Physical Review X

Circuit quantum electrodynamics (circuit QED) has recently emerged as a promising direction for solid-state based quantum computation. Using an on-chip structural platform, where an artificial photon-emitting atom, such as a superconducting qubit, is coupled to a one-dimensional electromagnetic-wave resonator, circuit QED creates and exploits coherent matter-light coupling between the qubit and the electromagnetic modes. Understanding comprehensively how the qubit interacts with the resonator and using that understanding to manipulate the qubit or the electromagnetic field of the resonator would constitute a major advance in this field. As nature would have it, however, the stronger the qubit-resonator coupling and the more interesting the underlying physics are, the more difficult it is to unravel the physics. What we propose in this paper is an indirect approach to circumvent the difficulty: to simulate an important circuit-QED model in the hard-to-access and hard-to-control regimes of very strong light-matter coupling, with another purposefully designed qubit-resonator system working in an easily accessible regime of weaker coupling.

The analogue system starts with a superconducting qubit coupled to the electric field of the single mode of the resonator in the strong-coupling regime. Our essential idea is to drive the qubit with two classical (i.e., unquantized) electromagnetic fields that are orthogonal to the axis of the one-dimensional resonator and are of different frequencies and strengths. This idea comes from the realization that a nontrivial mathematical transformation exists between the model describing this driven system and the original circuit-QED model. Remarkably, by tuning the frequencies and the amplitudes of the two driving fields, we can simulate the behavior of the original model across the full range of the qubit-resonator coupling with that of the driven analogue systems where the qubit-resonator coupling is only moderately strong. In particular, through numerical simulations of the analogue system we have verified the presence of photons in the ground state—a distinct property of the regime of ultrastrong light-matter interaction. Last, but not least, this proposal also allows us to simulate gedanken experiments in relativistic quantum mechanics.

Our proposed approach thus opens a path toward an ultimate understanding of physics in a broad range of regimes of light-matter coupling that are inaccessible in standard quantum optics by use of a quantum simulator.

We propose a method to get experimental access to the physics of the ultrastrong- and deep-strong-coupling regimes of light-matter interaction through the quantum simulation of their dynamics in standard circuit QED. The method makes use of a two-tone driving scheme, using state-of-the-art circuit-QED technology, and can be easily extended to general cavity-QED setups. We provide examples of ultrastrong- and deep-strong-coupling quantum effects that would be otherwise inaccessible.

Exposing a magnetic material to ultrafast (femtosecond) laser pulses apparently leads to a change in the magnetic state of the material on extremely fast time scales in the range of 100 femtoseconds, as was already discovered 15 years ago. Not surprisingly, intense research efforts have been invested in exploring the fundamental physics underlying such a phenomenon and in exploiting it to develop a new data-storage technology, as the speed of the current magnetic-material-based read-write technology reaches its fundamental limit. Despite such intense efforts, however, there is still no consensus on a microscopic theory of magnetism on femtosecond time scales. In this paper, our team, by combining a comprehensive set of experiments with theoretical modeling, makes important findings that will take the debate a big step forward.

Two particular questions of controversy are at the focus of our effort. Is the laser-induced ultrafast loss of magnetization in ferromagnetic metals a thermal process? Does the observation of two time scales in the ultrafast demagnetization in some ferromagnetic metals mean that the microscopic theory of femtosecond magnetism must involve two distinct mechanisms? To answer these questions, we have taken nickel, one of the best-known ferromagnetic materials, and investigated systematically how its ultrafast demagnetization dynamics changes with the ambient temperature and the intensity of the laser pulses. Our most striking finding is that the demagnetization dynamics of nickel shows a transition from a one-time-scale process to a two-time-scale process upon increasing the ambient temperature. Remarkably, we are able to reproduce both this seemingly complex phenomenon and the systematic dependence of the dynamics on the laser intensity with a microscopic theory that invokes a single mechanism: When laser-heated electrons transfer their energy to the lattice vibrations, their spins may switch directions. It is this spin switching that is responsible for the observed ultrafast demagnetization, the behavior of which is ultimately related to the temperature of the hot electrons.

In other words, the ultrafast loss of magnetization in nickel is indeed thermal, related to the thermalization of the spins with the electronic system; and a single microscopic mechanism can explain both single-time-scale and two-time-scale dynamics.

The microscopic mechanisms responsible for the ultrafast loss of magnetic order triggered in ferromagnetic metals by optical excitation are still under debate. One of the ongoing controversies is about the thermal origin of ultrafast demagnetization. Although different theoretical investigations support a main driving mechanism of thermal origin, alternative descriptions in terms of coherent interaction between the laser and the spin system or superdiffusive spin transport have been proposed. Another important matter of debate originates from the experimental observation of two time scales in the demagnetization dynamics of the 4f ferromagnet gadolinium. Here, it is still unclear whether it is necessary to invoke two distinct microscopic mechanisms to explain such behavior, or if one single mechanism is indeed sufficient. To uncover the physics behind these two unsolved issues, we explore the dependence of ultrafast-demagnetization dynamics in nickel through a survey of different laser intensities and ambient temperatures. Measurements in a large range of these external parameters are performed by means of the time-resolved magneto-optical Kerr effect and display a pronounced change in the maximum loss of magnetization and in the temporal profile of the demagnetization traces. The most striking observation is that the same material system (nickel) can show a transition from a one-step (one time scale) to a two-step (two time scales) demagnetization, occurring on increasing the ambient temperature. We find that the fluence and the temperature dependence of ultrafast demagnetization—including the transition from one-step to two-step dynamics—are reproduced theoretically assuming only a single scattering mechanism coupling the spin system to the temperature of the electronic system. This finding means that the origin of ultrafast demagnetization is thermal and that only a single microscopic channel is sufficient to describe magnetization dynamics in the 3d ferromagnets on all time scales.

When you want an image with 100×100-pixel resolution, how many imaging data points do you need? 10 000, the obvious answer seems to be. No, says compressed sensing, a branch of signal-processing science and computational statistics. This research branch develops methods of acquiring and reconstructing a signal that rely on fewer data-acquisition points or measurements than what was previously considered possible, and has enabled faster and more precise measurements in a wide range of applications, including cameras, computed tomography, magnetic-resonance imaging, and genome sequencing. Current techniques are, however, still not optimal, requiring more data points or measurements than necessary. In this interdisciplinary paper, we have designed and tested a new compressed-sensing strategy that allows us to go significantly beyond the known limits for data acquisition and signal reconstruction, by bringing methods and inspirations from statistical physics to bear on compressed sensing—a field that traditionally does not belong to physics.

Mathematically, the problem of compressed sensing is easy to formulate. The signal to be reconstructed is an N-component one, represented by a vectors (N can be viewed as the total number of pixels desired), and compressed sensing deals with signals that are “sparse,” with only a fraction (ρ₀) of the N components being nonzero. The actual acquired data is represented by an M-component vector, y, with M also being only a fraction α of N. y and s are connected linearly by a known mapping. Reconstruction of signal s then becomes a problem of performing the inverse mapping. Current strategies use linear-programming-based optimization to determine the right inverse mapping. But, they require more data points than would be absolutely necessary, i.e., α must be larger than ρ₀.

The strategy we have designed, called the seeded belief-propagation algorithm, approaches the signal acquisition and reconstruction in a radically different way. It views the ultimate signal to be reconstructed as the result of a sufficient sampling of a probabilistic Boltzmann-measure-like distribution, which depends on the mapping that corresponds to data-acquisition measurements and the acquired data points. Using a class of specially designed mappings that exploit the physics intuition about crystal nucleation and a sampling technique known to statistical physics as the belief-propagation algorithm, our new compressed-sensing protocols achieve, in a computationally efficient manner, exact reconstruction of the original signal, not only when α is larger than ρ₀, but also as α approaches ρ₀, in other words, as the number of acquired data points approaches the absolute minimum. In fact, the examples we have tested show that the gains are stunning.

We would like to think that our interdisciplinary approach breaks new ground in the field of compressed sensing, and we anticipate many further fundamental developments and practical applications.

Compressed sensing has triggered a major evolution in signal acquisition. It consists of sampling a sparse signal at low rate and later using computational power for the exact reconstruction of the signal, so that only the necessary information is measured. Current reconstruction techniques are limited, however, to acquisition rates larger than the true density of the signal. We design a new procedure that is able to reconstruct the signal exactly with a number of measurements that approaches the theoretical limit, i.e., the number of nonzero components of the signal, in the limit of large systems. The design is based on the joint use of three essential ingredients: a probabilistic approach to signal reconstruction, a message-passing algorithm adapted from belief propagation, and a careful design of the measurement matrix inspired by the theory of crystal nucleation. The performance of this new algorithm is analyzed by statistical-physics methods. The obtained improvement is confirmed by numerical studies of several cases.

Quantum computers are much more vulnerable to noise than classical computers, because the quantum states of the tiny qubits in them can be altered by the smallest noise, easily leading to errors. Error correction is then obviously an issue of paramount importance for the success of quantum computation. One very promising approach, indeed, currently the best candidate for practical implementations, uses topological error-correction codes. The error-correcting performance of a topological code is captured essentially by its error threshold: As long as the noise intensity is below the threshold, noise-induced errors can be fully corrected by well-designed manipulations that involve only a few qubits. For some of the landmark topological codes, however, working out what this threshold is for the most generic form of noise disturbance is a difficult technical challenge. In this paper, we have accomplished this feat, for several of such codes and for a very general form of noise, by recasting the study of their stabilities as the study of existence of ferromagnetic phases in certain classes of classical models of interacting spins.

In general, a quantum error-correction code works by first defining a set of error-identifying quantum measurements (or “check operators”), then making the measurements to identify the error (establishing a so-called “error syndrome”), and finally prescribing and executing a set of error-correction quantum operations on the qubits. A topological code is distinguished by two key features: First, all the quantum measurements needed for error correction are “local,” involving only a few qubits that can be viewed as “neighbors.” Second, no local operation on its own can change the encoded state of the whole computer. In its essence, “topological” signifies this robustness against local disturbances. The two families of topological codes that we have focused on in this work are the most-studied toric codes and the color codes. In the former, the physical qubits are placed on the square-lattice-like grid on the surface of a torus, and in the latter, on the vertices of a trivalent, e.g., hexagonal, lattice—the architectures of the qubit connectivity dictated by the nature of the quantum measurements involved in the codes.

A previous work by Dennis, Kitaev, Landahl, and Preskill in 2004 pioneered the conceptual approach of determining the error threshold by mapping the quantum problem onto a classical spin model. The form of the noise investigated there was, however, only one of the three possible fundamental types. Our study explores the case of the most generic noise form, which includes not only all three noise types, but also any correlations among them. Finding the mapping becomes, therefore, considerably more challenging technically. We have succeeded in demonstrating that, for the noise form we considered, the classical counterpart of the toric code is an 8-vertex spin model. The error threshold then corresponds to the point in the classical model where a magnetic ordering transition is lost due to the underlying disorder in the classical spins, which is equivalent to the presence of faulty qubits. Using Monte Carlo simulations and duality arguments, we are able to find the error threshold to be at 19% approximately—higher than what was previously thought. Remarkably, the mapping of the color codes leads to new types of classical 8-vertex models, but at the same time, their error thresholds are also at 19%.

We believe that this interdisciplinary effort should bring us a step closer toward the ultimate goal of building high-error-tolerance and large-scale quantum computers, and we also anticipate that this work will be of interest to the statistical mechanics community as well.

The inevitable presence of decoherence effects in systems suitable for quantum computation necessitates effective error-correction schemes to protect information from noise. We compute the stability of the toric code to depolarization by mapping the quantum problem onto a classical disordered eight-vertex Ising model. By studying the stability of the related ferromagnetic phase via both large-scale Monte Carlo simulations and the duality method, we are able to demonstrate an increased error threshold of 18.9(3)% when noise correlations are taken into account. Remarkably, this result agrees within error bars with the result for a different class of codes—topological color codes—where the mapping yields interesting new types of interacting eight-vertex models.

Examples of resonances abound in classical physics, from the oscillations of a loaded spring to the collapse of the Tacoma Narrows Bridge. In quantum mechanics, the principle of wave-function superposition leads to new manifestations of resonant behavior. This is demonstrated no more strikingly than by a class of phenomena called Fano resonances. A Fano resonance, in its simplest and most fundamental form, involves the interference of a discrete quantum state with a continuum of states in the same system. Although originally discovered in atomic scattering, in 1935 by Hans Beutler, and interpreted by Ugo Fano in the same year, Fano resonances have since been demonstrated in many other areas of physics, including optics, plasmonics, matter-wave scattering in ultracold atom systems, and charge transport through mesoscopic quantum dots. It is this last area that provides the specific context for this paper. Numerous works in the past have focused on the Fano resonance that arises from the interference of a single discrete state with a continuum. In this paper, we give a new and significant twist to this phenomenon by demonstrating, experimentally and for the first time, a novel multistate Fano resonance involving two single-electron states localized on two spatially remote quantum point contacts and a continuum of states of an intervening two-dimensional (2D) electron gas.

The existence of single-electron states localized on a single quantum point contact (or “bound states”) was already demonstrated in earlier experimental work, through a Fano resonance generated by nonlocal coupling between a single such state and a point-contact detector. In this work, we extend the earlier approach to generate and demonstrate a multistate Fano resonance that involves two bound states spatially separated by a distance of several hundred nanometers but interacting with each other through a 2D electron gas. The energy of these single-electron bound states can be controlled by suitable tuning of the voltages applied to the quantum-point-contact gates. By performing this tuning such that the two states are close to each other in energy and by measuring the charge transport through the device using the detector, we show that the two states experience a strong and coherent interaction—manifested as anticrossing of the gate-voltage-dependent positions of the two individual Fano resonances associated with the two different localized states. This interaction is, in fact, much stronger than that exhibited by two quantum dots that are so close that their discrete states overlap with each other.

Interpreting this counterintuitive observation with theoretical modeling, we show that the strong coherent interaction between the two remote states emerges from the fact that the interaction is mediated by all of the states of the intervening 2D electron gas. Therefore, our work suggests a new approach to use a continuum to engineer coherent, nonlocal coupling between nano structures in extended mesoscopic systems.

We demonstrate a fully tunable realization of a multistate Fano resonance, in which a pair of remote quantum states experience an effective coupling due to their mutual overlap with a continuum. Our mesoscopic implementation of this system exploits the ability of the semiconductor nanostructures known as quantum point contacts (QPCs) to serve, in the low-density limit close to pinch-off, as an on-demand localized state. By coupling the states formed on two separate QPCs, through a two-dimensional electron gas that serves as a continuum, we observe a robust effective interaction between the QPCs. To explain this result, we develop a theoretical formulation, based on the ideas of the Schrieffer-Wolff transformation, which is able to reproduce our key experimental findings. According to this model, the robust character of the interaction between the two remote states arises from the fact that the interaction is essentially mediated by a large number of degenerate continuum states. While the continuum is often viewed as a source of decoherence, our experiment therefore instead suggests the possibility of using this medium to support the interaction of quantum states, a result that may allow new approaches to coherently couple nanostructures in extended geometries.

A single-stranded DNA or RNA molecule can be driven through a nanoscale pore connecting two separated solution chambers under the action of an electric voltage across the pore. It turns out that this translocation can be detected in the changes of a tiny electric current passing through the nanopore and used to analyze the sequence of the translocated molecule. However, one major challenge in the quest for efficient sequencing techniques is noise, arising from the inherent random nature of the process and leading to irreproducibility of individual events observed. Almost all the approaches being developed focus on finding ways to reduce the effect of the noise. In this paper, we propose a radically different approach: Instead of trying to eliminate the effect of random noises, we actually take full advantage of the very stochasticity of the translocation process for gains towards fast and accurate sequencing of polynucleotides, by exploiting the idea of nanopore engineering and with fundamental theoretical understanding.

The inherent randomness of the translocation process leads to a range (or a distribution) of translocation times even for identical molecules. Desired information about the sequence of the translocating molecule is hidden in that distribution. Our idea is to engineer both the qualitative shape and the quantitative features of that distribution in such a way that the hidden information is brought to the forefront. How should such engineering be achieved? The approach we propose is to structurally pattern the inside of the pores with “spots” of sticky interactions with translocating molecules. Using a recently developed coarse-grained, but still microscopic, model for polymer translocation, we have shown that the translocation process is extremely sensitive to the detailed structure of such interaction patterns, with a faster-than-exponential dependence of the translocation time on the stickiness of the pore. Exploring two types of different pores with distinct translocation-time distributions, we demonstrate theoretically that the sequence of an unknown polynucleotide can be determined with arbitrary accuracy from the stochastic translocation-time readout obtained from the driven, sequential translocation of the polynucleotide through a sufficient number of pores with different interaction patterns.

We believe that our approach will open a new front in the development of the nanopore-based molecular sequencing techniques.

The effect of the microscopic structure of a pore on polymer translocation is studied using Langevin dynamics simulation, and the consequence of introducing patterned stickiness inside the pore is investigated. It is found that the translocation process is extremely sensitive to the detailed structure of such patterns with faster than exponential dependence of translocation times on the stickiness of the pore. The stochastic nature of the translocation process leads to discernible differences between how polymers with different sequences go through specifically patterned pores. This notion is utilized to propose a stochastic sensing protocol for polynucleotides, and it is demonstrated that the method, which would be significantly faster than the existing methods, could be made arbitrarily robust.

Oceanic rogue waves are relatively large surface waves that appear spontaneously far at sea. They can suddenly develop from very calm and apparently safe sea states, cause serious damage to ships or offshore structures, and then disappear without a trace. Where these apparent anomalies come from is still a puzzle for scientists. A simple theoretical model for describing the evolution of these waves is a nonlinear Schrödinger equation. This equation, due to its nonlinearity, has a set of hierarchically ordered solutions known as rational breathers, or evolving solitons growing out of, and amplifying, a small localized wave perturbation. Recently, we created the lowest order rational breather, also known as the Peregrine soliton, in a laboratory-scale water tank. One open question was then: Could higher-order strongly amplifying breathers, or super rogue waves, be generated also in such a water tank? In this paper, we combine an experiment with the theory to show that the answer is an affirmative “yes.”

To generate these solutions in an open water tank, we start with a carrier wave that is a wave-tank analogue of the stable small ocean waves. The mathematical initial conditions describing small localized perturbations for the generation of the higher-order breathers are simulated exactly with a computer controlled paddle and stage-wise experiments are carefully designed to remove artifacts or get around the constraints imposed by the limited size of the tank. Indeed, large localized waves with an amplification factor of 5—super rogue waves in this laboratory setting—grow out of the small perturbation in the manner predicted by the equation.

We believe that our work not only suggests an easily accessible platform for exploring extreme water-wave dynamics, but may also stimulate similar experimental studies on high-order breather solutions in other fields such as optics, plasma physics, and superfluidity where nonlinear dynamics rules.

Super rogue waves with an amplitude of up to 5 times the background value are observed in a water-wave tank for the first time. Nonlinear focusing of the local wave amplitude occurs according to the higher-order breather solution of the nonlinear wave equation. The present result shows that rogue waves can also develop from very calm and apparently safe sea states. We expect the result to have a significant impact on studies of extreme ocean waves and to initiate related studies in other disciplines concerned with waves in nonlinear dispersive media, such as optics, plasma physics, and superfluidity.

Photons are right at the center of many new quantum technologies, such as secure quantum communication and quantum information processing. Generation, detection, and manipulation of photons at the single-photon level have all seen proof-of-principle demonstrations. However, these demonstrations involve large-scale optical elements and rely on unscalable single-photon sources and detectors. The ultimate functional platform for quantum technologies will be single optical chips where miniature high-performance photon sources, detectors, and photonic circuits for routing photons and controlling their interactions are integrated together. One approach to this end that is currently being explored is to implant semiconductor quantum dots as single-photon emitters into a photonic crystal in which cavities and waveguides for photon manipulation through light-matter interactions can be created. In this experimental paper, we demonstrate that indeed, a single quantum dot placed inside, and coupled to, a photonic-crystal waveguide can be used to realize an efficient on-chip, highly directed single-photon source.

A photonic-crystal waveguide (PWG) is a channel-like structure that is opened inside the photonic crystal. When a light-emitting quantum dot is placed inside such a waveguide, the rate of its spontaneous emission is modified due to the coupling between the emitter and the waveguide. Does the waveguide not only channel the emitted photons, but also enhance the rate of the emission of the quantum dot in the channel direction at the same time? By measuring the time-averaged, time-resolved, and autocorrelated intensity of photoluminescence, we have demonstrated and quantified the enhancement of the photon emission in the direction of the waveguide. We have also proven the single-photon character of the emission. Taken together, these results show the high potential of this quantum-dot–PWG structure as a high-efficiency, broadband single-photon source for on-chip implementations of quantum photonics.

We investigate single-photon generation from individual self-assembled InGaAs quantum dots coupled to the guided optical mode of a GaAs photonic crystal waveguide. By performing confocal microscopy measurements on single dots positioned within the waveguide, we locate their positions with a precision better than 0.5  μm. Time-resolved photoluminescence and photon autocorrelation measurements are used to prove the single-photon character of the emission into the propagating waveguide mode. The results obtained demonstrate that such nanostructures can be used to realize an on-chip, highly directed single-photon source with single-mode spontaneous emission coupling efficiencies in excess of β_Γ∼85% and the potential to reach maximum emission rates >1  GHz.

Applying a magnetic field to magnetic material, you can switch the direction of the magnetic moment of the material. This concept is central to data-storage technologies based on magnetic materials. Naturally, fast switching speeds, which can lead to high data recording rates, are highly desired. Over the last few years, a number of methods have been demonstrated to achieve ultrafast switching on the time scales of subpicoseconds. One of the more promising of these experiments used a short laser pulse to induce the switching in an antiferromagnetic material and observed, unexpectedly, oscillatory magnetic switching that continued long after the pulse was turned off. What is then the mechanism behind this observation? The group who made the observation explained it by invoking both the notion of magnetic “inertia”—an analogue to the mass of a particle—and the importance of the antiferromagnetic nature of the material. In this theoretical paper, we reexamine the observed phenomenon at a microscopic level and provide an understanding that is both more fundamental and more general than the concept of magnetic inertia.

Our approach starts with a microscopic description of the dynamics of atomistic spins, the parameters in which are directly derived from first-principles calculations. The primary model system we have investigated is a synthetic antiferromagnet of a Fe/Cr/Fe trilayer. While the atomic spins in each of the layers couple to each other ferromagnetically, the spins in neighboring layers interact antiferromagnetically. Using atomistic spin-dynamics simulations, we have explored the relative roles, and the time evolutions, of the different magnetic-energy terms, in particular, the antiferromagnetic exchange interaction and the magnetic-anisotropy terms. Our results clearly indicate that a time-dependent redistribution of the energy imparted on the spins by the magnetic field pulse into these different energy terms under the inherent nonlinearity of the microscopic spin dynamics is responsible for the oscillatory magnetic switching observed. Moreover, this understanding, and our additional simulations of a ferromagnetic system, also lead to the conclusion that such transient switching dynamics is possible and should be observable in ferromagnetic systems, too, when the dynamic interplay between the different energy terms governing the magnetization dynamics becomes significant enough.

The magnetization dynamics of a synthetic antiferromagnet subjected to a short-magnetic-field pulse has been studied by using a combination of first principles calculations and atomistic spin-dynamics simulations. We observe switching phenomena on the time scale of tens of picoseconds, and inertia-like behavior in the magnetization dynamics. We explain the latter in terms of a dynamic redistribution of magnetic energy from the applied-field pulse to other possible energy terms, such as the exchange interaction and the magnetic anisotropy, without invoking concepts such as the inertia of an antiferromagnetic vector. We also demonstrate that such dynamics can also be observed in a ferromagnetic material where the incident-field pulse pumps energy to the magnetic anisotropy.