Imagine you have a row of toy boxes. Each box can either be empty or have one object inside (like a toy). Your job is to fill the boxes as fast as you can using a machine that can put toys into boxes.
When a qubit is in its low-energy state, it’s like an empty box. When it is in its higher-energy state, it’s like a full box. When all the qubits go from low- to high-energy, the whole setup stores some energy.
Now, if you have many boxes, scientists have found that you can fill them faster using a quantum trick, instead of filling each box one by one.
Say you have 12 boxes on a table. You point at box 1 and put in a toy. Then you point at box 2 and put in a toy. And so on. You can try to do it quickly but you’re still basically doing one box at a time.
Scientists recently reported an experiment that this simple tale is a metaphor for. It consisted of 12 qubits (short for ‘quantum bits’ — the smallest logical pieces of a quantum computer).
The experiment’s point was to drive each qubit locally, i.e. each one gets its own little push. The study called this the classical baseline.
Now, imagine you have a different machine that, instead of filling one box at a time, fills two neighbouring boxes together in a single move.
So it does something like fill boxes 1 and 2 together, then fill boxes 2 and 3 together, then fill boxes 3 and 4 together, and so on.
The machine still isn’t filling all 12 at once but because it can create pairs of fills together, the filling can become more collective, like a wave of filling through the row.
In the experiment, the scientists used a special kind of interaction where two excitations are created together on neighbouring qubits. As a result these two neighbours tended to flip together, going from empty-empty to full-full.
(To achieve this, the team used a technique called parametric modulation.)
Now, say two kids, A and B, are filling boxes in these two different ways.
You’re trying to check which kid fills all the boxes fastest.
If Kid B, who’s using the pair filling technique, only wins because their tool is stronger, that’s not interesting. The interesting claim is that even with fair tools, the pair-filling technique can store energy faster.
In a quantum device, not all stored energy is equally extractable as useful work. Instead the study uses a standard concept called ergotropy, which is the part of the energy that you can, in principle, extract as useful work with allowed operations.
For our metaphor, you can treat it as the amount of real charge you put in the boxes.
Then the scientists calculated the average charging power, i.e. how much useful energy got stored per unit time.
They did this for batteries of different sizes: 2 boxes, 3 boxes, … up to 12 boxes.
They found that the pair-filling, i.e. quantum, method could achieve more optimised charging power than the classical baseline and that the advantage tended to grow as the number of qubits increased.
They also reported that the optimal charging time window was very short, on the order of tenths of a microsecond.
This means Kid B has a short interval in which they can fill boxes very efficiently, and that interval stays around the same short length once there are several boxes.
But the scientists don’t just say their quantum way is faster. They also show that it’s faster for the reason they claim.
They measured the correlations between neighbours — i.e. whether excitations (or full boxes) appeared together more often than they’d expect if each box was independent.
In the classical way, they expected a neutral value, meaning no togetherness. For the quantum way, they expected more togetherness during the burst of charging.
They reported evidence consistent with the latter: in the quantum way, the neighbour-neighbour correlation indicator showed more paired behaviour in the same short window when the charging power peaked.
So where is the quantumness that provides this advantage?
In the normal world, a box is either empty or not empty. It’s one or the other.
In the quantum world, an object can also be in a special in-between condition — and not just because you don’t know what’s in the box. It’s a real physical kind of in-between that scientists call coherence.
When many quantum objects interact, they can also become linked in a way that makes their joint state not just the equivalent of ‘each box has its own toy’ but of ‘the whole set is described together’. This is called entanglement.
The study tried to show that during charging, the system wasn’t merely populating its excited states: it was also creating coherence and entanglement. A purely classical process can’t do this; quantumness must have been involved.
The scientists did this by measuring how many qubits were excited versus how many weren’t. Then they measured the total usable stored energy (ergotropy). Whatever was left after subtracting the plain part was the quantum-like part.
Finally, they checked whether the qubits were becoming entangled with each other, instead of acting independently. They did this by collecting measurement data, computing a quantity that has a clear rule, then saying from that whether the qubits could be entangled.
For instance, if the rule says no unlinked system can score above 10 on this test. So if the scientists measured 12, the system has to be entangled.
The scientists effectively showed that if they design the tool correctly and compare them fairly, the quantum way could flip more qubits than the classical way for up to 12 qubits, and not just by flipping the qubits one after the other faster.
So what’s the big deal?
I don’t know. It’s just fascinating.
The study was published in Physical Review Letters on February 9.
Featured image: A visual representation of the ‘quantum battery’ used in the study. It’s encoded in a 16-qubit lattice, 12 of which were activated for the experiment. Credit: arXiv:2602.08610v1.