Under Quantum Lock And Key

China leads the world in quantum key distribution (QKD) technology, which promises to lay the foundations for an ‘unhackable’ quantum internet.

AsianScientist (Oct. 5, 2021) – When it comes to securing privacy, you have to sometimes fight fire with fire. Currently, our online transactions and sensitive data are secured by public keys, which can be thought of as large numbers that are ‘unlocked’ by calculating their prime number factors.

As quantum computing moves into the mainstream, however, solving such problems will become trivial, leaving the lives that we lead on the internet vulnerable to hackers. The solution to the challenge posed by quantum computing, oddly enough, could be another form of quantum technology: quantum key distribution (QKD).

While countries such as the United States, Japan and Singapore have been interested in QKD for several decades, China is today the clear leader, having already deployed both long-distance fiber optic networks and satellite-based quantum communications. How did China develop its world-leading expertise and what does this mean for the technology moving forward?

The key to privacy

Before we investigate the implications of QKD and the eventual hope for a quantum internet, let’s take a step back to look at how QKD is supposed to work and why it has been so challenging to implement thus far.

One unique feature of quantum information is that, under certain conditions, ‘no-cloning theorems’ apply: specially prepared quantum states cannot be copied without being visibly and irrevocably changed. This makes the sharing of quantum states a perfect mechanism for exchanging secret keys. After all, an attacker copying secret key transmissions won’t be able to proceed without alerting you to the intrusion.

To enable this quantum secrecy, information can’t be sent using the usual electrical pulses handled by modern electronics. Instead, quantum information is sent using qubits, packets of photons which have been prepared to carry information in special quantum states, such as in their polarization angle.

The states that make up qubits come in special sets or ‘bases,’ measured in a certain way. When a qubit is measured the same way it was prepared, it is guaranteed to return the value of zero or one. However, when measured differently, the same qubit can return zero or one in a random fashion, with its prior state erased.

As such, qubits allow a sender and a receiver to be sure that they have received the same information simply by publicly comparing how their measurements were made, without ever publicly divulging what results those measurements had.

But if they wish to be completely certain that their communications have not been tampered with, they can also choose to compare a randomly chosen subset of their measurements. Any unusual randomization will indicate the presence of an eavesdropper in their system trying to measure the qubits in transit, irrevocably transforming the information they are carrying.

Using a theoretical understanding of qubits, quantum information theorists have been devising robust QKD protocols since the 1980s, such as Bennett and Brassard’s 1984 protocol or Arthur Ekert’s 1991 protocol, referred to in the literature as BB84 and E91 respectively. However, these theoretical possibilities far outstripped the technological capabilities of the time.

Qubit transmission requires a photon to pass unaltered from sender to receiver, but in real life, any transmission medium is ‘noisy,’ occasionally absorbing or modifying any signal that passes through it. Furthermore, this noise rises exponentially as the transmission distance increases.

Accordingly, any attempts to perform QKD over real-world distances, relevant to telecommunication, would have to wait two decades for the arrival of highly transparent fiber optics that could be used for quantum transmission of photons.

Shern Ren is studying towards a PhD degree in physics at the National University of Singapore. When he isn't working on the statistical mechanics of nanomachines and single-molecule systems, you may find him scratching his head over politics, education and the mathematics of Threes.

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