Laser beam orientation controls light's quantum properties

Dec 04, 2013
Quantum optics: Light finds the right mix
Development of quantum computers and quantum communication systems depends, in part, on establishing a technique for controlling the quantum state of photon pairs. Credit: agsandrew/iStock/Thinkstock

Quickly switching the orientation of a laser beam enables control over the quantum properties of light.

Classical computers process data as a series of ones and zeros known as bits; in contrast, quantum computers encode information into the physical properties of an information unit known as a quantum bit, or qubit. Two or more qubits are then able to link together to speed up the processing efficiency, enabling quantum computers to tackle mathematical problems beyond the capability of conventional machines. However, reliably creating the precise multiple qubits required for quantum-computer protocols remains a challenge.

Individual packets of light—photons—are quantum mechanical systems that could act as qubits. A photon stores information in its polarization—the plane in which its associated electric field oscillates. Recently, Leonid Krivitsky at the A*STAR Data Storage Institute in Singapore and co‐workers demonstrated a technique that can create pairs of photons within a broad class of quantum states.

"The ability to generate well-controlled quantum states of light is essential for quantum optics and quantum information experiments," says Krivitsky. "Whereas methods are well developed for generating arbitrary pure states of photons, there are limited approaches to creating mixed states."

Krivitsky and co-workers started by aiming a linearly polarized at a crystal. The nonlinear optical properties of the target meant that the incoming light generated pairs of photons in a known as a Bell state. The photons paired in a Bell state are either both horizontal or both vertical, with equal probability. Two such Bell states are achievable depending on the polarization of the laser light.

The researchers controlled the laser light using a device known as a liquid crystal retarder (LCR), which can quickly rotate optical polarization. By changing the shape of the voltage driving the LCR, the researchers blended the two Bell states into mixed states of any desired purity and entanglement.

Preparation of a quantum state with a specific mixture of probabilities of horizontal or vertical photons is crucial for some quantum algorithms, but previous approaches have involved complicated optical equipment. "Our method does not require a sophisticated optical setup," says Krivitsky. "It is versatile—it can be plugged into any existing setup—and is insensitive to mechanical vibrations."

The team is now working to find an equally simple detection method so that the results of a quantum calculation can be 'read out'. "We are developing an advanced protocol that can directly and quickly characterize these mixed states," says Krivitsky.

Explore further: Highly stable quantum light source for applications in quantum information developped

More information: Dai, J., Len, Y. L., Teo, Y. S., Krivitsky, L. A. & Englert, B.-G. Controllable generation of mixed two-photon states. New Journal of Physics 15, 063011 (2013).

add to favorites email to friend print save as pdf

Related Stories

Optics: Statistics light the way

May 22, 2013

Millions of years of evolution have molded our eyes into highly sensitive optical detectors, surpassing even many man-made devices. Now, Leonid Krivitsky and his co-workers at the A*STAR Data Storage Institute ...

In quantum computing, light may lead the way

Oct 08, 2013

(Phys.org) —Light might be able to play a bigger, more versatile role in the future of quantum computing, according to new research by Yale University scientists.

On-chip quantum buffer realized

Nov 13, 2013

Nippon Telegraph and Telephone Corp. has realized a quantum buffer integrated on an optical waveguide. The buffer is based on the "slow light effect", where the propagation speed of a pulsed light in a special ...

How losing information can benefit quantum computing

Nov 24, 2013

Suggesting that quantum computers might benefit from losing some data, physicists at the National Institute of Standards and Technology (NIST) have entangled—linked the quantum properties of—two ions ...

Recommended for you

Physicists discuss quantum pigeonhole principle

Jul 26, 2014

The pigeonhole principle: "If you put three pigeons in two pigeonholes at least two of the pigeons end up in the same hole." So where's the argument? Physicists say there is an important argument. While the ...

Unleashing the power of quantum dot triplets

Jul 24, 2014

Quantum computers have yet to materialise. Yet, scientists are making progress in devising suitable means of making such computers faster. One such approach relies on quantum dots—a kind of artificial atom, ...

Exotic state of matter propels quantum computing theory

Jul 23, 2014

So far it exists mainly in theory, but if invented, the large-scale quantum computer would change computing forever. Rather than the classical data-encoding method using binary digits, a quantum computer would process information ...

Quantum leap in lasers brightens future for quantum computing

Jul 22, 2014

Dartmouth scientists and their colleagues have devised a breakthrough laser that uses a single artificial atom to generate and emit particles of light. The laser may play a crucial role in the development of quantum computers, ...

Boosting the force of empty space

Jul 22, 2014

Vacuum fluctuations may be among the most counter-intuitive phenomena of quantum physics. Theorists from the Weizmann Institute (Rehovot, Israel) and the Vienna University of Technology propose a way to amplify ...

User comments : 0