Toward Coherent Control in the Nanoscale

Author: Tamar Seideman

Professor of Chemistry and Physics,
Northwestern University, Evanston, IL.
Link to Seideman Group>>

[This is an invited article from Prof. Seideman on the recent work published in J. Physics B which illustrated how the effect of the interaction of light with plasmons on the surfaces of metal nanoparticles could be utilized to create sources of coherent and polarized light -- A work that opens up lot of other possibilities.
-- 2Physics.com Team]

Coherent control has been applied in recent years to a wide variety of problems, ranging from atomic physics and gas-phase molecular dynamics through solid-state physics and semiconductor device technology to solution chemistry and biology [1]. The broad applicability of this method owes to its origin in a simple and very general concept, namely wave interference. As traditionally practiced, however, coherent control applies to large ensembles – laser beams are macroscopic objects whose focal spot size is constrained by the diffraction of light to the 10^-7-10^-4 m range.

Molecular-scale electronics, the application of single molecules or ordered networks of molecules to make electronic or electromechanical devices, has grown during the past two decades from an inspiring dream[2] to a technology-facing area of science. The use of electronic and nuclear molecular degrees of freedom to create transport, logic, or memory elements is conceptually intriguing, since it makes use of the quantum states of single or small collections of molecules, where coherence may be expected to have a special role. The application of coherent control concepts and tools to control electronic transport and molecular dynamics in the nano-constructs, however, requires the development of nanoscale light sources with controlled polarization and phase properties.

It is here that a third established and yet rapidly growing field, namely nanoplasmonics [3], presents an opportunity. The physics underlying plasmonic phenomena is familiar from related fields such as surface enhanced Raman spectroscopy and tip-assisted spectroscopies. Similar to metal surfaces and sharp tips, metal nanoparticles enhance light that is shine upon them at frequencies (the plasmon resonance frequencies) that depend sensitively on the particle shape and size and on the dielectric constants of both the system and the surrounding medium. The origin of the enhancement is collective coherent excitation of conductive electrons in the particles, which leads to buildup of polarization charges on the particle surface.

The possibility of using metal nanoparticles to focus and enhance incident light, and the sensitivity of the electromagnetic fields generated to the structural parameters and chemical composition of the material system, have been illustrated numerically and experimentally and are well understood. It is this sensitivity that is responsible for the application of nanoplasmonics to make sensors, medical diagnostics and markers. The question how to design nanoparticles and arrays thereof to have specific, predetermined optical properties and hence desired functionalities is thus both fundamentally interesting and potentially tied to applications.

Recent work by the Seideman group at Northwestern University has extended concepts and tools developed for coherent control of molecular dynamics to guide light in the nanoscale via metal nanoparticle arrays and to develop nanoplasmonics with predetermined functionalities [4]. Several simple elements in what the group envisions developing into coherently controlled nanoplasmonics are schematically illustrated in Fig. 1.

The T-junction of Fig. 1a was applied to guide electromagnetic energy traveling down the leg into one or the other of the two symmetry-equivalent arms of the junction. The challenge of inducing light to bend about corners was achieved by choice of the incident field polarization. The symmetry breaking was achieved by choice of the incident field phase. Figure 1b depicts a hybrid construct, which combines elements that provide local enhancement (such as nanospheres) with elements that provide long distance propagation (such as nanowires) in order to minimize losses. The structural parameters of the construct are optimized using a genetic algorithm. Fig. 1c depicts a plasmonic nanocrystal, developed to separate an incident plane wave into two frequency components and funnel each component in a different direction normal to the direction of incidence, in parallel to the surface plane. Elsewhere the group applies genetic algorithms to iteratively build-in other optical properties into metallic and hybrid metal-semiconductor nanoparticle arrays, designing elements such as lenses and antennas with predetermined properties.

The challenge of numerically developing coherent nanoscale light sources and applying them to control in the nanoscale is one of the topics of ongoing work of the Seideman group. By proper construction of single metal nanoparticles and arrays thereof, the group is able to produce spatially localized, time-resolved electromagnetic fields with predetermined phase and polarization properties. It is hoped that such sources will enable the extension of the machinery of coherent control to the nanoworld, with potential applications in the control of molecular-scale electronics, electromechanics and possibly spintronics.

References:
[1] S. A. Rice and M. Zhao, Optical Control of Molecular Dynamics, (John Wiley & Sons, 2000);
M. Shapiro and P. Brumer, Principles of the Quantum Control of Molecular Processes (Hoboken, N.J.:
Wiley-Interscience, 2003); H. A. Rabitz, M. M. Hsieh and C. M. Rosenthal, Science 303, 1998 (2004).
[2] A. Ariram and M.A. Ratner, Chem.Phys.Lett. 29, 277 (1974).
[3] S. A. Maier, M. L. Brongersma, P.G. Kik, S. Meltzer, A. A. G. Requicha, and H. A. Atwater, Adv. Mat. 13, 1501 (2001); S. Link and M. A. El-Sayed, Annu. Rev. Phys. Chem. 54, 331 (2003); E. Hutter and J. H. Fendler, Adv. Mat. 16, 1685 (2004).
[4] M. Sukharev and T. Seideman, J.Phys. B 40 S283 (2007); J.Chem.Phys. 126, 204702 (2007); NanoLett. 6, 715 (2006);

Symmetries, Horizons, and Black Hole Entropy

Author: Steve Carlip

Affiliation: Department of Physics, University of California at Davis

[This is an invited article from Prof. Steve Carlip who received this year's Gravity Research Foundation award for his essay on this topic. The award-winning essay will be published in future issue of General Relativity and Gravitation and International Journal of Modern Physics D.
-- 2Physics.com Team]

Drop a box of hot gas into a black hole. The initial state is gas plus a black hole; the final state is a slightly larger black hole, and nothing else. If the second law of thermodynamics -- which requires that entropy never decrease -- is to hold, the final black hole had better have enough entropy to account for the entropy of the gas it swallowed up.

Thirty-five years ago, Bekenstein used such thought experiments to show that a black hole should have an entropy proportional to the area of its event horizon in Planck units [1]. Soon afterwards,Hawking demonstrated that black holes are, indeed, thermodynamic objects, radiating as black bodies with characteristic temperatures and entropies that match Bekenstein's estimates [2]. In every other thermodynamic system we know, thermal properties reflect the statistical mechanics of underlying microscopic states. Entropy,for example, counts the microstates, while temperature measures their energy. Since the Bekenstein-Hawking entropy involves both Planck's constant and Newton's constant, a statistical mechanical description would have to involve quantum gravity, and might teach us something about the unsolved problem of how to quantize general relativity.

Until fairly recently, no one had a clear idea of the microscopic states responsible for black hole entropy. Today, we suffer the opposite problem: we have many explanations, each describing a different set of states but all agreeing on the final numbers. String theory, for instance, gives us three ways to count black hole states (as excitations of weakly bound branes, as horizonless "fuzzball" geometries, and as states in a dual field theory "at infinity"); loop quantum gravity provides two more; others come from induced gravity, causal set theory, holographic entanglement entropy, and global geometry [3]. None of these approaches is complete, but within its realm of applicability, each seems to work. The new puzzle -- the "problem of universality" -- is to understand why everyone gets the same answer.

One attractive possibility is that a hidden symmetry of classical general relativity controls the thermodynamic properties of black holes. Near the horizon, a black hole looks nearly scale-invariant (technically, conformally invariant) and nearly two-dimensional; quantities such as masses get red-shifted away, as do excitations transverse to the r-t plane. Cardy showed twenty years ago that the thermodynamic properties of a two-dimensional conformal field theory are completely determined by a few parameters that describe its symmetries [4]. Two-dimensional conformal descriptions of matter in a black hole background can be used to derive the spectrum of Hawking radiation [5]; perhaps similar reasoning can be applied to the degrees of freedom of the black hole itself.

To see whether such an explanation makes sense, we must first figure out what it means to ask a question about a black hole in quantum gravity. The uncertainty principle prevents us from simply saying,"A black hole is present." Instead, we must find a way to impose constraints strong enough to ensure the presence of a black hole,but weak enough to be allowed by quantum mechanics. My most recent work has focused on the possibility of introducing such "horizon constraints" as ordinary constraints in the Hamiltonian formulation of general relativity [6]. The results so far are promising: one can obtain the correct Bekenstein-Hawking entropy for a wide class of black holes from the constraints and the symmetry alone. Moreover, there is some evidence of "universality": at least one string theory approach can be understood as a special case of the horizon constraint method, and there are tantalizing hints of a connection with loop quantum gravity. If this explanation is really universal, the horizon constraints should be hidden in other derivations of blackhole entropy as well. We're looking...

A universal explanation of black hole thermodynamics should not, of course, give a complete description of the underlying microstates -- that would ruin its universal character. Still, the horizon constraint method suggests a new way of looking at the degrees of freedom of a black hole. The key point is that the horizon constraints break the fundamental symmetry of general relativity, general covariance (technically, diffeomorphism invariance). As a result, states that would normally be considered equivalent, differing only by a "gauge" transformation, are now physically distinct. This is roughly analogous to the Goldstone mechanism in condensed matter and particle physics, in which a broken symmetry gives rise to new degrees of freedom. In a few cases [7,8], this can be made explicit; it is an open question whether the description works more generally.

The other crucial open question is whether the horizon constraint method can also describe Hawking radiation and other thermodynamic properties of the black hole. To answer, we need to understand how the horizon constraints affect matter near the horizon. This is a hard question, but shouldn't be an impossible one.

References:
[1] J.D. Bekenstein, Phys. Rev. D7 (1973) 2333.
[2] S.W. Hawking, Nature 248 (1974) 30.
[3] For some references, see S. Carlip, J. Phys. Conference Series 67 (2007) 012022, arXiv:gr-qc/0702094.
[4] J.A. Cardy, Nucl. Phys. B270 (1986) 186.
[5] S. Iso, T. Morita, and H. Umetsu, arXiv:hep-th/0701272.
[6] S. Carlip, arXiv:gr-qc/0702107, to appear in Phys. Rev. Lett.
[7] S. Carlip, Class. Quant. Grav. 22 (2005) 3055, arXiv:gr-qc/0501033.
[8] R. Aros, M. Romo, and N. Zamorano, Phys. Rev. D75 (2007) 067501, arXiv:hep-th/0612028.

Entanglement and One-Way Quantum Computing

Prof. Anton Zeilinger [photo credit: Jacqueline Godany]

Authors: Robert Prevedel and Anton Zeilinger

Affiliation: Faculty of Physics, University of Vienna
and
Institute of Quantum Optics and Quantum Information, Austrian Academy of Science.

[This is an invited article based on some recent publications by the authors in various journals referred below.
-- 2Physics.com Team]

According to Erwin Schrödinger [1], one of the founding fathers of quantum mechanics, entanglement is the essence of quantum physics. It inspires fundamental questions about the principles of nature and is also the basis for emerging technologies [2] of quantum information processing such as quantum cryptography [3], quantum teleportation [4,5] and quantum computation [6].

Entangled particles possess correlations stronger than those allowed by classical physics, and can only be described by their joint behaviour, no matter how far the two particles are located from each other. Imagine two ‘entangled coins’, placed at opposite ends of a galaxy, each of which, when thrown individually, yields a random results (head or tails). However, owing to entanglement between the coins, the throw of the second coin always leads to an outcome that is fully determined by the result of rolling the first one, independent of their spatial separation or the time ordering of the throws.

This feature can be used to transmit information securely between two parties (quantum cryptography) or, when generalized to many particles, to process information faster and more efficiently than possible by any classical means. The latter has sparked the now increasingly growing research field of quantum computation [6]. Owing to the unique features of quantum mechanics, such as superposition and entanglement, quantum computers have the potential to perform tasks utterly intractable on any conceivable classical computing hardware.

While different theoretical approaches of how to realize these quantum computers in the laboratory exist, one particular model excites the community because of its simplicity for experiments. It is an entanglement based scheme widely known as “one-way” quantum computing [7]. In this scheme the computation or information processing is performed by measuring particles that are part of a large entangled network. As with the coins, the state of all the particles in the network will be influenced according to the outcome of the measurement at a particular site. This allows to manipulate large arrays of quantum particles (usually called “cluster states”) and therefore to utilize their powerful features to execute algorithms in a speed and fashion impossible with a classical processor. Depending on which and how the individual particles are measured, the remaining particles will occupy different states [7]. Reading out this state will provide the user with the output of his computation. Because of the irreversibility of the measurement process (it collapses the quantum superposition to a definite state) the term “one-way” quantum computer was introduced.

In the experiments, we employ the polarization degree of freedom of single photons as our information carriers. A photon that carries horizontal polarization thus represents a qubit which is in the logical “0” state, while vertical polarization embodies “1”. However, we can also prepare photons whose polarization is along 45 degrees, thus representing “0+1”, i.e. it exists in a superposition of both basis states.

After entangling a certain number of these photons (qubits), we start to run our one-way quantum computer by measuring the photons polarizations in a certain order and fashion [8]. By doing so, the input information, represented by the initial state of some photons, is altered and processed due to the measurement task. Reading out the quantum state of the remaining photons yields the output of the computation. The larger the initial, entangled state, the more measurements can be performed and the more complex and powerful the computation.

In proof-of-principle experiments, small versions of these quantum computers have already been built [8,9]. In the most successful realization, a process called spontaneous parametric down-conversion is employed in order to create photon pairs (also known as Bell states) which are entangled in their polarization state. To generate large networks of entangled states, these entangled photon pairs are further subject to operations that combine the individual Bell states to yield large cluster states.

Using single photons as qubits has the advantage that their quantum state remains basically unaltered during the experiment, owing to the typically weak coupling of photons to their environment. Moreover, single photons can be conveniently controlled with standard optical components.

In the actual experiment, we utilized two Bell states to create a cluster state made up of four photons. Depending on the type and order of measurements on this cluster, different one-qubit and two-qubit operations could be realized, therefore demonstrating the working principles of such one-way quantum computers and the potential to perform even more complex computations. With this set of operations, the successful realization of a quantum algorithm has also been shown. Quantum algorithms [6] can execute certain tasks such as searching in an unsorted database with less elementary steps than classically possible. Take e.g. a database with N elements. A classical algorithm needs to look, on average, N/2 times into the database to find a desired entry. Grover’s quantum algorithm [10] masters this search tasks with only steps. In our experiment, the database consists of 4 entries, each embodied by a photon. Intriguingly, in this case, Grover’s algorithm finds the corresponding entry with unit fidelity after only a single run [8-10]. This has been verified in the experiment and our demonstrations will hopefully pave the way for the realization of even more complex quantum algorithms, such as Shor’s algorithm [11] for the efficient factorization of large integers into prime numbers – a method on which most of today’s encryption protocols rely.

References:
[1] Schrödinger, E. Die gegenwärtige Situation der Quantenmechanik. Die Naturwissenschaften 49, 823-828 (1935).
[2] Prevedel, R. et al. Photonic entanglement as a resource in quantum computation and quantum communication. J. Opt. Soc. Am. B 24, 241-248 (2007).
[3] Ekert, A. K. Quantum Cryptography Based on Bell's Theorem. Phys. Rev. Lett. 67, 661-663 (1991).
[4] Bennett, C. H. et al. Teleporting an unknown quantum state via dual classical and EPR channels. Phys. Rev. Lett. 70, 1895-1899 (1993).
[5] Bouwmeester, D. et al. Experimental Quantum Teleportation. Nature 390, 575 (1997).
[6] Deutsch, D. & Ekert, E. Quantum computation. Phys. World 11, 47-52 (1998).
[7] Raussendorf R. & Briegel, H. J. A one-way quantum computer. Phys. Rev. Lett. 86, 5188-5191 (2001).
[8] Walther, P. et al. Experimental one-way quantum computing. Nature 434, 169-176 (2005).
[9] Prevedel, R. et al. High-speed linear optics quantum computing using active feed-forward. Nature 445, 65-69 (2007).
[10] Grover, L. K. Quantum mechanics helps in search for a needle in a haystack. Phys. Rev. Lett. 79, 325-328 (1997).
[11] Shor, P. W. Algorithms for quantum computation: Discrete logarithms and factoring. Proceedings of the 35th Annual Symposium on Foundations of Computer Science. Los Alamitos: IEEE Computer Society Press 124-134 (1994).

Upcoming Physics Conferences

Here is a selected list of forthcoming conferences in Physics. You are welcome to freely advertise Physics jobs or conferences in 2Physics by sending an email to 2Physics@gmail.com.

May 28-June 22: Theoretical advanced study institute (TASI) in elementary particle physics: "String Universe" (Boulder, Colorado, USA)
June 1-5: Central European workshop on quantum optics, 14th edition (Palermo, Italy)
June 2-9: ICSW 07: Dark Energy (Tehran, Iran)
June 4-7: 6th intl conference on nuclear and radiation physics (Almaty, Kazakhstan)
June 5-9: Annual APS Division of Atomic, Molecular and Optical Physics Meeting (Calgary, Canada)
June 10-13: From Quantum to Cosmos II -- Space-based Research in Fundamental Physics and Quantum Technologies (Bremen, Germany)
June 10-13: Intl conference on quantum information (Rochester, NY)
June 11-15: Quantum to Emergent Gravity (Trieste, Italy)
June 11-22: Summer school on particle physics (Trieste, Italy)
June 11-29: Physics at TeV colliders (Les Houches, France)
June 16-20: 4th intl workshop on quantum chromodynamics - theory and experiment (Bari, Italy)
June 18-20 SciNeGHE07: Fifth Workshop on Science with the New Generation of High Energy Gamma-ray Experiments (Villa Mondragone, Frascati, Rome, Italy)
June 18-22: School on attractor mechanism (Frascati, Italy)
June 18-22: 19th Petrov school -- summer school-seminar on recent problems in theoretical and mathematical physics (Kazan, Russia)
June 26-29: Physics in collision symposium on elementary and astro-particle physics (Annecy, France)
July 2-7: 13th intl symposium on particles, strings and cosmology (London, UK)
July 2-27: ESF school of theoretical physics: string theory and the real world (Les Houches, France)
July 7-10: Vienna symposium on the foundations of modern physics (Vienna, Austria)
July 8-14: 7th Edoardo Amaldi Conference on Gravitational waves (Sydney, Australia)
July 9-13: Cosmology and Strings (Trieste, Italy)
July 13-14: String and M theory approaches to particle physics and cosmology (Florence, Italy)
July 13-17: 'Cosmology and Strings' Workshop (ICTP, Trieste, Italy)
July 22-27: Cosmology, Hereaus School (Bad Honnef, Germany)
July 26-August 1: 15th intl conference on supersymmetry and the unification of fundamental interactions (Karlsruhe, Germany)
July 30-August 4: 25th intl symposium on lattice field theory (Regensburg, Germany)
July 30-August 11: Cosmology and particle physics beyond the standard models (Cargese, France)
August 7-17: Helmholtz intl summer school on nuclear theory and astrophysical applications(Dubna, Russia)
August 13-18: 23rd intl symposium on lepton and photon interactions at high energy (Daegu, Korea)
August 16-18: 11th Paris cosmology colloquium (Paris, France)
August 16-18: Windsor summer school on condensed matter theory: quantum transport and dynamics in nanostructures (Windsor, UK)
August 20-24: Exploring QCD: deconfinement, extreme environments and holography (Cambridge, UK)
August 23-29: 13th Lomonosov conferences on elementary particle physics (Moscow, Russia)
August 27-31: A century of cosmology (Venice, Italy)
August 29-September 1 : Asia-Pacific International Conference on Gravitation and Astrophysics (ICGA8) (Nara, Japan)
September 2-6: Photons, atoms, and qubits (Royal Society, London, UK)
September 2-6: Symposium on advanced methods of quantum chemistry and physics (Torun, Poland)
September 3-7: 3rd intl conference on physics and control (Potsdam, Germany)
September 2-7: Quantum Field Theory (Leipzig, Germany)
September 4-7: Initial Conditions in Cosmology (Wuerzburg, Germany)
September 10-14: Compact Objects (Sao Jose dos Campos, Brazil)
September 10-14: Intl workshop on topological quantum computing (Dublin, Ireland)
September 11-15: 19th intl conference on topics in astroparticle and underground physics (Sendai, Japan)
September 11-14: Recent advances in quantum integrable systems (Annecy-le-Vieux, France)
September 15-22: New trends in high-energy physics (Yalta, Ukraine)
September 17-21: Quantum Field Theory (Leipzig, Germany)
September 17-21: Electron transport in nanosystems (Yalta, Ukraine)
September 24-28: 6th intl Heidelberg conference on dark matter in astro & particle physics(Sydney, Australia)
September 24-28: 4th intl conference on flavor physics (Beijing, China)
October 1-5: Planets to Dark Energy (Manchester,UK)
October 8-14: Advanced string school (Bhubaneswar, India)
October 11-13: Algebra, geometry, and mathematical physics (Göteborg, Sweden)
October 28-November 2: 7th intl conference on complex systems (Boston, MA)
November 4-10: Noise, information and complexity at quantum scale (Erice, Sicily, Italy)
December 4-9: Intl conference on magnetic materials (Kolkata, India)
December 13-18: Topical conference on elementary particles, astrophysics, and cosmology (Miami, Florida)
May 12-16: New paths to quantum gravity (Holbaek Bay, Denmark)
June 15-20: 5th intl conference on new developments in photodetection (Aix-les-Bains, France)
June 16-20: LISA Symposium (Barcelona, Spain)
October 6-11: 18th Intl Spin Physics Symposium (Charlottesville, Virginia, USA)