What are these electronic nanostars in 2D-superconductors supposed to mean?

Physicists from France and Russia have discovered magnetic disturbances in two-dimensional (2D) layered superconductors, resembling small oscillating stars. These star-like electronic excitations are located around individual magnetic atoms put inside the superconducting material. This experimental observation, made by means of scanning tunneling spectroscopy at only 0.3 degrees above the absolute zero-temperature, is the direct confirmation of the famous theory by Yu-Shiba-Rusinov who predicted these quantum bound magnetic sates to exist. Moreover, it was found out that in the two-dimensional systems the magnetic excitations spread over longer distances as compared to ordinary three dimensional superconducting materials. This discovery opens a route for generating more complex quantum states, topologically protected from decoherence, out of chains or clusters of magnetic atoms in superconductors. Building and manipulating such protected states is a crucial step towards the long-awaited quantum computers. The results were published in Nature Physics this week.

Prof. Dmitri Roditchev from the Superior School of Industrial Physics and Chemistry (ESPCI ParisTech, Paris), Gerbold Ménard,Dr. Christophe Brun, Dr. Tristan Cren from the Institute of Nanosciences of Paris at Sorbonne University, Dr. Vasily Stolyarov from the Laboratory of Topological Quantum Phenomena in Superconducting Systems at MIPT, and their colleagues from Paris-Saclay University studied the emergence of Yu-Shiba-Rusinov (YSR) states bound around single magnetic atoms embedded in a two-dimensional superconductor. YSR states were theoretically predicted in the 1960s, but very few evidences for them have been experimentally revealed till now. In the present work it was found that in two-dimensional systems, magnetic excitations extend over a greater distance as compared to ordinary three-dimensional superconductors, and the emergent YSR quantum states are more stable, which makes them more suitable for developing a new generation of quantum electronics.

A crystal lattice of a layered superconducting material  -niobium diselenide - was used in the tests. With an ultra-low-temperature scanning tunnelling microscope built by Roditchev, the researchers were able to observe , for the first time, YSR states around single magnetic atoms of iron.  “We have demonstrated that the use of two-dimensional superconductors instead of the three dimensional ones results in an increase in the spatial extension of YSR states for several dozen nanometres, i.e. ten times further than in “normal” three-dimensional superconductors. And the area of excitation was shaped like a six-fold electronic “star” with its rays extending along the axis of the crystal lattice of niobium diselenide. The observed “stars” are more stable and more suitable to creating new topologically protected states. Non-Abelian anyons can be collected from the YSR state chains, and can then be used as elements in quantum computers of the future,” says Vasily Stolyarov, a co-author of the study and the head of the Laboratory of Topological Quantum Phenomena in Superconducting Systems at MIPT.

The experiments described in the article were conducted in Paris. Work is underway at MIPT’s Laboratory of Topological Quantum Phenomena in Superconducting Systems to create the experimental conditions necessary to obtain such high quality results. The Laboratory was set up in 2014 using funds from a mega grant awarded to Alexander Golubov, a professor  at the University of Twente (Netherlands). The main purpose of the Laboratory is to study the quantum properties of new superconductors and topologically protected materials, and also hybrid artificial systems based on these materials.

 The laboratory is being equipped in close cooperation with Roditchev’s and Cren’s groups and it is expected that the facilities at both laboratories will complement one another.

 Yu-Shiba-Rusinov states were predicted in the late 1960s by three physicists from China, the USSR, and Japan independently of one another. They suggested that magnetic atoms introduced into a superconductor must create special states of excitation around themselves – electron-hole standing waves named after their discoverers. Calculations show that areas of topological conductivity may form around these states, where the current is only able to flow in one direction. Until recently, however, it had not been possible to confirm this prediction experimentally.

For the last 20 years, scientists have been attempting to create quantum systems that will outperform traditional semiconductor-based computers, the development potential of which is now almost exhausted. A number of potential “candidate” systems to be used as a base to build the components of a quantum computer are currently being investigated. The main problem preventing the development of these computers is the high sensitivity of the nanoworld to external influences that destroy quantum states. One promising option is to use topologically protected electron states that are resistant to decoherence. Non-Abelian anyons may be perfect for this; they are not negative ions, but rather special excitations in two-dimensional quantum systems in a magnetic field. 

The theory predicts that such non-Abelian anyons may occur in a two-dimensional “liquid” of electrons in a superconductor under the influence of a local magnetic field. The electron liquid thus becomes degenerate, i.e. the electrons can have different states at the same energy level. The superposition of several anyons cannot be affected without moving them, therefore they are completely protected from disturbances.

Moscow Institute of Physics and Technology

Measuring the smallest vibration

absolute zero. This required building a sensor capable of resolving the smallest vibration allowed by quantum mechanics.

Quantum mechanics predicts that a mechanical object, even when cooled to absolute zero, produces small vibrations, called “zero-point fluctuations”. The reason we don’t observe these vibrations in everyday life is that for a tangibly sized object at room temperature, they are much smaller than the vibrations caused by thermal motion of atoms. EPFL researchers have overcome this challenge by coupling a micrometer-sized glass string to a very precise, optical displacement sensor. The sensor is so precise that it can, in principle, resolve the string’s zero-point fluctuations before they are obscured by thermal vibrations. Combining this low-noise readout with a feedback force, the scientists were able to suppress the string’s thermal vibrations to a magnitude only 10 times larger than their zero-point value – in effect realizing an extreme version of a noise cancellation headphone. The work is published in Nature.

Feedback at the quantum limit

Feedback is a ubiquitous tool in modern engineering, used in applications ranging from cruise control to atomic clocks. The basic paradigm uses a sensor to monitor the state of a system (e.g. a car's velocity) and an actuator (e.g. an engine throttle) to steer the system along a desired path (e.g. within the ed limit).

As sensor technology advances, it has been proposed to use such "feedback control" to prepare and stabilize delicate quantum states – for instance, the celebrated half-living, half-dead state of Schrodinger's Cat. Aside from their fundamental interest, the ability to cultivate quantum states is expected to play a crucial role in future technologies such as quantum computers.

The main challenge to quantum feedback control is something called "decoherence", which dictates that the behavior of a system in a quantum state is rapidly destroyed by its interaction with the thermal environment. This places stringent requirements on the speed and precision of the sensor. Successful demonstrations have therefore been limited to a small subset of well-isolated systems, like individual trapped atoms, photons, and superconducting circuits.

Shedding light on the problem

The lab of Tobias J. Kippenberg at EPFL has fabricated an extremely precise optical position sensor that may – surprisingly – extend quantum feedback control to engineered mechanical devices . In the "blink of an eye" (0.3 - 0.4 seconds) the sensor is capable of resolving a displacement 100 times smaller than the size of a proton. Making use of such a high-speed sensor, it is possible to capture an image in which the blur (or uncertainty) of an object's position is smaller than the uncertainty caused by the thermal motion of its constituent atoms.

Using a continuous stream of such "freeze frames", the researchers have used feedback to reduce the motion of a mechanical device – in this case the vibration of a micron-sized, glass string – to the value it would have if the device were cooled 0.001 degrees above absolute zero. The residual vibration of the string is only 10 times bigger than the minimum (``zero-point”) value allowed by quantum mechanics. This means that the string spends 10% of its time in its quantum “ground state.”

Kippenberg’s lab specializes in the field of “cavity optomechanics”. In optomechanics, much like in high-speed photography, the motion of a mechanical device is imaged using an intense beam of light. The challenge in this case was to focus the light into a very small spot, in order to maximize its interaction with the tiny string.

The researchers achieved this by confining the light and the string to a miniature “hall of mirrors” called an “optical microcavity”. Developed in the Center of MicroTechnology at EPFL, the optical microcavity is a small, disk-shaped piece of glass, above which the string is suspended by 50 nm. Laser light coupled into the disk circulates along its periphery ~10,000 times, each time reflecting off the string and incurring a small delay in proportion to the string’s vibration amplitude. This delay is measured using a technique called interferometry.

To cool the string’s vibration, the researchers took advantage of a well-known side-effect of optical measurement: namely, that each reflection of the circulating field also imparts a small force, called “radiation pressure”, on the string. Using a sequence of electronics, the researchers imprinted a measurement of the string’s vibration onto the intensity of a second laser field. As lead author Dal Wilson explains, “The radiation pressure applied by the second field, when appropriately delayed, exactly opposes the thermal motion of the string, like a noise-cancellation headphone”.

http://www.nanotechnologyworld.org/#!Measuring-the-smallest-vibration/c89r/55c8cde20cf2244af607fa2e

A New Starting Point for Atom Interferometry

Researchers have developed a new atom interferometer that has the potential to be the world’s most sensitive accelerometer.

Interferometers using atoms rather than light can measure acceleration and rotation to high precision. Because atoms are slower than light, atom interferometers have the potential to reach greater inertial sensitivity than their optical counterparts. However, one of the main limitations in atomic interferometry has been discriminating the different contributions to the interference signal. Bright/dark fringes at the output of an atom interferometer result from phase shifts induced by multiple inertial effects as well as interferometer imperfections. In Physical Review Letters, Susannah Dickerson and colleagues from Stanford University, California, now report on a new method that can directly resolve and characterize these phase shifts. They perform their “point source interferometry” on a 10-meter-high atomic fountain, where they can simultaneously measure rotation and acceleration with unprecedented sensitivity [1]. The results open new perspectives in the development of ultraprecise inertial navigation systems as well as future precision tests of general relativity.

The atoms inside an atom interferometer are controlled by beam splitters and mirrors, which in this case are light pulses tuned to particular resonance frequencies in the atoms. The interpretation in terms of matter waves follows from the analogy with optical interferometry. The first beam splitter that an incoming matter wave encounters separates the wave into two different paths. The accumulation of phase along the two paths leads to interference at the last beam splitter, whose two output channels produce complementary probability amplitudes for detecting atoms. The detection probability in each channel is then a sine function of the accumulated phase difference. Most generally, atom interferometers are based on the Mach-Zehnder design, in which two splitting processes are separated by a mirror that folds the paths back together.

These atom interferometers are not only impressive manifestations of quantum physics, but they can also be extremely sensitive inertial sensing devices. The first demonstration of matter-wave inertial sensors occurred about twenty years ago, and since then atom interferometers have evolved into instruments at the leading edge of precision measurements. They measure inertial or gravitational forces affecting the propagation of matter waves with sensitivities comparable to, or even better than, existing classical sensors for rotation as well as for acceleration [2]. With their present performance and technological maturity, these inertial quantum sensors have found a place in a variety of applied and fundamental applications such as navigation [3], geophysics, and gravitation. Current efforts are aimed at either pushing the limits in sensitivity with long baseline experiments [4] or developing compact, commercial devices [5].

Why can we build such sensitive inertial sensors with atom interferometers? “Classical” arguments can provide a basic understanding. When atoms are subject to acceleration or rotation along their trajectory, their speed along this trajectory is modified. This variation of atomic speed results in a variation of the atomic de Broglie wavelength, which itself leads to a dephasing between the two interferometer arms that changes the detection probability at each output channel. In general, atoms enter an interferometer with a wide range of velocities, which means the incoming matter wave has a large spread in de Broglie wavelengths. The outgoing matter wave blends together the effects of rotation and acceleration, as well as unwanted contributions from wave-front distortion and mirror vibrations. For navigation applications, the rotation response needs to be isolated from the acceleration response, and for precision measurements, the noise sources need to be fully understood and characterized. In practice, the different inputs are separated using multiple interferometers aligned along different directions [6].

The experiment reported by Dickerson et al. is admirable in many respects. It employs the longest baseline in atom interferometry, with a 10-meter-tall vacuum enclosure. It also manages to resolve the velocity-dependent phase shifts, allowing multiaxis inertial sensing with a single device. To achieve this, the Stanford team generated a small ultracold atom cloud just 200 micrometers in diameter as their device input. This “point source” contained a few million rubidium atoms at a temperature of less than 50nanokelvin. Using lasers, the team launched their cloud from the bottom of the vacuum enclosure and applied a three-pulse accelerometer sequence, which separates the cloud into two ballistic trajectories—one going slightly higher than the other [see Fig.1(a)]. With this apparatus, Dickerson et al. succeeded in realizing an experimental idea from 60 years ago [7].

Because the atoms are so cold, the cloud is still relatively small when the two paths recombine 3 seconds after the initial split [see Fig.1(b)]. The final cloud—with its interference fringes—is just a few millimeters across, allowing it to be fully imaged on two CCD cameras at the bottom of the enclosure. Nevertheless, the cloud is a factor of 30 larger than its initial size, and this expansion means that each pixel of the CCD cameras corresponds to the probability amplitude for a specific parabolic trajectory through the interferometer. It is as if the point source spreads out into multiple atom “lasers,” i.e., particularly well-collimated atomic wave packets with specific de Broglie wavelengths, and each pixel records the output of the interferometer from each of these lasers. As for classical optics, the resolution set by the pointlike source makes it possible to record directly on the CCD the full interference pattern, not just the probability amplitude. The methods developed by Dickerson and co-workers allow them to extract, at will, information about rotation, acceleration, and even interferometer imperfections such as optical wave-front distortion. They show that the potential acceleration sensitivity of their device is 6.7×10−12g, which is two orders of magnitude better than the previous limit.

This long baseline experiment brings atom interferometry into a new regime of quantum manipulation. To demonstrate this, the researchers imaged the atoms about 1 second after the first beam splitting pulse, at the peak of the fountain trajectory, where a CCD camera captured the atoms in a quantum superposition of 2-millimeter-wide clouds separated by 1.4 centimeters. Even though this wide separation is a natural consequence of coherent wave-packet manipulation (which can also be achieved with large momentum beam splitting), it is still fascinating to have a direct image of this quantum object. Such a long-lived (seconds) macroscopically separated wave packet raises new challenges for studying relativistic effects, or even decoherence induced by spacetime fluctuations [8].

What can happen next? Very long baseline atom interferometers are actively being studied in several ground-based experiments as well as for certain proposed space missions. The hope is that these precise quantum devices may eventually test the equivalence principle or detect gravitational waves [9]. In this ongoing development, one question concerns what type of atom source will achieve the greatest precision: incoherent “light bulbs” or coherent “lasers.” The achievement by Dickerson et al. argues that “light bulbs” can power extremely sensitive devices. However, other ongoing experiments with atom lasers [10] will certainly give other answers—or raise new questions.http://physics.aps.org/articles/v6/92?referer=rss