Due to noises in the telecommunication channel, the quality of quantum entanglement will gradually decrease with distance. That is to say, the entanglement between two particles will continuously decrease over distance, which is the fundamental bottleneck in quantum communication over long distances.

In the quantum communication process, the fragile protons are easily absorbed, leading to transmission errors, or even to transmission failures. To make up for the loss of data during transmission, repeaters are used to relay information and extend the transmission distance.

For some reason, the probability of producing two pairs of photons for entanglement is low, so the realization of a quantum RP repeater is internationally recognized as a huge challenge. Pan has studied quantum communication for years. In 1998 and 2003, he and his colleagues realized quantum entanglement swapping and entanglement purification; in 2002, he obtained the most intense source of entangled photon pairs; in 2007, they put forward theoretical plans for a highly efficient quantum repeater with storage capacity and a low error rate. Finally, they realized the quantum repeater, taking a crucial step toward long-distance quantum communication.

Pan and his team have also experimented with global quantum communication, which is to transmit quantum signals through satellites to receptors tens of thousands of km away. Such a global communication system will be secure.

Massive scale

"It would take the fastest computer in the world tens of billions of years to factorize a 400-digit number, while a quantum computer can accomplish this within a few minutes," Pan said.

Traditional computers use binary digits to represent information, while quantum computer uses quantum bits (qubits). The qubits are in a state that has an equal probability of having one of two extreme values until a measurement is made on it. A quantum computer could perform parallel calculations on a massive scale someday and bring about a great leap in information processing and storage.

Pan's group has made some breakthroughs in quantum computing. In 1997, the group realized the design of a one-way quantum computer, which broke the world record and marked a significant step in the physical realization of quantum computers.

In the same year, Pan's group was the first in the world to perform the Shor algorithm with a quantum computer. The algorithm was put forward by American scientist Peter Shor in 1995, and it proves that quantum computers can decipher the existing RSA encryption algorithm.

RSA algorithm is a public key encryption widely used in electronic commerce. The algorithm was proposed by Ron Rivest, Adi Shamir and Leonard Adleman of the Massachusetts Institute of Technology in 1977, and RSA are the initials of their last names.

The Shor algorithm is an internationally acknowledged problem. Pan and his group worked with researchers from Oxford University, and conducted an experimental demonstration of Shor's algorithm using four photonic qubits. The experiment represented an essential step toward full realization of Shor's algorithm and scalable linear optics quantum computation.

"The computer chips are getting smaller and smaller in size and faster and faster in speed. The classical electrodynamics theory is becoming less relevant, while quantum mechanics is getting more useful," Pan said. Compared with traditional computers, the quantum computer is very small and fast, has large storage and consumes little energy, Pan said. In other words, a super-fast computer will be small enough to be put in a pocket.

Nevertheless, quantum computers will not be on the market any time soon because they could be used to decrypt the widely-used existing encryption system, and hack the banking and electronic commerce systems.

The most potent weapon against such grave threats to the encryption system is quantum cryptography. That is because quantum state cannot be cloned. Guo Guangcan and his group are working on quantum cryptography, and have successfully distributed quantum key to the optical fiber between Beijing and Tianjin recently. A professor at the University of Science and Technology of China and an academician at the Chinese Academy of Sciences, Guo has been working on the subject since the early 1990s, and has kept abreast with frontier research in the field.

Time travel

In science fiction, people can travel back to the past and forward into the future, as well as from place to place in the blink of an eye. In reality, it is now no longer fiction to transport a particle from one place to another at a stunning speed. "Maybe in the future we can move a person from one place to another in an instant," Pan said.

In fact, Pan is a person who frequently moves from place to place, spending half of each typical year in China, and another half outside doing research.

Pan will turned 39 in March 2009. A decade ago his paper was honored by Nature as being among the "Twenty-one Classic Papers in a Hundred Years of Physics." Other papers to have received this honor include Wilhelm Conrad Roentgen's paper on X rays and Einstein's paper on the theory of relativity.

Pan was elected among the top 10 distinguished youths of China, and won China's national award for young scientists. In 2003, he was granted the Erich Schmid prize, the top biennial prize that the Austrian Academy of Sciences awards to physicists under 40 years old.