Remember Star Trek — and Emory Erickson, the fictional inventor of the “teleporting” transporter? Well, Erickson had better look out, for a real-life counterpart threatens to upstage him. And while Danish physicist Eugene Polzik may not be “beaming up” people from one planet to another yet — as it happens in the sci-fi series — his team in Copenhagen’s Niels Bohr Institute has demonstrated that it’s possible to do so with light and matter, at least over short distances.

In the world of reality, the feat has stupendous application potential: engineers, in the foreseeable future, can start thinking of next generation communication networks that transport much larger chunks of information without anyone ever being able to eavesdrop.

Think of an ultra secure communication network in a world where hackers abound.

Or, hyper-speed computers that can carry out the most complex mathematical calculations at significantly faster rates than the most powerful of classical supercomputers. That too, involving a large number of parameters.

Quantum teleportation, as it is called, is considered the Holy Grail of safe and secure information and communication technologies. A quantum actually means an object that cannot be divided further without disturbing its inherent properties. A decade ago, quantum teleportation was just an idea bordering on science fiction. But over the years, thanks to a train of theoretical and experimental successes by different scientific groups, particularly in the US and Europe, it has slowly crawled into the ambit of reality.

“This work is a significant step towards a practical quantum communication network. It will generate more active research in this area and bring new important advances,” says Boris Blinov, a University of Washington scientist who is credited with entangling a single atom with a single light particle for the first time in 2004.

Quantum communication

“Teleportation” exploits a basic but bizarre property of sub-atomic particles which scientists call “entanglement”. It allows two particles located at distant places to share an identical quantum state without any physical contact. As a result, a twiddling of one set of “entangled” particles creates a similar effect on the other set placed far away. Albert Einstein, who disliked but grudgingly admitted the significance of quantum mechanics (the physical theory that deals with matter and energy on the smallest possible scale) called this “spooky actions at a distance”.

But what makes Polzik and his students’ work stand out is the fact that they could “teleport” between objects of two different nature — light and matter (or atoms). While light serves as a carrier of information (for example, it is already in use in fibre optics technology), the other is an excellent storage medium. Scientists in the past could “teleport” with either of the two; light or a small number of atoms. Besides, those quantum teleportation experiments lasted a split second and the distance covered was in millimetres.

“Teleportation is one of the most important building blocks of the quantum communication network of the future that will revolutionise communication using computers,” says Polzik.

Polzik and colleagues could move information to about half a metre distance. “But the present approach is scalable to longer distances,” says Polzik, whose paper appeared in the October 5 issue of Nature. More importantly, they could for the first time involve a large number of atoms — about a thousand billion atoms — in the experiment. Collaborating with Polzik’s team was theoretical scientist Ignacio Cirac of the Max Plank Institute for Quantum Optics in Garching, Germany. Significantly, Cirac together with his former colleague at the University of Innsbruk in Austria, Peter Zoller, was the first researcher to propose the concept of quantum computers in 1995.

According to Blinov, using a huge number of atoms, instead of a single atom, has important implications. “For example, it is possible to store the quantum state of light (the light pulse) in the quantum state of matter (the atomic ensemble), and later retrieve it. This feature of atomic ensembles make them useful for quantum memory applications,” he told KnowHow. In other words, light particles, or photons, can be used for communication, while long-lived spin states of caesium atoms can store information for later retrieval.

Commenting on their work, Arvind, an Indian Institute of Technology Madras researcher who has specialised in quantum physics says, “The work is definitively a step forward in quantum teleportation.” But he is quick to add that much more work is required before the technique is exploited for real applications.

Entangled particles

With the help of entangled particles, successful teleportation can be achieved roughly as follows: A pair of entangled particles is created, one being transmitted to “Alice” and the other to “Bob”. (The names Alice and Bob describe the transmission of quantum information from A to B.) Alice now entangles the object of teleportation with her own particle and then measures the state of union. She sends the result to Bob in the usual manner, say, using a normal telephone line. He applies it to his particle and “conjures up” the teleportation object from it.

First the twin pair is produced by sending a strong light pulse to a glass tube filled with caesium gas (about a thousand billion atoms) and is kept at room temperature. The enclosure is surrounded by magnetic field installations so that the ensemble of caesium atoms have spins all pointing in the same direction and fluctuating according to their given quantum state. Since it is a laser, all its light particles (photons) are uniformly polarised and their electric field oscillates in just one direction. Under these 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 caesium atoms located at Bob’s site.

Alice mixes the arriving pulse by means of a beam splitter — equipment used to split light — 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 beam splitter are measured with photo detectors 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 a top spinning about its major axis.

To prove that quantum teleportation has been successfully performed, a second intense pulse of polarised 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 means total success; 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, for example, by communicating measured values by telephone, without the help of entangled particle pairs, the researchers said.

Admits Blinov, “Significant improvements in the fidelity of the entangled and the teleported state are needed to make this scheme useful for practical goals. Several other groups in the world are now working with similar systems. Many great results have come out of their research. With such a major effort taking place, it is reasonable to expect rapid improvements in fidelity, together with an increased versatility of the quantum devices.”

Another vital feature of quantum computing is that information will not be coded and computed as binary bits such as 0 and 1, like the conventional computers of the day. But, it can exist in three different super positions, 0, 1, and 2 simultaneously. This means that it can hold much more information. Besides, such quantum super positions, in principle, make data absolutely secure.

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 Danish scientists say.

Michail Lukin and Matthew Eisaman, who wrote an accompanying commentary in Nature say that apart from practical applications, the Danish experiment demonstrates an exceptional degree of quantum control over light and matter. Such control is much needed as the field of experimental quantum-information science matures, they feel.