Earlier this year, the Nature Physics editors started to think about ways to mark the journal’s 15th anniversary. Little did we know then that, by October, we would not be able to come together and raise a glass to the occasion, and so the celebration had to be confined to the pages of the journal. We knew early on that we wanted to give our past and present editors a chance to reminisce about their time at the journal, and that turned into a collection of memories of their favourite papers.
But how do you turn those assorted papers into a visual concept to make a cool cover? Once we started thinking about it, it struck us that it’s not unusual to see experimental methods, especially imaging methods, demonstrated with the help of numbers or letters as simple test objects. So we asked some of our authors if they had any images of a 15 (or a 1 and a 5) on their hard drives that we might use for the cover. We were deeply moved by the response: although nobody had the sort of thing we were looking for on file, they offered to take some data especially for us — in August, in the middle of a pandemic.
Our art editor then took four of these images and arranged them into a collage to create one big number 15. Bringing together methods from different areas of physics reflects the aim of Nature Physics itself to be a platform for the entire physics community.
What are the methods used to create the images that eventually made up the anniversary cover?
Quantum holography (Hugo Defienne & Daniele Faccio, University of Glasgow)
“Holography is a widely used imaging technique that can be applied to the full electromagnetic spectrum, from X-rays to radio waves and relies on the coherence properties of these waves to extract information from interference patterns.
We have recently extended holography to the case of intrinsically incoherent waves, so that no phase information can be retrieved from a classical interference measurement. Instead, the phase information is now encoded and decoded using entanglement. Entangled photon pairs are used to probe complex objects of which amplitude and phase components are retrieved by imaging the spatial structure of entanglement. As an example, the image on the cover shows the quantum holographic image of the number 15 imprinted onto a spatial light modulator. See also the preprint for more details”
Self-assembly (Serim Ilday, Bilkent University – UNAM)
“These are microscopy images. Each dot forming the number ‘15’ is a laser beam. Laser pulses that get absorbed by the liquid heat it. The rest, untouched by the laser pulses, remains cold. The liquid starts flowing from the hot to the cold regions, just like in a steam engine. The flows carry polystyrene spheres (red image) and E. coli bacterial cells (green image) towards the beam spots. When they exceed a threshold number, particles and cells slow down the flow the same as the water slows down when you drain it over a sieve. Then, their numbers grow further and write ‘15’.
The recipe? Couple an ultrafast laser to a microscope through a series of optical elements, including a spatial light modulator, which divides a single beam into multiple beams. Cinema projectors have at least one of these for precisely the same reason. Sandwich a thin liquid layer containing the material of interest between two glass slides. Put it under the microscope and shine the laser. Record using a camera. Enjoy!”
Quantum gas microscope (Immanuel Bloch, Max Planck Institute of Quantum Optics)
“The ‘birthday candles’ forming the ’15’ are individual atoms fluorescing in ultra-high vacuum. Lithium-6 atoms are cooled down to around a billionth of a degree above absolute zero and trapped using laser beams. By interfering three pairs of beams, an optical lattice is created which forces the atoms onto a micrometre-spaced regular grid.
An additional custom-shaped laser-pattern coaxes them into the shape of the ’15’. Visible light is then scattered off the atoms and collected with a microscope objective and a single-photon sensitive camera. During illumination, the atoms need to be hindered in heating up via continuous laser cooling. The resulting black-and-white photo is finally coloured. When the atoms are not sending special birthday greetings, they simulate the quantum mechanical behaviour of complex many-body systems.”