‘Spooky action at a distance’ – a beginner’s guide to quantum entanglement and why it matters in the real world

Many governments and tech companies are investing heavily in quantum technologies. In New Zealand, the recently announced Institute for Advanced Technology is also envisioned to focus on this area of research.

As quantum technologies develop, we argue quantum literacy becomes essential for informed discussions and policy on their potentially profound societal implications.

Quantum technologies build on quantum mechanics, a fundamental theory that explains the structure of matter and has enabled the design of many useful devices such as transistors, microchips and lasers.

The term “quantum” comes from German physicist Max Planck, who proposed that energy can only come in discrete packets, or quanta.

When atoms absorb or emit energy quanta, they transition between quantised energy levels. New technologies use the quantum nature of such levels to develop super-fast computers, precision sensors and improved encryption.

One of the key ingredients in almost any kind of quantum tech is the phenomenon known as “quantum entanglement”. It has really bizarre implications which Albert Einstein once called “spooky action at a distance”. Among non-physicists, it typically raises consternation or fascination.

Concepts of quantum mechanics are sometimes incorporated – and in the process occasionally misappropriated – in popular culture.

Entanglement has not been spared this fate. Some science fiction writers are using it as a device for making the impossible seem plausible.

For example, in Liu Cixin’s 2008 novel Three Body Problem, an alien civilisation uses pairs of entangled particles to maintain faster-than-light (super-luminal) communications with Earth. To be clear, this is impossible.

Quantum entanglement can’t beat the speed-of-light limit, but it can still make some wild things work. This includes quantum-enhanced sensors to improve applications in medicine and environmental monitoring, and in precision measurements such as the gravitational wave detector LIGO in the United States.

Quantum computers could also crack certain problems that are practically unsolvable on a classical computer, such as modelling the mechanics of how proteins fold.

And quantum cryptography would protect information better by providing eavesdropper-proof encryption protocols – while also being able to detect earthquakes on the side.

The wild quantum world

Entanglement works only with quantum things and emerges most clearly when there are only two energy levels.

Classical computers store information in bits, where each bit can be either 0 or 1. In a quantum computer, the bits are replaced by “qubits”, each having two energy levels which are usually denoted as |0⟩ and |1⟩.

Unlike the classical bit, a qubit can be in a “superposition”, meaning it can be both |0⟩ and |1⟩, until an observer checks the qubit state.

This measurement yields either 0 or 1, depending on the relative share of the states |0⟩ and |1⟩ in the superposition. If the result is 0, the qubit state after the measurement becomes |0⟩. Likewise, if the result is 1, the state becomes |1⟩.

To discuss entanglement, we need to consider at least two qubits in an entangled state. We use the state described mathematically as |Φ⁺⟩ (see figure below).

Let’s imagine two quantum engineers, who we named Alice and Bob in our illustration. Each takes one qubit from the pair and travels somewhere far apart. When they measure their qubits, they’ll both obtain a 0 or a 1 with equal probability.

If they repeat this experiment with many other entangled qubit pairs prepared in the same |Φ⁺⟩ state and record their results, both will find a random series of 0s and 1s.

But when they compare their lists, they will find something astounding: every time Alice measures a 0, Bob will have also measured 0 for his corresponding qubit, and vice versa. The results are perfectly correlated, even though both their states are undetermined prior to the measurement.

It is as if, when Alice makes her measurement, Bob’s qubit instantaneously “knows” and changes into the same state.

Einstein was so bothered by this non-intuitive behaviour that he strongly believed quantum mechanics must be incomplete, and that a better theory would contain hidden variables that determine the outcome of the measurements before the pair is even separated.

However, experiments in the 1980s have definitively ruled out such local hidden-variable theories. For their demonstration that Einstein was wrong, three physicists were awarded the Nobel Prize in 2022.

New Zealand’s contribution

We have illustrated entanglement using pairs of qubits. But fundamentally, entanglement can occur between all kinds of physical systems, and this is where New Zealand researchers are making significant contributions.

Superconductors are materials that have zero electrical resistance when cooled below a certain temperature and at the same time expel magnetic fields. They are useful for making strong magnets.

To make a metal superconducting, the electrons form entangled pairs, known as Cooper pairs. A research team involving one of us has recently proposed a scheme to extract entangled electron pairs from the superconductor and transfer their entanglement onto photons, the quanta of light.

Another research group has successfully entangled two atoms cooled to almost absolute zero.

To expand research and build an industry based on quantum technologies, we need targeted investment to establish a quantum-ready workforce. Not only must we actively contribute to and capitalise on the global quantum effort, we also have to lift quantum literacy at all levels of society – starting in school.