There's an old adage among tech journalists like me - you can either explain quantum accurately, or in a way that people understand, but you can't do both.

That's because quantum mechanics - a strange and partly theoretical branch of physics - is a fiendishly difficult concept to get your head around.

It involves tiny particles behaving in weird ways. And this odd activity has opened up the potential of a whole new world of scientific super power.

Its mind-boggling complexity is probably a factor in why quantum has ended up with a lower profile than tech's current rockstar - artificial intelligence (AI).

This is despite a steady stream of recent big quantum announcements from tech giants like Microsoft and Google among others.

Broadly speaking, we tend to think about quantum more commonly in the form of hardware like sensors and computers, while AI is more software-based – it requires hardware to operate.

Put them together, and we might one day have a new form of technology that's more powerful than anything we have ever created… although the word "might" is doing some heavy lifting in that particular prediction, warns Brian Hopkins, VP and principal analyst in emerging tech at research firm Forresters.

"The potential is there, but the jury is still out," he says.

"Initial experiments suggest promise, but they all indicate that we require much more powerful quantum computers and further innovative research to effectively apply quantum effects to AI."

In terms of their value, both are lucrative. The quantum sector could be worth up to $97bn (£74bn) by 2035, according to market research group McKinsey.

Meanwhile, AI's value is forecast in the trillions. But they both live under the shadow of hype and the bursting of bubbles.

"I used to believe that quantum computing was the most-hyped technology until the AI craze emerged," jokes Mr Hopkins.

In mid-October analysts warned some key quantum stocks could fall by up to 62%, while mutterings about an AI bubble grow ever louder.

Quantum and AI have one more thing in common - errors. While we are largely familiar now with the "hallucinations" of generative AI tools, quantum is plagued by a different kind of error.

These are caused because the state in which the particles have to operate is so fragile. The slightest change to the environment, including light and noise, can disrupt them.

It's tricky to sustain such an environment. This week Elon Musk suggested on X that quantum computing would run best on the "permanently shadowed craters of the moon".

Quantum computers don't look anything like a traditional machines. There is no design blueprint, but they are currently very big.

They exist in laboratories, and the most commonly adopted format seems to include a kind of jellyfish-inspired shape.

They require extremely cold temperatures and lasers. It's not the sort of thing you're likely to have in your home, let alone in your pocket.

They're also a bit bling - researchers have found that using synthetic diamonds to create qubits, which are the building blocks of quantum computers, enables them to work much closer to room temperature.

The luxury jeweller De Beers has a subsidiary company called Element Six, which claims to have launched the world's first general-purpose quantum-grade diamond in 2020. And it has worked with Amazon Web Services on optimising artificial diamonds for future networks of quantum machines.

These machines are all in their infancy right now, there are believed to be around 200 of them in the whole world (China however has not disclosed how many it has) – this doesn't stop quantum experts making bold claims about their potential.

"We as consumers will touch the impacts of quantum computing in almost every walk of our lives," said Rajeeb Hazra, the boss of Quantinuum, a firm recently valued at $10bn. He was talking to the BBC's Tech Life podcast.

"The area of quantum computing is, in my mind, when you look at the applications, as big if not bigger than AI."

Prof Sir Peter Knight is one of the UK's top quantum experts. "Things that could take the age of the universe to calculate, even on the most powerful supercomputer, could be performed probably in seconds," he told Dr Dr Jim Al-Khalili on BBC Radio 4's The Life Scientific.

So what exactly are these gigantic, life-changing things that the machines might do once they're ready?

As with AI, there's a lot of quantum research directed towards improving healthcare.

Quantum computers could one day be able to effortlessly churn through endless combinations of molecules to come up with new drugs and medications – a process that currently takes years and years using classical computers.

To give you an idea of that scale - in December 2024, Google unveiled a new quantum chip called Willow, which it claimed could take five minutes to solve a problem that would currently take the world's fastest super computers 10 septillion years - or 10,000,000,000,000,000,000,000,000 years – to complete.

Hazra says this could pave the way for personalised medication, where instead of getting a standard prescription, you get a specific drug tailormade for your individual body, that's most likely to work for you.

And that applies to wider chemical processes too, such as new ways to produce fertilizers more efficiently, potentially a huge boost for global farmers.

Quantum sensors, which use the principles of quantum mechanics to measure things incredibly precisely, already exist and are found in atomic clocks.

In 2019, scientists at Nottingham University put them in a prototype device the size of a bike helmet, and used them in a new system to conduct non-intrusive brain scans on children with conditions such as epilepsy.

"The foundations for human cognition are laid down in the first decades of life, but there have always been limited ways to study them due to restrictions in brain scanning technology," said researcher Ryan Hill at the time.

"A particular problem has always been movement and the fact that the large traditional fixed scanners have always required patients to stay completely still.

"Not only does this fail to give an accurate picture of the brain operating in a natural environment, but it also places severe restrictions on who can be scanned, with children representing the biggest challenge."

Last year, scientists at Imperial College, London trialled an alternative to GPS satellite navigation, dubbed a "quantum compass", on the city's underground Tube network.

GPS doesn't work underground but this does – the idea is that it could more accurately track and pinpoint objects anywhere in the world, either above or below ground, unlike GPS signals which can be blocked, jammed and affected by the weather.

"The UK economy relies on GPS to the tune of £1bn per day, position, navigation and timing – this is often labelled a defence requirement - but all our financial transactions require a timestamp for authentication," says Dr Michael Cuthbert, director of the UK's National Quantum Computing Centre.

"Using quantum clocks, gyroscopes and magnetometers enables us to create a resilience against jamming and spoofing of our vital navigational systems."

The National Grid is investing in quantum research to see if it can help with what's known as "load shedding" - how to maximise the output of thousands of generators from various energy sources as demand rises and falls in real time, preventing blackouts.

And Airbus partnered with the UK quantum firm IonQ to trial quantum-based algorithms designed to load cargo more efficiently onto aircraft. An aircraft can use thousands of kilos of extra fuel if its centre of gravity shifts by just a small amount.

So far, so good – but we also need to talk about secrets.

It is widely accepted that current forms of encryption – the way in which we store both personal data and official secrets – will one day be busted by quantum technology being able to churn through every single possible combination in record time, until the data becomes unscrambled.

Nations are known to be already stealing encrypted data from each other with a view to being able to decode it one day.

"It's called harvest now, decrypt later," says Prof Alan Woodward, a cybersecurity expert from Surrey University.

"The theory of how to break current forms of public key encryption await a truly operational quantum computer," he adds.

"The threat is so high that it's assumed everyone needs to introduce quantum-resistant encryption now."

The moment a such a computer exists is sometimes referred to as Q-day. Estimates of when it might arrive vary, but Brian Hopkins at Forrester says it could be soon - around the year 2030.

Companies like Apple and the secure messaging platform Signal have already rolled out what they believe to be post-quantum encryption keys, but they cannot be applied retrospectively to current data encrypted in the traditional way.

And that's already a problem. In October, Daniel Shiu, the former head of cryptographic design at GCHQ, the UK's intelligence, security and cyber agency, told the Sunday Times it was "credible that almost all UK citizens will have had data compromised" in state-sponsored cyber attacks carried out by China – with that data stockpiled for a time when it can be decrypted and studied.