Originally published 2019-07-28.
University of Glasgow
In July 2019 a team at the University of Glasgow released what they described as the first image showing quantum entanglement, the link between particles that Albert Einstein famously called "spooky action at a distance". The picture, a pair of fuzzy grey crescents, was published in the journal Science Advances on 12 July 2019 in a paper titled "Imaging Bell-type nonlocal behavior", led by Dr Paul-Antoine Moreau of the university's School of Physics and Astronomy.
It is not a snapshot of a glowing thread joining two particles, and it does not show anything travelling faster than light. It is a carefully built image of correlations: patterns that appear only when two entangled photons are recorded together. Here is what the image shows, how it was made and why physicists cared.
What quantum entanglement is
When two particles are entangled, they share a single quantum state. Measure a property of one, such as its polarisation or phase, and you can predict the result of the same measurement on the other, however far apart they are. Before measurement, neither particle has a definite value on its own; only the pair does.
The idea troubled Einstein. In 1935 he, Boris Podolsky and Nathan Rosen published the famous EPR paper, arguing that quantum mechanics must be incomplete. Erwin Schrödinger replied the same year and gave the phenomenon its name, Verschränkung, which he translated as "entanglement". Einstein's remark about spooky action came later, in a letter to Max Born dated 3 March 1947.
One point is often garbled in popular articles. Entanglement does not let anyone send a message faster than light. Each individual measurement result is random, so the person holding one particle cannot choose what the other side sees. The correlation only becomes visible when the two sets of results are compared, which requires an ordinary, light-speed channel.
How the Glasgow team made the image
The experiment starts with a laser shone into a crystal of beta-barium borate (BBO). Occasionally a laser photon splits inside the crystal into two lower-energy photons that are entangled, a process called spontaneous parametric down-conversion. It is one of the most common ways to make entangled light.
Moreau et al., Science Advances, 2019
In the setup above, light enters from the bottom left. A beam splitter separates each pair. One photon travels to a spatial light modulator showing a "phase object", a simple pattern that shifts the phase of the light, and then to a single-photon detector. Its twin passes through a second modulator carrying one of four phase filters (0°, 45°, 90° and 135°), then through a delay line to an intensified camera sensitive enough to register individual photons.
The key trick is timing. The camera only records a frame when the detector on the other arm fires at the same moment, so the image is built entirely from photons whose entangled partner was also caught. Uncorrelated light simply never appears.
Reading the four-panel picture
Moreau et al., Science Advances, 2019
Each of the four panels corresponds to one of the four filter settings. The bright crescents show where coincident photons landed, and the colour bar on the right gives the photon counts per pixel. As the filter angle changes, the crescents rotate. The shape the camera sees depends on a setting applied to the other photon of the pair, which never reached the camera.
By combining these images, the team could calculate a quantity used in Bell tests and show that it exceeded the limit allowed by any classical, "local" explanation. In other words, the picture is a visual form of a Bell-inequality violation. Moreau described it to the University of Glasgow as an elegant demonstration of a fundamental property of nature, "seen for the very first time in the form of an image".
Why Bell tests matter
In 1964 the Northern Irish physicist John Stewart Bell showed that the EPR debate could be settled by experiment. If particles carried hidden instructions from the start, the correlations between them would have to stay below a certain limit, now called a Bell inequality. Quantum mechanics predicts correlations that break that limit.
Experiments since the 1970s have repeatedly found the quantum prediction to be right. In 2015 three independent teams closed the main remaining loopholes at the same time, and in 2022 Alain Aspect, John Clauser and Anton Zeilinger shared the Nobel Prize in Physics for their work on entangled photons and Bell tests. The Glasgow image adds no new law of physics; its contribution is showing the same effect spatially, across a whole picture rather than as a single number.
Is this really a photo? Common misunderstandings
- It is not one snapshot. The image is accumulated from many detection events, each triggered by a coincidence between the two arms.
- It does not show particles "talking". Nothing visible passes between the photons; the picture displays correlations.
- It is not the first image related to entanglement at all. In 2014 Gabriela Barreto Lemos and colleagues in Vienna imaged objects using photons that never touched them, relying on entangled partners. Glasgow's claim was narrower: the first image showing a Bell-type violation.
What it could be used for
Entanglement is a working resource in quantum technology. It underpins quantum key distribution schemes such as Artur Ekert's E91 protocol, quantum teleportation (first demonstrated in 1997), and many approaches to quantum computing. Moreau told the university that the result could help advance quantum computing. Imaging techniques that rely on correlated photons are also being studied for low-light and low-damage imaging, where every photon counts.
The work was funded by the European Union's Marie Skłodowska-Curie Actions, the European Research Council, the Leverhulme Trust and the UK's EPSRC.
Sources: University of Glasgow, 12 July 2019: Scientists unveil the first-ever image of quantum entanglement; Moreau et al., Science Advances 5(7), 2019; Wikipedia: Quantum entanglement; Wikipedia: Bell's theorem. Images: University of Glasgow; Moreau et al., Science Advances (2019).
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