Anderson in an interview

He is certainly among the most creative physicists of last century. I find myself resonating with his style in doing physics, with all modesty. This is an excerpt from an interview [http://www.aip.org/history/ohilist/23362_1.html]:

How about quantum electro-dynamics?

ANDERSON:

It was too hard for me. I'm not a formalist. I did listen to some of those lectures. I found the formalism forbidding. I only later came to understand that kind of formalism. I'm lazy. I'm mentally lazy. I believe in using only the tools that are necessary for the job. If I've got a serious problem and it's my problem that I have to solve, I'll go on inventing formalism until I find the answer. But in general I avoid the formalism if I possibly can.

KOJEVNIKOV:

Among other physicists, who do you think is the closest to you in style?

ANDERSON:

Well some of them are experimentalists. I've always thought that Nicolaas Bloembergen and I were very similar. He's half a theorist, or say six tenths experimentalist and four tenths theorist, and I'm more like six tenths theorist and four tenths experimentalist. So that's fairly close. This is immodest of me, and I say this with all modesty, but I think Fermi worked the way I worked. He really was focused on the experimental question. And then he would do formalism if he had to solve it. And Van very much so. Van was no formalist. The reason I was rescued from complete ignorance of formalism was that I had these courses from Schwinger, and so I learned the Green's function formulas and some of that aspect of physics. But that was another thing about the nuclear physics. The Schwinger group of students and the few students who were working with Furry, they were focused on formalism. And I wanted to really explain experimental facts. So when Van gave me this problem—

Accoustic diode

This is a device that allows one-way propagation of sound. Obviously, such device must break time reversal symmetry. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.084301]

The device consists of a two-dimensional sonic crystal arranged in a mesh of square steel rods. By rotating the steel rods, Li et al. are able to manipulate the unit cell of the sonic crystal element to turn the diode on (sound waves only propagate one way) and off (sound waves can move back and forth). Furthermore, Li et al. make their device entirely from linear acoustic materials, which allows them to control sound propagation with a simpler and more efficient process, over a broader bandwidth, and with lower power consumption, compared to existing nonlinear sonic-crystal-based devices.

Anderson and Scalapino talk on Science

This is not a latest news but still interesting for those who are working with cuprate superconductors, which are widely thought to be a central problem faced with by contemporary physicists. The two argue about the possibility of a paring glue. [http://www.sciencemag.org/content/316/5832/1705/reply]

[Anderson]:

I do not, however, accept that Scalapino’s calculations, refined as they are, come near to settling the question he has raised. The numerical analysis of Scalapino’s reference 7, and the analysis of experiments in his references 8 and 9, all have, logically, two pieces. To take ref 7 for definiteness, Scalapino’s group does two computations. The first, while difficult, is logically unimpeachable: It is a carefully designed simulation of the properties of the Hubbard model which we both agree is by far the main Hamiltonian candidate. Sure enough, they find d-wave superconductivity and other physical properties that agree with experiments—and also with much simpler mean field theories (1).

In step 2, they attempt to derive from the measured quantities a number of theoretical constructs, such as the "pairing interaction vertex," assuming that the underlying theory is the conventional Feynman-Dyson diagram theory as adapted for condensed matter problems in the 1960s. Thus, the procedure, far from being a purely direct computational result, is a theoretical construct with a very relevant input of unproven assumptions. The interaction vertex which is derived does not look at all like the J term in the Hamiltonian; it is much smaller than it should be, at high energies, growing to its full strength only at the lowest energies.

This is an unlikely result. It says that somehow all the low-frequency spin fluctuations have killed this giant interaction at the high-frequency end, but left it intact at low frequency to do its work on the pairing gap. It amounts to replacing my "elephant" J with almost nothing but its indirect consequences. It also contradicts the simplest mean field theory of the t-J model (ref 1, called because of its simplicity the "Plain Vanilla" theory).

If I found a result which so blatantly did not make physical sense, I might have questioned the method rather than attacking those of us who seem to have found the right answers by doing things otherwise. It is very attractive to abandon these particular methodological assumptions, because the same set of assumptions, applied in the normal state of the same materials, have had zero success in describing its unique and very anomalous properties.

[Scalapino]:

While there is a growing consensus that superconductivity in the high T_c cuprates arises from strong short-range Coulomb interactions between electrons rather than the traditional electron-phonon interaction, the precise nature of the pairing interaction remains controversial (1). This is the case even among those who agree that the essential physics of the cuprates is contained in the Hubbard model (Perspectives, "Is there glue in cuprate superconductors?", by P. W. Anderson, 22 June 2007, p. 1705). For example, both Anderson’s resonating-valence-based (RVB) theory (2) and the spin-fluctuation exchange theory (3, 4) lead to a short-range interaction which forms d _x²−y² pairs. However, the dynamics of the two interactions differ.

In the RVB picture, the superconducting phase is envisioned as arising out of a Mott-liquid of fluctuating singlet pairs. These pairs are bound by a superexchange interaction J which is proportional to t²/U. Here t is the effective hopping matrix element between adjacent sites and U is an onsite Coulomb interaction. J is determined by the virtual hopping of an electron of a given spin to an adjacent site containing an electron with an opposite spin (5). Thus the dynamics of J involves virtual excitations above the Mott gap which is set by U, and the pairing interaction is essentially instantaneous. In this case, as Anderson recently discussed (6), one would not speak of a pairing glue.

In the spin-fluctuation exchange picture, the pairing is viewed as arising from the exchange of particle-hole spin 1 fluctuations whose dynamics reflect the frequency spectrum seen in inelastic magnetic neutron scattering. This spectrum covers an energy range which is small compared with U or the bare bandwidth 8t. In this case, the pairing interaction is retarded and in analogy to the traditional phonon mediated pairing, one says that the spin-fluctuations provide the pairing glue.

Thus, the question of whether there is pairing glue in the cuprates is a question about the dynamics of the pairing interaction. It offers a way of distinguishing different theories. For the Hubbard model, recent numerical calculations (7) have shown that the strongest pairing occurs for U of order the bandwidth 8t. This is also thought to be the parameter regime appropriate to the cuprates. In this regime, these calculations find that the dynamic dependence of the pairing interaction is the same as that of the dynamic spin susceptibility. Thus, there is pairing glue in the Hubbard model.

Of course, the ultimate question is: What does the experiment tell us about the dynamics of the pairing interaction? Just as the spatial structure of the pairing interaction can be determined from the k-dependence of the superconducting gap, the dynamics of the interaction is reflected in the frequency dependence of the gap. In addition, if the interaction is retarded, that is delayed for a time of order ћ/(2J), the gap will have both a real and an imaginary component. This frequency structure of the gap is reflected in a variety of experiments and analysis of structure in the angular resolved photoemission spectrum (8), and the infrared conductivity (9) have suggested that the dynamics is indeed determined by spin-fluctuations. However, as opposed to the Hubbard model, real materials have phonons and alternative explanations have also been proposed (10). Thus, while there is pairing glue in the Hubbard model, more experimental work is needed to settle the question of whether there is glue in the cuprate superconductors.

Superfluid Exists in the core of a neutron star

This is a wonderful discovery. A neutron star 11,000 light years far from us was observed cooling down extrodinarily fast (4% in the past 10 years), which has been deciphered as a signature of the existence of such superfluid. The pressure exerted by gravity on the core of a neutron star is huge and neutrons, which are fermions, were predicted to form boson-like pairs under such high pressure circumstances. Such pairs condense and move coherently and make a superfluid, which is frictionless. More neutrinoes shall be released, and hence more energy shall be taken away, resulting in the faster cooling rate, explain these authors. [http://www.wired.com/wiredscience/2011/02/superfluid-neutron-star/]

Quantum control using unsharp measurements

"For the purpose of controlling a system, two facts appear self-evident. First, the more information one can obtain about the system, the better one can control it. Second, one needs to do more than just obtain information in order to control the system. In the quantum world, however, self-evidence cannot be trusted. Writing in Physical Review A, Ashhab and Nori¹ refute the two 'facts' just given and show that a quantum system can be quickly driven to any desired state using a fixed type of measurement. Although various schemes have been proposed^{2, 3} for driving a quantum system from one state to another using only quantum measurements, this is the first time it has been shown to be achievable using repetitions of a given measurement. Crucially, the authors' proposal requires the measurement to be unsharp. That is, one must avoid obtaining too much information about the system."[http://www.nature.com/nature/journal/v470/n7333/full/470178a.html?WT.ec_id=NATURE-20110210]
[1]Ashhab, S. & Nori, F. Phys. Rev. A 82, 062103 (2010)

Couterflow

This Letter addresses a practical problem that is encountered in, for example, a recent oil gushing in Mexico Gulf. To 'top kill' the oil well, one may pump "mud" from above to suppress the welling from bottom. However, in this "top kill", one needs consider an issue as described in the mentioned Letter, the so-called "Helmholtz instability", namely, "Dense fluids, i.e., mineral suspensions called ‘‘mud’’ [1,2], are introduced into oil wells to provide hydrostatic pressure to offset hydrocarbon (oil and gas) fluid pressure in deep formations, stopping upward flow and reducing the fluid pressure at the surface to near ambient. If hydrocarbon is flowing upward in the well, there is a counterflow between the upwelling hydrocarbon and the descending mud. Successful top kill requires that the mud descend despite this counterflow. However, upwelling at speeds > 1m/s, as in the uncontrolled Macondo well ‘‘blowout’’ in the Gulf of Mexico in 2010, may lead to a Kelvin- Helmholtz instability [3]."[PRL 106, 058301 (2011)] How to avoid the instability ? Suggested to use viscoelastic materials.

Strong interaction makes better clocks !

I have not read this report in Science [331:1043 (2011)], but it catches my attention simply because of its claim ! Anyway, it should be funny to read. "Optical lattice clocks with extremely stable frequency are possible when many atoms are interrogated simultaneously, but this precision may come at the cost of systematic inaccuracy resulting from atomic interactions. Density-dependent frequency shifts can occur even in a clock that uses fermionic atoms if they are subject to inhomogeneous optical excitation. However, sufficiently strong interactions can suppress collisional shifts in lattice sites containing more than one atom. We demonstrated the effectiveness of this approach with a strontium lattice clock by reducing both the collisional frequency shift and its uncertainty to the level of 10⁻¹⁷. This result eliminates the compromise between precision and accuracy in a many-particle system; both will continue to improve as the number of particles increases. "

BEC is like laser, with perfect coherence: from an experimental point of view

In this work, BEC has been shown possessing the same kind of coherence properties of laser, as expected. The researchers measure directly the third order correlation functions regarding atom bunching (for bosons), g(0,t1,t2), which shows the likelihood of having three particles detected at moments 0, t1 and t2, respectively. For thermal bosons, this function features a broad peak, while for BEC it is rather uniform. This was just observed [Science, 331:3046(2011)] !

A major advance in understanding the behavior of light was to describe the coherence of a light source by using correlation functions that define the spatio-temporal relationship between pairs and larger groups of photons. Correlations are also a fundamental property of matter. We performed simultaneous measurement of the second- and third-order correlation functions for atoms. Atom bunching in the arrival time for pairs and triplets of thermal atoms just above the Bose-Einstein condensation (BEC) temperature was observed. At lower temperatures, we demonstrated conclusively the long-range coherence of the BEC for correlation functions to third order, which supports the prediction that like coherent light, a BEC possesses long-range coherence to all orders.

Faculty members in conflict with the university president

This story is definitely a reminder of similar things. It involves a famous materials scientist in Tohoku University, Inoue, who also serves as the president of this university. The clouds originate at four papers bearing his name in the 1990s, and could not abate even today, as told by Nature [http://www.nature.com/news/2011/110223/full/470446a.html?WT.ec_id=NATURE-20110224]:

The controversy has been simmering for more than three years, during which critics have repeatedly questioned the results of the four papers^1–4 by Inoue. An internal committee at the university assessed the criticisms and ruled that a formal investigation was not warranted. In the committee's December 2007 report, senior officials questioned whether the criticism was motivated by "malice" and "divorced from a pure concern for academic development". Since then, university faculty members have repeated the criticisms and raised others. But Inoue, a prolific specialist in an unusual form of alloy called metallic glasses, told Nature that his team has unique skills and experience in producing the alloys, which could explain why other scientists have failed to reproduce some of his lab's results.
.......

But in May 2007, a series of anonymous letters began arriving at Tohoku University and other places alleging that the four papers^1–4 co-authored by Inoue in the 1990s contained inconsistencies in the way that the data were presented. The letters also alleged that others in the field had been unable to reproduce the results.

In response to these allegations, Tetsuo Shoji, the university's executive vice-president for research affairs at the time, formed a five-person committee to decide whether a full-scale investigation was warranted. In December 2007, the committee issued a 12-page report that said there were "no rational grounds" for a full investigation. On the problem of irreproducibility, the report said: "various factors including the purity of materials … the cooling method, the protocols, temperature control, moisture control and time control can bring about differences in results, making it easy to imagine how problems in reproducing work might exist among researchers."

Nine metallic-glass experts outside Japan contacted by Nature generally lauded Inoue's contribution to the field. At least one, however, had specifically tried to produce some of the metallic glasses described in the papers^1–4 under discussion but had failed to achieve the large dimensions reported by Inoue. Two others were unable to reproduce other metallic-glass results from Inoue's laboratory.

..........

Omura and other critics also point out, however, that the committee included three people (Shoji, Makoto Watanabe and Keiichi Noe) who had been promoted to their current positions as executive vice-presidents by Inoue since he became university president in 2006. Inoue denies any influence over the committee's make-up. Shoji says that he and other committee members discussed the problem of conflict of interest but decided they would "be able to fairly evaluate the situation". Some critics believe that actual fairness is not enough, and that such a committee must also be seen to be free of any potential conflicts of interest.

In April 2008, university officials rebuffed Omura's letter, defending both the committee's operations and its conclusions.

But in July 2008, 11 researchers from Tohoku's Institute of Multidisciplinary Research for Advanced Materials sent a petition to the university calling for further explanation of the investigating committee's initial report. Attached to the petition was a covering letter by the institute's director, Fumio Saito (who had not signed the petition himself). Then, just over a month later, Saito sent a letter to all university divisions apologizing for implying in his covering letter that the petition expressed "the collective opinion of the institute". Saito's apology added to the controversy.

In yet another twist, in April 2009 a university committee chaired by Inoue temporarily postponed the granting of emeritus status to two retiring faculty members who had been involved with Omura on a website that collects information about the dispute. Emeritus status is customarily granted to retiring professors who have been at the university for at least seven years, as both had been. According to Yukihisa Kitamura, an executive vice-president at Tohoku University, it was a "rare" measure taken while the two were under investigation for allegedly "dishonouring the university" by being involved with the website. In June, with the investigation still under way, the university granted the two emeritus status.

The unrest looks set to continue. During April and May last year, three key science-funding agencies — the science and education ministry, the Science and Technology Agency and the New Energy and Industrial Technology Development Organization — all asked Tohoku University to evaluate accusations raised by Omura and three others. Citing confidentiality rules, a lawyer representing Inoue and Tohoku University declined to say what actions the university would take in response to the requests. Zhang has not responded to Nature 's request for further comment.

..............

Japan has no external body, akin to the US Office of Research Integrity, for investigating alleged scientific misconduct, despite calls for one from some quarters¹⁶. Only the minister of education has authority over a university president, and Kosaku Okada, a representative from the ministry's division familiar with the case, says that the ministry will not get involved. "We see the president as just another scientist and so we leave it up to the university to do any investigation," he says. But an independent investigation may be the only way to silence the critics.

Nonlinear dynamics are not easy !!

This review [http://www.nature.com/nature/journal/v470/n7335/full/470475a.html?WT.ec_id=NATURE-20110224#/references] explains why I claim that !

The formal problem of the stability of rotating flow was first addressed by Lord Rayleigh in the late nineteenth century⁷. Rayleigh found that if the rotational velocity of a fluid decreases more rapidly with radius than the reciprocal of the distance from the axis of rotation, such a system is unstable to infinitesimal perturbations. Astrophysical disks, by this criterion, should be stable. But Rayleigh's analysis was restricted to vanishingly small disturbances, and the geometrical shape of the perturbations was in the form of rings with cylindrical symmetry. It is still not known what types of flow that are formally stable by this Rayleigh criterion might still be unstable to more general forms of disturbance; it is known, however, that some types of Rayleigh-stable flow certainly can be destabilized^{4, 8}. The issue of interest is whether the rotation of an astrophysical gas disk about a central mass falls into this unstable category.

This problem can be investigated in the laboratory by studying what is known as Couette flow. In a Couette apparatus, water is confined to flow in the space between two coaxial cylinders. There should be no motion along the central axis, only rotational flow about the axis. The cylinders rotate independently of one another, so that small frictional viscous forces near the cylindrical walls will set up a hydrodynamical flow in which the rotational velocity depends on the distance from the rotation axis. By choosing the rotational velocities of the rotating cylinders appropriately, a small section of an astrophysical disk can be mimicked in the laboratory. In such a disk, the flow velocity is inversely proportional to the square root of the distance from the centre, a pattern known as Keplerian flow. The question to be answered is whether Keplerian flow, formally stable by the Rayleigh criterion, actually breaks down into turbulence.
......

It is this question that Paoletti and Lathrop¹ have sought to address. When a Couette flow becomes turbulent, one of the consequences is a greatly enhanced outward flux of angular momentum, which is imparted to the outer cylinder in the form of a torque. In their experiment, the authors measure this torque directly. An earlier investigation⁹ had claimed to detect this torque, but the new experiment¹ was conducted under conditions in which (undesirable) viscous effects were more effectively minimized.

Close on the heels of Paoletti and Lathrop's claim, however, comes a report by Schartman et al.¹⁰ on a related experiment. These investigators found no transition to turbulence for Keplerian flow with the same controlled level of viscosity. This null result was first reported³ in 2006, and the most recent paper maintains its original conclusion that there is no evidence of a turbulent breakdown of Keplerian-like laminar flow for very small values of the viscosity.

Trapped ions realize coupled harmonic oscillators

I highlight this work just because it was done at nearly the same time by two distant groups, one in US and the other in Austria. Both published their work in Nature. They demonstrated the potential of trapped ions in quantum computing.

[doi:10.1038/nature09800] More than 100 years ago, Hertz succeeded in transmitting signals over a few metres to a receiving antenna using an electromagnetic oscillator, thus proving the electromagnetic theory¹ developed by Maxwell. Since this seminal work, technology has developed, and various oscillators are now available at the quantum mechanical level. For quantized electromagnetic oscillations, atoms in cavities can be used to couple electric fields^{2, 3}. However, a quantum mechanical link between two mechanical oscillators (such as cantilevers^{4, 5} or the vibrational modes of trapped atoms⁶ or ions^{7, 8}) has been rarely demonstrated and has been achieved only indirectly. Examples include the mechanical transport of atoms carrying quantum information⁹ or the use of spontaneously emitted photons¹⁰. Here we achieve direct coupling between the motional dipoles of separately trapped ions over a distance of 54 micrometres, using the dipole–dipole interaction as a quantum mechanical transmission line¹¹. This interaction is small between single trapped ions, but the coupling is amplified by using additional trapped ions as antennae. With three ions in each well, the interaction is increased by a factor of seven compared to the single-ion case. This enhancement facilitates bridging of larger distances and relaxes the constraints on the miniaturization of trap electrodes. The system provides a building block for quantum computers and opportunities for coupling different types of quantum systems.

[doi:10.1038/nature09721] The harmonic oscillator is one of the simplest physical systems but also one of the most fundamental. It is ubiquitous in nature, often serving as an approximation for a more complicated system or as a building block in larger models. Realizations of harmonic oscillators in the quantum regime include electromagnetic fields in a cavity^{1, 2, 3} and the mechanical modes of a trapped atom⁴ or macroscopic solid⁵. Quantized interaction between two motional modes of an individual trapped ion has been achieved by coupling through optical fields⁶, and entangled motion of two ions in separate locations has been accomplished indirectly through their internal states⁷. However, direct controllable coupling between quantized mechanical oscillators held in separate locations has not been realized previously. Here we implement such coupling through the mutual Coulomb interaction of two ions held in trapping potentials separated by 40 μm (similar work is reported in a related paper⁸). By tuning the confining wells into resonance, energy is exchanged between the ions at the quantum level, establishing that direct coherent motional coupling is possible for separately trapped ions. The system demonstrates a building block for quantum information processing and quantum simulation. More broadly, this work is a natural precursor to experiments in hybrid quantum systems, such as coupling a trapped ion to a quantized macroscopic mechanical or electrical oscillator.

Geometric frustration occurs to (Ba,Sr)TiO3 ferroelectrics

Both BT and ST are old systems and a lot, but still much to be unveiled, has been learned about their lattice dynamics. However, exotic phenomena might happen when these two compounds are mixed compositionally. According to this ab initio study [Nature, 470:513(2011)], some geometric frustration has been spotted in this BST system. And this gives rise to a number of new phases such as stripes. What this story suggests is that, much more can be lavished about these common systems, and only imagination can limit.

Geometric frustration is a broad phenomenon that results from an intrinsic incompatibility between some fundamental interactions and the underlying lattice geometry^{1, 2, 3, 4, 5, 6, 7}. Geometric frustration gives rise to new fundamental phenomena and is known to yield intriguing effects such as the formation of exotic states like spin ice, spin liquids and spin glasses^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17}. It has also led to interesting findings of fractional charge quantization and magnetic monopoles^{5, 6}. Mechanisms related to geometric frustration have been proposed to understand the origins of relaxor and multiferroic behaviour, colossal magnetocapacitive coupling, and unusual and novel mechanisms of high-transition-temperature superconductivity^{3, 4, 5, 12, 16}. Although geometric frustration has been particularly well studied in magnetic systems in the past 20 years or so, its manifestation in the important class formed by ferroelectric materials (which are compounds with electric rather than magnetic dipoles) is basically unknown. Here we show, using a technique based on first principles, that compositionally graded ferroelectrics possess the characteristic ‘fingerprints’ associated with geometric frustration. These systems have a highly degenerate energy surface and display critical phenomena. They further reveal exotic orderings with novel stripe phases involving complex spatial organization. These stripes display spiral states, topological defects and curvature. Compositionally graded ferroelectrics can thus be considered the ‘missing link’ that brings ferroelectrics into the broad category of materials able to exhibit geometric frustration. Our ab initio calculations allow deep microscopic insight into this novel geometrically frustrated system.

Engineering open quantum systems

The effects of an infinite environment on a finite physical system under interest are usually deciphered with equilibrium or non-equilibrium thermodynamics, where the central concept are temperature and entropy as well as Onsager-type relations. However, there exist more elaborate and accurate methods, which are essential in engineering and control. The pioneering work along this line is represented by Boltzman's equation and his H-theorem. In that treatment, the effects are carefully accounted in terms of scattering. In recent years, some general schemes, such as master equation approach and Langevin approach, have been developed to take care of the real time dynamics of open systems. These methods are widely used in laser design and quantum de-coherence and quantum measurement. The latter delves fundamentally in quantum computing. On the other hand, the coupling to environment may be utilized to good ends. Common sense may say that such couplings are not easily manipulated and usually detrimental to quantum control. However, this situation changes and these couplings becomes tractable and can be engineered. Look at this work [Nature, 470:486(2011)]:

The dynamics of an open quantum system S coupled to an environment E can be described by the unitary transformation , with ρ_SE the joint density matrix of the composite system S + E. Thus, the reduced density operator of the system will evolve as ρ_S = Tr_E(Uρ_SEU^†). The time evolution of the system can also be described by a completely positive Kraus map

with E_k operation elements satisfying , and initially uncorrelated system and environment³¹. If the system is decoupled from the environment, the general map (1) reduces to , with U_S the unitary time evolution operator acting only on the system.

Control of both coherent and dissipative dynamics is then achieved by finding corresponding sequences of maps (1) specified by sets of operation elements {E_k} and engineering these sequences in the laboratory. In particular, for the example of dissipative quantum-state preparation, pumping to an entangled state |ψ reduces to implementing appropriate sequences of dissipative maps. These maps are chosen to drive the system to the desired target state irrespective of its initial state. The resulting dynamics have then the pure state |ψ as the unique attractor, . In quantum optics and atomic physics, the techniques of optical pumping and laser cooling are successfully used for the dissipative preparation of quantum states, although on a single-particle level. The engineering of dissipative maps for the preparation of entangled states can be seen as a generalization of this concept of pumping and cooling in driven dissipative systems to a many-particle context. To be concrete, we focus on dissipative preparation of stabilizer states, which represent a large family of entangled states, including graph states and error-correcting codes³².

We start by outlining the concept of Kraus map engineering for the simplest non-trivial example of ‘pumping’ a system of two qubits into a Bell state. The Hilbert space of two qubits is spanned by the four Bell states defined as and . Here, |0 and |1 denote the computational basis of each qubit, and we use the short-hand notation |00 = |0₁|0₂, for example. These maximally entangled states are stabilizer states: the Bell state |Φ⁺, for instance, is said to be stabilized by the two stabilizer operators Z₁Z₂ and X₁X₂, where X and Z denote the usual Pauli matrices, as it is the only two-qubit state that is an eigenstate of eigenvalue +1 of these two commuting observables, that is, Z₁Z₂|Φ⁺ = |Φ⁺ and X₁X₂|Φ⁺ = |Φ⁺. In fact, each of the four Bell states is uniquely determined as an eigenstate with eigenvalues ±1 with respect to Z₁Z₂ and X₁X₂. The key idea of pumping is that we can achieve dissipative dynamics which pump the system into a particular Bell state, for example , by constructing two dissipative maps, under which the two qubits are irreversibly transferred from the +1 into the −1 eigenspaces of Z₁Z₂ and X₁X₂.

The dissipative maps are engineered with the aid of an ancilla ‘environment’ qubit^{25, 33} and a quantum circuit of coherent and dissipative operations. The form and decomposition of these maps into basic operations are discussed in Box 1. The pumping dynamics are determined by the probability of pumping from the +1 into the −1 stabilizer eigenspaces, which can be directly controlled by varying the parameters in the employed gate operations. For pumping with unit probability (p = 1), the two qubits reach the target Bell state—regardless of their initial state—after only one pumping cycle, that is, by a single application of each of the two maps. In contrast, when the pumping probability is small (p 1), the process can be regarded as the infinitesimal limit of the general map (1). In this case, the system dynamics under a repeated application of the pumping cycle are described by a master equation³⁴:

Here H_S is a system Hamiltonian, and c_k are Lindblad operators reflecting the system–environment coupling. For the purely dissipative maps discussed here, H_S = 0. Quantum jumps from the +1 into the −1 eigenspace of Z₁Z₂ and X₁X₂ are mediated by a set of two-qubit Lindblad operators (see Box 1 for details); here the system reaches the target Bell state asymptotically after many pumping cycles.

Competing gaps scenario for underdoped cuprates

The normal state properties are definitely more exotic than the superconducting properties of the engimatic quasi-2D d-wave cuprate superconductors. Under special inspection is the psudogap state, which is signalled with a loss in quasi-particle spectral weight above a typical temperature T*, which is much higher than Tc, the SC critical temperature. A basic question concerns the nature of this gap opening. One idea precasts that the pseudogap senses the preformed cooper pairs, which will condense into a coherent superfluid at Tc. Another idea relates this gap to a different order, which might compete with the SC order. Up to now, evidences regarding these two ideas are elusive. This terrific work [Nature Physics, 7: 21–25 (2011) doi:10.1038/nphys1851] comes to give a real boost. They study Bi2212 with various dopings. They focus on the temperature dependence of the weight loss associated with the antinodal point in the 1BZ. As the sample is cooled across T*, a linear tempeature dependence was spotted around T*, but a very fast deviation sets in at lower temperatures, T**>Tc. This deviation motivates the authors to conceive a two-gap scenario: the proper pseudogap opens at T*, but the cooper pair forms at T**, and SC coherence comes at Tc. According to this proposition, the pseudogap has a separate origin that should be concerned with a different order. They are able to find out a remarkably accurate scaling behaviors regarding the dependence of the cooper paring spectral weight loss on the T**. This scaling law holds for all samples they prepared. Read this paper !! Very inspiring !!

Touching from a distance

One frequently encounters electronic circuits like this one, but how does it work ? Try to work it out ! It is an instructive exercise !
[http://physics.aps.org/articles/v4/11]
(Top) Optical micrograph of the device used by Wang et al. [1] to entangle spatially separated photons stored in superconducting resonators. A and B label the waveguide resonators, which are entangled in the experiment. The phase qubits q₀ and q₁ are Josephson junction phase qubits, while C is a coupling resonator used to mediate the interaction between the qubits. (Bottom) Equivalent circuit schematic for the device.

Why does it grow in the observed way ?

Crystal growth has been all the time an intriguing but complicated problem. The macroscopic shape depends in a subtle way on several factors, such as the properties of the surroundings. Indebted to the increasing power of computers, physicists are enabled to simulate a real growth. On the other hand, experiments also provide important insights. "Despite the many parameters involved, theorists have predicted that icicles, as well as other natural features like stalactites, should all converge to the same shape as they grow. In a paper appearing in Physical Review E, Antony Chen and Stephen Morris at the University of Toronto, Canada, describe an experimental setup that allows them to image icicles as they grow under controlled conditions, and test these predictions. They mounted a camera through a slot in the side of a refrigerator, within which icicles formed as water dripped from a nozzle and onto a rotating wooden support. Rotating the support helps even out the effects of drafts and temperature gradients." [http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.026307]

Negative Linear Compressibility and Massive Anisotropic Thermal Expansion in Methanol Monohydrate

Materials with these properties are rare. These authors found just one in a simple way: "The vast majority of materials shrink in all directions when hydrostatically compressed; exceptions include certain metallic or polymer foam structures, which may exhibit negative linear compressibility (NLC) (that is, they expand in one or more directions under hydrostatic compression). Materials that exhibit this property at the molecular level—crystalline solids with intrinsic NLC—are extremely uncommon. With the use of neutron powder diffraction, we have discovered and characterized both NLC and extremely anisotropic thermal expansion, including negative thermal expansion (NTE) along the NLC axis, in a simple molecular crystal (the deuterated 1:1 compound of methanol and water). Apically linked rhombuses, which are formed by the bridging of hydroxyl-water chains with methyl groups, extend along the axis of NLC/NTE and lead to the observed behavior." [Science, 331 (6018): 742-746]

Electronic correlations are crucial in 2DEG based on STO

I have highlighted a number of studies on the 2DEG that were created about the interfaces based on SrTiO3 compounds. The 2DEG thus obtained have been shown with various interesting ground states including superconducting ones. Here comes a new work [Science, 331 (6019): 886-889] that demonstrates the importance of electronic correlations in determining the transport properties of this 2DEG. This time the 2DEG was introduced by inserting a RO layer, R=La, Pr,Nd,Sm and Y, in the SrTiO3 matrix. It turns out that, the electronic properties of this 2DEG are crucially hinging on the R element. For La, Pr and Nd, it is conducting while for the rest it is insulating.

The formation of two-dimensional electron gases (2DEGs) at complex oxide interfaces is directly influenced by the oxide electronic properties. We investigated how local electron correlations control the 2DEG by inserting a single atomic layer of a rare-earth oxide (RO) [(R is lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), or yttrium (Y)] into an epitaxial strontium titanate oxide (SrTiO₃) matrix using pulsed-laser deposition with atomic layer control. We find that structures with La, Pr, and Nd ions result in conducting 2DEGs at the inserted layer, whereas the structures with Sm or Y ions are insulating. Our local spectroscopic and theoretical results indicate that the interfacial conductivity is dependent on electronic correlations that decay spatially into the SrTiO₃ matrix. Such correlation effects can lead to new functionalities in designed heterostructures.

Helical DNA filters spins

Spin filters might be salient devices in spintronics. One example of such filters is a ferromagnetic layer, which allows a spin in one direction to pass while blocks it in the other. The selectivity has been shown up to about 25%. Now a study using DNA makes a big difference, where the selectivity can be achieved of as high as 60%. In this case, periodic helice plays an essential role, but how this is so has not been understood yet [Science 18 February 2011: 894-897]. In general, it raises an interesting question on how an electron interacts with chiral supramolecules.

Göhler et al. (3) describe a surprisingly efficient method for electronic spin filtering. They have studied how electrons emitted by a gold substrate, upon absorption of light, pass through a self-assembled DNA monolayer on the gold surface. In particular, they have studied the spin of the electrons after their passage through the DNA layer and have found that one spin type passes through much more easily, meaning that this layer acts as a spin filter, strongly hindering the passage of the other spin type. This filter effect is observed only if the DNA is assembled on the gold surface as a closely packed ordered array of helices, and is stronger if the helices are longer, reaching selectivities of 60%. For chaotic assemblies of floppy DNA chains on the gold surface, the spin filter effect was not observed. [http://www.sciencemag.org/content/331/6019/864.full]

flaming tornado

It is always a pleasure to watch simple but 'wonder' provoking experiments like this one. It plays with flames. Places some fuel in a plate and ignite it and then trap it with a web cage. Now you spin the cage, and you'll see the flames grow tall and thin into a flame tornado. Ever wondered why ? It is actually easy to explain. Try it yourself. Cool !

A movie on Einstein

Hi, man, as a fans of him, I'm excited to read this ! I have never seem a film acting this big man's real life. So, it is really a wonderful thing to wait this thing to come. I read a lot of things about Einstein, and I would like to wipe eyes to test my memory.

HSI Films will handle worldwide sales on “Einstein” and immediately introduce the project to distributors in Berlin.

Said Eric Christenson, “Coming aboard a project that teams Wayne Wang and Ron Bass is tremendously exciting, and we have something special on our hands with ‘Einstein.’ People know the name and the theories, yet most people don't know the fascinating details of his life's story.”

Koldo Eguren added, “People don't know about his struggles with poverty, his dyslexia, his love for music, his relationships with the women in his life, his persecution by the Nazis and his battle to deal with living in the public eye and being under constant scrutiny. Ron's screenplay peels back the layers and allows us to see past Einstein the scientist and Einstein the celebrity, showing us Einstein the man.”

How do you get yourself less wet if you are caught in a rain ?

This is a daily problem and is funny enough and thus deserves a journal paper [Eur. J. Phys. v.32, p.355 (2011)] to deal with it. Unfortunately, I could not say more, as I'm not able to access it freely. Only the abstract is posted: "The question whether to walk slowly or to run when it starts raining in order to stay as dry as possible has been considered for many years—and with different results, depending on the assumptions made and the mathematical descriptions for the situation. Because of the practical meaning for real life and the inconsistent results depending on the chosen parameters, this problem is well suited to undergraduate students learning to decide which parameters are important and choosing reasonable values to describe a physical problem. Dealing with physical parameters is still useful at university level, as students do not always recognize the connection between pure numbers and their qualitative and quantitative influence on a physical problem. This paper presents an intuitive approach which offers the additional advantage of being more detailed, allowing for more parameters to be tested than the simple models proposed in most other publications."

Investigating into natural lighting

Lighting accompanies thunderstorms, during which large amounts of energy are released without being leased. Despite the ostensible familiarity, lighting is still enigmatic, as described in this interview:

We learn about lightning in school, I thought it was a closed case?
Well, we know lightning initiates up inside a thunderstorm but we're not sure how it initiates – how it gets started. In fact there are still three big questions. The first is the initiation. The second is how does it propagate, sometimes through miles of air? And the third is, when it reaches the ground – how does it choose to strike this object and not the other object?

But isn't it just an electrical discharge between thunderclouds and the ground?
In a sense, but the big problem is that to get a spark, air needs to break down. It needs to stop being an insulator and start being a conductor. We commonly experience this if you touch a doorknob and you get a spark between your finger and the doorknob. What happens is the charges get concentrated into your fingertip and you get a big electric field. Then, as your finger approaches, the conventional breakdown field is reached, which is about 3 million volts per metre – and then air sparks.

The problem is if you look up inside thunderclouds, the breakdown field that you need to make a spark is never found. People have been launching balloons for decades, they've been flying airplanes, they've been launching rockets...but the fields they record are not even close to this strength.

Back from vacation

Just came back from vacation. Time needed to catch up with what happened in the last two weeks. haha...