Quantum control protocols could lead to more accurate, larger scale quantum computati

[ScienceRocks]

Quantum control protocols could lead to more accurate, larger scale quantum computations
April 4, 2012
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The quantum circuit used in the demonstration is a 3mm x 3mm chip with a 1mm x 1mm diamond in the middle. Credit: Delft University of Technology/UC Santa Barbara.

A protocol for controlling quantum information pioneered by researchers at UC Santa Barbara, the Kavli Institute of Nanoscience in Delft, the Netherlands, and the Ames Laboratory at Iowa State University could open the door to larger-scale, more accurate quantum computations. Their findings, in a paper titled "Decoherence-protected quantum gates for a hybrid solid-state spin register," are published in the current issue of the journal Nature.
"Although interactions between a quantum bit ('qubit') and its environment tend to corrupt the information it stores, it is possible to dynamically control qubits in a way that facilitates the execution of quantum information-processing algorithms while simultaneously protecting the qubits from environment-induced errors," said UCSB physicist David Awschalom. He and his group were responsible for developing the electron and nuclear spins used as the quantum bits –– the quantum version of the computer bit –– in their demonstration and for helping to analyze the results.

Awschalom is director of UCSB's Center for Spintronics & Quantum Computation, professor of physics, electrical and computer engineering, and the Peter J. Clarke Director of the California NanoSystems Institute.

Dynamical protection of quantum information is essential for quantum computing as the qubits used for information processing and storage are highly susceptible to errors induced by interactions with atoms in the qubits'
environment. The scientists' previous research has shown that quantum information stored in qubits can be effectively protected through successive control operations (rotations) on a qubit that filter out these unwanted interactions. However, these control operations also filter out the interactions between qubits that are essential for the realization of logic gates for quantum information processing. Thus, until recently, quantum information stored in protected qubit states could not be used for quantum computations.


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A 20 micron x 20 micron magnification of the diamond chip, showing an integrated diamond lens above the single particle spins where the calculations take place. Credit: UCSB

The research team, which also included members from the University of Southern California, showed that by precisely synchronizing the rotations of an electron spin with the rotation of a nearby nuclear spin, they could realize dynamical protection of both qubits from the environment while maintaining the interactions between the two spins that are necessary for quantum information processing. As a proof of principle, the researchers demonstrated the high-fidelity execution of a quantum search algorithm using this two-qubit system. Quantum search algorithms, if executed on a larger number of qubits, could provide search results of certain databases considerably faster than search algorithms performed on a classical computer.

The results of this study point to greater possibilities for quantum computers that overcome, according to Awschalom, the perception that spin qubits in semiconductors, such as those used in this work, suffer from too strong of environmental interactions to be useful qubits. These solid state spin systems also offer the added benefit of operating at room temperature, in contrast to other candidate qubit systems which operate at only at a fraction of a degree above absolute zero.

"This demonstration of performing a quantum algorithm at the subatomic level with single spins suggests a pathway to build increasingly complex quantum machines, using qubit control protocols that circumvent the expected limitations from real materials," said Awschalom.
Quantum control protocols could lead to more accurate, larger scale quantum computations

 

[ScienceRocks]

Physicists control quantum tunneling with light for the first time
April 5, 2012
Scientists at the Cavendish Laboratory in Cambridge have used light to help push electrons through a classically impenetrable barrier. While quantum tunnelling is at the heart of the peculiar wave nature of particles, this is the first time that it has been controlled by light. Their research is published today, 05 April, in the journal Science.



Particles cannot normally pass through walls, but if they are small enough quantum mechanics says that it can happen. This occurs during the production of radioactive decay and in many chemical reactions as well as in scanning tunnelling microscopes.

According to team leader, Professor Jeremy Baumberg, "the trick to telling electrons how to pass through walls, is to now marry them with light".

This marriage is fated because the light is in the form of cavity photons, packets of light trapped to bounce back and forth between mirrors which sandwich the electrons oscillating through their wall.

Research scientist Peter Cristofolini added: "The offspring of this marriage are actually new indivisible particles, made of both light and matter, which disappear through the slab-like walls of semiconductor at will."

One of the features of these new particles, which the team christened 'dipolaritons', is that they are stretched out in a specific direction rather like a bar magnet. And just like magnets, they feel extremely strong forces between each other.

Such strongly interacting particles are behind a whole slew of recent interest from semiconductor physicists who are trying to make condensates, the equivalent of superconductors and superfluids that travel without loss, in semiconductors.

Being in two places at once, these new electronic particles hold the promise of transferring ideas from atomic physics into practical devices, using quantum mechanics visible to the eye.
Physicists control quantum tunneling with light for the first time

 

[waltky]

Quantum Magnetism Simulated In Europe...

Quantum magnetism simulated using ultracold fermions
May 24, 2013 > Quantum magnetism has been mimicked – or simulated – using ultracold fermionic atoms for the first time. Researchers in Switzerland and France placed atoms on a 2D square lattice created by criss-crossing laser beams. By controlling the interactions between atoms, the team put pairs of atoms into antiferromagnetic configurations. While quantum magnetism plays an important role in a range of solid-state phenomena, it can be difficult to calculate its effect on materials such as high-temperature superconductors. As a result, quantum simulations should lead to better theoretical models of a range of solids.

Quantum magnetism involves a subtle effect called the exchange interaction. This is a quantum interaction between pairs of identical fermions – such as electrons – that tends to prevent neighbouring fermions from having their spin magnetic moments pointing in the same direction. As well as being responsible for the magnetic properties of everyday materials such as iron, quantum magnetism is also believed to play an important role in high-temperature superconductivity and other exotic states of matter such as spin liquids.

Criss-crossing laser beams

Quantum simulations using ultracold atoms allow physicists to create artificial materials in which the atoms play the role of electrons in a solid. However, unlike real materials, where it can be difficult to vary the interactions between electrons, the forces between atoms in a quantum simulator can be fine-tuned by adjusting lasers and magnetic field.

These latest simulations were done by Tilman Esslinger and colleagues at ETH Zürich and the University of Bordeaux. The team began with an ultracold cloud of potassium-40 atoms, which are fermions. The cloud is a mixture in which half of the atoms are in the –9/2 spin state and the other half in the –7/2 state. This two-state system simulates –1/2 and 1/2 spin states of the electron. The criss-crossing laser beams are then switched on, creating a 2D square lattice wherein each lattice site contains one potassium-40 atom. The exchange interaction is then simulated by applying a magnetic field to the lattice, which makes atoms with the same spin repel each other.

The next experimental step involves solving a thermodynamics problem. Even at the extremely low lattice temperatures there is too much entropy – or disorder – for quantum magnetism to emerge. To get round this problem, Esslinger and colleagues came up with a way of "stashing" entropy at the edges of the lattice so that quantum magnetism could emerge in the centre.

This is done by tweaking the properties of the optical lattice so that the interactions between nearest-neighbour atoms alternate between strong and weak in the x and y directions. An atom with a strong interaction with a nearest neighbour will form a pair (or dimer) in which the spins point in opposite directions – and the lattice of 5000 atoms becomes a collection of antiferromagnetic dimers.

Merging dimers