I can't understand it, but they tell it is secure!

The experiment's with entangled photons was carried out in this setup. Such apparatus could be miniaturised using techniques from integrated optics scaling down to a few mm. Credit: IQC, University of Waterloo

Schematic diagram of random oblivious transfer (ROT) experiment setup. Credit: C. Erven et al./arXiv

Long, long time ago I did my post doc in elementary particle physics and quantum properties where on the menu. Still, I remember I had problem in really understanding quantum properties and I used to console myself with the words of Niels Bohr: "If quantum mechanics hasn't profoundly shocked you, you haven't understood it".

Today the situation has not improved and I still have problems when I try to really understand quantum mechanics. However, if I content myself of a glimpse into it (which is a long shot from understanding it) I can appreciate the magic, as it is the case in this research result of a team at the Center for quantum Technologies of the National University of Singapore in cooperation with a team at the University of Waterloo, Canada.

In a nutshell the researchers have worked out a more sophisticated method for ensuring a secure transaction exploiting a quantum property, the photon entanglement. A pair of photons generated in a certain way will have opposite spin, one will have an up spin and the other a down spin, like a coin that has two sides. Now if you flip the coin and you see "head", it is clear that the other side will be "tail", no need to check! Similarly, if you intercept one photon in the pair and it has an up spin you can be sure that the other has a down spin. Now things get a bit more complex though. To see the photon spin you need to interact with it and this interaction flips its spin. If it was up, now it is down. Since this photon is entangled with the one that was created with it, the flipping of its spin results in the splitting of the other spin, so that now you know that this photon had an up spin (but that knowledge has reversed its spin to down) the other photon has an up spin (it was down, but by reading and flipping the spin of the photon you instantaneously flipped the other too).

This very strange property (the more you try to make sense of it the foggier it gets..) can be used to control if someone is tampering the communication, sending the information with one photon as market and checking the other photon used as the control that is following a different path. We already have products using this mechanism to ensure security, known as QKD: Quantum Key Distribution.

What the researchers did was to push the envelope a bit further to create a security mechanism still based on entanglement where the two parties don't trust each other and the verification takes place through a two party secure computation. 

When you enter your PIN in an ATM machine the bank gets your PIN and verify that it is the correct one. What if the ATM is a fake and you key in your PIN? Well, in that case you just gave away your precious PIN to a malicious party (it has already happened through fake ATM ...). On the other hand, if the bank reveals you its matching pattern you could be the malicious party that will use that information to fabricate a PIN to cheat the system.

Today you have to trust the bank, and the bank doesn't trust you! What the researchers did was to invent a mechanisms where both you and your bank can distrust each other, and therefore are not prepared to share information and still you can execute a verifiable, secure transaction.

To do that you perform a ROT: Random Oblivious Transfer, and you leverage on quantum entanglement properties of the photons, as shown in the schematics. TO get more details (that might not increase your understanding of the mechanisms, I am afraid) take a look at this report.

At the EIT ICT Labs we have an area studying cryptographic techniques to bring them to the market and make the life of thieves a bit more complicated....

Author - Roberto Saracco

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