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  • Entangled Light Beams Store Quantum Info
Nov 2010
COPENHAGEN, Denmark, Nov. 10, 2010 — Quantum information has been stored using two entangled light beams, researchers at the Niels Bohr Institute at the University of Cophenhagen report. Quantum memory or information storage is a necessary element of future quantum communication networks.

Illustration of two quantum memories. Each memory consists of a glass cell filled with cesium atoms, which are shown as small blue and red balls. The light beam is sent through the atoms and the quantum information is transferred from the light to the atoms. (Images: Quantop)

Quantum networks will be able to protect the security of information better than the current conventional communication networks. The cornerstone of quantum communication is a phenomenon called entanglement between two quantum systems, for example, two light beams. Entanglement means that the two light beams are connected to each other, so that they have well defined common characteristics, a kind of common knowledge. According to the laws of quantum mechanics, a quantum state can't be copied, making it ideal for the secure transfer of data.

In professor Eugene Polzik's research group Quantop at the institute researchers have now been able to store the two entangled light beams in two quantum memories. The research is conducted in a laboratory where a forest of mirrors and optical elements such as wave plates, beamsplitters, lenses etc. are set up on a large table, sending the light around on a more than 10-m-long labyrinthine journey. Using the optical elements, the researchers control the light and regulate the size and intensity to get just the right wavelength and polarization the light needs to have for the experiment.

The two entangled light beams are created by sending a single blue light beam through a crystal where the blue light beam is split up into two red light beams. The two red light beams are entangled, so they have a common quantum state. The quantum state itself is information.

Experimental setup for a quantum information experiment. The table is filled with optical elements such as mirrors, lenses and wave plates, which are used to guide and manipulate infrared light. Inside each of the two metal cylinders is a glass cell with cesium atoms. The two cylinders are magnetic shields which protect the atoms from magnetic fields. Shown in the upper right corner is a pair of detectors which are used to make measurements of the infrared light.

The two light beams are sent on through the labyrinth of mirrors and optical elements and reach the two memories, which in the experiment are two glass containers filled with a gas of cesium atoms. The atoms' quantum state contains information in the form of a so-called spin, which can be either "up" or "down." It can be compared with computer data, which consists of the digits 0 and 1. When the light beams pass the atoms, the quantum state is transferred from the two light beams to the two memories. The information has thus been stored as the new quantum state in the atoms.

"For the first time such a memory has been demonstrated with a very high degree of reliability. In fact, it is so good that it is impossible to obtain with conventional memory for light that is used in, for example, internet communication. This result means that a quantum network is one step closer to being a reality," said Polzik.

The new findings are published in Nature Physics.

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With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
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