Unbreakable encrypted messages a step closer
Let us assume that Alice wants to send an encrypted message to Bob. She uses the machine to generate photons that travel through free space and into a spatial light modulator (SLM) that alters the properties of the individual photons (for example, amplitude, tilt) to properly encode the message into flat but tilted wavefronts that can be focused to unique points dictated by the tilt. But the SLM does one more thing: it distorts the shapes of the photons into random patterns, such that the wavefront is no longer flat which means it no longer has a well-defined focus. Alice and Bob both know the keys which identify the implemented scrambling operations, so Bob is able to use his own SLM to flatten the wavefront, re-focus the photons, and translate the altered properties into the distinct elements of the message.
Along with modifying the shape of the photons, Lum and the team made use of the uncertainty principle, which states that the more we know about one property of a particle, the less we know about another of its properties. Because of that, the researchers were able to securely lock in six bits of classical information using only one bit of an encryption key — an operation called data locking.
“While our device is not 100 percent secure, due to photon loss,” said Lum, “it does show that data locking in message encryption is far more than a theory.”
The ultimate goal of the quantum enigma machine is to prevent a third party — for example, someone named Eve — from intercepting and deciphering the message. A crucial principle of quantum theory is that the mere act of measuring a quantum system changes the system. As a result, Eve has only one shot at obtaining and translating the encrypted message — something that is virtually impossible, given the nearly limitless number of patterns that exist for each photon.
The paper by Lum and Howell was one of two papers published simultaneously on the same topic. The other paper, “Quantum data locking,” was from a team led by Chinese physicist Jian-Wei Pan.
“It’s highly unlikely that our free-space implementation will be useful through atmospheric conditions,” said Lum. “Instead, we have identified the use of optic fiber as a more practical route for data locking, a path Pan’s group actually started with. Regardless, the field is still in its infancy with a great deal more research needed.”
— Read more in Daniel J. Lum et al., “Quantum enigma machine: Experimentally demonstrating quantum data locking,” Physical Review A 94, no. 2 (August 2016) (DOI:http://dx.doi.org/10.1103/PhysRevA.94.022315); and Yang Liu et al., “Experimental quantum data locking,” arXiv:1605.04030 [quant-ph] (2016) (DOI: 10.1103/PhysRevA.94.020301)
