NIST’s Super Quantum Simulator ‘Entangles’ Hundreds of Ions

NIST physicists have built a quantum simulator made of trapped beryllium ions (charged atoms) that are proven to be entangled, a quantum phenomenon linking the properties of all the particles. The spinning crystal, about 1 millimeter wide, can contain anywhere from 20 to several hundred ions.

Credit: NIST

Physicists at the National Institute of Standards and Technology (NIST) have “entangled” or linked together the properties of up to 219 beryllium ions (charged atoms) to create a quantum simulator. The simulator is designed to model and mimic complex physics phenomena in a way that is impossible with conventional machines, even supercomputers. The techniques could also help improve atomic clocks.

The new NIST system can generate quantum entanglement in about 10 times as many ions as any previous simulators based on ions, a scale-up that is crucial for practical applications. The behavior of the entangled ions rotating in a flat crystal just 1 millimeter in diameter can also be tailored or controlled to a greater degree than before.

Described in the June 10, 2016, issue of Science, NIST’s latest simulator improves on the same research group’s 2012 version by removing most of the earlier system’s errors and instabilities, which can destroy fragile quantum effects.

“Here we get clear, indisputable proof the ions are entangled,” NIST postdoctoral researcher Justin Bohnet said. “What entanglement represents in this case is a useful resource for something else, like quantum simulation or to enhance a measurement in an atomic clock.”

In the NIST quantum simulator, ions act as quantum bits (qubits) to store information. Trapped ions are naturally suited to studies of quantum physics phenomena such as magnetism.

Quantum simulators might also help study problems such as how the universe began, how to engineer novel technologies (for instance, room-temperature superconductors or atom-scale heat engines), or accelerate the development of quantum computers. According to definitions used in the research community, quantum simulators are designed to model specific quantum processes, whereas quantum computers are universally applicable to any desired calculation.

Quantum simulators with hundreds of qubits have been made of other materials such as neutral atoms and molecules. But trapped ions offer unique advantages such as reliable preparation and detection of quantum states, long-lived states, and strong couplings among qubits at a variety of distances.

In addition to proving entanglement, the NIST team also developed the capability to make entangled ion crystals of varying sizes—ranging from 20 qubits up to hundreds. Even a slight increase in the number of particles makes simulations exponentially more complex to program and carry out. The NIST team is especially interested in modelling quantum systems of sizes just beyond the classical processing power of conventional computers.

“Once you get to 30 to 40 particles, certain simulations become difficult,” Bohnet said. “That’s the number at which full classical simulations start to fail. We check that our simulator works at small numbers of ions, then target the sweet spot in this midrange to do simulations that challenge classical simulations. Improving the control also allows us to more perfectly mimic the system we want our simulator to tell us about.”

The ion crystals are held inside a Penning trap, which confines charged particles by use of magnetic and electric fields. The ions naturally form triangular patterns, useful for studying certain types of magnetism. NIST is the only laboratory in the world generating two-dimensional arrays of more than 100 ions. Based on lessons learned in the 2012 experiment, NIST researchers designed and assembled a new trap to generate stronger and faster interactions among the ions. The interaction strength is the same for all ions in the crystal, regardless of the distances between them.

The researchers used lasers with improved position and intensity control, and more stable magnetic fields, to engineer certain dynamics in the “spin” of the ions’ electrons. Ions can be spin up (often envisioned as an arrow pointing up), spin down, or both at the same time, a quantum state called a superposition. In the experiments, all the ions are initially in independent superpositions but are not communicating with each other. As the ions interact, their spins collectively morph into an entangled state involving most, or all of the entire crystal.

Researchers detected the spin state based on how much the ions fluoresced, or scattered laser light. When measured, unentangled ions collapse from a superposition to a simple spin state, creating noise, or random fluctuations, in the measured results. Entangled ions collapse together when measured, reducing the detection noise. 

Crucially, the researchers measured a sufficient level of noise reduction to verify entanglement, results that agreed with theoretical predictions. This type of entanglement is called spin squeezing because it squeezes out (removes) noise from a target measurement signal and moves it to another, less important aspect of the system. The techniques used in the simulator might someday contribute to the development of atomic clocks based on large numbers of ions (current designs use one or two ions). 

“The reduction in the quantum noise is what makes this form of entanglement useful for enhancing ion and atomic clocks,” Bohnet said. “Here, spin squeezing confirms the simulator is working correctly, because it produces the quantum fluctuations we are looking for.”

The work was funded in part by the National Science Foundation, Army Research Office and Air Force Office of Scientific Research.

Reference: 

J.G. Bohnet, B.C. Sawyer, J.W. Britton, M.L. Wall, A.M. Rey, M. Foss-Feig, J.J. Bollinger. 2016. Quantum spin dynamics and entanglement generation with hundreds of trapped ions. Science, June 10, 2016. DOI: 10.1126/science.aad9958 

NIST

A new quantum memory on the horizon

Memory candidate with a bright future: Max Planck
researchers have addressed individual praseodymium
ions in the crystal of an yttrium orthosilicate using
resourceful microscopy and laser technologies.
This opens up the possibility of storing quantum
information in these ions, which have several advantages
compared to other memory candidates.
© MPI for the Science of Light

Sensitive measurements can be used to detect signals from an individual ion in a crystal

A promising material is lining itself up as a candidate for a quantum memory. A team at the Max Planck Institute for the Science of Light in Erlangen is the first to succeed in performing high-resolution spectroscopy and microscopy on individual rare earth ions in a crystal. With the aid of ingenious laser and microscopy technology they determined the position of triply charged positive praseodymium atoms (Pr3+) in an yttrium orthosilicate to within a few nanometres and investigated their weak interaction with light. In addition to its impact on fundamental studies, the work may make an important contribution to the quantum computers of the future because the ions investigated are suitable for storing and processing quantum information.

Around the globe, numerous researchers are working on components for the quantum computers of the future, which will be able to process information significantly faster than today. The key elements of these super-computers include quantum systems with optical properties similar to those of an atom. This is why many researchers are currently focusing their attention on different systems such as light-emitting crystal defects (“colour centres”) in diamond or on semiconductor quantum dots. However, so far there has been no ideal solution. “Some of the light sources lose their brightness or flicker in an uncontrollable way,” explains Vahid Sandoghdar, who heads the Nano-Optics Department at the Max Planck Institute for the Science of Light in Erlangen. “Others are greatly affected by the environment into which they are embedded.”

Researchers observe the signals of an individual ion

It has long been known that the rare earth ions such as neodymium or erbium do not suffer from these problems – which is also why they play a key role in lasers or laser amplifiers. They emit only weakly, however, and are therefore difficult to detect. This is precisely what Tobias Utikal, Emanuel Eichhammer and Stephan Götzinger from Sandoghdar’s Group in Erlangen have succeeded to do: after more than six years of intensive research they were able to detect individual praseodymium ions, pinpoint them with an accuracy of a few nanometres, and measure their optical properties with an accuracy never achieved before.

The triply charged, positive ions were embedded in tiny microcrystals and nanocrystals of yttrium orthosilicate (YSO). Their energies varied only slightly depending on their position in the crystal. In other words, they reacted to slightly different frequencies. The scientists used this to excite individual ions in the crystals with a laser and to observe how they emit the energy after some time in form of light. “Because rare earth ions are not strongly affected by the thermal and acoustic oscillations of the crystal, some of their energy states are unusually stable,” says Sandoghdar. “It takes more than a minute before they make the transition into the ground state again – a million times longer than for most of the other quantum systems that have been investigated so far.”

The aim is for the signals of the ions to be even easier to observe in the future. Since an individual ion responds with less than 100 photons per second at the moment, the Erlangen-based scientists want to employ nano-antennas and microcavities to amplify the praseodymium signal by a hundred or a thousand times.

http://www.mpg.de/8202685/quantum-ion-crystal

Ion beams pave way to new kinds of valves for use in spintronics

Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have tested a new approach to fabricating spin valves. Using ion beams, the researchers have succeeded in structuring an iron aluminium alloy in such a way as to subdivide the material into individually magnetizable regions at the nanometer scale. 

The prepared alloy is thus able to function as a spin valve, which is of great interest as a candidate component for use in spintronics. Not only does this technology use electron charge for purposes of information storage and processing, it also draws on its inherent magnetic properties (that is, its spin). Spintronics holds great potential for magnetic storage media. For example, with magnetic random access memories a computer's time-consuming start-up phase may cease to be an issue – as in that case it would be operational as soon as it is switched on.

Typically, a spin valve is made up of successive non-magnetic and ferromagnetic layers. This layering is a very involved process and getting these components to connect reliably presents a major challenge. This is why HZDR researcher Dr. Rantej Bali and his colleagues are taking an entirely different approach. “We’ve built structures with lateral spin valve geometry where the different magnetic regions are organized one next to the other as opposed to in layers one on top of the other,” explains Bali. The idea behind this new geometry is to facilitate working in parallel on larger surfaces while keeping fabrication costs low.

First, the scientists annealed a thin layer of an iron aluminum alloy (Fe₆₀Al₄₀) at 500 degrees C. This resulted in formation of a highly ordered structure, where every other atomic layer was made up exclusively of iron atoms. According to the researchers’ expectations, this substance behaved as a paramagnetic material – in other words, the magnetic moments became disordered. After this, the scientists coated the alloy with a protective polymer resist so that a striped pattern was produced on its surface. The resist-free regions were alternatingly 2 and 0.5 micrometers wide, and crucially, were separated from each other by 40 nanometer wide strips of resist.

Next, the material was irradiated with neon ions at the HZDR’s Ion Beam Center – with important consequences. The scientists were able to demonstrate that the irradiated material exhibits very interesting properties. Beneath the protective resist strips, the material remains paramagnetic while the resist-free narrow and wide stripes actually become ferromagnetic. “A spin valve is switched via the magnetic field. Changing the spins’ alignment – parallel or antiparallel – changes the electrical resistance. We’re interested in the magnitude of the effect,” says Bali. An externally applied magnetic field aligns the spins within these regions. Depending on the magnetic field’s strength, they can be adjusted to run in parallel or antiparallel. This magnetization is permanent and is not lost if the outer field is switched off.

The reason for this behavior lies in the fact that the ion beam changes the alloy’s structure. “The ions destroy the iron layers’ highly ordered structure. They knock the atoms out of position and other atoms take their place, and, as a result, the iron and aluminum atoms become randomly distributed,” explains Sebastian Wintz, a Ph.D. student who was part of the team of researchers. A small dose of ions is enough to play this atomic-level game of tag. Wintz characterizes the process as follows: “It’s a cascade, really. A single ion is capable of displacing up to 100 atoms." The regions beneath the polymer resist stripes, on the other hand, are impenetrable to the ions – which is why these regions remain paramagnetic and separate out the ferromagnetic stripes.

Working closely with researchers at the Helmholtz Center Berlin (HZB), the HZDR scientists were able to visualize the material’s magnetic structure using the special SPEEM (spin-resolved photoemission microscope) at the HZB’s BESSY II synchrotron. The microscopic images showed the existence of regions with paramagnetic and ferromagnetic order demonstrating the high level of spatial resolution that can be realized by the structuring process using ion beams.

Additional experiments will allow Rantej Bali and his colleagues to investigate the properties of these magnetically structured materials. The researchers are also trying to figure out the limits to miniaturization of magnetic nanostructures.

Source: http://www.hzdr.de/db/Cms?pNid=473&pOid=41055

An atom trap consisting of both rapidly rotating electric fields and static laser fields keeps ions securely locked into place in a latticelike potential

Over the past 60 years, atomic physicists have developed sophisticated techniques to trap and isolate single neutral atoms and ions. Now they are tweaking these techniques so they can re-assemble isolated particles into pairs, triplets, and larger arrays. These carefully controlled multiparticle systems could be used to simulate the behavior of solids, or function as prototype quantum bits for storing information and performing computations. An important technological step toward preparing ions for these types of experiments is now reported in Physical Review Letters by Leon Karpa and colleagues at the Massachusetts Institute of Technology, Cambridge [1]. The team combined two key trapping technologies—optical lattices and Coulomb potentials—into a hybrid trap that keeps ions anchored into one position for up to 100 times longer than existing lattice traps, even in the presence of external or stray electric fields. Ultimately, this highly stable trap could be used to capture arrays of ions to explore many-body solid-state effects.

Techniques for storing charged particles were first developed in the 1950s, when Wolfgang Paul and Hans Dehmelt independently designed and developed ion traps. To this day, Paul’s quadrupole trap, which uses a combination of alternating (ac) and static (dc) fields to confine strings of ions, is a key component of mass spectrometers. A common type of quadrupole trap is the linear Paul trap, in which ac fields generate a two-dimensional, saddle-shaped potential that rotates at tens of millions of revolutions per second (radio frequencies.) The rotation is too fast for the ions to follow, so they experience an effective bowl-shaped potential that traps them at the minimum. Static fields along the third dimension provide additional confinement.

By the early 1980s, physicists had confined single barium and magnesium ions in quadrupole traps. (Simultaneous advances in laser cooling, which slows ions and neutral atoms as they are loaded into a trap, played an essential role in these early trapping experiments.) Over the following decades, researchers demonstrated they could manipulate and read out the quantum states of individual ions with ever-increasing precision. Today, the world’s most accurate clock is based on the frequency of a trapped aluminum ion; and many proposed quantum-computing platforms rely on trapped ions.

Given this success, why are ion trappers turning to new traps? An important goal is to gain more control over the shape of the trapping potential, especially on short length scales. Achieving this would offer greater freedom to position the ions in tailored configurations and explore different interactions between ions. While quadrupole traps create deep trapping potentials, on the order of 10 electron volts (105 kelvin), they only have a simple parabolic structure. Shaping the dc fields can produce more complex trap geometries. But the length scales of these potentials are still on the order of hundreds of micrometers, which limits how closely together two ions can be positioned if each is in its own local minimum.

In contrast, an optical lattice trap, which is formed by superimposing two counterpropagating laser beams to make a standing wave, can trap atoms a few hundreds of nanometers apart. In these traps, the particles are locked by dipolar forces into the local minima of the standing wave generated by the laser field. The trap shapes can be further tailored by tuning the angles and wavelengths of the beams. These traps work particularly well for isolating and manipulating neutral ions. But with ions, the dipole forces from the laser fields are generally too weak to overcome the effects of stray electric fields on the charged particles. Researchers at the Max Planck Institute for Quantum Optics and the University of Freiburg in Germany have successfully trapped an ion in a focused Gaussian beam for a few milliseconds (ms) [2]. In 2012, the same group trapped an ion for up to 100 microseconds (μs) in an all-optical lattice by first catching the particle in a standard quadrupole trap, then gradually turning off the radio-frequency fields while ramping up the laser beams [3].

Hybrid traps combine the stability of quadrupole traps with the flexibility of optical traps. Moreover, the radio-frequency and optical fields produce a combination of Coulomb and dipole forces, respectively, on the ion, which can be used to simulate many-body Hamiltonians. For example, one candidate model, the Frenkel-Kontorova Hamiltonian, describes nearest-neighbor interactions within a sinusoidal potential and exhibits both classical and quantum phase transitions [4]. There have already been a few experiments with hybrid traps. A team at Aarhus University in Denmark, for example, tried leaving their quadrupole trap on and showed the ion stayed within a single well of the optical lattice [5]. Several experiments used separate, but overlapping, quadrupole and laser traps for ions and atoms, respectively, to explore collisions between the charged and neutral particles at ultracold temperatures where quantum effects are dominant [6, 7, 8, 9]. The idea is that if a single ion is trapped in a sea of atoms, its fluorescence can be a sensitive probe of how it interacts chemically with the surrounding particles. These experiments, however, grappled with the effects of micromotion, which is the residual motion of the ion at the radio frequency used to drive the quadrupole trap.

Like the researchers at Aarhus University, the MIT team uses a quadrupole trap in combination with an optical trap. The radio-frequency fields confine an ion tightly in two dimensions, while along the third axis the static fields are relaxed so that the ions see a relatively flat landscape. Karpa et al. introduce a one-dimensional lattice along this axis by driving an optical resonator with a laser field (Fig. 1.) The key step forward is that their hybrid trap locks an ion into more localized positions for a longer period of time, thanks to a built-in cooling mechanism. Specifically, an additional laser field removes one vibrational quanta after another from the trapped ion, a technique known as Raman sideband cooling. The cooling cycle leaves the ion in its vibrational ground state. The cooler ion stays spatially localized to within about 4nanometers (nm) and is pinned at a valley of the standing-wave structure. The team also shows they can use the same cooling and trapping method for up to three ions at a time.

To prove that their trap was stiff—meaning an ion wouldn’t be easily disrupted by local variations in the electric field—Karpa et al. pushed on the ions with a time-varying electric field. In the absence of the optical lattice, the ions swung back and forth with the field over hundreds of nm, but with the lattice on, they remained fixed in place. Only when Karpa et al. slowed down the oscillations in the field could the ions respond to the force. The researchers determined that the single-site pinning lasted for up to 10ms, which is 104 times longer than the trap’s vibrational period. The vibrational period—classically, the time it takes an ion to roll back and forth in its potential well—sets the minimum time scale for which it makes sense to say that ion is trapped

In addition to other challenges, researchers working with hybrid traps will need to address the ion micromotion at radio frequencies. Although all-optical traps offer a cleaner option, they are presently limited to 100μs trapping times because laser fluctuations heat the ions [3]. Karpa et al. have already proposed one potential solution to micromotion: a hybrid trap design that uses static, as opposed to radio-frequency, fields. Exploring proposals such as this one could open the door to new approaches in a host of fields, ranging from ultracold quantum chemistry to quantum computing architectures to simulations of solid-state physics.

Source: http://physics.aps.org/articles/v6/113

Opposite charges attract and like charges repel. This is a universal scientific truth – except when it isn’t.

At some point in elementary school you were shown that opposite charges attract and like charges repel. This is a universal scientific truth – except when it isn’t. A research team led by Berkeley Lab chemist Richard Saykally and theorist David Prendergast, working at the Advanced Light Source (ALS), has shown that, when hydrated in water, positively charged ions (cations) can actually pair up with one another.

“Through a combination of X-ray spectroscopy, liquid microjets and first principles’ theory, we’ve observed and characterized contact pairing between guanidinium cations in aqueous solution,” Saykally says. “Theorists have predicted this cation-to-cation pairing but it has never been definitively observed before. If guanidinium cations can pair this way, then other similar cation systems probably can too.”

Guanidinium is an ionic compound of hydrogen, nitrogen and carbon atoms whose salt – guanidinium chloride – is widely used by scientists to denature proteins for protein-folding studies. This practice dates back to the late 19th century when the Czech scientist Franz Hofmeister observed that cations such as guanidinium can pair with anions (negatively charged ions) in proteins to cause them to precipitate. The Hofmeister effect, which ranks ions on their ability to “salt-out” proteins, became a staple of protein research even though its mechanism has never been fully understood.

In 2006, Kim Collins of the University of Maryland proposed a “Law of Matching Water Affinities” to help explain “Hofmeister effects”. Collins’s proposal holds that the tendency of a cation and anion to form a contact pair is governed by how closely their hydration energies match, meaning how strongly the ions hold onto molecules of water. Saykally, who is a faculty scientist in Berkeley Lab’s Chemical Sciences Division and a professor of chemistry at the University of California Berkeley, devised a means of studying both the Law of Matching Water Affinities and Hofmeister effects. In 2000, he and his group incorporated liquid microjet technology into the high-vacuum experimental environment of ALS beamlines and used the combination to perform the first X-ray absorption spectroscopy measurements on liquid samples. This technique has since become a widely used research practice.

Berkeley Lab’s Rich Saykally has spent much of his career investigating the amazing chemistry of water.

“The XAS spectrum is generally sensitive to the changes in the local solvation environment around each atom, including potential effects of ion-pairing,” Saykally says. “However, the chemical information that one can extract from such experimental data alone is limited, so we interpret our spectra with a combination of molecular dynamics simulations and a first principles theory method.”

Development of this first principles theory method was led by Prendergast, a staff scientist in the Theory of Nanostructures Facility at Berkeley Lab’s Molecular Foundry. Computational resources were provided by the National Energy Research Scientific Computing Center (NERSC). The Molecular Foundry and NERSC, as well as the ALS, are all U.S. Department of Energy national user facilities hosted at Berkeley Lab.

With the liquid microjet technology, a sample rapidly flows through a fused silica capillary shaped to a finely tipped nozzle with an opening only a few micrometers in diameter. The resulting liquid beam travels a few centimeters in a vacuum chamber and is intersected by an X-ray beam then collected and condensed out. In analyzing their current results, which were obtained at ALS Beamline 8.0.1, the Berkeley Lab researchers concluded that the counterintuitive cation-cation pairing observed is driven by water-binding energy, as predicted by theory.

Orion Shih, a recent graduate of Saykally’s research group, is the lead author of a paper describing this study in the Journal of Chemical Physics. The paper is titled “Cation-cation contact pairing in water: Guanidinium.” Saykally is the corresponding author. Other co-authors are Alice England, Gregory Dallinger, Jacob  Smith, Kaitlin Duffey, Ronald Cohen and Prendergast.

“We found that the guanidinium ions form strong donor hydrogen bonds in the plane of the molecule, but weak acceptor hydrogen bonds with the pi electrons orthogonal to the plane,” Shih says. “When fluctuations bring the solvated ions near each other, the van der Waals attraction between the pi electron clouds squeezes out the weakly held water molecules, which move into the bulk solution and form much stronger hydrogen bonds with other water molecules. This release of the weakly interacting water molecules results in contact pairing between the guanidinium cations. We believe our observations may set a general precedent in which like charges attract becomes a new paradigm for aqueous solutions.”

Source: http://newscenter.lbl.gov/science-shorts/2013/09/18/in-water-likes-can-attract/