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2Physics Quote:
"Extraterrestrial material in the form of interstellar dust can also enter the solar system and may be deposited on Earth. Their nucleosynthetic history is locked in its isotopic signatures. Interstellar matter will contain stable isotopes but also freshly produced radionuclides. Thus, the existence of fresh radionuclides in the interstellar medium serves as radioactive clocks for their recent production."
-- Anton Wallner (Read full article: "Radioactive Iron - An Astrophysical Clock for Nucleosynthesis").

Sunday, May 17, 2015

A Current Out Of Fluctuations

(From left to right) Pierre Pfeffer, Fabian Hartmann, Sven Höfling, Martin Kamp, Lukas Worschech.

Authors: Pierre Pfeffer1, Fabian Hartmann1, Sven Höfling1,2, Martin Kamp1, Lukas Worschech1

1Technische Physik, Universität Würzburg, Physikalisches Institut and Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Würzburg, Germany.
2SUPA, School of Physics and Astronomy, University of St. Andrews, United Kingdom.

In a recent experiment [1] we demonstrated the conversion of voltage fluctuations into a directed current on the nanoscale in a system consisting of two Coulomb-coupled quantum dots. The use of external gates allows a control of the current magnitude and a switching of its direction. The work was inspired by theoretical proposals by Sánchez et al. [2] and Sothmann et al. [3]. Possible future applications include energy harvesting as well as local cooling on the nanoscale, both vital to the further development of autonomous and energy-saving electronics.

Since heat and fluctuations are especially challenging factors in miniaturizing electronic circuits [4], the design of heat engines and rectifiers has become a central point of research in nanoelectronics and led to concepts and devices such as Brownian- and Büttiker-Landauer-motors [5,6] phonon rectifiers [7,8] and piezoelectric nanogenerators [9-10]. Despite being diverse concepts, they have in common an interplay of non-linearity, a certain form of symmetry breaking and the presence of fluctuations. For instance, Brownian motors enable a unidirectional particle flow even when there is no net average force in one direction, if the device’s potential is asymmetric. In this context, Sánchez et al. [2] and Sothmann et al. [3] theoretically examined rectifying devices which stand out because of their conception as three terminal devices, which allows a decoupling of the directions of energy flow and charge current.

Figure 1: Illustration of the energy transfer. Coulomb-coupling transfers energy from higher-energy electrons in the lower cavity to lower-energy electrons in the upper one.

The basic operating principle of the Coulomb coupled rectifier is illustrated in Fig. 1. Energy is transferred locally between two Coulomb-coupled cavities. If the electrons in one cavity (lower sphere in Fig. 1, in the following named QDb ) have on average higher energies than the electrons in the other cavity (upper sphere, QDt ), electrons in the first cavity can change into a lower-energy state and transfer the energy difference δE to electrons in the second cavity by means of Coulomb-interaction. No particle exchange takes place between the two cavities.

If QDt is coupled to two reservoirs by means of two quantum point contacts (QPC), a rectified current through the upper cavity occurs if three essential requirements are met: The amount of fluctuations in the upper and lower cavity have to differ, the transmissions of the QPCs have to be energy-dependent, and the two QPCs have to be asymmetric (Fig. 2a). This is for instance possible by applying an external electric field and enables a switching of the current direction.
Figure 2: a) Schematic operating principle. i): QDt is connected to the outer reservoir by two transport barriers. ii): Asymmetric configuration with charge current flow to the right. iii): Asymmetric configuration with charge current flow to the left. b) Electron microscopy image of the sample (side view) together with a schematic of the applied voltages. Vgl and Vgr allow a switching between the conditions displayed in a) ii) and a) iii).

To realize this device, we grew a GaAs/AlGaAs semiconductor heterostructure with a high mobility two-dimensional electron gas by molecular beam epitaxy. Electron beam lithography and dry chemical etching techniques were used to define the structural layout. Fig. 2b shows a side view electron microscopy image of the sample and is overlain with a schematic representation of the QDs indicating the two separate parts of the system (upper subsystem where the resulting current flows in blue and lower subsystem where the fluctuations occur in red). Moreover, the connected voltages are schematically added to the picture. Vgl and Vgr are the gate voltages applied to the upper side gates which allow us to individually manipulate the transmission coefficients of the two QPC’s. For instance, applying a negative voltage to Vgl and a positive one to Vgr opens the left but closes the right QPC and vice versa. In order to keep the central electrostatic potential of QDt constant while asymmetrically controlling the transmission of the left and right QPC we varied the voltages in push-pull configuration with Vgl = -Vgr. The lower side gates remain unused in the presented experiments. The static gate voltage Vgb further allows to tune the transmission of the top current carrying system. A noise voltage Vnoise is supplied to the lower reservoir. All experiments reported here were conducted at 4.2 K in the dark by immersing the sample in liquid helium.
Figure 3: a) Dependence of output current on the voltages applied to the two laterally defined side gates for increasing noise from 7.6 to 144 mV in steps of 15 mV. The current direction and magnitude can be altered by the gate voltage configuration and noise amplitude, respectively. b) Output powers P against the counteractive voltage Vlr for different noise amplitudes increasing from 7.6 to 150 mV in steps of 15 mV. c) Maximum output powers P max versus σnoise.

Fig. 3a shows the rectified current through QDt for different noise amplitudes from 7.6 to 144 mV when changing the gate voltages Vgl and Vgr. Notably, without noise, no current could be measured. Starting with a gate voltage configuration of Vgl = − Vgr = − 2 V and increasing the push-pull voltage initially increases the current from zero to a maximal value which depends on the applied noise amplitude. Thereafter, I decreases again and vanishes completely at around Vgl = 0.01 V (independently on the noise amplitude). A further increase of Vgl changes the current direction. I therefore becomes negative, reaches a minimum and finally increases back to zero again.

In order to harvest useful work from the rectifier, it has to power a load or, equivalently, the generated current has to flow against an applied voltage difference. Consequently, to measure the output power P, a voltage difference Vlr counteracting the current was applied between the channels. The output power is the product of the current and the counteracting applied voltage. Fig. 3b shows the voltage dependent output power for different noise amplitudes. The output power has a parabolic dependency on the applied voltage and vanishes at two particular points: at Vlr = 0 V (maximal current) and at the stopping voltage Vst (no current) which depends on σnoise. Fig. 3c presents the maximum output powers, obtained at half the stopping voltage, depending on the noise amplitude. There, a quadratic dependency on the noise amplitude can be seen with a maximum output power value of Pmax = 24 pW.

In conclusion, we demonstrated a rectification mechanism via Coulomb-coupled quantum dots. The presented findings are a step towards reducing the power consumption of electronic devices and may allow a further miniaturization of electronic circuits and thus pave the way towards sustainable, efficient and autonomous electronics.

[1] F. Hartmann, P. Pfeffer, S. Höfling, M. Kamp, L. Worschech, "Voltage Fluctuation to Current Converter with Coulomb-Coupled Quantum Dots". Physical Review Letters, 114, 146805 (2015). Abstract.
[2] Rafael Sánchez, Markus Büttiker, "Optimal energy quanta to current conversion". Physical Review B, 83, 085428 (2011). Abstract.
[3] Björn Sothmann, Rafael Sánchez, Andrew N. Jordan, Markus Büttiker, "Rectification of thermal fluctuations in a chaotic cavity heat engine". Physical Review B, 85, 205301 (2012). Abstract.
[4] Mehdi Asghari, Ashok V. Krishnamoorthy, "Silicon photonics: Energy-efficient communication". Nature Photonics, 5, 268 (2011). Abstract.
[5] R. Dean Astumian, Peter Hänggi, "Brownian Motors". Physics Today, 55, 33 (2002). Link.
[6] M. Büttiker, "Transport as a consequence of state-dependent diffusion". Zeitschrift für Physik B, 68, 161 (1987). Abstract.
[7] Rolf Landauer, "Motion out of noisy states". Journal of Statistical Physics, 53, 233 (1988). Abstract.
[8] C. W. Chang, D. Okawa, A. Majumdar, A. Zettl, "Solid-State Thermal Rectifier". Science, 314, 1121 (2006). Abstract.
[9] Nan Zeng, Jian-Sheng Wang, "Mechanisms causing thermal rectification: The influence of phonon frequency, asymmetry, and nonlinear interactions". Physical Review B, 78, 024305 (2008). Abstract.
[10] Zhong Lin Wang, Jinhui Song, "Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays", Science, 312, 242 (2006). Abstract.

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Sunday, April 26, 2015

Core-shell Hybrid Nanostructure Based High Performance Supercapacitor Electrode

Ashutosh Kumar Singh (left) and Kalyan Mandal

Authors: Ashutosh Kumar Singh, Kalyan Mandal

Affiliation: Department of Condensed Matter Physics and Material Sciences,
S.N. Bose National Centre for Basic Sciences
, Kolkata, India.


Since last decade, energy crisis has been one of the vital problems in the society due to the excessive use of fossil-fuel resources and environmental pollution. Therefore, the development of very light-weight and environment-friendly proficient energy storage devices has become the priority of the researchers and scientists for satisfying the demand of modern consumer’s hybrid electric and portable electronics devices [1,2]. In this progression, researchers have developed new type of energy storage devices called supercapacitors, also known as electrochemical capacitors which offer high power and eneergy density, high rate capability as well as superb cycle stability as compared to conventional battery and capacitors [3,4].

The supercapacitors are classified in two groups based on their charge storage method. The first group is called pseudocapacitor which involves the redox reactions of the electrode materials at the interface of electrode and electrolyte, whereas the second group is known as electric double layer capacitor which holds charge separation at the interface of electrode and electrolyte [5,6]. By changing the morphology of the electrode materials, one can manipulate the performance of a supercapacitor. The performance and quality of the supercapacitors are very much dependent on the materials and morphology used in the preparation of their electrodes. Recently, many metal oxide based materials (like RuO2, NiO, Fe2O3, MnO2, Co3O4, TiO2, etc.) have been used for the fabrication of pseudocapacitor electrodes; among them NiO and Fe2O3 have been widely used as redox active materials for the fabrication of supercapacitor electrodes of different morphologies, such as Fe2O3-nanotube, Fe2O3-thinfilm, electrospun Co3O4-nanostrtuctures, porous Fe2O3-nanostrtuctures, NiO–nanobelts, NiO–nanoballs, NiO–nanoflowers, NiO–nanoflakes. The reasons behind the extensive use of NiO and Fe2O3 as supercapacitor electrode materials are: they are very stable in nature, they are non-toxic as well as environment friendly and they are very cheap and easily available.


After having so many of supportive properties for being used as electrode materials in supercapacitors, still their reported specific capacitance values are very low compared to their own theoretical specific capacitance value and other metal oxide based electrodes. The only problem restricts them to be used as an electrode material for high performance supercapaitor is their bad electrical conductivity and we all are aware of the fact that electrode material must have high electrical conductivity for high performance supercapacitor.


In the recent development process of supercapacitor performance, it has been found that the electrical conductivity could be improved by introducing impurities via doping of one metal oxide material with other metal oxide material. This doping process enhances the charge movement which affects the reactions at the interface of electrode and electrolyte. So far in the literature we have not found any work based on NiO and Fe2O3 as mixed component transition metal oxides for supercapacitor electrodes. However, keeping all the above research facts in the mind, still there exist a plenty of remarkable opportunities to enhance the electrochemical properties of NiO and Fe2O3 based electrodes.
Unique features of the approach:

Therefore, we report a simple fabrication technique and unique electrochemical properties of the electrode based on core/shell Fe-Ni/Fe2O3-NiO hybrid nanostructures (HNs). This core-shell HNs have very high aspect ratio with a porous thin nanolayer of redox active oxides which would provide a very large surface area for redox reactions at the interface of electrode and electrolyte. This would contribute to the enhancement of the ion and electron movement and performance of the supercapacitor. In addition, the core material consists of conductive FeNi nanowires (NWs) which provides the expressway for the electrons to transport to the current collector via core material.

This would automatically improve the rate capability and power density of the supercapacitor. The electrical conductivity of the electrode could be improved by introducing impurities via doping of one metal oxide (NiO) material with another metal oxide (Fe2O3) material. The unique feature of this electrode fabrication technique is that it doesn’t contain any extra binder material. As a result, there would be enhancement in the charge transfer kinetics [7]. This kind of fabrication technique could be applied in the fabrication process of electrodes of all energy storage devices in general.

Significant Results:

According to our anticipations, the core/shell Fe-Ni/Fe2O3-NiO hybrid nanostructure shows high quality supercapacitive performance in terms of specific capacitance (1415 F/g), energy density (27.6 Wh/kg), power density (10.3 kW/kg), cycling stability (remain 95% of initial specific capacitance after 3000 charge/discharge cycle) and rate capability [7]; these profound results made it a very good and unique alternative for the next generation supercapacitor electrodes.

[1] Patrice Simon, Yuri Gogotsi, "Materials for electrochemical capacitors". Nature Materials, 7, 845 (2008).  Abstract.
[2] John R. Miller, Patrice Simon, Electrochemical Capacitors for Energy Management". Science 321, 651 (2008). Abstract.
[3] Zhibin Lei, Li Lu, X.S. Zhao, "The electrocapacitive properties of graphene oxide reduced by urea". Energy & Environmental Science, 5, 6391 (2012). Abstract.
[4] Sheng Chen, Junwu Zhu, Xiaodong Wu, Qiaofeng Han, Xin Wang, "Graphene Oxide−MnO2 Nanocomposites for Supercapacitors". ACS Nano 4, 2822 (2010). Abstract.
[5] Wei Chen, R.B. Rakhi, Liangbing Hu, Xing Xie, Yi Cui, H.N. Alshareef, "High-Performance Nanostructured Supercapacitors on a Sponge". Nano Letters, 11, 5165 (2011). Abstract.
[6] Raghavan Baby Rakhi, Wei Chen, Dongkyu Cha, H. N. Alshareef, "Nanostructured Ternary Electrodes for Energy-Storage Applications". Advanced Energy Materials, 2, 381 (2012). Abstract.
[7] Ashutosh K. Singh, Kalyan Mandal, "Engineering of high performance supercapacitor electrode based on Fe-Ni/Fe2O3-NiO core/shell hybrid nanostructures". Journal of Applied Physics, 117, 105101 (2015). Abstract.

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Sunday, April 19, 2015

Percolation in Laser Filamentation

Wahb Ettoumi

Author: Wahb Ettoumi
Affiliation: GAP-Biophotonics, University of Geneva, Switzerland.
Other coauthors of the PRL paper: Jérôme Kasparian (left) and Jean-Pierre Wolf.

The discovery of laser filamentation can be attributed to M. Hercher [1], who observed damage tracks along the laser path in crystals. Later, the filamentation phenomenon was shown for a laser propagating in air (For a review, see Ref.[2]). For the first time, the optical power at hand could allow one to witness a new type of light propagation based on the Kerr effect, a non-linear phenomenon which acts as a focusing lens and overcomes the beam natural diffraction. As a consequence, the propagation medium is ionized, and produces a plasma filament of tens of microns wide, which can be sustained over meters in air.

The beam collapse is eventually stopped by this newly created plasma, which acts as a defocusing lens, and counter-balances the Kerr effect. This subtile equilibrium is broken when the energy losses along the propagation cause the Kerr effect to be negligible again, and the beam finally diffracts.

Image 1

For powers largely exceeding the critical power needed for the observation of a single filament, the initial beam inhomogeneities seed the emergence of many single filaments, as if many small beamlets were each undergoing filamentation. In 2010, an experimental campaign in Dresden [3] was aimed at characterizing the number of filaments with respect to the initial power (Image 1). However, we only noticed until recently the similarity between the laser burns obtained there on photographic paper and the numerical simulations of systems relevant to the statistical physics community. More particularly, we decided to probe the resemblance of the experimental recordings with percolation patterns.

Initially, the laser beam exhibits a noisy profile, but with rather small fluctuations around an average fluence. As the laser propagates, the Kerr effect drives the light to concentrate more and more around the peaks of the highest amplitude, leading to the clustering of light into islands of different sizes, each one potentially holding one or multiple filaments.

Image 2

At larger distances, typically of several meters in usual experimental setups, the energy flux towards the inner cores of the multiple filaments causes the fluence islands to shrink in size, destroying the previously well held light clusters into smaller, disconnected parts (Image 2). At higher distances, the losses due to the medium's absorption eventually wipe out the smallest clusters.

Because of the lack of experimental data, we turned to the numerical simulation of the non-linear Schrödinger equation, well-known for its remarkable agreement with real filamentation experiments. We showed [4] that the precise way light clusters depending to each other is a phase transition: we measured a set of seven critical exponents governing the pattern dynamics at the vicinity of the transition between a fully connected state and a non-connected one. The similarity with the percolation universality class is striking, but the clusters' size distribution in the laser case exhibits a finite cut-off physically associated to fluence islands withholding a single filament (their area is approx. 2 mm2).

An interesting issue subsists, however. The finite-size scaling techniques we used are intrinsically equilibrium methods, so that we implicitely assumed that each slice during the laser propagation could be treated as a statistical equilibrium of a given system. But the laser obviously evolves in time, and is not trapped into a quasi-stationary state, nor a fluctuating equilibrium. A hand waving argument can be drawn by saying that the evolution is quasi-static, but a correct theoretical argument remains to be found.

[1] M. Hercher, "Laser-induced damage in transparent media". Journal of Optical Society of America, 54, 563 (1964).
[2] A. Couairon, A. Mysyrowicz, "Femtosecond filamentation in transparent media". Physics Report, 441, 47-189 (2007). Abstract.
[3] S. Henin, Y. Petit, J. Kasparian, J.-P. Wolf, A. Jochmann, S. D. Kraft, S. Bock, U. Schramm, R. Sauerbrey, W. M. Nakaema, K. Stelmaszczyk, P. Rohwetter, L. Wöste, C.-L. Soulez, S. Mauger, L. Bergé, S. Skupin, "Saturation of the filament density of ultrashort intense laser pulses in air". Applied Physics B, 100, 77 (2010). Abstract.
[4] W. Ettoumi, J. Kasparian, J.-P. Wolf, "Laser Filamentation as a New Phase Transition Universality Class". Physical Review Letters, 114, 063903 (2015). Abstract.


Sunday, April 12, 2015

Study of Fractal Behavior of Cell Surface Gives a Hint to a New Way of Attack on Cancer

Igor Sokolov (left) and Maxim E. Dokukin

Authors: Igor Sokolov1,2.3, Maxim E. Dokukin1

1Department of Mechanical Engineering, Tufts University, Medford, MA, USA
2Department of Biomedical Engineering, Tufts University, Medford, MA, USA
3Department of Physics, Tufts University, Medford, MA, USA.

Fractal is one of the most intriguing geometry in nature. Fractal surface keeps repeating itself through different scales. Fractal typically occurs from chaos or far from equilibrium processes (which are actually quite similar to chaos as well) [1]. Sedivy and Mader were two of the first scientists who published -- back in 1997 -- a hypothesis about possible connection between cancer and fractal [2]. The reason behind this idea was in the observation that cancer typically behaves rather randomly and chaotically. Cancer-specific fractal behavior of tumors at the macroscale was found when analyzing the tumor perimeters [3, 4]. Similar behavior was found in the analysis of geometry of vascular system in tumors (antiangiogenesis) [5, 6]. However, when going down to the smaller scale of single cells, the analysis did not show the expected transition to fractal behavior when cells become cancerous.

Both cancer and normal cells demonstrated almost ideal fractal behavior [7, 8], although with different fractal dimension (the degree of roughness of the fractal surface). More precise measurements done with the help of atomic force microscopy (AFM) [9] did show a better segregation between cancer and normal cells based on the use of fractal dimension. The issue is that fractal dimension can be calculated for any surface, no matter whether or not it’s fractal. Although we noticed that cancer cells seemed to be closer to ideal fractals than normal, there was no actual analysis done of how close the cell surfaces were to fractal.

In our recently published paper [10], we finally did study the question of how good the fractal approximation is for cancer and normal cells. Specifically, we looked at human cervical epithelial cells. The cells were so-called primary cells, which were derived from cervical tissue of either healthy humans or tumors of cancer patients. In addition to cancer and normal cells, we added an intermediate stage of cell progression towards cancer, immortal or precancerous cells (these cells were genetically mutated normal cells). Furthermore, we looked at cells at different number of cell population doubling (cell passage, or cell age). This is important because there are various evidence that immortal cells start demonstrating malignant behavior with the increase of population doubling. Similarly, cancerous cells increase their aggressiveness with the number of cell divisions. So, we assumed that the cells are aligned towards cancer with the number of their population doubling.

To see how close the cell surface is to fractal, we need to describe how fractal is defined mathematically. To find if the surface is fractal or not, one needs to calculate so-called self-correlation function, and see how this function depends on the size of the surface features [11]. Sometimes it is easier to calculate the Fourier transform of the surface as a function of the inverse (reciprocal) feature size. A simple power dependence of these functions is the definitive property of fractal. The deviation from that simple power dependence is the deviation from fractal.

Figure 1 shows typical examples of fractal surfaces as well as the image of a cell of study; its zoomed image is recorded with AFM. The magnitude of the Fourier transform as a function of the reciprocal feature size is shown in Fig.1 at the top right column. The power law in this log-log scale should be a straight line. One can see that normal and cancer cells can be approximated by two straight lines rather than one. The immortal cells of this example demonstrate the behavior closest to a straight line, i.e., fractal. To characterize the deviation from fractal, we introduced a new surface parameter, multi-fractality. The multi-fractality is the difference between the two slopes of the Fourier magnitude shown in Fig. 1. If multi-fractality is zero the cell surface is fractal.
Figure 1 (To view with higher resolution, click on the image): Examples of fractal objects (right column). The middle column: An actual image of a cell obtained by means of scanning electron microscope (SEM) and a high-resolution image obtained by an atomic force microscope (AFM). The right column: (Top) The definition of fractal as the straight line of the magnitude versus reciprocal size plotted in log-log scale; (Bottom) Dependence of the multi-fractality parameter on the stage of progression towards cancer. Fractal is reached when the multi-fractality parameter is zero.

Figure 1 (the bottom right column) shows essentially the main result of the work, the dependence of the multi-fractality parameter on the stage of progression towards cancer, from normal through immortal to malignant behavior. Within each cell type, the results are ordered by the number of population doubling. One can see that contrary to the previous expectations, cancer is not a pure fractal. Moreover, its behavior deviates from fractal further with the cancer progression. It seems that the ideal fractal (zero multi-fractality) is reached at a particular moment of transition between precancerous (immortal) cells to cancer.

Based on these results, we can speculate that it votes in favor of theories which consider cancerous cells as another state of cell functioning which is pretty deterministic rather than chaotic. Moreover, it is known that the development of chaotic behavior is typically associated with some bifurcation points. Having effective influences on such points, one could thus prevent chaos, and maybe cancer, from development. Thus, the search of such bifurcation points in the biochemical pathways responsible for the morphology of the cell surface, and effectively influencing those bifurcation points might be a new way to attack on cancer.

We gratefully acknowledge partial funding for this work by Tufts Collaborates! Grant an NSF CMMI-1435655 (IS).

[1] Joseph L. McCauley, "Chaos, dynamics, and fractals : an algorithmic approach to deterministic chaos". Cambridge nonlinear science series 2, xxi, 323 p. (Cambridge University Press, 1993). 
[2] Roland Sedivy, Robert M. Mader, "Fractals, chaos, and cancer: do they coincide?" Cancer investigation, 15, 601 (1997). Abstract.
[3] G.A. Losa, et al.  "Fractals in biology and medicine" Volume. Mathematics and biosciences in interaction. Vol. IV. (Basel ; Boston: Birkhäuser, 2005).
[4] A. Mashiah, O. Wolach, J. Sandbank, O. Uziel, P. Raanani, M. Lahav, "Lymphoma and leukemia cells possess fractal dimensions that correlate with their biological features". Acta Haematologica, 119, 142 (2008). Abstract.
[5] Joanne R. Less, Thomas C. Skalak, Eva M. Sevick, Rakesh K. Jain, "Microvascular architecture in a mammary carcinoma: branching patterns and vessel dimensions". Cancer Research, 51, 265 (1991). Full Article.
[6] James W. Baish, Rakesh K. Jain, "Fractals and cancer". Cancer Research, 60, 3683 (2000). Abstract.
[7] Willem Jan Meerding, Heleen Doornewaard, Marjolein van Ballegooijen, Anita Bos, Yolanda van der Graaf, Jan G. van den Tweel, Yvonne T. van der Schouw, J. Dik F. Habbema, "Cost analysis of PAPNET-assisted vs. conventional Pap smear evaluation in primary screening of cervical smears". Acta Cytologica, 45, 28-35 (2001). Abstract.
[8] Gérald E. Piérard, Nadia Ezzine-Sebai, Bécima Fazaa, Nazli Nikkels-Tassoudji, Claudine Piérard-Franchimont, "Karyometry of malignant melanoma cells present in skin strippings". Skin Research and Technology, 1, 177-179 (1995). Abstract.
[9] M. E. Dokukin, N. V. Guz, R. M. Gaikwad, C. D. Woodworth, I. Sokolov, "Cell surface as a fractal: normal and cancerous cervical cells demonstrate different fractal behavior of surface adhesion maps at the nanoscale". Physical Review Letters, 107, 028101 (2011). Abstract.
[10] M E Dokukin, N V Guz, C D Woodworth, I Sokolov, "Emerging of fractal geometry on surface of human cervical epithelial cells during progression towards cancer". New Journal of Physics, 17, 033019 (2015). Full Article.
[11] Benoit B. Mandelbrot, "Is nature fractal?" Science, 279(5352), 783c (1998). DOI:10.1126/science.279.5352.783c.

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Sunday, April 05, 2015

Efficient Photon Collection from a Nitrogen Vacancy Center in a Circular Bullseye Grating in Diamond

[From left to right] Luozhou Li, Edward Chen and Dirk Englund.

Authors: Luozhou Li, Edward Chen, Dirk Englund

Affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, USA.

Link to Quantum Photonics Laboratory >>

The nitrogen-vacancy center (NV) [1] behaves much like an atom trapped in the diamond lattice. Because of the high band gap and the mostly spin-free composition of the diamond host, the NV is well isolated from the environment, so it shows well-behaved atom-like properties. Most importantly, it’s possible to optically prepare and measure the long-lived states of the associated electron and nuclear spins. NVs are potentially promising building blocks for a large-scale quantum network where the optical addressability of the NV allows flying qubits, or photons, to connect nodes of this network together. One of the fundamental bottlenecks for this to be made into a reality is the flux of photons collected from an NV, which determines how quickly the NV’s spin state can be measured and compared: the more fluorescent photons that are collected, the faster new connections can be made. The same photon collection limitation is also true for using the NV as a highly sensitive quantum sensor, where the sensitivity to electric, magnetic and temperature fields increase with increased photon collection. Thus, higher photon detection of the NV’s photoluminescence is of central importance to many NV quantum technologies, such as communication, computing, and even sensing.

In our recent work [2], we introduce a circular “bullseye” grating in diamond (Figure 1), which enables record-high photon collection from the nitrogen-vacancy (NV) color center. The bullseye grating consists of concentric slits etched into a diamond membrane [3], which itself is about half of a wavelength in thickness. The grating period satisfies the second-order Bragg condition, giving rise to the scattering of light out of the membrane. The scattered light from each grating interferes constructively out of the plane and into the far field, thereby enabling significantly higher collection efficiency. With this circular grating, we have shown that it’s possible to collect about an order of magnitude more fluorescence than is possible from an NV in un-patterned diamond.
Figure 1: (a) Illustration of an array of diamond bullseye gratings adjacent to a microwave strip line. (b) Schematic of the circular grating. ‘a’ denotes the lattice constant and ‘gap’ the air spacing between circular gratings. (c) Simulated electric field intensity (log scale) in the x = 0 plane with air above and glass below the diamond. A dipole emitter was placed in the center of the bullseye grating, and was oriented along the horizontal direction.

Achieving higher collection efficiency from the NV impacts several applications such as improved sensing of static or dynamic electromagnetic fields just outside the diamond, higher luminosity room-temperature single photon sources, and better quantum memories for quantum computing and networking. For example, NV researchers [4] have recently shown that the NV is even sensitive to changes of single proton spins, paving the way for magnetic resonance imaging of individual molecules in liquid — and this application would be improved by better fluorescence collection from the NV.

The efficient photon collection should allow for a range of new measurements, such as non-demolition measurements of NV spins — i.e., you could make a measurement and then act back on the NV spin state. We’re also using the efficient collection for medium-scale quantum registers, which would contain on the order of tens of qubits each, and for quantum sensing.

[1] Marcus W. Doherty, Neil B. Manson, Paul Delaney, Fedor Jelezko, Jörg Wrachtrup, Lloyd CL Hollenberg, "The nitrogen-vacancy colour centre in diamond." Physics Reports, 528, 1-45 (2013). Abstract.
[2] Luozhou Li, Edward H. Chen, Jiabao Zheng, Sara L. Mouradian, Florian Dolde, Tim Schröder, Sinan Karaveli, Matthew L. Markham, Daniel J. Twitchen, and Dirk Englund, "Efficient photon collection from a nitrogen vacancy center in a circular bullseye grating." Nano letters, 15, 1493 (2015). Abstract.
[3] Luozhou Li, Igal Bayn, Ming Lu, Chang-Yong Nam, Tim Schröder, Aaron Stein, Nicholas C. Harris, Dirk Englund. "Nanofabrication on unconventional substrates using transferred hard masks." Scientific reports, 5, Article number 7802 (2015). Article.
[4] A. O. Sushkov, I. Lovchinsky, N. Chisholm, R. L. Walsworth, H. Park, M. D. Lukin, "Magnetic resonance detection of individual proton spins using quantum reporters." Physical Review Letters, 113, 197601 (2014). Abstract.

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Sunday, March 29, 2015

Photo-activated Biological Processes As Quantum Measurements

Birgitta Whaley (left) and Atac Imamoglu

Authors: Atac Imamoglu1, Birgitta Whaley2

1Institute of Quantum Electronics, ETH Zurich, Switzerland,
2Berkeley Quantum Information and Computation Center, Department of Chemistry, University of California, Berkeley, USA.

Image credit: Ilya Sinayskiy
Our current understanding of the physical world around us is based on quantum mechanics. It is natural in this framework to argue that at the molecular level, biological processes are also governed by the laws of quantum mechanics. This reasoning in turn implies that at short enough length and/or time scales the dynamics can exhibit counter-intuitive features, such as the molecule/system being in a coherent superposition of distinguishable states. A key question of fundamental interest is whether there are biological systems for which these microscopic dynamical quantum features help or enable the biological function [1-3]. Two processes that have been extensively studied in this context are light harvesting in photosynthesis and sensing of the inclination of the earth’s magnetic field by migrating birds.

Past 2Physics article by Atac Imamoglu :
December 23, 2012: "Observation of Entanglement Between a Quantum Dot Spin and a Single Photon" by Wei-bo Gao, Parisa Fallahi, Emre Togan, Javier Miguel-Sanchez, Atac Imamoglu.

Our work brings a new perspective to the analysis of these processes by embedding them in a quantum measurement context, where the biological system is modelled as a measurement device that is subject to the laws of quantum mechanics (i.e., a quantum meter) [4]. The function of this quantum meter is to measure an external classical stimulus, which is thereby equivalent to the biological sensing of this stimulus. We have analyzed several photo-activated biological processes within this formulation and find that these processes fall into two distinct classes.

In the first category, the measurement interaction induces changes in the system state at a rate that is proportional to the strength of the external stimulus. In this case, we find that while the presence of quantum coherence during the measurement interaction may result in a small enhancement of the rate that increases at most linearly with the increasing coherence time, it is however not essential for the biological function that results from the sensing of this stimulus.

By contrast, in the second category, the measurement interaction does not directly lead to an excitation rate that is proportional to the strength of the external stimulus. Instead, it is the quantum coherent evolution after the optical excitation that controls the sensitivity of the biological system to the stimulus. Most importantly, in this category, unless there is some quantum coherent dynamics after the photoactivation, there is vanishing sensitivity to the signal to be measured. Another difference is that depending on the specific nature of this coherent evolution, more detailed information about the signal than just its strength can be transmitted to the biological receptors.

The extensively studied process of photosynthesis [see, e.g., 1-3, 5-7] as well as the process of human vision [8,9] both belong to the first category. In contrast, the proposed hypothesis of a radical pair mechanism [10,11] for sensing of the inclination of the earth’s magnetic field by migratory birds belongs to the second category. An essential component of this mechanism is the coherent oscillation between singlet and triplet radical pairs in which the paired electrons are separated by several nanometers and are thus formally entangled over non-trivial distances. The chemical reactivity of the radical pair is different in the singlet and triplet states, resulting in a chemical signature of the non-equilibrium quantum dynamics induced by the quantum coherent dynamics.

While much indirect chemical evidence exists for this hypothesis, experimental validation in birds is challenging and, despite many plausibility arguments, no clear evidence for the validity of this hypothesis in migratory birds has been established thus far. It therefore remains an intriguing and open question today, as to whether there are biological sensing processes that can function only if quantum coherence is preserved on some extended time scale.

[1] Graham R. Fleming, Gregory D. Scholes, Yuan-Chung Cheng, “Quantum effects in biology”, Procedia Chemistry, 3, 38 (2011). Abstract.
[2] Neill Lambert, Yueh-Nan Chen, Yuan-Chung Cheng, Che-Ming Li, Guang-Yin Chen, Franco Nori, “Quantum biology”, Nature Physics, 9, 10 (2013). Abstract.
[3] M. Mohseni, Y. Omar, G. Engel, M. Plenio (Eds.), "Quantum effects in biology" (Cambridge University Press, 2014). 
[4] A. Imamoglu, K. B. Whaley, “Photo-activated biological processes as quantum measurements”, Physical Review E, 91,022714 (2015). Abstract.
[5] Rienk van Grondelle, Vladimir I. Novoderezhkin, “Quantum effects in photosynthesis”, Procedia Chemistry, 3, 198 (2011). Abstract.
[6] Konstantin E. Dorfman, Dmitri V. Voronine, Shaul Mukamel, Marlan O. Scully, “Photosynthetic reaction center as a quantum heat engine”, Proceedings of the National Academy of Sciences of USA, 110, 2746 (2013). Abstract.
[7] Aurélia Chenu, Gregory D. Scholes, “Coherence in energy transfer and photosynthesis”, Annual Review of Physical Chemistry, 66, 69 (2015). Abstract.
[8] F. Rieke, D. A. Baylor, “Single-photon detection by rod cells of the retina”, Review of Modern Physics, 70, 1027 (1998). Abstract.
[9] Philipp Kukura, David W. McCamant, Sangwoon Yoon, Daniel B. Wandschneider, Richard A. Mathies,”Structural observation of the primary isomerization in vision with femtosecond-stimulated Raman”, Science, 310, 1006 (2005). Abstract.
[10] Thorsten Ritz, Salih Adem, Klaus Schulten, “A model for photoreceptor-based magnetoreception in birds”, Biophysical Journal, 78, 707 (2000). Full Article.
[11] Kiminori Maeda, Alexander J. Robinson, Kevin B. Henbest, Hannah J. Hogben, Till Biskup, Margaret Ahmad, Erik Schleicher, Stefan Weber, Christiane R. Timmel, P.J. Hore, “Magnetically sensitive light-induced reactions in cryptochrome are consistent with its proposed role as a magnetoreceptor”, Proceedings of the National Academy of Sciences, 109, 4774 (2012). Abstract.

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Sunday, March 22, 2015

Quantum Teleportation of Multiple Properties of A Single Quantum Particle

Jian-Wei Pan (left) and Chao-Yang Lu

Authors: Chao-Yang Lu, Jian-Wei Pan

CAS Centre for Excellence and Synergetic Innovation Centre in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, China.

The science fiction dream of teleportation [1] is to transport an object by disintegrating in one place and reappearing intact in another distant location. If only classical information is of interest, or if the object could be fully characterized by classical information—which can in principle be precisely measured—the object can be perfectly reconstructed (copied) remotely from the measurement results. However, for microscopic quantum systems such as single electrons, atoms or molecules, their properties are described by quantum wave functions that can be in superposition states. Perfect measurement or cloning of the unknown quantum states is forbidden by the law of quantum mechanics.

In 1993, Bennett et al. [2] proposed a quantum teleportation scheme to get around this roadblock. Provided with a classical communication channel and shared entangled states as a quantum channel, quantum teleportation allows the transfer of arbitrary unknown quantum states from a sender to a spatially distant receiver, without actual transmission of the object itself. Quantum teleportation has attracted a lot of attention not only from the quantum physics community as a key element in long-distance quantum communication, distributed quantum networks and quantum computation, but also the general audience, probably because of its connection to the scientific fiction dream in Star Trek. An interesting question is frequently asked: “would it be possible in the future to teleport a large object, say a human?” Before attempting to seriously answer that question, let us take steps back, look at where we actually are and think about a much, much easier and fundamental question: have we teleported multiple, or all degrees of freedom (DOFs) that fully describe a single particle, thus truly teleporting it intact? The answer is NO.

Past 2Physics article by Chao-Yang Lu and/or Jian-Wei Pan :

January 04, 2015: "Achieving 200 km of Measurement-device-independent Quantum Key Distribution with High Secure Key Rate" by Yan-Lin Tang, Hua-Lei Yin, Si-Jing Chen, Yang Liu, Wei-Jun Zhang, Xiao Jiang, Lu Zhang, Jian Wang, Li-Xing You, Jian-Yu Guan, Dong-Xu Yang, Zhen Wang, Hao Liang, Zhen Zhang, Nan Zhou, Xiongfeng Ma, Teng-Yun Chen, Qiang Zhang, Jian-Wei Pan

June 30, 2013: "Quantum Computer Runs The Most Practically Useful Quantum Algorithm" by Chao-Yang Lu and Jian-Wei Pan.

Although extensive efforts have been undertaken in the experimental demonstrations of teleportation in various physical systems, including photons [3], atoms [4], ions [5], electrons [6], and superconducting circuits [7], all the previous experiments shared one fundamental limitation: the teleportation only transferred one degree of freedom (DOF). This is insufficient for complete teleportation of an object, which could naturally possess many DOFs. Even in the simplest case, for example, a single photon, the elementary quanta of electromagnetic radiation, has intrinsic properties including its frequency, momentum, polarization and orbital angular momentum. A hydrogen atom—the simplest atom—has principle quantum number, spin and orbital angular momentum of its electron and nuclear, and various couplings between these DOFs which can result in hybrid entangled quantum states.

Complete teleportation of an object would require all the information in various DOFs are transferred at a distance. Quantum teleportation is a linear operation applied to the quantum states, thus teleporting multiple DOFs should be possible in theory. Experimentally, however, it poses significant challenges in coherently controlling multiple quantum bits (qubits) and DOFs. Hyper-entangled states—simultaneous entanglement among multiple DOFs—are required as the nonlocal quantum channel for teleportation. Moreover, the teleportation also necessitates unambiguous discrimination of hyper-entangled Bell-like states from a total number of 4N (N is the number of the DOFs). Bell-state measurements would normally require coherent interactions between independent qubits, which can become more difficult with multiple DOFs, as it is necessary to measure one DOF without disturbing another one. With linear operations only, previous theoretical work has suggested that it was impossible to discriminate the hyper-entangled states unambiguously.

We have taken a first step toward simultaneously teleporting multiple properties of a single quantum particle [8]. In the experiment, we teleport the composite quantum states of a single photon encoded in both the polarization—spin angular momentum (SAM) — and the orbital angular momentum (OAM). We prepare hyper-entangled states in both DOFs as the quantum channel for teleportation. By exploiting quantum non-demolition measurement, we overcome the conventional wisdom to unambiguously discriminate one hyper-entangled state out of the 16 possibilities. We verify the teleportation for both spin-orbit product states and entangled state of a single photon, and achieve an overall fidelity of 0.63 that well exceeds the classical limit.
Figure 1: Scheme for quantum teleportation of the spin-orbit composite states of a single photon. Alice wishes to teleport to Bob the quantum state of a single photon 1 encoded in both its SAM and OAM. To do so, Alice and Bob need to share a hyper-entangled photon pair 2-3. Alice then carries out an h-BSM assisted by teleportation-based QND measurement with an ancillary entangled photon pair.

Figure 1 illustrates our linear optical scheme for teleporting the spin-orbit composite state. The h-BSM is implemented in a step-by-step manner. First, the two photons, 1 and 2, are sent through a polarizing beam splitter (PBS). Secondly, the two single photons out of the PBS are superposed on a beam splitter (BS, see Fig.1a). Only the asymmetric Bell state will lead to a coincidence detection where there is one and only one photon in each output, whereas for the three other symmetric Bell state, the two input photons will coalesce to a single output mode. In total, these two steps would allow an unambiguous discrimination of the two hyper-entangled Bell states. To connect these two interferometers, we exploit quantum non-demolition (QND) measurement—seeing a single photon without destroying it and keeping its quantum information intact. Interestingly, quantum teleportation itself can be used for probabilistic QND detection. As shown in Fig.1 left inset, another pair of photons entangled in OAM is used as ancillary. The QND is a standard teleportation itself.
Figure 2: Experimental setup for teleporting multiple properties of a single photon. Passing a femtosecond pulsed laser through three type-I β-barium borate (BBO) crystals generates three photon pairs, engineered in different forms. The h-BSM for the photons 1 and 2 are performed in three steps: (1) SAM BSM; (2) QND measurement; (3) OAM BSM.

Figure 2 shows the experimental setup for the realization of quantum teleportation of the spin-orbit composite state of a single photon. We prepare five different initial states to be teleported (see Fig. 3 left inset), which can be grouped into three categories: product states of the two DoFs in the computational basis, products states of the two DoFs in the superposition basis, and a spin-orbit hybrid entangled state. To evaluate the performance of the teleportation, we measure the teleported state fidelity
defined as the overlap of the ideal teleported state (|φ >) and the measured density matrix. The teleportation fidelities for |φ >A, |φ >B, |φ >C, |φ >D and |φ >E yield 0.68±0.04, 0.66±0.04, 0.62±0.04, 0.63±0.04, and 0.57±0.02, respectively. Despite these experimental noise, the measured fidelities of the five teleported states are all well beyond 0.40—the classical limit, defined as the optimal state estimation fidelity on a single copy of a two-qubit system. These results prove the successful realization of quantum teleportation of the spin-orbit composite state of a single photon. Furthermore, for the entangled state |φ >E, we emphasize that the teleportation fidelity exceeds the threshold of 0.5 for proving the presence of entanglement, which demonstrates that the hybrid entanglement of different DoFs inside a quantum particle can preserve after the teleportation.
Figure 3: Experimental results for quantum teleportation of spin-orbit entanglement of a single photon. The fidelities are above the classical limit and entanglement limit.

Our methods can in principle be generalized to more DOFs, for instance, involving the photon’s momentum, time and frequency. The efficiency of teleportation can be enhanced by using more ancillary entangled photons, quantum encoding, embedded teleportation tricks, and high-efficiency single-photon detectors. The multi-DOF teleportation protocol is by no means limited to this system, but can also be applied to other quantum systems such as trapped electrons, atoms, and ions, which can be expected to be tested in the near future. Besides the fundamental interest, the developed methods in this work on the manipulation of quantum states of multiple DOFs will open up new possibility in quantum technologies.

[1] Anton Zeilinger, "Quantum teleportation". Scientific American, 13, 34–43 (2003). Link.
[2] Charles H. Bennett, Gilles Brassard, Claude Crépeau, Richard Jozsa, Asher Peres, William K. Wootters, "Teleporting an unknown quantum state via dual classic and Einstein-Podolsky-Rosen channels". Physical Review Letters, 70, 1895–1899 (1993). Abstract.
[3] Dik Bouwmeester, Jian-Wei Pan, Klaus Mattle, Manfred Eibl, Harald Weinfurter, Anton Zeilinger, "Experimental quantum teleportation". Nature, 390, 575–579 (1997). Abstract.
[4] Xiao-Hui Bao, Xiao-Fan Xu, Che-Ming Li, Zhen-Sheng Yuan, Chao-Yang Lu, Jian-Wei Pan, "Quantum teleportation between remote atomic-ensemble quantum memories", Proceedings of the National Academy of Sciences of the USA, 109, 20347–20351 (2012). Abstract.
[5] M. D. Barrett, J. Chiaverini, T. Schaetz, J. Britton, W. M. Itano, J. D. Jost, E. Knill, C. Langer, D. Leibfried, R. Ozeri, D. J. Wineland, "Deterministic quantum teleportation of atomic qubits". Nature, 429, 737–739 (2004). Abstract.
[6] W. Pfaff, B. J. Hensen, H. Bernien, S. B. van Dam, M. S. Blok, T. H. Taminiau, M. J. Tiggelman, R. N. Schouten, M. Markham, D. J. Twitchen, R. Hanson, "Unconditional quantum teleportation between distant solid-state quantum bits". Science, 345, 532–535 (2014). Abstract.
[7] L. Steffen, Y. Salathe, M. Oppliger, P. Kurpiers, M. Baur, C. Lang, C. Eichler, G. Puebla-Hellmann, A. Fedorov, A. Wallraff, "Deterministic quantum teleportation with feed-forward in a solid state system". Nature. 500, 319–322 (2013). Abstract.
[8] Xi-Lin Wang, Xin-Dong Cai, Zu-En Su, Ming-Cheng Chen, Dian Wu, Li Li, Nai-Le Liu, Chao-Yang Lu, Jian-Wei Pan, "Quantum teleportation of multiple degrees of freedom of a single photon". Nature, 518, 516-519 (2015). Abstract.