Researchers load a quantum debit card with unforgeable 'quantum money'

Unforgeable “quantum money” is made a big step from the pages of science fiction into the real world. A new lab test shows how a tiny store of quantum information can act like a debit card you can load, keep for a moment, and then spend without leaving room for fakes, forgeries, or counterfeits.

The trick behind quantum money leans on the no-cloning theorem – a rule in quantum physics that says you cannot make an exact copy of an unknown quantum state.

The team added secure storage to the transaction, and the paper validates that the money can be checked later without opening a door to counterfeiting.

Understanding quantum money

This fascinating concept of quantum money uses the rules of quantum mechanics to make unbreakable currency.

Instead of relying on paper bills or digital codes, quantum money would store information in quantum states – like the spin of an electron or the polarization of a photon.

The core idea traces back to Stephen Wiesner’s 1983 proposal called conjugate coding. It works because the theorem blocks perfect copying, so a counterfeiter who tries to clone the states would trip a built in error check.

In short, if someone tries to counterfeit quantum money, the act of measuring or copying the quantum information will disturb it and reveal the fraud instantly.

Think of it like this: if you had a magic coin whose shine changed whenever someone else touched it, you’d always know if it was genuine.

That’s how quantum money works – it’s designed so only the issuing bank can check its authenticity without disturbing the quantum states.

The experiment was led by Julien Laurat, at the Kastler Brossel Laboratory at Sorbonne University and CNRS in Paris.

His group pushed the concept from on the fly checks into a version that lets a user hold value briefly without breaking security.

How quantum memory works

The bank in this scheme sends very weak pulses of light, single photons on average, each carrying a bit-like quantum state called qubits.

Those states live in the polarization of light, and a user loads them into a quantum memory that can return the same states on demand.

That memory is a cloud of cesium atoms cooled to a few millionths of a degree above absolute zero.

The atoms are arranged to reach high optical depth, then an added control beam uses electromagnetically induced transparency (EIT) to map the light into the atoms with high efficiency.

Earlier results showed how to build a high efficiency polarization memory in cold atoms.

The Paris team folds that know-how into a full transaction that covers storage, retrieval, and verification with strict error checks.

How verification catches cheats

During payment, the vendor measures each retrieved state in one of two possible bases and later tells the bank which basis was used.

The bank compares only the cases where its original basis matches the vendor’s, which gives a clean error rate tied to any tampering or noise.

If someone tried to split or copy the light to pay twice, the mismatch would raise that error rate. The protocol sets a threshold where honest noise is tolerated but double spending of quantum money fails with high probability.

What the experiment measured

The device reached an average storage and retrieval efficiency near 77 percent and kept polarization errors below 1 percent for several light levels.

The run with one photon on average per pulse reported an error of about 0.78 percent while staying within the security region set by the theory.

“This protocol imposes stringent requirements on storage efficiency and noise level to reach a secure regime. An efficiency above 50% is required to have a possible range of secure operations,” wrote Hadriel Mamann, the lead author.

Why time and size matter

Right now the memory holds the states for about 1 microsecond in the main demonstration and remains secure up to around 6 microseconds under the team’s conditions.

That secure window equals roughly 0.75 miles of light travel in fiber, which is enough to show the idea works but not enough for a pocket device.

Other platforms are stretching lifetimes in different ways.

One recent work shows a millisecond scale integrated memory for photonic qubits, which hints at pathways to longer buffers that could, in time, pair with high efficiency devices like the one used here.

City scale quantum networks are already being tested.

One 46 node network ran for 31 months, which makes clear there is an environment where a quantum debit card could plug in once storage stretches beyond millisecond scales.

Future of quantum money

If efficiency keeps creeping up and lifetime grows by three or four orders of magnitude, users could store value long enough to route payments across a city without exposing the states to forgery.

The same memory parts would also help stitch small quantum processors into a larger system that can pass fragile states without losing them to noise.

This is why the result matters to quantum communications.

It shows that a high efficiency cold atom memory can meet the strict noise and loss limits of a transactional test, not just a lab benchmark, and do so while keeping errors under the threshold once thought out of reach.

The study is published in Science Advances.

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