No one is going to claim that the movie made by the Chinese physicists Chao-Yang Lu, Jian-Wei Pan and their colleagues at the University of Science and Technology of China in Hefei is a work of art. Its crudely pixellated cartoon images and lettering recall the earliest days of computer graphics in games like Pong and Space Invaders.
What makes the animations startling, however, is that each pixel is a single atom. The researchers used AI-controlled software to drag each atom into position in real time in a grid, creating a series of frames. The resulting movie depicts the famous thought experiment described in 1935 by Erwin Schrödinger involving a cat in a box that might or might not be killed by a quantum-mechanical event.
This isn’t just a display of technical virtuosity. Moving and positioning single atoms is central to a scheme for performing quantum computation that has become one of the most favoured candidates for creating large-scale quantum computers.
Quantum computation involves encoding information in the states of quantum bits (qubits), which are governed by quantum rules. This means each bit can encode not just a binary 1 or 0, as in conventional computers, but quantum “mixtures” of the two, called superpositions.
By entangling many such qubits, meaning that their quantum states become interdependent – if one of a pair of qubits is found to be in a 1 state, say, the other must be a 0 – it becomes possible in principle to use groups of qubits to carry out some types of calculation much faster than can be done on any conventional computer.
One big question for quantum computing is what to use as the qubits. The devices made by Google and IBM use tiny loops of superconducting material. Other prototype quantum computers, already being made commercially, encode the information in the configurations of electrons in individual charged atoms (ions) trapped by electromagnetic fields. But for the devices explored in the work by the Hefei team, the qubits are electrically neutral atoms of rubidium held in the focus of laser light.
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These laser-beam traps, called optical tweezers, can be moved to drag each atom into position on a two-dimensional grid. The grid measures just 230 thousandths of a millimetre square and can host up to 2,024 individual atoms. The idea in neutral-atom quantum computing is that the atoms will be shuffled between various reservoirs: taken from a holding pen, say, to a space where they are allowed to interact and perform the calculation. Each qubit atom will stay in its specified quantum state for just a short time – but by continually replenishing qubits from the store, a computation might be extended for as long as you like.
To conduct a viable quantum computation, these rearrangements need to happen fast. The whole movie lasts just 60 thousandths of a second, and so it can only be followed by eye when slowed down by a factor of 30 or so. This kind of light-based manipulation of neutral-atom qubits has been demonstrated before, but the team of Lu and Pan (the leading exponents of quantum information technologies in China) has now taken it to new heights.
My only complaint is that their movie of Schrödinger’s cat is a little misleading. It begins with a “cat in box” next to a radioactive source. The radioactive decay of a single atom was, in Schrödinger’s experiment, considered to trigger the release of a poison that would kill the cat.
But the decay is a quantum event, the timing of which can’t be predicted; we can only say that, for example, after an hour there’s a 50:50 chance of it having happened. Schrödinger objected to the view, promoted by Niels Bohr, that only when we open the box to observe the state of the cat can we say whether it is truly alive or dead. “What is the state of the cat before we open the box?”, the Chinese team asks, in letters spelt out with rubidium atoms.
Their answer, in the movie’s final frame, is “alive and dead” – illustrated with a cat that is half fat and smiling, and half skeletal and glum. This, Schrödinger argued, is an absurd scenario: a cat cannot be both alive and dead at the same time. Therefore, he said, Bohr’s interpretation must be somehow incomplete.
But according to a recent survey conducted by Nature, almost 50% of physicists now say that a question like this about the cat doesn’t really have a clear meaning. For my money, that’s the right answer.
