Quantum computers are set to revolutionise the face of online security. They are the ultimate code breakers and makers, writes Tom Stace.
Secret codes evoke scenes of cold war agents in trench coats writing shopping lists on top of secrets written in lemon juice, but it is here in the field of cryptography that the quantum revolution began.
The counterintuitive world of quantum mechanics has attracted curious minds for over a century.
Today, physicists are on the path to building quantum technologies that will be used in the next generation of clocks to keep GPS satellites synchronised; in ultra-high-precision sensors for medical devices and navigation; and to systematically design new medicines and materials down to the atom.
But the advent of these technologies will also challenge the fabric of online security which we rely on to protect the electronic passwords we use every day.
You might recall that the prime factors of 15 are 5 and 3: 5×3=15. How would you react if I told you that I had a machine that could find the prime factors of extremely large numbers? Perhaps with indifference, or maybe you'd crack your favourite geek joke ("an atom walks into a bar…")?
The prudent response would be to visit your bank, in person, and withdraw your savings immediately. You see, factoring is the digital master key to every virtual bank vault on the planet — fast factoring would crack open encrypted online financial transactions.
Currently, the security of all online transactions rests on the exponential difficulty of finding prime factors. The computational resources a thief would require to crack modern encryption in a reasonable time vastly outstrips the combined capabilities of all the conventional computers on the planet.
However, the nascent quantum revolution will upset this status quo.
In 1994, Peter Shor of the Massachusetts Institute of Technology invented an algorithm that solves this precise mathematical problem. For now, this is not a threat to online banking: Shor's algorithm can't run on any conventional computer. Rather, it only runs on a quantum computer — one that utilises the peculiar properties of quantum mechanics.
Quantum technologies are in their infancy. At present, the largest quantum computers have only about 10 operating quantum bits. They are also exquisitely sensitive to errors: each quantum bit will work reliably for just a few thousand basic manipulations. They are far less advanced than the computer inside your smartphone, which can process its half-a-trillion digital bits at the rate of several billion per second.
While quantum computers will eventually make modern encryption redundant, quantum mechanics also paves the way to provably secure communications beyond what we have now.
After millennia of code-making and breaking, there remains one encryption scheme that is provably unbreakable, even by quantum computers: the one-time key protocol (see box). Its main ingredient is a list of random numbers secretly shared between the communicating parties, which form the key to encoding and decoding a secret.
But the one-time key only defers the problem: how is the key shared in the first place?
The parties must either share it in private at some earlier time, or arrange a trusted agent to disseminate the key. This presents problems if they've never met, if the secret is too important to trust to anyone else, or if they've used up their original one-time key.
Amazingly, quantum mechanics provides a mechanism to distribute a shared, random key over an untrusted communication channel without prior collusion or trusted agents.
Using a protocol known as quantum key distribution, parties exchange carefully prepared photons, the quantum particles of light (for example over optical fibre). As photons are sent, received and then measured, the intrinsic randomness of quantum measurement generates a truly random key shared between the parties, allowing them to communicate securely and indefinitely.
In an effort to discover the random key, a would-be-eavesdropper might try to surreptitiously intercept and measure the photons. But quantum mechanics says that the measurement intrinsically changes the photons' state, leaving behind tell-tale evidence of the eavesdropper's interference.
Just as a shy singer performs confidently in the shower but becomes fearful in front of a listening audience, the state of a quantum system changes when it is observed. This ensures parties know whether their shared random key is indeed secret, or if it is compromised and cannot be trusted.
Quantum key distribution systems are now commercially available. The optic fibre they use is incredibly transparent, but light still gets absorbed within a few dozen kilometres, setting an upper limit on the distances over which these systems can operate.
These are useful for a secure link between, say, the White House and the Pentagon, which is both short and highly sensitive. But widespread use of this technology will require household optical-fibre connections and quantum repeaters — miniature quantum computers — spaced at regular intervals, to enable quantum randomness to be securely transmitted over thousands of kilometres.
Australian efforts are at the forefront of research into quantum repeaters, with diamond nano-particles and atomic gases representing some of the most promising platforms.
In the end, quantum mechanics gives us a way to break modern encryption algorithms, but it also leads to new and provably secure communications, and will enable a variety of other technologies.
But perhaps more importantly, as we solve the challenge of engineering the foundations of quantum technology, we will inevitably learn profound things about the quantum world and the nature of our universe.
The philosophical questions that arise from the principles used in quantum cryptography are profound, and go to the core of our understanding of reality.