Why should Bob do the heavy lifting when you can have Charlie do all the work?
While it would have been very easy to miss over the last month or so, as dealing with Covid-19 and rapidly switching to working from home to protect our fellow citizens and health service has taken priority, one of my students – @Henry Semenenko – very successfully defended his PhD and quietly published an important demonstration addressing a potential weakness in quantum security.
Picked up by Science Daily, Henry’s paper on Chip-Based Measurement-Device-Independent Quantum Key Distribution shows how to remove a potential vulnerability due to the single photon detector systems used in quantum key distribution (QKD). QKD is a method that allows us to distribute extremely secure digital encryption keys to two parties. It allows a first party (ideally a big VC) to lock a message (ideally a digital cheque with lots of 0’s) before sending it over a network to a second party (ideally KETS) allowing them to unlock it (and cash it!) once it arrives.
So, what is the problem? Well quantum security is still in its earlier days, similar to the early days of the internet – think back to when your dial-up modem used to routinely drop your connection. We are still working out some of the kinks, and the single photon detectors – because they were purpose built for light detection, not security – have some potential vulnerabilities if we are not careful.
Usually in QKD the VC would send single qubits in the form of photons (single particles of light) to KETS with one bit of the key stored in each photon sent. Then it would be up to KETS to measure the photons and learn the key. But, as mentioned, detectors can be vulnerable – there has been several demonstrations of people blinding detectors and manipulating their detection efficiencies and timing, ultimately allowing them to learn the digital encryption key without the legitimate parties noticing. That is where measurement-device-independent (MDI) QKD comes in.
In MDI-QKD, both the VC and KETS would send qubit photons to a third party to be jointly measured. This joint measurement on both photons together can then be viewed as creating entanglement between the VC and KETS backwards in time. Entanglement in this context means the VC and KETS effectively share a stream of highly correlated pairs of photons from which they can generate their key. Moreover, one can show that this correlation is only shared by the VC and KETS, and not with anyone else in the universe. In other words, the third party asked to make the measurements could be the VC’s biggest rival!
While there has been some excellent early demonstrations of this technique, key to Henry’s result is accomplishing this with chip-based devices.
It is this chip-based approach that is ultimately going to enable quantum communications and security at scale in real world commercial applications.
However, this chip-based approach also made the experiment much more challenging… something Henry learned the hard way. The earlier experiments were generally done with bulk fibre optic systems that are inherently modular. When one component does not work, it is easily swapped out for another. Need another modulator to make your life easier and give you an additional degree of freedom? Just splice it in. But integrated chips generally have far fewer knobs to turn to make something work and the whole point of them is their design is fixed. That is where they get their stability from, their monolithic structure (i.e. one-piece construction). All of this is wonderful once you have a robust design, but far more challenging when doing the world’s first chip-based MDI-QKD experiment!
Lucky for us, Henry persevered and demonstrated a wonderful experiment that is now shared with the world as part of our collective scientific consciousness. The next phase of the work is being discussed within QETLabs with the suitably mysterious project title of “The Node”. As for Henry, after years of understanding how to eke out the best performance and ridiculous control of integrated quantum photonic chips, he is now unsurprisingly developing them with IMEC, one of the world-leading fabrication foundries, as a Quantum Photonics Research Engineer. While here at KETS, we are working on this and many other cutting-edge ways to keep your information and data safe from quantum computing technology.
Chris Erven, May 2020