Quantum key distribution for a post-quantum world


The advent of quantum computers and their ability to solve calculations at incredible speeds by exploiting the fundamental properties of quantum mechanics could revolutionize our world. But what does this quantum future mean for data security?

As quantum computing evolves from the test lab to the real world, this unprecedented new form of computing power is having massive implications for current forms of encryption and public-key cryptography (PKC), such as Rivest-Shamir-Aleman (RSA) and elliptic curve cryptography (ECC) . Given the processing capabilities of quantum computers, which can analyze massive amounts of data orders of magnitude faster than current digital computers, these forms of encryption essentially become vulnerable to bad actors.

In the coming post-quantum future, cryptographic solutions built on the rules of quantum physics are essential to ensure that sensitive digital information is distributed safely and securely over the upcoming quantum internet. One of the pillars of this more secure future of quantum computing is called Quantum Key Distribution (QKD), which uses fundamental properties of physics to derive encryption keys for secure encryption between two sites simultaneously.

Unleash the power of photons

At the physical level, the data bits sent during the key exchange for today’s popular encryption techniques, such as RSA and ECC, are encoded using large pulses of photons, or voltage changes. With QKD, everything is encoded on a single photon, which relies on quantum mechanical properties that enable detection and prevent successful eavesdropping. Quantum objects exist in a superposition state, in which the value for a property of the object can be described as a set of probabilities for different values.

The coded photons are transmitted via the so-called quantum channel. A separate channel, called the classic channel, established between the two endpoints handles clock synchronization, key sifting, or some other data exchange. this channel could be any conventional data communication channel.

Several varieties of QKD

As technology advances, a number of implementations and protocols for QKD emerge. For example, discrete variable QKD (DV-QKD) is used in many commercial QKD systems today. A DV-QKD system consists of two endpoints: a transmitter and a receiver. The quantum connection between these endpoints could be free space or dark fiber. In this case, the transmitter encodes a bit value, 0 or 1, on a single photon by controlling the photon’s phase or polarization. A separate data link between the two endpoints is used to communicate information about the quantum measurements and timing.

While initial QKD implementations consisted of separate dedicated fibers for the quantum and data channels, new versions can use separate wavelengths for each channel on the same fiber, resulting in lower cost deployments and efficiencies.

Other implementations include continuous variable QKD (CV-QKD) and entanglement. In CV-QKD, the transmitter applies a random data source to modulate the position and momentum quantum states of the transmission. Entanglement QKD, on the other hand, uses quantum phenomena in which two quantum particles are created in a way in which they share quantum properties; no matter how far apart they may later be, a measurement of a property on each will yield the same values.

Challenges for QKD

Distance remains a limitation in implementing QKD over fiber as the individual transmitted photons are absorbed over distance. The laser power is attenuated to produce the individual photons and standard telecommunications equipment cannot be used to repeat or amplify the signal. Generally, between 60 miles and 90 miles is the practical limit.

Methods of extending the distance include Trusted Exchange, Twin Field QKD, and Quantum Repeater.

  • Trusted exchanges act as repeaters – they receive the optical signals, convert them to digital, and then convert them back to optical. Trusted exchanges must be secured to prevent an intruder from reading the transmission while it is in digital form.
  • Twin Field QKD adds a midpoint node that receives signals from both endpoint nodes, increasing the distance between endpoints to potentially hundreds of kilometers.
  • Quantum repeaters could eventually break the distance barriers of QKD over fiber and provide a function similar to repeaters in today’s telecommunications: to amplify, or regenerate, data signals so they can be transmitted from one end device to another.

Advances in single-photon sources and low-noise detectors will further improve the feasible distances for QKD.

What’s next for QKD

QKD has significant value in a quantum world due to its ability to enable symmetric key sharing between endpoints and to detect when quantum channel eavesdropping is occurring. However, before QKD can be widely implemented by carriers, it must be able to be supported in a carrier environment and provide the availability and reliability that their customers expect.

For example, an interruption in the quantum channel can result in the loss of real-time key material; However, secure key storage coupled with QKD allows further distribution of key material while investigation of quantum channel failure takes place. This also means developing approaches and skills to troubleshoot and manage QKD devices and services.

Since QKD is based on quantum mechanics, the state of observation affects the quantum system, which in turn poses challenges for debugging and management. As technology evolves and improves, QKD implementations on smaller mobile devices such as drones may eventually become possible. Regardless of how QKD evolves, it appears to be a promising solution for securing communications on the quantum internet.


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