atoms are found in nature—fermi-ons, which are made of an odd number of sub-atomic components (protons and neutrons), and

bosons, made of an even number of subatomic components. The JILA group combined potas-sium and rubidium, which are different classes of atoms (a slightly negative fermion and a slightly positive boson, respectively).

The resulting molecules exhibit a permanent and significant differential in electric charge, which, along with the ultracold temperatures and high density, allows the molecules to exert strong forces on each other.

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Ultra-Cold Polar Molecules, continued

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The molecules are in the lowest possible vibra-tional energy state and are not rotating, so they are relatively stable and easy to control. They also have what is considered a long lifespan for the quantum world, lasting about 30 millisec-onds (thousandths of a second).

JILA’s ultracold polar gas has a density of 10 quadrillion molecules per cubic centimeter, a temperature of 350 nanoKelvin (billionths of a degree) above absolute zero, and a measurable separation of electric charge.

The process for making the molecules begins with a gas mixture of very cold potassium and rubidium atoms confined by a laser beam. By sweeping a precisely tuned magnetic field across the atoms, scientists create large, weakly bound molecules containing one atom of each type. (This technique was pioneered by Jin in her 2003 demonstration of the world’s first Fermi pair condensate.)

At this stage the molecules are very large and possess a high amount of internal energy, which allows them to decay and heat up rapidly, both undesirable effects for practical applications. The scientists faced the considerable challenge of ef-ficiently converting atoms that are far apart into tightly bound molecules, without allowing the released binding energy to heat the gas.

In a process that Jin describes as “chemistry with-out explosions,” scientists used two lasers operat-ing at different frequencies—each resonating with a different energy jump in the molecules—to convert the binding energy into light instead of heat.

The molecules absorb near-infrared laser light and release red light. In the process, more than 80 percent of the molecules are converted, through an intermediate energy state, to the lowest and most stable energy level.

A key to success was the development of de-tailed theory for the potassium/rubidium mol-ecule’s energy states to identify the appropriate

intermediate state and select the laser colors for optimal control. In addition, both lasers were locked to an optical frequency comb, a precise measurement tool invented in part at NIST and JILA, synchronizing the two signals perfectly.

[For an informative look at optical frequency combs and how they work, see :http://jilawww.colorado.edu/outreach/JILA_Gems/optical_frequency_comb/index.html]

The research described in Science Express is part of a larger NIST/JILA effort to develop techniques to understand and control the complex features of molecules and their interactions.

Practical benefits could include new chemical reactions and processes for making designer materials and improving energy production, new methods for quantum computing using charged molecules as quantum bits, new tools for precision measurement such as optical mo-lecular clocks or molecular systems that enable searches for new theories of physics beyond the Standard Model, and improved understanding of condensed matter phenomena such as colossal magnetoresistance (for improved data storage and processing) and superconductivity (for per-fectly efficient electric power transmission).

JILA researchers are now working to improve the efficiency of producing tightly bound polar molecules and extend molecule lifetimes. They also plan to apply the new molecules to explore new scientific directions.

The research was supported by the National Sci-ence Foundation, NIST, Air Force Office of Scien-tific Research and W.M. Keck Foundation.

*K.K. Ni, S. Ospelkaus, M.H.G. de Miranda, A. Pe’er, B. Neyenhuis, J.J. Zirbel, S. Kotochigova, P.S. Juli-enne, D.S. Jin, J. Ye. 2008. “A High Phase-Space-Density Gas of Polar Molecules. “ Science Express. (On-line, Sept. 18, 2008.)

Quantum Dots in a Whole New Light, from page 1a very specific set of photon frequencies that it can absorb and emit.

Although QDs contain tens of thousands of atoms, they have some atom-like behaviors because of the peculiar nature of quantum-me-chanical objects. In the quantum world, where particles can act like waves, each object (such as an electron) has an associated wavelength.

Structures of the right size can confine a certain set of wavelengths just as a clarinet or oboe can play a certain set of notes whose wavelengths have the right mathematical relationship with the length of the horn. The longer oboe can resonate with longer confined wavelengths, and thus can play lower notes: Frequency depends on size. The same is true for quantum dots. And just as a musician can choose a particular instru-ment to play a desired set of notes, nanoscien-tists can customize dots for different purposes.

In the case of QDs, the wavelengths of inter-est belong to a two-part entity that behaves as a single “quasiparticle”: an electron and a “hole.” (A hole is the absence of an electron in the semiconductor material’s lattice that results when an electron is excited out of its ground state. Holes behave like, and can be treated as, positively charged objects.) The electron-hole pair is called, appropriately, an exciton because it can be viewed as an excitation in the crystal, and because they release energy in the form of a photon when they recombine.

Additional quanta of energy can produce paired excitons, called biexcitons. Like every other quantum-mechanical entity, excitons and biex-citons have associated wavelengths. QDs are the right size to confine those wavelengths in three dimensions.

As a result, QDs promise to serve as a minutely controllable source of photons for signal process-ing, and for generating “entangled” pairs of pho-tons to use in transmitting secure keys to encrypt-ed messages, among other uses. There are many advantages: Dots produce a dependable volume of photons with a high degree of wavelength

accuracy, and their semiconductor materials are familiar to scientists and engineers.

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Inside the cryostat (top), the semiconductor con-taining the dots is placed in a special holder (two photos, above left) and mounted under a lens device (right). The bottom left image, made by an atomic force microscope, shows dots in white.

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When excitons or biex-citons recombine, they emit photons or pairs of photons in a “radiative cascade,” dropping from one energy level to the next lowest as they shed energy.

In the quantum world, only certain energy lev-els, or quanta, are permit-ted. So there are only a few allowed sequences, or paths, for this transi-tion process, and each path results in emission of photons of particular distinctive wavelengths.

Sometimes each mem-ber of a photon pair has opposite polarization. That could make them ideal subjects for entanglement because, in the absence of information about which photon was polarized in which direction, taking a measure-ment of one photon would instantly determine the other's polarization.

Exploiting those prop-erties in a practical device, however, will require extremely sen-sitive understanding of exactly how QDs emit certain kinds of pho-tons in specific condi-tions. So Solomon and colleagues set out to determine how com-pletely the full emission spectrum of a QD could be controlled and measured.

Their technique involved two separate lasers trained on the dot. The first was used to “dress” the dot by using the strong laser field to alter the

quantum dot states. Then a second laser beam was applied to inject carriers into the QD states.

As these carriers relax to lower energies, they emit photons of various particular wavelengths depending on which of the allowed pathways

they followed.

The group made its QDs of indium ar-senide embedded in a surrounding crystal "sandwich" of gallium arsenide and alumi-num arsenide.

To minimize thermal “noise” effects, they cooled the sample to 4.2 degrees above

absolute zero with liquid helium. Then they focused on emissions from a single “dressed” QD about 30 nanometers (billionths of a meter) wide and 5 nm high.

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Quantum Dots in a Whole New Light, continued

The quantum dot lies embedded between layers of gallium arsenide that act as a waveguide for the control beam, which is attached via fiber-optic connection to the edge of the sample. The pump laser beam (using visi-ble-red wavelengths of either 633 or 730 nm) enters through a lens from the top, and the emission signal emerges at the same location.

This green laser is used to pump the titanium-sapphire laser that excites the dot.

Quantum Dots in a Whole New Light, continued

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If the control (“dressing”) laser beam and the pump laser beam were in the same plane, opti-cal “noise” from scattered control photons would degrade the output signal. So the team attached an optical fiber carrying the control beam at right angles to the pump beam. (See illustration.)

Under the experimental conditions, the QD exciton and biexciton states have five or 10 ways to emit pho-tons of various wavelengths. By using dif-ferent tunings of the lasers, the researchers were able to prompt and re-cord every pos-sible permitted emission from the dot, nota-bly including the “fine struc-ture” that is well known in theory, but difficult to observe.

In all cases, the experimental data were in excel-lent agreement with simulations produced by theoretical calculations.

The work constitutes the first instance in which the complete exciton-biexciton emission spec-trum of a dressed QD has been recorded. But it has considerable further significance, for two reasons. First, the group was able to produce any desired emission by controlling the intensity and

“detuning” of the control laser. (Detuning is the process of shifting the frequency of one wave so that it is out of resonance with the target QD transition.) Whether this can eventually be ac-complished in a practical device at room tem-perature is unknown. But if so, it could provide the basis for exquisite control over photons used in numerous forms of quantum information pro-cessing.

That sort of regu-lation could com-pensate for the in-herent problems that arise when QDs -- which are often fabricated by spraying atoms onto a surface in a vacuum, a nec-essarily inexact method -- emerge with asymmetrical proportions that in turn affect their optical properties.

Second, the JQI group demonstrated that the output of the dressed QD could be generated one photon at a time by using a pulsed laser source for the pump beam. That ability will be important in any eventual functional equipment based on the phenomena.

In the next stage of the research, the group will tackle the problem of making photons using their optical techniques when the original QD asymmetries forbid them.

Cross-section scanning tunneling microscope (STM) image shows indium arsenide quantum dot regions embedded in gallium arsenide. Each 'dot' is approximately 30 nanometers long. Faint lines are individual rows of atoms. (Color has been added for clarity.) Credit: J.R. Tucker

Part of the laser-beam input line from the bench to the dot.

Bill Phillips gave an address and a colloquium on Oct. 1 and 2 at the University of Oregon. The public lecture was titled "Time, Einstein and the Coolest Stuff in the Universe."

On Oct. 23 and 24, Ian Spielman will present at the 5th New Laser Scientist Conference, a 1½ day mini-conference in conjunction with the October 2008 FiO/LS meeting of the OSA/DLS in Rochester, NY. He will also give a talk titled "Two-dimensional bosons in an optical lattice" at Georgetown University on Oct. 14.

Sankar Das Sarma gave an invited presentation on Topological Quantum Computing -- and his

work was featured in two other talks -- at ICPS 2008 this summer.

Elsewhere down south, Garnett Bryant gave a keynote talk at NFO-10 in Rio. The abstract is available at http://www.jqi.umd.edu/outreach/

NFO10abstract_gwb_v3.pdf.

Chris Monroe will give the Math-ematics and Physical Sciences Distinguished Lecture on Oct. 20 at the National Science Foundation.

Alan Migdall , Bill Phillips, San-kar Das Sarma and Chris Monroe were featured speakers at the Army Research Laboratory Fel-lows Symposium in September. See http://www.jqi.umd.edu/outreach/confagenda.html.

Entangled States

Joint Quantum InstituteDepartment of Physics, Univ. of MarylandCollege Park, MD 20742E-mail: jqi_info@squid.umd.eduTelephone: (301) 405-6129

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.

POSTDOC FELLOWSHIP: Qudsia Quraishi has been awarded a multi-year "Intelligence Community Postdoctoral Fellowship," granted by the National Geospatial Agency.

APPLICATION DEADLINES: The application peri-ods for JQI's Postdoctoral Fellowships and Gradu-ate Assistantship close on Dec. 1, 2008 and Jan. 16, 2009 respectively.

For instructions and general guidelines, see:

Postdocs: http://www.jqi.umd.edu/working/post-doc.html

Graduate Students: http://www.jqi.umd.edu/work-ing/grad.html.

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Kudos and Deadlines