Imagine a world where encrypted, secret files are suddenly cracked open - something known as "the quantum apocalypse".
Put very simply, quantum computers work completely differently from the computers developed over the past century. In theory, they could eventually become many, many times faster than today's machines.
That means that faced with an incredibly complex and time-consuming problem - like trying to decrypt data - where there are multiple permutations running into the billions, a normal computer would take many years to break those encryptions, if ever.
But a future quantum computer, in theory, could do this in just seconds.
Such computers could be able to solve all sorts of problems for humanity. The UK government is investing in the National Quantum Computing Centre in Harwell, Oxfordshire, hoping to revolutionise research in the field.
But there is also a dark side.
A number of countries, including the US, China, Russia and the UK, are working hard and investing huge sums of money to develop these super-fast quantum computers with a view to gaining strategic advantage in the cyber-sphere.
Every day vast quantities of encrypted data - including yours and mine - are being harvested without our permission and stored in data banks, ready for the day when the data thieves' quantum computers are powerful enough to decrypt it.
"Everything we do over the internet today," says Harri Owen, chief strategy officer at the company PostQuantum, "from buying things online, banking transactions, social media interactions, everything we do is encrypted.
"But once a functioning quantum computer appears that will be able to break that encryption... it can almost instantly create the ability for whoever's developed it to clear bank accounts, to completely shut down government defence systems - Bitcoin wallets will be drained."
It's a prognosis echoed by Ilyas Khan, chief executive of the Cambridge and Colorado-based company Quantinuum. "Quantum computers will render useless most existing methods of encryption," he says.
"They are a threat to our way of life."
Seriously? That does sound completely apocalyptic, so why haven't we heard more about this?
The answer is that yes, OK, this would indeed be the case if no precautions were being taken. "If we weren't doing anything to combat it then bad things would happen," says a Whitehall official who asked not to be named.
In practice, mitigation efforts are already in train and have been for some years. In the UK, all government data classified as "top secret" is already "post-quantum" - that is, using new forms of encryption which researchers hope will be quantum-proof.
Tech giants like Google, Microsoft, Intel and IBM are working on solutions, as well as more specialist companies like Quantinuum and Post-Quantum.
Most importantly, there is currently something of a post-quantum cryptography "beauty parade" taking place at the US National Institute for Science and Technology (NIST) just outside Washington DC. The aim is to establish a standardised defence strategy that will protect industry, government, academia and critical national infrastructure against the perils of the quantum apocalypse.
All of this will not be cheap.
Quantum computing is expensive, laborious and generates large amounts of heat. Developing quantum-safe algorithms is one of the major security challenges of our time.
But experts say the alternative - doing nothing - is simply not an option.