.comment-link {margin-left:.6em;}

2Physics

2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team

(Read Full Article: "Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko"
)

Sunday, April 15, 2012

How Quantum Physics Could Make 'The Matrix' More Efficient

Mile Gu of CQT, Singapore

Researchers have discovered a new way in which computers based on quantum physics could beat the performance of classical computers. The work -- by researchers from Centre for Quantum Technologies (CQT), Singapore and University of Bristol, UK -- implies that a 'The Matrix'-like simulation of reality [1] would require less memory on a quantum computer than on a classical computer. It also hints at a way to investigate whether a deeper theory lies beneath the quantum theory. The finding is published in Nature Communications dated March 27th, 2012 [2].


Karoline Wiesner of the University of Bristol, UK -->


The finding emerges from fundamental consideration of how much information is needed to predict the future. Mile Gu, Elisabeth Rieper and Vlatko Vedral at CQT, with Karoline Wiesner from the University of Bristol, UK, considered the simulation of 'stochastic' processes (see Figure 1), where there are several possible outcomes to a given procedure, each occurring with a calculable probability. Many phenomena, from stock market movements to the diffusion of gases, can be modeled as stochastic processes.


















Figure 1: A simulator for a Stochastic Process can be thought of a physical system that stores select information about past outputs, and uses them to generate the require statistics for the future. Ideally, we want to construct a simulator that is as simple as possible, such that the amount of information it requires about the past is minimized.


The details of how to simulate such processes have long occupied researchers. The minimum amount of information required to simulate a given stochastic process is a significant topic of study in the field of complexity theory. In scientific literature of that field, it is referred to as 'statistical complexity' [3].

Elisabeth Rieper of CQT, Singapore -->

Researchers know how to calculate the amount of information transferred inherently in any stochastic process, a quantity known as the excess entropy. Theoretically, this sets the lowest amount of information needed to simulate the process. In reality, however, classical simulations of stochastic processes require more storage than this.

Mile, Karoline, Elisabeth and Vlatko showed that quantum simulators need to store less information than the optimal classical simulators. This was done by isolating the source of inefficiency within classical simulators. When we attempt to simulate any stochastic process that has a binary property ‘X’ , which affects the future evolution of the process, then the value ‘X’ must be stored. This is unavoidable; even when all future observations of the process do not guarantee that one can deduce the value of ‘X’. The researchers showed that this implied that classical simulators erase information; they contain a source of irreversibly that cannot be removed.

Quantum simulators, however, have greater potential freedom. Instead of allocating a full bit to store the value of ‘X’, one can store the conditions ‘X= 0’ and ‘X=1’ in non-orthogonal states. Consequently, the simulator saves memory, as it was never sure what state the property was in the first place. Nevertheless, Mile et al showed that it is often possible to engineer dynamics such that the simulator can still replicate the dynamics of our desired reality.

Vlatko Vedral of CQT, Singapore

This result has significant implications. Stochastic processes play a ubiquitous role in the modeling of dynamical systems that permeate quantitative science, from climate fluctuations to chemical reaction processes. Classically, the statistical complexity is employed as a measure of how much structure a given process exhibits. The rationale is that the optimal simulator of such a process requires at least this much memory. The fact that this memory can be reduced quantum mechanically implies the counter-intuitive conclusion that quantizing such simulators can reduce their complexity beyond this classical bound, even if the process they're simulating is purely classical. Many organisms and devices operate based on the ability to predict and thus react to the environment around them. The possibility of exploiting quantum dynamics to make identical predictions with less memory implies that such systems need not be as complex as one originally thought.






















Figure 2: The researchers calculated the information-storage requirements for a set of stochastic models describing a perturbed coin - a coin that flips at each time-step with some probability p. The figure shows how a classical simulation (a) compares to a quantum simulation (b) and the expected ideal (c) for different probabilities p. The quantum model is closer to the ideal than classical, but there's still a gap.

What surprised the researchers is that the quantum simulations are still not as efficient as they could be: they still have to store more information than the process would seem to need (see figure 2). Could an even more general probability theory -- with even more bizarre correlations -- side-step this restriction?

"What's fascinating to us is that there is still a gap. It makes you think, maybe here's a way of thinking about a theory beyond quantum physics," says Vlatko.

Reference
[1] The Matrix (1999 American science fiction action film written and directed by Larry and Andy Wachowski). Wikipedia Page.
[2] Mile Gu, Karoline Wiesner, Elisabeth Rieper and Vlatko Vedral, "Quantum mechanics can reduce the complexity of classical models." Nature Communications 3, 762 (2012). Abstract. arXiv:1102.1994.
[3] Cosma Rohilla Shalizi and James P. Crutchfield, "Computational mechanics: pattern and prediction, structure and simplicity." Journal of Statistical Physics, 104, 817–879 (2001). Abstract.

Labels: ,


Sunday, April 08, 2012

Strong Coupling and Its Dynamic Control of Distant Nanocavities

Susumu Noda (left) and Yoshiya Sato (right)



Authors: Susumu Noda and Yoshiya Sato

Affiliation: Department of Electronic Science and Engineering, Kyoto University, Japan

Noda's Quantum Optoelectronics Laboratory >>


The dynamic manipulation of photons in nanostructures is essential for various applications including advanced photonic circuits, stopping light, and quantum information processing. In particular, the formation and dynamic control of a coupled state among on-chip, photonic nanoelements at arbitrary positions should have great impact on these directions. However, when we couple individual nanocavities, they must be placed in close proximity since the light is confined so tightly in each cavity that their evanescent fields extends only a few microns. With this limitation, a nanocavity can only couple to adjacent nanocavities, which restricts the architecture of the system and makes it difficult to achieve on-demand dynamic control of the coupled states.

In our recent work, we have obtained strong coupling between nanocavities even though they are separated by a large distance (> 80 μm) and achieve dynamic control over their coupling to freeze the photon state on demand. Photonic states can now be separated without being isolated, opening the door to the development of advanced functional photonic circuits in scalable classical and quantum information processing. These results have recently been published in the journal "Nature Photonics"[1]





















Fig. 1.a,
Schematic model of two indirectly coupled photonic nanocavities through a waveguide with reflecting boundary walls. b, The resonant spectrum of the isolated individual nanocavities A and B (red line) and their resonant spectrum when coupled to an open waveguide (dashed green line). The green line spectrum has a line width of δin, which corresponds to the coupling bandwidth between the nanocavities and the waveguide. c, The resonant spectrum of the bounded waveguide discretized by Fabry-Perot (FP) resonant effect.

At first, we discuss how to realize strong coupling between distant nanocavities. We employ a system as shown in Fig. 1. Two nanocavities (A and B) are connected by a waveguide, which is terminated on both sides by reflecting walls (C and D). The individual nanocavities each have a single resonant mode with the same frequency (see Fig. 1b, red line). The bounded waveguide has many standing wave modes due to Fabry-Perot (FP) resonance (see Fig. 1c). To realize strong cavity-cavity coupling, we theoretically investigated the system in detail and found that all FP waveguide modes should be detuned far from the nanocavity modes (by more than the coupling bandwidth δin between the nanocavities and the waveguide) as shown in Fig. 1b and c. Even under such a condition, the nanocavities can still couple to each other indirectly through a forced oscillation of the FP waveguide modes, while concentrating photons in either nanocavity, not the waveguide.

Fig. 2. a, Overall view of the fabricated silicon-based photonic crystal observed by SEM. Two photonic crystal nanocavities (A & B) are placed 202a apart with a line defect waveguide nearby. b, Magnified image of a nanocavity. This is based on a multi-step hetero structure with a1 of 415 nm and a2 of 420 nm. c, Magnified image of the waveguide and a partial reflector. d, e Spectra of the vertically emitted light observed at cavities A (d) and B (e) respectively, obtained by introducing a tunable continuous-wave laser through partial reflector C. f, Time resolved amplitude of vertically emitted light from cavities A & B. A pulse laser with duration of 4 ps and a centre wavelength of ~ 1539.45 nm (width = 1 nm) was introduced through the partial reflector C, and vertically emitted light from each cavity was observed in the time domain by a cross-correlation method.

According to this scheme, we fabricated a silicon-based photonic crystal sample as shown in Fig. 2a-c. Two multistep-hetero nanocavities[2], A and B, with original Q factors of ~ 1 million, were placed 83µm apart, with a line defect waveguide nearby. Both ends of the waveguide were bound by partial reflectors (C and D), formed by narrowing the waveguide’s width. Figure 2d, e show resonant spectra of the fabricated sample observed from nanocavity A and B, respectively. Two resonant peaks with similar intensities were observed from both cavities with exactly the same wavelengths (1539.39 and 1539.54 nm). These peaks correspond to the coupled nanocavity modes. The splitting of the peaks (150 pm) is 50 times larger than the resonant peak’s width (~3pm), indicating that the system is within the strong coupling regime.

Next, we carried out time domain measurements. We excited the coupled cavity modes by introducing a short optical pulse through partial reflector C. The results are shown in Fig. 2f. Clear exchange of photons between the distant nanocavities was observed with a period of ~54 ps. This exchange is seen to continue more than 400 ps, demonstrating the long coherence time of photons in this system. Note that the photon lifetime is much larger than that of the FP waveguide modes (~40ps). This indicates that the photons are predominantly concentrated in the nanocavities rather than in the waveguide when the coupled nanocavity modes are excited.























Fig. 3.
Dynamic control of the coupling state between the nanocavities.
a, Schematic of the experimental set-up. A control pulse is irradiated into cavity B, causing a dynamic wavelength shift by the carrier plasma effect. b, Time-resolved amplitude of emitted light from cavities A and B, where the control pulse was irradiated into cavity B when the photons populated only cavity A.

The next important step was to demonstrate dynamic control over these coupling states. Because the nanocavities are sufficiently far apart, it is possible to induce a dynamic change in either cavity, completely independently, to control the coupling state. Here, we attempted to induce a wavelength shift of cavity B using a control pulse with duration of 4 ps, and a wavelength of 770 nm at a specified time during the photon exchange. (See Fig. 3a) The control pulse is absorbed by the cavity, generating free carriers, which lowers the refractive index and induces a blueshift of the resonant wavelength [3]. This blueshift cuts off the coupling between cavities A and B, and this decoupled state continues for ~1 ns due to the long carrier lifetime in silicon. Figure 3b shows the results obtained for the control pulse irradiation. The control pulse is irradiated onto the cavity B with the photons populating only cavity A. This figure clearly shows that the photon exchange was stopped successfully, and froze the photon population in the state at the moment of control pulse irradiation. This finding suggests that the behaviour of photons can be controlled even in regions where they are not present which therefore enables remote control of photons. We have also observed similar phenomena even at single photon power levels.

The results obtained in this work are expected to be applicable to various nanophotonic circuits that require distant coupling of on-chip cavities and will become a fundamental building block for areas including the stopping (or slowing) of light and even photonic quantum information processing.

References
[1] Y. Sato, Y. Tanaka, J. Upham, Y. Takahashi, T. Asano, and S. Noda “Strong coupling between distant photonic nanocavities and its dynamic control”. Nature Photonics. 6, 56-61 (2012). Abstract.
[2] Yasushi Takahashi, Yoshinori Tanaka, Hiroyuki Hagino, Tomoyuki Sugiya, Yoshiya Sato, Takashi Asano, and Susumu Noda, "Design and demonstration of high-Q photonic heterostructure nanocavities suitable for integration". Optics Express. 17, 18093-18102 (2009). Abstract.
[3] Yoshinori Tanaka, Jeremy Upham, Takushi Nagashima, Tomoaki Sugiya, Takashi Asano & Susumu Noda, "Dynamic control of the Q factor in a photonic crystal nanocavity". Nature Materials, 6, 862-865 (2007). Abstract.

Labels: ,


Sunday, April 01, 2012

Unveiling the Unconventional Pairing in Iron-based Superconductor: Direct Observation of the Nodal Gap Structure in Ferropnictide Superconductor

Yan Zhang (left) and Zirong Ye (right), leading authors of the Nature Physics paper





Authors: Yan Zhang, Dong-Lai Feng


Affiliation: State Key Laboratory of Surface Physics, Advanced Materials Laboratory, and Department of Physics, Fudan University, Shanghai 200433, China

Link to Feng Group: Research Group of Complex Quantum Systems >>

Pairing symmetry is a pivotal characteristic of a superconductor. In the conventional BCS superconductors, the formation of Cooper pairs is due to the attractive interaction between electrons mediated by the electron-phonon interaction. Such pairing interaction results in an isotropic s-wave pairing symmetry, which is manifested as finite-sized energy gap called superconducting gap in single particle excitations throughout the entire Fermi surface. However, for many unconventional superconductors, since Coulomb repulsive interaction between electrons is often rather strong, Cooper pairs favor a non-zero angular momentum to minimize the total energy. For example, the cuprate high temperature superconductors take the d-wave pairing symmetry, which would cause superconducting gap diminishes at certain locations called nodes on the Fermi surface. Such gap nodes will have significant effects on the low temperature properties.

Four years after the discovery of the iron-based superconductors in 2008, the mechanism of this new class of high temperature superconductors is still under debate, because of their diversified structure, composition, and electronic structure. Scientists are still struggling to construct a unified picture for the basic phenomenology of different iron-based superconductors. One central issue is the exact nature of the superconducting gap. For example, in the superconducting state, some iron-based superconductors, such as Ba1-xKFe2As2, BaFe2-xCoxAs2, KxFe2-ySe2, FeTe1-xSex, etc., exhibit a nodeless behavior [1, 2], while others like LaOFeP, LiFeP, KFe2As2, BaFe2(As1-xPx)2, BaFe2-xRuxAs2, and FeSe exhibit a nodal behavior with zero energy excitations [2].

This discrepancy on superconducting gap raises serious challenges, questions and debates. For example, one could ask whether the nodal behavior is due to d-wave pairing; and if so, why there are two types pairing symmetries or mechanisms in iron-based superconductors? Many theories have been proposed to address these fundamental questions, but no consensus has been reached. The main obstacle is that all the previous measurements do not provide detailed information of the gap structure in the momentum space, and are somewhat indirect. We have no knowledge on the location of the nodes, as to which band does it belong, and where is it in the Brillouin zone, etc.

Figure 1. (a) The three-dimensional Fermi surface of BaFe2(As0.7P0.3)2. (b) kz dependence of the symmetrized spectra measured on the α hole FSs. (c) The superconducting gaps on the α FSs as a function of kz.

Recently, these mysteries regarding the nature of the superconducting gap in iron-based superconductors have been resolved by our angle resolved photoemission spectroscopy (ARPES) study [3]. We have successfully determined the nodal gap structure of BaFe2(As1-xPx)2, which is a prototypical iron-based superconductor with nodal behaviors established by many transport studies. As shown in Fig. 1a, the Fermi surface of BaFe2(As0.7P0.3)2 consists of three hole Fermi surface sheets (FSs) (α, β and γ) surrounding the central Γ–Z axis of the Brillouin zone, and two electron FSs (δ and η) around the corner. Detailed survey on the electron FSs found a nodeless superconducting gap with little kz dependence. However, for the α hole FSs, the experimental data clearly showed a zero superconducting gap or nodes located around the Z point (Fig. 1b and 1c).
























Figure 2. False-color plots of the gap distribution on the Fermi surface of BaFe2(As0.7P0.3)2.


The gap distribution of BaFe2(As0.7P0.3)2 is summarized in Fig. 2. The node is located on a ring around Z, which immediately rules out the d-wave pairing symmetry, since it would give four vertical line nodes in the diagonal directions (θ = ± 45°, ± 135°) as in the cuprates. The horizontal ring node around Z is not forced by symmetry, as it is fully symmetric with respect to the point group. Therefore, the node is an “accidental” one under the s-wave pairing symmetry, which is likely induced by the strong three-dimensional nature of the α band, and its sizable d3z2−r2 orbital character near Z. This finding provides a general explanation as to why the gap is nodal for certain compounds and nodeless for others, and thus helps build a universal picture of the pairing symmetry in iron-based superconductors.

References:
[1] Y. Zhang, L. X. Yang, M. Xu, Z. R. Ye, F. Chen, C. He, H. C. Xu, J. Jiang, B. P. Xie, J. J. Ying, X. F. Wang, X. H. Chen, J. P. Hu, M. Matsunami, S. Kimura, and D. L. Feng, "Nodeless superconducting gap in AxFe2Se2 (A=K,Cs) revealed by angle-resolved photoemission spectroscopy". Nature Materials, 10, 273–277 (2011). Abstract.
[2] J. Hirschfeld, M. M. Korshunov, and I. I. Mazin, "Gap symmetry and structure of Fe-based superconductors". Reports on Progress in Physics, 74, 124508 (2011). Abstract.
[3] Y. Zhang, Z. R. Ye, Q. Q. Ge, F. Chen, Juan Jiang, M. Xu, B. P. Xie and D. L. Feng, "Nodal superconducting-gap structure in ferropnictide superconductor BaFe2(As0.7P0.3)2". Nature Physics, doi:10.1038/nphys2248 (Published online Mar 04, 2012). Abstract.

Labels: