Quantum teleportation: Transfer of flying quantum bits at the touch of a button

Deterministic quantum teleportation of a photonic quantum bit. Each qubit that flies from the left into the teleporter leaves the teleporter on the right with a loss of quality of only around 20 percent, a value not achievable without entanglement. (Credit: Ill./©: University of Tokyo) 

By means of the quantum-mechanical entanglement of spatially separated light fields, researchers in Tokyo and Mainz have managed to teleport photonic qubits with extreme reliability. This means that a decisive breakthrough has been achieved some 15 years after the first experiments in the field of optical teleportation. The success of the experiment conducted in Tokyo is attributable to the use of a hybrid technique in which two conceptually different and previously incompatible approaches were combined.

 "Discrete digital optical quantum information can now be transmitted continuously – at the touch of a button, if you will," explained Professor Peter van Loock of Johannes Gutenberg University Mainz (JGU). As a theoretical physicist, van Loock advised the experimental physicists in the research team headed by Professor Akira Furusawa of the University of Tokyo on how they could most efficiently perform the teleportation experiment to ultimately verify the success of quantum teleportation. Their findings have now been published in the prestigious specialist journal Nature.

Optical configuration of the teleportation experiment at the University of Tokyo. Laser sources and non-linear optical processes supplied the quantum bit and the required entanglement. Several mirrors and beam splitters facilitated complete teleportation.

Quantum teleportation involves the transfer of arbitrary quantum states from a sender, dubbed Alice, to a spatially distant receiver, named Bob. This requires that Alice and Bob initially share an entangled quantum state across the space in question, e.g., in the form of entangled photons. Quantum teleportation is of fundamental importance to the processing of quantum information (quantum computing) and quantum communication. Photons are especially valued as ideal information carriers for quantum communication since they can be used to transmit signals at the speed of light. A photon can represent a quantum bit or qubit analogous to a binary digit (bit) in standard classical information processing. Such photons are known as 'flying quantum bits'.

The first attempts to teleport single photons or light particles were made by the Austrian physicist Anton Zeilinger. Various other related experiments have been performed in the meantime. However, teleportation of photonic quantum bits using conventional methods proved to have its limitations because of experimental deficiencies and difficulties with fundamental principles.

What makes the experiment in Tokyo so different is the use of a hybrid technique. With its help, a completely deterministic and highly reliable quantum teleportation of photonic qubits has been achieved. The accuracy of the transfer was 79 to 82 percent for four different qubits. In addition, the qubits were teleported much more efficiently than in previous experiments, even at a low degree of entanglement.

The concept of entanglement was first formulated by Erwin Schrödinger and involves a situation in which two quantum systems, such as two light particles for example, are in a joint state, so that their behavior is mutually dependent to a greater extent than is normally (classically) possible. In the Tokyo experiment, continuous entanglement was achieved by means of entangling many photons with many other photons. This meant that the complete amplitudes and phases of two light fields were quantum correlated. Previous experiments only had a single photon entangled with another single photon – a less efficient solution. "The entanglement of photons functioned very well in the Tokyo experiment – practically at the press of a button, as soon as the laser was switched on," said van Loock, Professor for Theory of Quantum Optics and Quantum Information at Mainz University. This continuous entanglement was accomplished with the aid of so-called 'squeezed light', which takes the form of an ellipse in the phase space of the light field. Once entanglement has been achieved, a third light field can be attached to the transmitter. From there, in principle, any state and any number of states can be transmitted to the receiver. "In our experiment, there were precisely four sufficiently representative test states that were transferred from Alice to Bob using entanglement. Thanks to continuous entanglement, it was possible to transmit the photonic qubits in a deterministic fashion to Bob, in other words, in each run," added van Loock.

Earlier attempts to achieve optical teleportation were performed differently and, before now, the concepts used have proved to be incompatible. Although in theory it had already been assumed that the two different strategies, from the discrete and the continuous world, needed to be combined, it represents a technological breakthrough that this has actually now been experimentally demonstrated with the help of the hybrid technique. "The two separate worlds, the discrete and the continuous, are starting to converge," concluded van Loock.

Source : ETH Zurich.
By Science and Universe

Teleported by Electronic Circuit: Physicists 'Beam' Information

A flash of light. ETH-researchers cannot "beam" objects or humans of flesh and blood through space yet, a feat sometimes alluded to in science fiction movies. They managed, however, to teleport information from A to B -- for the first time in an electronic circuit, similar to a computer chip. (Credit: © iscatel / Fotolia)

ETH-researchers cannot "beam" objects or humans of flesh and blood through space yet, a feat sometimes alluded to in science fiction movies. They managed, however, to teleport information from A to B -- for the first time in an electronic circuit, similar to a computer chip.

Physicists at ETH Zurich have for the first time successfully teleported information in a so-called solid state system. The researchers did it by using a device similar to a conventional computer chip. The essential difference to a usual computer chip is that the information is not stored and processed based on the laws of classical physics, but on those of quantum physics. In a study, which is published in the current issue of the scientific journal Nature, the researchers were able to teleport information across a distance of about six millimetres, from one corner of a chip to the opposite one. This was shown to be possible without transporting the physical object carrying the information itself from the sender’s to the receiver’s corner.

<<Image : The ant depicted on top of the superconducting circuit illustrates the relative proportions of the macroscopic electrical circuits for the first time used for teleportation. (Photo: Jonas Mlynek / ETH Zurich)

”Usually, in telecommunication information is transmitted by electromagnetic pulses. In mobile communications, for example, microwave pulses are used, while in fibre connections it is optical pulses,“ explains Andreas Wallraff, Professor at the Department of Physics and head of the study. In contrast, quantum teleportation does not transport the information carrier itself, but only the information. This is possible due to the quantum mechanical properties of the system, in particular the entanglement established between the sender and the receiver. For non-physicists, entanglement constitutes a ”magic“ link between the two parties which exploits the laws of quantum physics.

”Like beaming”

Microscope image of one of the quantum bits used in the teleportation experiments. The sample was fabricated at ETH Zurich’s clean room FIRST. (Photo: Arkady Fedorov, Lars Steffen / ETH Zurich)

As a prerequisite for quantum teleportation, an entangled state is created between the sender and the receiver. After that the two parties can be physically separated from each other while preserving their shared entangled state. In the present experiment the physicists program a bit of quantum information into their device at the sender. Because the two parties are entangled, this information can be read out at the receiver. ”Quantum teleportation is comparable to beaming as shown in the science fiction series Star Trek,“ says Wallraff. ”The information does not travel from point A to point B. Instead, it appears at point B and disappears at point A, when read out at point B.”

The scientists at ETH Zurich have shown that more information is transported from the sender to the receiver in their experiments than possible by classical means. In this way they could prove that the information was indeed relayed by quantum teleportation making use of the laws of quantum physics.

In the experiments of the ETH-physicists, sender and receiver are realised as small superconducting circuits on a seven by seven millimetre sized chip. The scientists created the entanglement between the two parties using controlled pulses of microwave-photons. To access the quantum properties of the system, the scientists had to cool the chip to temperatures very close to the absolute zero using Helium.

High data rates

The distance of six millimetres over which the ETH Zurich-researchers have teleported seems to be short in comparison with other teleportation experiments. A year ago, for example, Austrian scientists managed to teleport information by more than one hundred kilometres between the Canary Islands of La Palma and Tenerife. That and other similar experiments, however, were fundamentally different from the one performed at ETH Zurich, because they use visible light in an optical system for teleportation. The ETH-researchers, however, managed to teleport information for the first time in a system which consists of electronic circuits. ”This is interesting, because such circuits are an important element for the construction of future quantum computers,“ says Wallraff.

Another advantage of the system of the ETH-scientists: It is extremely fast and much faster than most previous teleportation systems. In this system approximately 10,000 quantum bits can be teleported per second. A quantum bit is a unit of quantum information.

Correct transfer of information, always

In addition, the researchers were able to find a solution for an obstacle of quantum information transfer: in teleportation sender and receiver can be entangled in four different ways – in four so-called Bell-states. Depending on the specific Bell-state the information transmitted to the receiver has to be read out in a specific manner. If that is not done, the correct information is read out correctly only in a single one of the four cases.

The device created by the ETH-researchers can deal with this problem. After the information has been teleported from the sender to the receiver, the two parties exchange data about their shared entangled state. In this way the correct information can always be read out in all four possible cases.

“Key future technology”

In a next step, the researchers plan to increase the distance between the sender and receiver in their device. The scientists say, they will try to teleport information from one chip to another. And in the long term the goal will be to explore whether quantum communication can be realised over longer distances with electronic circuits, more comparable to those achieved today with optical systems.

”Teleportation is an important future technology in the field of quantum information processing,“ says Wallraff. For example, it may be possible to transmit information from one location to another one in a future quantum device or processor. Compared to today's information and communication technologies, which are based on classical physics, quantum information processing has the advantage that the information density is much higher: In quantum bits more information can be stored and more efficiently processed than in classical bits.

Source : ETH Zurich.

By Science and Universe