Jörg Schwartz, j.schwartz@europhotonics.com

LONDON – The entanglement of photons is key to developing quantum computers and cryptography devices. However, understanding exactly what entanglement means is tricky, as is generating entangled photons at defined points in time. Researchers at Imperial College London have now come up with an idea for a system that can shoot out large numbers of entangled photons when needed – and they call it a “photonic machine gun.”

Quantum computing is a concept that uses the laws of quantum mechanics, such as superposition and entanglement, to perform operations on data. Quantum computers use qubits rather than the bits used by today’s computers; although somewhat similar to a classical bit, qubits are quite different. Like a bit, a qubit can have two possible values – normally 0 or 1 – but while a bit must be either 0 or 1, a qubit can be 0, 1 or a superposition of both. This means that qubits are described by probabilities – which are not 100 percent as with bits – and measuring them destroys their quantum mechanical nature.

The other difference, also a result of superposition, is that two qubits can be in any quantum superposition of four states, and three qubits in any superposition of eight states, etc. This means that far fewer qubits than bits are needed to compute a large number of states. A quantum computer operates by manipulating those qubits via quantum logic gates – that is, the quantum algorithm – until it is terminated. At that point, the quantum state is collapsed down to classical (probability) values and the result being read.

The need for relatively few qubits for complicated operations is the good news, but the bad news is that all of them must be entangled – i.e., they need to know about each other in a quantum-mechanical sense. “When looking for possible realization of qubits, photons don’t look very promising at the first glance,” said Dr. Terry Rudolph, a researcher at Imperial College. Although photons can be used for quantum communication (See “Alice and Bob talk quantum encryptedly across Vienna,” October/November 2009 *EuroPhotonics*), entangling them for quantum computing is not straightforward because photons do not interact with each other.

One way around this involves using the fact that photons do interact with other particles, such as atoms, and achieving the entanglement indirectly through interaction with such a mediator – for example, in a microresonator (See “Catching photons in a bottle,” October/November 2009 *EuroPhotonics*). But another, more direct, approach is making the photons entangled at the time when they are generated, which is central to Rudolph’s new proposal with colleague Netanel H. Lindner from Technion-Israel Institute of Technology in Haifa.

Rudolph and Lindner suggest using a quantum dot, a special energy-level structure that is included in many semiconductor devices these days, at low temperature. In a process referred to as “creation and subsequent decay of a charged exciton (trion),” electrons in the quantum dot are excited by a light pulse. Upon relaxation, the energy is released as a photon that is entangled with the electron. Under the right circumstances, the electron “remembers” this, and the next electron being released is also entangled with the electron – creating two photons that are entangled via their joint relationship to the electron. And repeating the process yields a third photon, also entangled with the previous two photons, and so on.

The other good news is that the generation of these entangled “clusters” can be triggered, unlike other methods of generating entangled photons – via interferometers, for instance, in which case generation is based on statistics, so a lot of luck is needed to make more than two entangled photons. The only bad news about the “machine gun” proposal presented in the Sept. 11, 2009, edition of *Physical Review Letters* is that it is purely theoretical so far. However, “all the pieces are there, and I know that quite a few people are out there trying to make this happen – as there is almost certainly a Nature paper in it,” Rudolph said.