A magnetic vortex to control electron spin

Researchers coupled a diamond nanoparticle with a magnetic vortex to control electron spin in nitrogen-vacancy defects. @ Case Western Reserve University

Researchers at Case Western Reserve University have developed a way to swiftly and precisely control electron spins at room temperature.

The technology, described in Nature Communications, offers a possible alternative strategy for building quantum computers that are far faster and more powerful than today's supercomputers.

"What makes electronic devices possible is controlling the movement of electrons from place to place using electric fields that are strong, fast and local," said physics Professor Jesse Berezovsky, leader of the research. "That's hard with magnetic fields, but they're what you need to control spin."

Other researchers have searched for materials where electric fields can mimic the effects of a magnetic field, but finding materials where this effect is strong enough and still works at room temperature has proven difficult.

"Our solution," Berezovsky said, "is to use a magnetic vortex."

Berezovsky worked with physics PhD students Michael S. Wolf and Robert Badea.

The researchers fabricated magnetic micro-disks that have no north and south poles like those on a bar magnet, but magnetize into a vortex. A magnetic field emanates from the vortex core. At the center point, the field is particularly strong and rises perpendicular to the disk.

The vortices are coupled with diamond nanoparticles. In the diamond lattice inside each nanoparticle, several individual spins are trapped inside of defects called nitrogen vacancies.

The scientists use a pulse from a laser to initialize the spin. By applying microwaves and a weak magnetic field, Berezovsky's team can move the vortex in nanoseconds, shifting the central point, which can cause an electron to change its spin.

In what's called a quantum coherent state, the spin can act as a quantum bit, or qubit--the basic unit of information in a quantum computer.

In current computers, bits of information exist in one of two states: zero or one. But in a superposition state, the spin can be up and down at the same time, that is, zero and one simultaneously. That capability would allow for more complex and faster computing.

"The spins are close to each other; you want spins to interact with their neighbors in quantum computing," Berezovsky said. "The power comes from entanglement."

The magnetic field gradient produced by a vortex proved sufficient to manipulate spins just nanometers apart.

In addition to computing, electrons controlled in coherent quantum states might be useful for extremely high-resolution sensors, the researchers say. For example, in an MRI, they could be used to sense magnetic fields in far more detail than with today's technology, perhaps distinguishing atoms.

Controlling the electron spins without destroying the coherent quantum states has proven difficult with other techniques, but a series of experiments by the group has shown the quantum states remain solid. In fact, "the vortex appears to enhance the microwave field we apply," Berezovsky said.

The scientists are continuing to shorten the time it takes to change the spin, which is a key to high-speed computing. They are also investigating the interactions between the vortex, microwave magnetic field and electron spin, and how they evolve together.

Case Western Reserve University

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Improved Stability of Electron Spins in Qubits

Calculation with electron spins in a quantum computer assumes that the spin states last for a sufficient period of time. Physicists at the University of Basel and the Swiss Nanoscience Institute have now demonstrated that electron exchange in quantum dots fundamentally limits the stability of this information. Control of this exchange process paves the way for further progress in the coherence of the fragile quantum states. The report from the Basel-based researchers appears in the scientific journal Physical Review Letters.

The basic idea of a quantum computer is to replace the ones and zeros used in today’s bits with quantum states, or qubits. Qubits are units of information that not only assume the values zero and one, but in which zero and one are possible at the same time, and in any chosen combination, in the form of a quantum superposition. Qubits can, for example, be implemented using the spins of individual electrons held in nanoscale structures made of semiconducting material, known as quantum dots. By exploiting quantum-mechanical principles such as superposition, a quantum computer can achieve enormous processing speeds – but only if the electron spins persist for long enough.

In recent years, it has been possible to extend this so-called coherence time to over a millisecond, thanks to the successful reduction of interference caused by nuclear spins. Thus, the search for other factors that affect the stability of the electron spins increased in importance.

Discovery of electron exchange

Physicists at the University of Basel and the Swiss Nanoscience Institute have now established that qubits’ coherence is limited by a process in which individual electrons are exchanged between a quantum dot and an external reservoir. The reservoir represents a type of electrode that is in contact with the quantum dot and is required for the measurements.

The researchers, led by Professor Dominik Zumbühl, observed that thermal excitation prompts an electron to jump from the quantum dot into the reservoir, and that shortly thereafter an electron jumps from the reservoir into the quantum dot.

This exchange creates a short-lived charge state, which the researchers in Basel have now been able to demonstrate for the first time with a charge sensor. The exchange process also leads to a randomizing of the electron spins, through which quantum information is lost.

Fundamental process for coherence

Based on the experimental observations, the researchers were able to significantly extend the existing theoretical description of double quantum dots, which can contain two electrons. They also succeeded in influencing the intensity of the temperature-dependent exchange process by cooling the electrons down to 60 millikelvins. At the same time, the process was slowed and the stability of the spins prolonged by changing the voltages at the entrances, or gates, to the quantum dot.

An understanding and control of this exchange process, which is fundamental to quantum dots, paves the way for further progress in qubit coherence. At the same time, it opens the way to a quick generation of desired spin states in quantum dots.

Implementation of a theoretical concept with Basel roots

This approach, whereby quantum dots in semiconductors are exploited in order to use the spin of an individual electron as a qubit, can be traced back to Prof. Daniel Loss of the University of Basel and the American physicist David DiVincenzo. Their concept, which they originally presented in 1998, has the potential to allow the creation of quantum computers with a large number of connected spin qubits. The current study was carried out in collaboration with researchers from the University of St Andrews (GB) and the University of California, Santa Barbara (US).

University of Basel

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Donuts, math, and superdense teleportation of quantum information

In superdense teleportation of quantum information, Alice (near) selects a particular set of states to send to Bob (far), using the hyperentangled pair of photons they share. The possible states Alice may send are represented as the points on a donut shape, here artistically depicted in sharp relief from the cloudy silhouette of general quantum state that surrounds them. To transmit a state, Alice makes a measurement on her half of the entangled state, which has four possible outcomes shown by red, green, blue, and yellow points. She then communicates the outcome of her measurement (in this case, yellow, represented by the orange streak connecting the two donuts) to Bob using a classical information channel. Bob then can make a corrective rotation on his state to recover the state that Alice sent.


Putting a hole in the center of the donut—a mid-nineteenth-century invention—allows the deep-fried pastry to cook evenly, inside and out. As it turns out, the hole in the center of the donut also holds answers for a type of more efficient and reliable quantum information teleportation, a critical goal for quantum information science.

Quantum teleportation is a method of communicating information from one location to another without moving the physical matter to which the information is attached. Instead, the sender (Alice) and the receiver (Bob) share a pair of entangled elementary particles—in this experiment, photons, the smallest units of light—that transmit information through their shared quantum state. In simplified terms, Alice encodes information in the form of the quantum state of her photon. She then sends a key to Bob over traditional communication channels, indicating what operation he must perform on his photon to prepare the same quantum state, thus teleporting the information.

Quantum teleportation has been achieved by a number of research teams around the globe since it was first theorized in 1993, but current experimental methods require extensive resources and/or only work successfully a fraction of the time.

Now, by taking advantage of the mathematical properties intrinsic to the shape of a donut—or torus, in mathematical terminology—a research team led by physicist Paul Kwiat of the University of Illinois at Urbana-Champaign has made great strides by realizing “superdense teleportation”. This new protocol, developed by coauthor physicist Herbert Bernstein of Hampshire College in Amherst, MA, effectively reduces the resources and effort required to teleport quantum information, while at the same time improving the reliability of the information transfer.

With this new protocol, the researchers have experimentally achieved 88 percent transmission fidelity, twice the classical upper limit of 44 percent. The protocol uses pairs of photons that are “hyperentangled”—simultaneously entangled in more than one state variable, in this case in polarization and in orbital angular momentum—with a restricted number of possible states in each variable. In this way, each photon can carry more information than in earlier quantum teleportation experiments.

At the same time, this method makes Alice’s measurements and Bob’s transformations far more efficient than their corresponding operations in quantum teleportation: the number of possible operations being sent to Bob as the key has been reduced, hence the term “superdense”.

Kwiat explains, “In classical computing, a unit of information, called a bit, can have only one of two possible values—it’s either a zero or a one. A quantum bit, or qubit, can simultaneously hold many values, arbitrary superpositions of 0 and 1 at the same time, which makes faster, more powerful computing systems possible.

“So a qubit could be represented as a point on a sphere, and to specify what state it is, one would need longitude and latitude. That’s a lot of information compared to just a 0 or a 1.”

“What makes our new scheme work is a restrictive set of states. The analog would be, instead of using a sphere, we are going to use a torus, or donut shape. A sphere can only rotate on an axis, and there is no way to get an opposite point for every point on a sphere by rotating it—because the axis points, the north and the south, don’t move. With a donut, if you rotate it 180 degrees, every point becomes its opposite. Instead of axis points you have a donut hole. Another advantage, the donut shape actually has more surface area than the sphere, mathematically speaking—this means it has more distinct points that can be used as encoded information.”

Lead author, Illinois physics doctoral candidate Trent Graham, comments, “We are constrained to sending a certain class of quantum states called ‘equimodular’ states. We can deterministically perform operations on this constrained set of states, which are impossible to perfectly perform with completely general quantum states. Deterministic describes a definite outcome, as opposed to one that is probabilistic. With existing technologies, previous photonic quantum teleportation schemes either cannot work every time or require extensive experimental resources. Our new scheme could work every time with simple measurements.”

This research team is part of a broader collaboration that is working toward realizing quantum communication from a space platform, such as the International Space Station, to an optical telescope on Earth. The collaboration—Kwiat, Graham, Bernstein, physicist Jungsang Kim of Duke University in Durham, NC, and scientist Hamid Javadi of NASA’s Jet Propulsion Laboratory in Pasadena, CA—recently received funding from NASA Headquarter's Space Communication and Navigation program (with project directors Badri Younes and Barry Geldzahler) to explore the possibility.

“It would be a stepping stone toward building a quantum communications network, a system of nodes on Earth and in space that would enable communication from any node to any other node,” Kwiat explains. “For this, we’re experimenting with different quantum state properties that would be less susceptible to air turbulence disruptions.”

The team’s recent experimental findings are published in the May 28, 2015 issue of Nature Communications, and represent the collaborative effort Kwiat, Graham, and Bernstein, as well as physicist Tzu-Chieh Wei of State University of New York at Stony Brook, and mathematician Marius Junge of the University of Illinois.

This research is funded by NSF Grant No. PHY-0903865, NASA NIAC Program, and NASA Grant No. NNX13AP35A. It is partially supported by National Science Foundation Grants DMS-1201886, No. PHY 1314748, and No. PHY 1333903.
______________________

Contact: Siv Schwink, communications coordinator, Department of Physics, 217/300-2201.

Paul Kwiat, Department of Physics, University of Illinois at Urbana-Champaign.

Image by Precision Graphics, copyright Paul Kwiat, University of Illinois at Urbana-Champaign.

Source: http://engineering.illinois.edu/news/article/11151?

The monopoly of Aluminium is broken

©TUDelft/Tremani

Discovering Majorana’s was only the first step, but utilizing it as a quantum bit (qubit) still remains a major challenge. An important step towards this goal has just been taken, as shown by researchers from TU Delft in today’s issue of Nature Physics. It is an almost thirty years old scientific problem that has just been resolved: demonstrating the difference between the even and odd occupation of a superconductor in high magnetic fields. Thus far, this was only possible in aluminium which is however incompatible with Majorana’s. This result enables the read out and manipulation of quantum states encoded in prospective Majorana qubits.

Qubit

Qubits store information similarly to normal (digital) bits. While a bit represents either 0 or 1, a qubit utilizes the laws of quantum mechanics, making it possible to be in the state of 0 and 1 at the same time. This enables solving several mathematical problems much faster than the most capable supercomputers ever built. Several research groups and companies around the globe pursue the development and prototyping of such a powerful quantum computer, including QuTech at the Delft University of Technology.

Majorana’s

A qubit encoded by Majorana’s is a promising building block for a practical quantum computer. However, until now, it was a major challenge to read out such a Majorana qubit. In order to do so, one needs to determine whether the number of the electrons is even or odd, or, in other words, what the parity state is.  The measurement of the parity of superconductors has been performed for the last thirty years, however, successful experiments were exclusively done on aluminium while all attempts addressing different superconducting materials, such as vanadium or niobium, have failed. This is a major issue for Majorana research as superconductivity is required to survive up to high magnetic fields, at which aluminium ceases to be a superconductor.

An alternative of aluminium

The research group at TU Delft has succeeded in determining the parity in a different superconductor: niobium titanium nitride (NbTiN). Most importantly, this material remains superconducting at high magnetic fields, which is an essential property to create Majorana’s. “The most beautiful outcome is that not only can we distinguish between the even and odd number of electrons, but a prepared, say, even state remains the same for more than one minute” as David van Woerkom from QuTech explains. “Since we typically work with timescales of micro- or even nanoseconds, one minute is essentially an eternity”.

Quasiparticles

Majorana’s are a special kind of so-called (quasi-)particles: all measurable properties of Majorana’s are zero, meaning that this particle is its own antiparticle. A pair of Majorana’s are insensitive to local perturbations and is therefore a very promising candidate to become the building block of quantum computation. Majorana’s were first postulated in the 1930’s by the young Italian scientist Ettore Majorana, who disappeared under mysterious circumstances not more than one year after his groundbreaking work. The first experimental evidence of the Majorana particle was reported in 2012 by the group of Leo Kouwenhoven in Delft.

The Quantum Technology Institute

QuTech, founded by TU Delft and TNO is an institute developing quantum computers. The current research reported today in Nature Physics has been performed in collaboration by QuTech and Microsoft Corporation Station Q. QuTech became a ‘Nation Icon’ of the Dutch government in 2014.

Source link: http://www.tudelft.nl/en/current/latest-news/article/detail/wetenschappers-doorbreken-monopolie-aluminium/

Stirring-up atomtronics in a quantum circuit

Atomtronics is an emerging technology whereby physicists use ensembles of atoms to build analogs to electronic circuit elements. 

Modern electronics relies on utilizing the charge properties of the electron. Using lasers and magnetic fields, atomic systems can be engineered to have behavior analogous to that of electrons, making them an exciting platform for studying and generating alternatives to charge-based electronics.

Using a superfluid atomtronic circuit, JQI physicists, led by Gretchen Campbell, have demonstrated a tool that is critical to electronics: hysteresis. This is the first time that hysteresis has been observed in an ultracold atomic gas. This research is published in the February 13 issue of Nature magazine, whose cover features an artistic impression of the atomtronic system.

Lead author Stephen Eckel explains, “Hysteresis is ubiquitous in electronics. For example, this effect is used in writing information to hard drives as well as other memory devices.  It’s also used in certain types of sensors and in noise filters such as the Schmitt trigger.” Here is an example demonstrating how this common trigger is employed to provide hysteresis.  Consider an air-conditioning thermostat, which contains a switch to regulate a fan. The user sets a desired temperature. When the room air exceeds this temperature, a fan switches on to cool the room. When does the fan know to turn off? The fan actually brings the temperature lower to a different set-point before turning off. This mismatch between the turn-on and turn-off temperature set-points is an example of hysteresis and prevents fast switching of the fan, which would be highly inefficient.

In the above example, the hysteresis is programmed into the electronic circuit. In this research, physicists observed hysteresis that is an inherent natural property of a quantum fluid. 400,000 sodium atoms are cooled to condensation, forming a type of quantum matter called a Bose-Einstein condensate (BEC), which has a temperature around 0.000000100 Kelvin (0 Kelvin is absolute zero). The atoms reside in a doughnut-shaped trap that is only marginally bigger than a human red blood cell. A focused laser beam intersects the ring trap and is used to stir the quantum fluid around the ring.

While BECs are made from a dilute gas of atoms less dense than air, they have unusual collective properties, making them more like a fluid—or in this case a superfluid.  What does this mean? First discovered in liquid helium in 1937, this form of matter, under some conditions, can flow persistently, undeterred by friction. A consequence of this behavior is that the fluid flow or rotational velocity around the team’s ring trap is quantized, meaning it can only spin at certain specific speeds. This is unlike a non-quantum (classical) system, where its rotation can vary continuously and the viscosity of the fluid plays a substantial role.

Because of the characteristic lack of viscosity in a superfluid, stirring this system induces drastically different behavior. Here, physicists stir the quantum fluid, yet the fluid does not speed up continuously. At a critical stir-rate the fluid jumps from having no-rotation to rotating at a fixed velocity. The stable velocities are a multiple of a quantity that is determined by the trap size and the atomic mass.

This same laboratory has previously demonstrated persistent currents and this quantized velocity behavior in superfluid atomic gases. Now they have explored what happens when they try to stop the rotation, or reverse the system back to its initial velocity state. Without hysteresis, they could achieve this by reducing the stir-rate back below the critical value causing the rotation to cease. In fact, they observe that they have to go far below the critical stir-rate, and in some cases reverse the direction of stirring, to see the fluid return to the lower quantum velocity state.

Controlling this hysteresis opens up new possibilities for building a practical atomtronic device. For instance, there are specialized superconducting electronic circuits that are precisely controlled by magnetic fields and in turn, small magnetic fields affect the behavior of the circuit itself. Thus, these devices, called SQuIDs (superconducting quantum interference devices) are used as magnetic field sensors. “Our current circuit is analogous to a specific kind of SQuID called an RF-SQuID”, says Campbell. “In our atomtronic version of the SQuID, the focused laser beam induces rotation when the speed of the laser beam “spoon” hits a critical value. We can control where that transition occurs by varying the properties of the “spoon”. Thus, the atomtronic circuit could be used as an inertial sensor.”

This two-velocity state quantum system has the ingredients for making a qubit. However, this idea has some significant obstacles to overcome before it could be a viable choice. Atomtronics is a young technology and physicists are still trying to understand these systems and their potential. One current focus for Campbell’s team includes exploring the properties and capabilities of the novel device by adding complexities such as a second ring.

This research was supported by the NSF Physics Frontier Center at JQI. 

Source: http://jqi.umd.edu/news/stirring-atomtronics-quantum-circuit#sthash.nQMN1xYP.dpuf