Beyond Majorana: Ultracold gases as a platform for observing exotic robust quantum states

The quantum Hall effect, discovered in the early 1980s, is a phenomenon that was observed in a two-dimensional gas of electrons existing at the interface between two semiconductor layers. Subject to the severe criteria of very high material purity and very low temperatures, the electrons, when under the influence of a large magnetic field, will organize themselves into an ensemble state featuring remarkable properties.

Many physicists believe that quantum Hall physics is not unique to electrons, and thus it should be possible to observe this behavior elsewhere, such as in a collection of trapped ultracold atoms. Experiments at JQI and elsewhere are being planned to do just that. On the theoretical front, scientists* at JQI and University of Maryland have also made progress, which they describe in the journal Physical Review Letters. The result, to be summarized here, proposes using quantum matter made from a neutral atomic gas, instead of electrons. In this new design, elusive exotic states that are predicted to occur in certain quantum Hall systems should emerge. These states, known as parafermionic zero modes, may be useful in building robust quantum gates.

For electrons, the hallmark of quantum Hall behavior includes a free circulation of electrical charge around the edge but not in the interior of the sample. This research specifically relates to utilizing the fractional quantum Hall (FQH) effect, which is a many-body phenomenon. In this case, one should not consider just the movement of individual electrons, but rather imagine the collective action of all the electrons into particle-like “quasiparticles.” These entities appear to possess fractional charge, such as 1/3.

How does this relate to zero modes? Zero modes, as an attribute of quantum Hall systems, come into their own in the vicinity of specially tailored defects. Defects are where quasiparticles can be trapped. In previous works, physicists proposed that a superconducting nanowire serve as a defect that snags quasiparticles at either end of the wire. Perhaps the best-known example of a composite particle associated with zero-mode defects is the famed Majorana fermion.

Author David Clarke, a Postdoctoral Research Scholar from the UMD Condensed Matter Theory Center, explains, “Zero modes aren’t particles in the usual sense. They’re not even quasiparticles, but rather a place that a quasiparticle can go and not cost any energy.”

Aside from interest in them for studying fundamental physics, these zero modes might play an important role in quantum computing. This is related to what’s known as topology, which is a sort of global property that can allow for collective phenomena, such as the current of charge around the edge of the sample, to be impervious to the tiny microscopic details of a system. Here the topology endows the FQH system with multiple quantum states with exactly the same energy. The exactness and imperturbability of the energy amid imperfections in the environment makes the FQH system potentially useful for hosting quantum bits. The present report proposes a practical way to harness this predicted topological feature of the FQH system through the appearance of what are known as parafermionic zero-modes.

These strange and wonderful states, which in some ways go beyond Majoranas, first appeared on the scene only a few years ago, and have attracted significant attention. Now dubbed ‘parafermions,’ they were first proposed by Maissam Barkeshli and colleagues at Stanford University. Barkeshli is currently a postdoctoral researcher at Microsoft Station Q and will be coming soon to JQI as a Fellow. Author David Clarke was one of the early pioneers in studying how these states could emerge in a superconducting environment. Because both parafermions and Majoranas are expected to have unconventional behaviors when compared to the typical particles used as qubits, unambiguously observing and controlling them is an important research topic that spans different physics disciplines. From an application standpoint, parafermions are predicted to offer more versatility than Majorana modes when constructing quantum computing resources.

What this team does, for the first time, is to describe in detail how a parafermionic mode could be produced in a gas of cold bosonic atoms. Here the parafermion would appear at both ends of a one-dimensional trench of Bose-Einstein Condensate (BEC) atoms sitting amid a larger two-dimensional formation of cold atoms displaying FQH properties. According to first author and Postdoctoral Researcher Mohammad Maghrebi, “The BEC trench is the defect that does for atoms what the superconducting nanowire did for electrons.”

Some things are different for electrons and neutral atoms. For one thing, electrons undergo the FQH effect only if exposed to high magnetic fields. Neutral atoms have no charge and thus do not react strongly to magnetic fields; researchers must mimic this behavior by exposing the atoms to carefully designed laser pulses, which create a synthetic field environment. JQI Fellow Ian Spielman has led this area of experimental research and is currently performing atom-based studies of quantum Hall physics.

Another author of the PRL piece, JQI Fellow Alexey Gorshkov, explains how the new research paper came about: “Motivated by recent advances in Spielman's lab and (more recently) in other cold atom labs in generating synthetic fields for ultracold neutral atoms, we show how to implement in a cold-atom system the same exotic parafermionic zero modes proposed originally in the context of condensed-matter systems.”

“We argue that these zero modes, while arguably quite difficult to observe in the condensed matter context, can be observed quite naturally in atomic experiments,” says Maghrebi. “The JQI atmosphere of close collaboration and cross-fertilization between atomic physics and condensed matter physics on the one hand and between theory and experiment on the other hand was at the heart of this work.”

“Ultracold atoms play by a slightly different set of rules from the solid state,” says JQI Fellow Jay Sau. Things which come easy in one are hard in the other. Figuring out the twists in getting a solid state idea to work for cold atoms is always fun and the JQI is one of the best places to do it.”

(*)  The PRL paper has five authors, and their affiliations illustrate the complexity of modern physics work.  Mohammad Maghrebi, Sriram Ganeshan, Alexey Gorshkov, and Jay Sau are associated with the Joint Quantum Institute, operated by the University of Maryland and the National Institute for Standards and Technology.  Three of the authors---Ganeshan, Clarke, and Sau---are also associated with the Condensed Matter Theory Center at the University of Maryland physics department.  Finally, Maghrebi and Gorshkov are associated with the Joint Center for Quantum Information and Computer Science (usually abbreviated QuICS), which is, like the JQI, a University of Maryland-NIST joint venture.

JQI

Nanotechnology World Association

IBM Scientists Demonstrate Quantum Phenomenon for the First Time Using a Plastic Film

For the first time, scientists at IBM Research (NYSE: IBM) have demonstrated a complex quantum mechanical phenomenon known as Bose-Einstein condensation (BEC), using a luminescent polymer (plastic) similar to the materials in light emitting displays used in many of today's smartphones.

This discovery has potential applications in developing novel optoelectronic devices including energy-efficient lasers and ultra-fast optical switches — critical components for powering future computer systems to process massive Big Data workloads. The use of a polymer material and the observation of BEC at room temperature provides substantial advantages in terms of applicability and cost. 

IBM scientists around the world are focused on an ambitious data centric exascale computing program, which is aimed at developing systems that can process massive data workloads fifty times faster than today. Such a system will need optical interconnects capable of high-speed processing of Petabytes to Exabytes of Big Data. This will enable high-performance analytics for: energy grids, life sciences, financial modelling, business intelligence and weather and climate forecasting. 

The complex phenomenon IBM scientists demonstrated at room temperature is named after the renown scientists Satyendranath Bose and Albert Einstein who first predicted it in the mid-1920s and only later experimentally proven in 1995. 

A Bose-Einstein Condensate is a peculiar state of matter which occurs when a dilute gas of particles (bosons) are cooled to nearly absolute zero (-273 Celsius, -459 Fahrenheit). At this temperature intriguing macroscopic quantum phenomena occur in which the bosons all line up like ballroom dancers. 

In 1995 this was demonstrated for the first time at these extreme temperatures, but today in a paper appearing in Nature Materials, IBM scientists have achieved the same state at room temperature using a thin non-crystalline polymer film developed by chemists at the University of Wuppertal in Germany. 

In the experiment, a thin polymeric layer is placed between two mirrors and excited with laser light. This thin plastic film is approximately 35 nanometers thick, for comparison a sheet of paper is about 100,000 nanometers thick. The bosonic particles are created through interaction of the polymer material and light which bounces back and forth between the two mirrors. 

The phenomenon only lasts for a few picoseconds (one trillionth of a second), but the scientists believe this is already long enough to use the bosons to create a source of laser-like light and/or an optical switch for future optical interconnects. These components are important building blocks to control the flow of information in the form of zeroes and ones between future chips and can significantly speed up their performance while using much less energy. 

"That BEC would be possible using a polymer film instead of the usual ultra-pure crystals defied our expectations," said Dr. Thilo Stoferle, a physicist, at IBM Research. "It's really a beautiful example of quantum mechanics where one can directly see the quantum world on a macroscopic scale." 

The next step for scientists is to study and control the extraordinary properties of the Bose-Einstein Condensate and to evaluate possible applications including analog quantum simulations. Such simulations could be used to model very complex scientific phenomena such as superconductivity, which is difficult using today's computational approaches.

The research was funded under the European Union's FP7 Project named ICARUS. The goal of ICARUS is to create and characterize new hybrid-semiconductor systems and then implement them in photonic and optoelectronic devices. For more information visitwww.icarus.group.shef.ac.uk 

This research was conducted in the Binnig and Rohrer Nanotechnology Center at IBM Research - Zurich. 

The scientific paper entitled "Room-temperature Bose–Einstein condensation of cavity exciton–polaritons in a polymer"   by Johannes D. Plumhof, Thilo Stoferle, Lijian Mai, Ullrich Scherf and Rainer F. Mahrt, appears in Nature Materials, DOI: 10.1038/NMAT3825

Source: http://www-03.ibm.com/press/us/en/pressrelease/42710.wss

A SQUID Analog with a Bose-Einstein Condensate

Figure 1: (a) Superposition of horizontal light sheet and a rapidly moving vertical
beam is used to create attractive potentials for BECs. By translating the
vertical beam in the horizontal plane, Ryu et al. can “paint” an arbitrary trapping
configuration. (b) Using this technique, Ryu et al. created a potential in the form
of a loop interrupted by two thin barriers. (c) Absorption image of a BEC in this
potential, resembling a dc-SQUID-like configuration.

Macroscopic quantum effects in Bose-Einstein condensates permit new kinds of ultrasensitive detectors.

Superconducting Quantum Interference Devices (specifically, dc-SQUIDs) are the world’s most sensitive sensors for magnetic flux. What lies at the heart of this quantum interference technology is the Josephson effect, which is a striking example of a macroscopic quantum phenomenon [1]. When two macroscopically coherent quantum systems such as superconductors are weakly coupled together, through a thin insulating layer for example, a direct current can appear across the junction with no applied voltage. This is known as the dc Josephson effect. As if that were not strange enough, if one tries to put a constant voltage across the junction, the direct current disappears and counterintuitively an alternating current now appears. This is known as the ac Josephson effect. By setting up a loop of a quantum system interrupted by a pair of such junctions, we can exploit these phenomena for interferometry. Magnetic flux acting on the electrons through a superconducting loop causes a phase shift between electric currents that flow across the two junctions and this interference alters the amplitude of the overall oscillation. As a result, dc-SQUIDs find application as ultrasensitive magnetometers [2].

In a neutral system, the role of magnetic field can be played by other effects such as rotational dynamics, and dc-SQUID analogs that work as rotation sensors have been implemented using superfluid helium—an uncharged fluid analog of superconductors [3, 4]. Now, in a paper in Physical Review Letters, Changehyun Ryu and colleagues from Los Alamos National Laboratory, New Mexico, report the creation of a dc-SQUID analog using a Bose-Einstein condensed (BEC) atomic gas [5]. The result should not only encourage studies on BEC properties in complex arrangements but also accelerate the development of an array of cold atom “devices” that may be applied to various other investigations.

How can an ultracold gas act like a dc-SQUID? The Josephson effect relies on the sinusoidal nature of the relation between the current that flows and the relative phase that exists across the junction. In conventional dc-SQUIDs the flow is an electric current, whereas in BECs it is a mass current, and the relative phase is the difference in phase factors of the wave function on either side of the junction. In these macroscopic quantum systems, the relative phase evolves in time in response to the applied potential differences (voltage difference for superconductors, pressure and temperature differences for superfluid helium, and population difference between the two condensates for BECs). A constant potential difference, for example, leads to a relative phase that increases linearly in time, and through the sinusoidal current-phase relation, this then gives rise to an oscillating current. The nonlinear current-phase relation is a key signature of the leakage and the weak coupling of the two macroscopic wave functions.

To achieve this weak coupling, for superconducting systems, researchers employ junctions that are either a tunneling type (where electrons tunnel through a thin nonsuperconducting element) or a constriction type (where the junction is a superconductor, but its physical dimension is made small). For superfluid helium, dc-SQUID analogs are made with a toroidal container interrupted with a pair of constriction-type junctions.

In BEC systems, researchers have observed the Josephson phenomena with tunneling-type junctions made of thin barriers in single-weak-link geometries [6, 7]. Wright et al. recently reported a vividly illuminating experiment that utilized a toroidal BEC interrupted with a single constriction-type junction [8] (see 10 January 2013 Synopsis). In that geometry, no interference takes place, but researchers investigated rich physics in a regime just outside of the Josephson regime where the atomic current as a function of relative phase was hysteretic but not purely sinusoidal.

In paving a way towards the BEC version of the dc-SQUID, Ryu et al. used a technique that they developed several years ago for creating arbitrary potentials for static trapping and dynamical manipulation of BECs [9]. By rapidly moving a laser beam over a static light sheet that provides tight confinement, they effectively “paint” an attractive optical potential in an appropriate geometry (Fig. 1). Here the spatial resolution of the potential is on the order of a few micrometers, which allowed them to create a toroidal potential interrupted by a pair of tunneling-type Josephson junctions. They then moved the junctions circumferentially towards each other, a clever scheme proposed by Giovanazzi et al. [10].

If the junctions are moved slowly enough, atoms tunnel through them and keep the potential difference between the two sectors of the toroidal atomic cloud approximately zero. This is in close analogy to the dc Josephson effect, where a direct current appears across the junction with no driving potential difference. However, as the junction speed is ramped up, the tunneling current cannot respond quickly enough, and a finite potential difference develops at some critical velocity. The atomic current oscillation should then be driven by this energy difference with a frequency proportional to it, which is the ac Josephson effect.

In the absence of dissipation, no net current flows in this ac regime, as the current is only alternating. Hence for speeds any higher than the critical velocity, atoms simply become compressed on one side and expanded on the other side, which appears as a sudden change in the relative population difference in the two sectors of the toroid. Since the Josephson oscillation itself was too small to directly detect, the transition between the dc and ac Josephson effects was sought after to study the Josephson dynamics.

Ryu et al. measured the relative population difference as a function of the total atom number for several fixed rotational speeds of the junctions. Their data from the absorption images of a BEC indeed show dynamic behavior consistent with the system transitioning from dc to ac Josephson regimes, and the overall atom number dependence of atomic current at such transitions is in good agreement with the predictions of the Josephson dynamics. Although it is done indirectly, the current-phase relation is found to be sinusoidal through a simulation on the experiment.

The work of Ryu et al., along with that of Wright et al. [8], offers an exciting opportunity towards sophisticated atom circuits designed and implemented as meaningful tools. In various configurations with or without the Josephson junctions, the cold-atom system has the potential to form an ideal testing ground for phenomena seen in other analogous systems but now with more tunability. That is clear from this result, which advances the fundamental and thought-provoking analogy between the macroscopic quantum physics of superconductivity, superfluids, and BEC gases.

So what remains to be done? A lot, actually. One immediate step is simply more measurements, in particular, obtaining the junctions’ current-phase relation directly. With dynamic variations in the coupling strength, the relation between the atomic current and the relative phase across the junction should exhibit a smooth transition from a linear to sinusoidal shape as the system enters the Josephson regime. This might emerge in a series of destructive interference fringe measurements as the two sections of the BEC toroid are allowed to expand and overlap physically. Another truly revealing measurement would be to directly observe the Josephson oscillation itself in this multijunction system, as done in single-junction experiments, and to demonstrate that the oscillation frequency is proportional to the applied potential difference.

When a beam of light or matter is split and recombined while enclosing a finite area, physical rotation of the instrument leads to a difference in the path lengths in the two directions and gives rise to a rotation-induced phase shift. Through this celebrated Sagnac effect, combined with the quantization condition that the total phase change along a loop must be an exact integer multiple of 2π, a BEC version of the dc-SQUID would function as a rotation sensor with potential applications in navigation, seismology, and geodesy. To explore such possibilities, the demonstration of double-path quantum interference via externally applied rotation may be in order. Studies along those lines should not only reveal intrinsic noises of the system configured as such a device but may also lead to the discovery of various nonlinear phenomena to enhance its sensitivity and utility.

Source: http://physics.aps.org/articles/v6/123?referer=rss