Quantum physicists have been sending qubits through the atmosphere encoded in individual photons for years now. The work is the foundation of a new type of quantum communication that is perfectly secure from eavesdropping.
But there are challenges in setting up a global system of quantum communication. Not least is the problem of decoherence, in which noise destroys the quantum nature of the information as it travels though the atmosphere. This has limited the distance record for this kind of transmission to 144km (although longer distances are possible through optical fibres).
The obvious way around this is to send the signals through space via a satellite. When sent straight up, the photons need only travel through 8 kilometres of atmosphere and so are much less likely to decohere.
On Friday, Anton Zeilinger’s group in Vienna announced that they had taken the first step in this direction by bouncing single photons off an orbiting satellite soome 1400km above the Earth.
The team used a 1.5 metre telescope called the Matera Laser Ranging Observatory in Italy to bounce single photons off the Ajisai geodetic satellite, an orbiting disco ball that is used for laser ranging measurements.
Quantum communication with entangled photons can only be done by sending and detecting them one at a time so the experiment is a crucial step in making space-based quantum communication possible.
However, the team also tried bouncing photons off several other disco balls such as Lageos II, without success.
But give them their due. The experiment proves that it is possible to use existing laser ranging equipment to send and receive single photons from orbiting satellites.
“Our findings strongly underline the feasibility of Space-to-Earth quantum communication with available technology,” says the team.
Of course, this isn’t a demonstration of quantum communication itself in space. That will require an orbiting source of entangled photons.
So all they need now is somebody to build and launch a satellite that can produce and transmit entangled photons. Any takers?
Ref: arxiv.org/abs/0803.1871: Experimental Verification of the Feasibility of a Quantum
Channel between Space and Earth