‘You’re not wrong,’ I smiled. ‘That is hard.’

‘But not,’ he said, ‘if you’re using a quantum computer. That’s what I’m writing my algorithm for.’

‘A what?’

‘A quantum computer – one that uses qubits and not, you know, just bits.’ I shook my head. ‘Am I speaking Punjabi?’ He looked at me, earnestly.

‘No, we’re still in English. I think.’

‘OK, it’s just sometimes I forget. It drives my teachers crazy, huna?’

‘Quite.’

‘OK, well a, classical, er, normal computer uses binary digits. It doesn’t matter how big the computer is or how powerful, everything comes down to ones and zeros. You can have a very fast machine, but it can only do one task after another – sequentially. Never mind the travelling salesman problem – a ten-character password would take even a supercomputer three years to crack. But a quantum computer is completely different. It uses qubits. They’re, like, subatomic. They can be either ones, or zeros, or a simultaneous superposition of the two.’

I held my hands up. ‘In English, please.’

‘Ah, sorry. I mean they can be both one and zero at the same time.’ He scratched his beard. ‘Mystical, really.’

‘That’s one word for it.’

‘So,’ he continued, ‘two qubits can be in any quantum superposition of four states; three qubits, eight states. But a normal computer can only ever be in one of those states at any one time.’

‘And what does that mean?’

‘It means you can do every step in a big calculation simultaneously.’

‘Like your travelling salesman … thing?’

‘Yes, although there are already algorithms exactly for …’

‘OK,’ I cut him off. ‘I get it. With enough qubits you could solve really hard problems really fast.’ He nodded. ‘Like adding up on a calculator rather than your fingers.’

‘Yes,’ he agreed. ‘It’s the difference between illuminating a room with a candle and a stadium with a floodlight.’

I didn’t understand anything about the physics, or the maths, of what he was saying. But the implications were obvious. ‘So, cracking the ten-digit password?’

‘With a quantum computer? Instantaneous,’ he grinned.

‘And they exist, these computers?’

He snorted and gulped down the last of his tea.

‘No way! That’s just sci-fi. Last month IBM revealed a working fifty-qubit quantum computer. But get this: it can only maintain its quantum state for ninety microseconds. There are loads of issues to resolve. Decoherence, scalability, noise …’

I held my hand up to stop him.

‘The main issue to resolve right now, Baaz, is that I have literally no idea what any of those things mean, OK?’

‘OK.’

‘So, let’s keep it simple. If anyone could resolve those issues and build a working machine …’

‘If anyone could demonstrate quantum supremacy, and build a proper machine – with, say, ten thousand qubits?’

‘They’d break all known cryptography. Right?’ Confusing as it was, I felt I was at least beginning to understand the basics.