1. The 'unjammable' quantum radar that could
render ALL stealth planes useless
U.S. researchers have employed the quantum properties of photons to create an unjammable
radar signal.
Conventional radar is vulnerable to a range of technologies, ranging from dropping chaff to
create false reflections to drowning the radar frequency with noise.
More sophisticated radar can deal with such ploys, but the most sophisticated radar jammers are
able to intercept the the signals and send back false information.
A U.S. Air Force B-2A stealth bomber: Researchers have managed to develop a new kind of
radar that is able to see through the counter-measures deployed as a ruse to fool anti-aircraft
systems
However, a team from the University of Rochester, New York have shown how the quantum
properties of photons can be used to outsmart this advanced stealth technology.
The new radar concept relies on the fact that any attempt to measure a photon always destroys its
quantum properties, MIT's Technology Review explains.

2. Physicists have exploited the quantum properties of photons to create the first imaging system
that is unjammable
Schematic of the quantum-secured radar: If a stealth aircraft attempts to intercept the photons and
resend them in a way that disguises its position, it would inevitably change the photons' quantum
properties
Jamming radar signals is an increasingly sophisticated affair. There are various techniques such
as drowning the radar frequency with noise or dropping chaff to create a false reflection. But the
most advanced radar systems can get around these ruses.
So a more sophisticated idea is to intercept the radar signal and modify it in a way that gives
false information about the target before sending it back. That’s much harder to outsmart.
But today, Mehul Malik and pals at the University of Rochester in New York state demonstrate a
way to do it.
These guys base their technique on the quantum properties of photons and in particular on the
fact that any attempt to measure a photon always destroys its quantum properties.
So their idea is to use polarised photons to detect and image objects. Reflected photons can of
course be used to build up an image of the object. But an adversary could intercept these photons
and resend them in a way that disguises the object’s shape or makes it look as if it is elsewhere.

3. However, such a process would always change the quantum properties of the photons such as
their polarisation. And so it should always be possible to detect such interference. “In order to
jam our imaging system, the object must disturb the delicate quantum state of the imaging
photons, thus introducing statistical errors that reveal its activity,” say Malik and co.
That’s more or less exactly how quantum key distribution for cryptography works. The idea here
is that any eavesdropper would change the quantum properties of the key and so reveal his or her
presence. The only difference in the quantum imaging scenario is that the “message” is sent and
received by the same person.
Malik and co have tested their idea by bouncing photons off an aeroplane-shaped target and
measuring the polarisation error rate in the return signal. Without any eavesdropping the system
easily imaged the aeroplane.
But when an adversary intercepted the photons and modified them to send back an image of a
bird, the interference was easy to spot, say Malik and co.
That’s an impressive demonstration of the first imaging system that is unjammable thanks to
quantum mechanics.
That’s not to say the technique is perfect. It suffers from the same limitations that plague early
quantum cryptographic systems, which are theoretically secure but crackable in practice.
For example, instead of sending single photons, the quantum imaging system sends photon
pulses which contain several photons. One or more of these can easily be siphoned away and
analysed by an adversary without anybody else being any the wiser.
However, there are an increasingly wide range of fixes for these problems for quantum key
distribution that could help make this quantum imaging system more secure.
Perhaps best of all, this kind of system could easily be put to work now. The techniques are well
known and widely used in optics labs all over the world. So there’s no reason, this security
cannot be added relatively quickly and cheaply to existing imaging systems.
Ref: arxiv.org/abs/1212.2605: Quantum-Secured Imaging
The technology works in a similar way to quantum key distribution for cryptography, where any
eavesdropper would change the quantum properties of the key by listening in, revealing his or
her presence.
Mehul Malik, who led the team that carried out the research at Rochester's Institute of Optics,
tested the concept by bouncing photons off a stealth bomber-shaped target and measuring the
return signal's polarisation error rate.

4. The system easily imaged the war plane without any eavesdropping, but when the adversary
intercepted the signal and modified it to send back the image of a bird, the radar was easily able
to see through the ruse.
No fooling us: When there is no jamming attack, the received image faithfully reproduces the
actual object, shown left. If the target attempts to send a 'spoof' image like the one on the right,
the imaging system can detect the presence of the jamming attack, because of the large error rate
in the received polarization
However, the researchers admit that their novel radar system is still not perfect. As MIT's
blogger explains, it suffers from the same limitations that plagued early quantum cryptographic
systems.
The quantum radar sends photons in pulses that contain several of the quantum particles, one or
more of which could be easily siphoned away and replicated to tune the signal sent back to the
same state as the one sent.
'Further, a sophisticated jammer may use quantum teleportation to teleport the polarisation state
of our querying photons onto photons carrying false position or time information,' says the study.
However, while the equipment needed to carry out such sophisticated jamming is readily
available in labs worldwide, it is not thought yet to be deployed by the military.