A Little More Quantum Computing

There’s another domain in Quantum Computing (QC) which periodically attracts headlines – Quantum Key Distribution (QKD). I thought this worth covering because it does not depend on the ability to do parallel computation on superposition states, so may not be as much at the mercy of limited coherence times. And ultimately it may also find much wider application, at least in cryptography.

The Core Principle
This starts with entanglement, which a lot of writers delight in presenting as mysterious, but which isn’t so difficult to understand with a very cursory understanding of quantum physics. Entanglement is another way to describe states (such as spin or polarization) of two or more particles that are related. The electrons in an atom are entangled simply because they can’t take on arbitrary independent states. For example, in a helium atom in the ground state total electron spin must be zero – the spin states are coupled in a combined state.

Entangled states don’t have to be constrained within an atom. Basic laws of conservation require that when free particles are created from a single interaction, energy, momentum and angular momentum (AM) must be conserved. If two particles are created from a state with zero AM, the AM of the created pair must be zero. If you measure the AM of one particle, the other must measure with opposite AM, no matter how far apart they get. (Some implications of doing this at a distance seem mysterious, but that’s another topic.)

There are multiple ways this basic idea can be used for QKD – I want this to be a quick read so I’ll stick to a simple method. Put a device which generates entangled pairs (typically photons, so we’re measuring polarization rather than spin) in-between two parties A and B who want to communicate. The device shoots out a sequence of pairs with each member of a pair travelling in opposite directions, so A gets one set, B gets the other set.

Both measure polarization of the photons they receive. Per pair, what they measure will be randomly distributed, but they are guaranteed, by entanglement, to measure opposite values. They synchronize, then each measures a pre-agreed fixed number of photons, giving them each a random string which they can use as an encryption key since they each know their key is the inverse of the other’s key.

The Value of This Approach to Secrecy
This use of entanglement benefits secrecy in several ways. Most obviously, this method gives a physically-based random number generator for encryption keys. Also, A and B can change keys frequently, making the approach similar to one-time pad encryption, which is known to be superior to any other method.

Most importantly, quantum physics ensures that if anyone attempts to eavesdrop on key generation, they will disrupt the entangled states, because any measurement will destroy the entanglement. You can’t even get around this limitation by making a copy of the state and measuring the copy – the no-cloning theorem ensures that copying will also destroy the state. So any attempt at eavesdropping can be detected by A and B. The mechanics of checking for this involves something called Bell inequality tests which I won’t try to explain here.

Implementation
An early method for producing entangled photons was (and maybe still is) spontaneous parametric down-conversion, where certain types of crystal will produce entangled pairs when pumped with a laser beam. More recently, advances in GaAs semiconductors with InAs quantum dots are now producing entangled pairs with much higher efficiency (although these methods require ultra-low temperatures).

The photons can be transmitted through fiber or free-space. There is attenuation with distance in each case through progressive loss of entanglement, more so in fiber than in free-space. There are various methods to correct for and improve on this, including quantum repeaters and the very cool-sounding but actually rather mundane quantum teleportation.

Using these methods, effective key distribution has already been demonstrated over >300km in optical fiber and to orbital levels in free-space. At least some of these approaches do not seem to require low temperatures, so lack some of the more obvious drawbacks associated with quantum computing.

Summary
While there’s always a danger in making absolute statements about security, it feels fairly safe to assert that any method of cracking this kind of encryption is inconceivable in the following sense: one-time key usage/frequent changes make statistical attacks impossible and any attempt by an eavesdropper to read the key directly or indirectly will be detectable because it will break the entanglement, unless 100 years of heavily-tested quantum physics is wrong. Of course this will just shift hacking to attacking cleartext data at source or destination, but hey, one step at a time.

There are already several commercial operations in this space: ID Quantique, QuintessenceLabs and SeQureNet and several of the big semis have research programs. The NSA is actively working in this area, as is the Chinese government who plan to launch a satellite to support QKD in the near future.

QKD is still pretty expensive – a tool for government encryption rather than personal use, but at least so far it doesn’t seem there are fundamental barriers to optimizing the technology and costs. This may ultimately be a much more practical application of quantum technologies in electronic systems than the much-hyped ultra-fast quantum computers.

You can read more about key distribution over fiber HERE, the Chinese government’s plans HERE, US government efforts HERE and an introduction to quantum teleportation HERE.

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