By Frey Wilson, PhD

… Explaining the basics of quantum technologies and debunking some of the common misconceptions.

“So, what does quantum mean?”

The literal meaning of the word quantum is simply ‘the smallest possible finite amount of stuff’… and the unexpected effects produced on this scale. We call the usual, everyday effects, ‘classical’.

“Okay then, what are quantum technologies?”

We can harness these unexpected effects to solve challenging problems or do novel things. There are 4 key areas where new technologies are emerging – computers (including machine learning and simulation), imaging, sensing & measurement, and communications. Time synchronization fits into the last of these.

“So quantum technologies are a thing of the future?”

Several technologies which have been around for a while use quantum effects – LEDs, MRI machines, lasers, and even the transistors which our current computers rely on. The development of those technologies is now considered to be part of the ‘first quantum revolution ’. The ‘second quantum revolution’ refers to those now under development. Some of these are even available today!

¨It seems that ‘quantum’ technologies basically mean faster/better…”

There is a misconception that a quantum solution to an engineering challenge (for example, quantum computing) means that is, therefore, superior in every way. For example, quantum computers are not ‘ultra-fast versions of normal computers. They usually meet a specific need – quantum computers solve problems which are unable to be solved with the usual computing logic (which revolves around addition and multiplication, etc.). The existence of quantum computers would not mean that it would be quicker to start up your laptop and open Microsoft Word! Quantum cryptography solves the unique challenge of preventing quantum computers from undoing current cryptographic standards. Quantum time synchronization overcomes current limitations with the current global time sync methods.

“Quantum physics is basically indistinguishable from magic!”

On the scale of ‘the smallest finite possible amount’, there are a few key effects which we can use to our advantage. Whilst these are very challenging to understand compared to our usual grasp of how the world works, we can predict and explain these effects well.

“What are the key features of quantum physics?”

We can distil these down to about 6 central tenets…

1. Discrete units: Anything that can be considered ‘quantum’ is called a ‘quantum state’ – and these states are ‘discrete units’ meaning that they come in set amounts. For example, 1, 2, 4, 193, or another integer amount of photons in a beam of light. This

applies to lots of different properties – the amount of energy that an electron in an atom has or the amount of charge a particle has. These discrete amounts are called ‘quanta’.

2. Wave-particle duality: Quantum states sometimes look and behave like waves, and sometimes look and behave like particles. Even both at the same time.

3. Uncertainty principle: Measurement of quantum particles is hard. Certain related properties (e.g. energy and time, or position and momentum) cannot both be known with an exact precision simultaneously. The limit for this precision is related to a value called ‘Planck’s constant’.

4. Superposition: Not only is measurement hard, but a quantum particle that isn’t measured will behave differently to one that we do observe and measure. Up until we measure it, it behaves as though it is doing all possible things at once. We call this superposition. Because of this, we can only ever know a probability that a quantum state will behave a certain way. This concept is where the famous ‘Schrödinger’s cat’ thought experiment comes from.

5. No-cloning theorem: Partly because measurement is so hard, it is impossible to create an identical and separate copy of an arbitrary unknown quantum state. A consequence of this is that, if you encode some information on a quantum state (this is called a ‘qubit’, short for quantum bit), then it ensures information cannot be exactly copied. It is this tenet which forms the basis for security in many proposed quantum protocols.

6. Entanglement: Under some conditions, groups of quantum states can be generated such that their properties are correlated beyond what is possible in classical physics. It can be thought of as an extension of superposition, with multiple quantum states. This means that, even at a great distance or when separated by barriers, two entangled quantum states would have related properties. If you changed a property in one, the other would also be affected. This is the key property that we leverage for our time synchronization method.

“How do we use this for time synchronization?”

We can use the last of these properties, entanglement, to communicate easily with a remote party, perhaps on the other side of the globe. If we both have a source of entangled photon pairs (and a receiver to detect them with) we can send one half of the pair to the other party (and measure for ourselves the other half), and vice versa. We can both now check when the sent photons were detected at our receiver. Armed with this information, and comparing correlated properties, we can negotiate the time difference between the clocks at our receivers. As a bonus, the no-cloning theorem means that we could put checks in place to check for spoofing.

“Isn’t time synchronization already ‘done’?”

Currently, time synchronization is performed by two parties sending classical radio signals to each other. The two compare measured properties, such as the phase, and use this to calculate the distance between themselves (and other reference points). From this, they can calculate their relative timing and positioning. This is how GPS (global positioning system) currently works. The limitations on this method have a proportional effect on how accurate GPS can be. Harnessing quantum effects to synchronize clocks means that we get picosecond precision (10-12) with a global reach. This translates to around millimeter precision for GPS. Another limitation with GPS is that it is relatively straightforward to spoof – the no-cloning theorem of quantum particles means that this is much harder.

“Quantum physics is basically indistinguishable from magic!”

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