Scientists have figured out how to find a quantum "cat" without scaring it away

Researchers at the University of New South Wales have offered an explanation using a cat and boxes. Imagine a dark room with eight identical boxes. In one of them sits a cat. You need to figure out where it has hidden, but if you check too persistently, the cat will get scared and jump to another place.

In the real experiment, of course, there was no cat. His role played a fragile quantum state of the nucleus of the antimony atom in the silicon chip. The new method made it possible to more accurately determine this state while disrupting it less. The work is published in PRX Quantum.

Details

The usual way to find a cat would look like this: pour water on each box in turn and listen to where a disgruntled "meow" would come from. But the room is noisy. You can make the mistake of hearing a "meow" where the cat isn't, or miss the real signal.

To increase confidence, you can repeat the test over and over again. But this is dangerous: the more often you disturb the boxes, the higher the chance that the cat will get scared and change places.

The quantum system has a similar problem. Scientists need to measure its state to see if there's an error. But the measurement itself can affect that state. In other words, the test can cause a new error.

The new method is trickier. After the first signal, the scientists don't keep "touching" the supposed cat box over and over again. Instead, they start checking the other boxes. If it's quiet, then the first guess was probably right.

The point is simple: sometimes information can be gleaned from the absence of a signal, not just the signal. If all the other boxes are quiet, it reinforces the certainty that the cat is sitting exactly where it was first heard.

In the physics experiment, the "boxes" were the eight possible quantum states of the nucleus of an antimony atom. Such a system is called a qubit: unlike the usual qubit, it has not two states, but more. The researchers worked with an eight-dimensional nuclear kudit in silicon.

They used an electron to check the state. It could be added to the atom and removed, but this operation could disturb the nucleus and force it into a different state. The new adaptive protocol reduced such unnecessary interventions: after the first positive result, the system switched to checking the other states.

The result was noticeable. The reading accuracy increased from 98.93% to 99.61%, and the total measurement time was reduced by about three times. To the average reader, the difference seems small, but for quantum computers it's important: such checks need to be performed many times, and even fractions of a per cent can accumulate into a serious problem.

Why it matters

Quantum computers promise to solve problems that are too complex for conventional machines: modelling molecules, searching for new materials, helping with optimisation and complex calculations. But they have a major enemy: errors.

Quantum information is very sensitive. It can be disturbed by noise, heat, external influences and even the measurement procedure itself. That is why large quantum computers need error correction: the system must constantly check itself and correct failures.

But here a paradox arises. To correct an error, it must be detected. And to detect it, you need to measure the system. And measurement can ruin everything.

This new approach helps to make such verification softer. It doesn't solve all the problems of quantum computers at once, but it shows how you can get more useful information and disturb the system less.

Background

The image of the cat refers to Erwin Schrödinger's famous thought experiment. In it, a cat in a closed box is linked to a quantum system and appears as if in a strange mixed state prior to observation. In popular culture, it has become a symbol of quantum fragility and strangeness.

In the new study, the cat is only a metaphor. It helps us understand the main challenge: how to test the system without destroying what you're trying to learn.

Scientists call such checks quantum non-demolition measurements. Simply put, they are measurements that are supposed to report the state of the system but not break it. In reality, they're not always perfect, so researchers are looking for ways to make them more accurate.

The authors emphasise that their approach may not just be useful for one particular system with an antimony atom. Similar problems arise in different quantum computer architectures, so an adaptive measurement strategy could be useful for other labs as well.

Source

Research: Arjen Vaartjes et al, "Maximizing the Nondemolition Nature of a Quantum Measurement Via an Adaptive Readout Protocol", PRX Quantum, 2026.