Quantum key distribution offers a way to transmit data with security guaranteed by the laws of physics themselves. While these communications cannot be hacked, my colleagues and I found that they can be jammed.

The security of our modern communications systems, from emails to banking, from militaries to private individuals, rely on the immense difficulty of undoing the mathematics used to perform the encryption. The encryption can easily be made strong enough that even the world’s most powerful supercomputer would take thousands of years to crack the code. However, quantum computers threaten this security.

Quantum computing technology is still in its infancy, but huge effort is being put into their development around the world because they promise massive advantages over ordinary computers for some problems, including searching vast quantities of information, or simulating chemical interactions accurately. Quantum computers, once they are large and reliable enough, will be able to easily break modern encryption methods, threatening high-value communications for finance, government operations, and defence.

Quantum key distribution offers a way to transmit data that even a quantum computer cannot hack.

The Micius QKD satellite on the front cover of the journal Science.

Quantum key distribution (QKD), essentially, allows two people to send an encryption key (used to encrypt and decrypt the data) to each to each other. If there is an eavesdropper on the line trying to intercept the transmission, they’re presence will be given away by the noise they add to the quantum states. The security of the encryption key is guaranteed by the laws of physics, and not even a quantum computer can hack in.

While QKD is also a relatively immature technology, satellite-mediated QKD has already been demonstrated between China and Austria using the Chinese Micius satellite, and satellite QKD is likely to be an extremely important technology in the future. My colleagues and I set out to find out if, while QKD cannot be hacked, can it be jammed?

The QKD ground station tracks the Micius satellite.

QKD receivers, because they are trying to measure the quantum states of the incoming photons, are extremely sensitive to loss of photons during the transmission, or noise from background photons. We found a way to inject extra photons into the QKD channel, creating enough noise to overwhelm the detector, and prevent a secure key from being transmitted.

Micius is currently the only working QKD satellite. The QKD transmitter is onboard the satellite, transmitting to a QKD receiver on the ground. We found that by shining a bright laser from the ground at the QKD satellite, we can bounce enough photons off the satellite, that are then picked up by the QKD receiver, that the QKD transmission will be overwhelmed, and no secure key can be obtained.

Satellite quantum communications could be disrupted by bouncing a laser off the QKD satellite.

This could be a huge problem for both current and future QKD satellites because the equipment needed to create this disruption could be purchased commercially for only around $200,000USD. This means that the equipment could even be available to malicious parties (e.g. terrorists) with a relatively small amount of resources, making it a potentially significant threat to the high-value communications that QKD will be used for in the future.

Fortunately, this knowledge means that we can look for ways to prevent this form of disruption, including changes to the design of the QKD receiver, and changes in the layout of the satellite that will make it much harder for the disrupting laser to inject enough noise into the QKD system.

For more information, you can download the full paper on this work, for free, here.

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