GARCHING, Germany, and COPENHAGEN, Denmark, Oct. 10, 2006 -- Researchers in Germany and Denmark have discovered that, for the first time, it is possible to teleport between stationary atoms, which can store quantum states, and light, needed to transmit information over great distances. While the work won't help bring the transporter from "Star Trek" any closer to being a reality, the scientists said it is an important step toward achieving quantum cryptography, or safe communication over long distances, and even making quantum computing practical.

---

Artist's rendering of the web of entanglement connecting two objects in the teleportation experiment between matter and light. (Image: Mette Høst, Niels Bohr Institute, Denmark)

---

A group of researchers from the Max Planck Institute of Quantum Optics (MPQ) in Garching and the Niels Bohr Institute (NBI) at the University of Copenhagen, headed by professor Eugene Polzik of NBI, said they have taken an important step by teleporting between light and atoms, or quantum teleportation. The group's work appears this month in the journal Nature.

The aim of the research in teleportation is not to develop a new method of transportation, but to develop the future's quantum commnication network, which will be based on teleportation of information. It would be a novel type of comunication network that will be able to transmit much larger amounts of information absolutely securely, the researchers said.

The concept of quantum teleportation -- the disembodied complete transfer of the state of a quantum system to any other place -- was first experimentally realized between two different light beams. Later it also became possible to transfer the properties of a stored ion to another object of the same kind. A team of scientists headed by professors Ignacio Cirac at MPQ and Polzik have now shown that the quantum states of a light pulse can also be transferred to a macroscopic object, an ensemble of 10¹² atoms. This is the first case of successful teleportation between objects of a different nature -- the one representing a "flying" medium (light), the other a "stationary" medium (atoms).

---

The target of teleportation is a cloud of Cesium atoms (glowing in the centre) enclosed in a cell surrounded by magnetic field installations. (Photo: Eugene Polzik, Niels Bohr Institute, Denmark)

---

Since the beginning of the 1990s, research into quantum teleportation has been booming with theoretical and experimental physicists. Transmission of quantum information involves a fundamental problem: According to Heisenberg’s uncertainty principle, two complementary properties of a quantum particle, e.g. location and momentum, cannot be precisely measured simultaneously. The entire information of the system thus has to be transmitted without being completely known. But the nature of the particles also carries with it the solution to this problem: the possibility of "entangling" two particles in such a way that their properties become perfectly correlated. If a certain property is measured in one of the "twin" particles, this determines the corresponding property of the other automatically and with immediate effect.

With the help of entangled particles, successful teleportation can be achieved roughly as follows, the scientists said: An auxiliary pair of entangled particles is created, the one being transmitted to "Alice" and the other to "Bob". (The names Alice and Bob were adopted to describe the transmission of quantum information from A to B.) Alice now entangles the object of teleportation with her auxiliary particle and then measures the joint state (Bell measurement). She sends the result to Bob in the usual manner. He applies it to his auxiliary particle and "conjures up" the teleportation object from it. The great challenge to theoretical physicists is to devise concepts which can also be put into practice. The experiment described here has been conducted by a research team headed by Polzik. It follows a proposal made by Cirac, managing director at MPQ, and his collaborator, Klemens Hammerer (also at MPQ at that time, now at University of Innsbruck, Austria).

First the twin pair is produced by sending a strong light pulse to a glass tube filled with caesium gas (about 10¹² atoms). The magnetic moments of the gas atoms are aligned in a homogenous magnetic field. The light also has a preferential direction: It is polarised, i.e. the electric field oscillates in just one direction. Under theses conditions the light and the atoms are made to interact with one another so that the light pulse emerging from the gas that is sent to Alice is "entangled" with the ensemble of 10¹² caesium atoms located at Bob’s site.

Alice mixes the arriving pulse by means of a beamsplitter with the object that she wants to teleport: a weak light pulse containing very few photons. The light pulses issuing at the two outputs of the beamsplitter are measured with photodetectors and the results are sent to Bob.

The measured results tell Bob what has to be done to complete teleportation and transfer the selected quantum states of the light pulse, amplitude and phase, onto the atomic ensemble. For this purpose he applies a low-frequency magnetic field that makes the collective spin (angular momentum) of the system oscillate. This process can be compared with the precession of a spinning top about its major axis: the deflection of the spinning top corresponds to the amplitude of the light, while the zero passage corresponds to the phase.

To prove that quantum teleportation has been successfully performed, a second intense pulse of polarized light is sent to the atomic ensemble after 0.1 milliseconds and "reads out" its state. From these measured values theoretical physicists can calculate the so-called fidelity, a quality factor specifying how well the state of the teleported object agrees with the original. (A fidelity of 1 is equivalent to a perfect agreement, while the value 0 indicates that there has been no transfer at all.) In the present experiment the fidelity is 0.6, which is well above the value of 0.5 that would at best be achieved by classical means, e.g. by communicating measured values by telephone, without the help of entangled particle pairs, the researchers said.

Unlike the customary conception of "beaming" as it is known from science fiction, this work does not involve a particle disappearing from one place and reappearing in another, the scientists said.

For more information, visit: www.mpq.mpg.de/mpq.html