Photon research breaks new boundaries

26 Mar 2008

Three independent research groups have demonstrated how the quantum nature of individual photons could boost the performance of both imaging and data transmission systems.

Perhaps the most intriguing is the suggestion by Seth Lloyd at the Massachusetts Institute of Technology (MIT) that the physics of quantum entanglement could be harnessed to build an ultralow-noise imaging system. According to Lloyd, a quantum imaging system could yield an exponential improvement in signal-to-noise ratio over conventional optical imaging techniques.

That's because a conventional detector has no way of distinguishing between a photon of noise and a photon that has been reflected back from the object of interest. Lloyd's solution is to create pairs of photons in entangled quantum states, which causes the photons to retain a "memory" of being created as a pair.

One entangled signal photon is directed at the object of interest, while the other (the ancilla) is retained at the imaging device for future reference. If a signal photon reflects from the object and returns to the imager, it can be compared to the ancilla, which retains a memory of its entangled partner. If the partnership is verified the photon is used to build an image of the object. However, if the ancilla has no memory of the photon, it is rejected as noise.

The challenge that remains, however, is how to compare the signal and ancilla photons. In principle, Lloyd believes that this could be achieved by allowing the two photons to recombine to create a single high-energy photon by firing them into a photonic crystal. In this case, the two photons are more likely to recombine to make a single higher-energy photon, which can be detected, if they retain a memory of entanglement.

But this requires a system that can put both reflected signal and ancilla at the same place at the same time, which Lloyd admits is no mean feat. However, he points out that there is no reason why it couldn't be done – and that it should be relatively easy to achieve compared to other quantum-information processes that physicists are currently trying to develop.

Single photons trek through space
While Lloyd's proposal remains in the realm of thought experiments, researchers at the University of Padova in Italy and the University of Vienna in Austria have demonstrated that single photons can be sent through space from a satellite to a receiving station on Earth. The work paves the way for a network of satellites sending quantum encrypted messages around the world.

The work was carried out at the Matera Laser Ranging Observatory in Italy, which has been designed to measure the time it takes for laser pulses to return to the observatory after having been reflected off a passing satellite. The European team employs the same basic technique, but make the beam deliberately weak so less than one photon from each pulse returns to Earth.

By bouncing the beam off the Japanese Ajisai satellite, which orbits at an altitude of about 1500 km, the researchers calculate that they receive an average of just 0.4 photons per pulse. By precisely calculating when each pulse is to return to the observatory, they are able to show that these detected photons are those transmitted by the telescope and not stray photons from background sources.

"Not only have we shown that it is possible to detect single photons from a satellite, but we have also demonstrated that we can do this using existing technology," said Paolo Villoresi of the University of Padova. "We are very happy about that."

Entangled photons send more messages
Last but not least, scientists at the University of Illinois have shown how "hyper-entanglement" can be used to transmit more information than ever before – although they admit that the scheme is likely to be limited to use in satellite-to-satellite data links.

In classical coding, photons are entangled in polarization or spin alone, which means that a single photon will convey only one of two messages, or one bit of information. But the Illinois team entangles pairs of photons in both polarization and in orbital angular momentum. This hyper-entanglement means that a single photon can convey one of four messages, or two bits of information.

Simultaneous entanglement in polarization and orbital angular momentum is achieved through a process of spontaneous parametric down conversion in a pair of nonlinear crystals. The team then encodes a message in the polarization state by applying birefringent phase shifts with a pair of liquid crystals.

While this technique of hyper-entanglement enables the transmission of two bits with a single photon, the group admits limitations to its application on Earth. "Atmospheric turbulence can cause some of the quantum states to easily decohere, thus restricting their likely communication application to satellite-to-satellite transmissions," concluded Julio Barreiro, a graduate student at the University of Illinois.