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Quantum Entanglement

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A new paradigm that should allow scientists to observe quantum behavior in small mechanical systems has been realized by researchers at California Institute of Technology (Caltech).

Their ideas offer a new means of addressing one of the most fascinating issues in quantum mechanics: the nature of quantum superposition and entanglement in progressively larger and more complex systems.

A quantum superposition is a state in which a particle, such as a photon or atom, exists simultaneously in two locations. Entanglement, which Albert Einstein called “spooky action at a distance,” allows particles to share information even if they are physically separated.

A key challenge in observing quantum behavior in a small mechanical system is suppressing interactions between the system and its noisy environment – i.e., the surrounding material supporting the system or any other external contact. The random thermal vibrations of the system’s surroundings, for example, can be transferred to the mechanical object and destroy its fragile quantum properties. To address this issue, a number of groups worldwide have begun to use cryogenic setups in which the immediate environment is cooled down to a very low temperature to reduce the magnitude of these random vibrations.

The Caltech team suggests a fundamentally different approach – using the forces imparted by intense beams of light to “levitate” the entire mechanical object, thereby freeing it from external contact and material supports. This approach, the researchers show, can dramatically reduce environmental noise, to the point where diverse manifestations of quantum behavior should be observable even when the environment is at room temperature.

Among the scientists involved in the work are Darrick Chang, a postdoctoral scholar at Caltech’s Institute for Quantum Information; Oskar Painter, associate professor of applied physics; and H. Jeff Kimble, Caltech’s William L. Valentine professor and professor of physics.

The idea of using optical forces to trap or levitate small particles is actually well established. It was pioneered by Arthur Ashkin of Bell Laboratories in the 1970s and 1980s and has since formed the basis for scientific advances such as the development of “optical tweezers” – which are frequently used to control the motion of small biological objects – and the use of lasers to cool atoms and trap them in space. These techniques provide an extremely versatile toolbox for manipulating atoms and have been employed to demonstrate a variety of quantum phenomena at the atomic level.

In the new work, Chang and his colleagues demonstrate theoretically that similar success can be achieved when an individual atom is replaced by a much more massive – but still nanoscale – mechanical system. A related scheme has been presented simultaneously by a group at Max Planck Institute of Quantum Optics in Garching, Germany.

The system proposed by the Caltech team consists of a small sphere made out of a highly transparent material such as fused silica. When the sphere comes into contact with a laser beam, optical forces naturally push the sphere toward the point where the intensity of light is greatest, trapping the sphere at that point. The sphere typically spans about 100 nm in diameter, or roughly a thousandth the width of a human hair. Because of its small size, the sphere’s remaining interactions with the environment – any that don’t involve direct contact with another material, because the sphere is levitating – are sufficiently weak that quantum behavior should emerge easily.

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For such behavior to appear, however, the sphere also must be placed inside an optical cavity, which is formed by two mirrors located on either side of the trapped sphere. The light that bounces back and forth between the mirrors both senses the motion of the sphere and is used to manipulate that motion at a quantum-mechanical level.

The researchers describe how this interaction can be used to remove energy from, or cool, the mechanical motion until it reaches its quantum ground state – the lowest energy allowable by quantum mechanics. A fundamental limit to this process is set by the relative strengths of the optical cooling and the rate at which the environment tends to heat (return energy to) the motion, bringing it back to the ambient temperature.

In principle, the motion of the well-isolated sphere can be cooled starting from room temperature down to a final temperature that is 10 million times lower; in that supercooled state, the center of mass of the sphere moves by only the minimum possible amount set by intrinsic quantum fluctuations.

The researchers also propose a scheme to observe a feature known as entanglement, which lies at the heart of quantum mechanics. Two remotely located systems that are quantum entangled share correlations between them that are stronger than classically allowed. In certain circumstances, entanglement can be a very valuable resource; it forms the basis for proposals to realize improved metrology and more powerful (quantum) computers.

The proposed scheme consists of sending a pair of initially entangled beams of light – the production of which was first accomplished by Kimble’s group at Caltech in 1992 – into two separate cavities, each containing a levitated sphere. Through a process known as quantum-state transfer, all of the properties of the light – in particular, the entanglement and its associated correlations – can be mapped onto the motion of the two spheres. 

Although the sizes of these nanomechanical objects still are very far from those we associate with everyday experience, the Caltech researchers believe that their proposal presents an exciting opportunity to realize and control quantum phenomena at unprecedented scales – in this case, for objects containing approximately 10 million atoms.

Other researchers involved in the work include graduate student Dalziel Wilson and postdoctoral scholars Cindy Regal and Scott Papp at Caltech; Jun Ye, a fellow at JILA, a joint institute of the University of Colorado at Boulder and the National Institute of Standards and Technology; and Peter Zoller, a professor at the University of Innsbruck in Austria. The work was initiated while Ye and Zoller were visiting as Gordon and Betty Moore Distinguished Scholars at Caltech.

The work was supported by the Gordon and Betty Moore Foundation, the National Science Foundation, the Army Research Office, Northrop Grumman Space Technology, the Austrian Science Fund and European Union Projects.

For more information, visit: www.caltech.edu

Published: December 2009
Glossary
einstein
A unit of energy equal to the amount of energy absorbed by one molecule of material undergoing a photochemical reaction, as determined by the Stark-Einstein law.
metrology
Metrology is the science and practice of measurement. It encompasses the theoretical and practical aspects of measurement, including the development of measurement standards, techniques, and instruments, as well as the application of measurement principles in various fields. The primary objectives of metrology are to ensure accuracy, reliability, and consistency in measurements and to establish traceability to recognized standards. Metrology plays a crucial role in science, industry,...
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
optical tweezers
Optical tweezers refer to a scientific instrument that uses the pressure of laser light to trap and manipulate microscopic objects, such as particles or biological cells, in three dimensions. This technique relies on the momentum transfer of photons from the laser beam to the trapped objects, creating a stable trapping potential. Optical tweezers are widely used in physics, biology, and nanotechnology for studying and manipulating tiny structures at the microscale and nanoscale levels. Key...
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum entanglement
Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become correlated to such an extent that the state of one particle instantly influences the state of the other(s), regardless of the distance separating them. This means that the properties of each particle, such as position, momentum, spin, or polarization, are interdependent in a way that classical physics cannot explain. When particles become entangled, their individual quantum states become inseparable,...
quantum mechanics
The science of all complex elements of atomic and molecular spectra, and the interaction of radiation and matter.
atomsBasic ScienceBiophotonicsCaltechEinsteinentangled beams of lightMax Planck Institute of Quantum OpticsmetrologynanoNews & Featuresoptical forcesoptical tweezersphotonicsphotonics.comphotonsquantum computersquantum entanglementquantum mechanicsResearch & TechnologysilicasuperpositionLasers

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