No more spooks
The concept of quantum teleportation was first experimentally realized between two different light beams. This was later also achieved with other objects. Now another important step followed: the transfer of the quantum states of a light pulse to a material object.
Since the early 1990s, research into quantum teleportation has been booming among theoretical and experimental physicists. A fundamental problem arises in the transmission of quantum information: According to Heisenberg's uncertainty principle, two complementary properties of a quantum particle - such as position and momentum - cannot be precisely measured at the same time. All the information in the system must therefore be transmitted without being fully known. But the nature of the particles also holds the solution to this problem: it lies in the possibility of "entangling" two particles with one another in such a way that their properties are perfectly correlated. If you measure a certain property on one of the "twin particles", the corresponding property of the other is automatically determined with immediate effect.
With the help of entangled particles, a successful quantum teleportation can be carried out roughly as follows: You create an auxiliary pair of entangled particles, which are sent to "Alice" and "Bob" respectively. The terms "Alice" and "Bob" have become common to describe the sending of quantum information from A to B. Alice now entangles the object she wants to teleport with one of the auxiliary particles and then measures the joint state (Bell measurement). She sends the result to Bob in the classic way. He applies it to his auxiliary particle and "conjures up" the teleportation object from it.
Are these "instructions for use" just mind games? The great challenge for theoretical physicists is to develop concepts that can also be put into practice. Thought and done: A research team led by Eugene Polzik at the Niels Bohr Institute in Copenhagen successfully carried out an experiment that went back to a suggestion by Ignacio Cirac from the Max Planck Institute for Quantum Optics and his colleague Klemens Hammerer (now the University of Innsbruck).
First the "pair of twins" is generated by sending a strong light pulse onto a glass tube filled with cesium gas (about 1012 atoms). The magnetic moments of the gas atoms are aligned in a homogeneous magnetic field. Light also has a preferred direction: It is polarized, so the electric field only oscillates in one direction. Under these conditions, light and atoms interact with each other, so that the emerging light pulse, after traversing the gas and sent to Alice, is "entangled" with the ensemble of 1012 cesium atoms located at Bob's location.
Alice uses a beam splitter to mix the incoming pulse with the object she wants to teleport - a faint pulse of light containing only a few photons. The resulting light pulses at the two outputs of the beam splitter are measured with photodetectors and the measurement results are sent to Bob.
Because of the measurement results, Bob knows what needs to be done to complete the teleportation and transfer the selected quantum states of the light pulse, amplitude and phase, to the atomic ensemble. To do this, he applies a low-frequency magnetic field that causes the collective spin (angular momentum) of the system to oscillate. This process can be compared to the precession of a gyroscope around its main axis: the deflection of the gyroscope corresponds to the amplitude of the light, while the zero crossing corresponds to the phase.
To prove that the teleportation was successful, after 0.1 milliseconds a second strong pulse of polarized light is sent to the atomic ensemble, which "reads" its state, so to speak. From these measured values, the theoretical physicists can calculate the so-called "fidelity", a figure of merit that indicates how well the state of the teleported object matches the original. A figure of merit of 1 corresponds to a perfect transmission, while a value of zero means that no transmission took place at all. In the present experiment, the figure of merit is 0.6 and is thus well above the value of 0.5, which could at best be achieved in the classical way, for example by transmitting the measured values by telephone, without the involvement of entangled particles.
Contrary to the common notion of "beaming", not one particle has disappeared from one place and reappeared in another place."Quantum teleportation is about communication methods used in quantum cryptography, the encryption of data, and not about new types of traffic routes," emphasizes Hammerer. "The importance of the experiment lies in the fact that, for the first time, teleportation between atoms, which represent stationary quantum storage, and light, which is needed for the transmission of information over long distances, has been successful. An important step has been taken with this, quantum cryptography, i.e. absolutely secure To enable communication over long distances, for example between Munich and Copenhagen." © Max Planck Society
The Max Planck Society (MPG) is a basic research institution funded primarily by the federal and state governments. It operates around eighty Max Planck Institutes.
