Seeing the light
The Imperial scientists exploring fundamental physics with quantum measurements
For decades, physicists have revealed the fundamental building blocks and forces of nature by building ever larger and more energetic particle colliders. Many believe that the next set of discoveries will come by pushing to even higher energy with even larger machines.
Not everyone believes that. Many physicists argue that now we have identified the family of fundamental particles and forces that make up the so-called Standard Model, the time has come to think differently and that we may need to move away from hugely expensive large-scale collaborations to smaller, less costly experiments.
Imperial has a foot in both camps. It is involved in large collaborations like the CMS and LHCb experiments on CERN’s Large Hadron Collider (LHC) and the forthcoming neutrino experiment DUNE at Chicago’s Fermilab.
Imperial physicists are also working on relatively modest “tabletop” experiments which, despite their compact footprints, could shed light on some of the most interesting questions in fundamental physics: why is there more matter than antimatter in the universe? Does dark matter exist and what form does it take if so? What can we learn from gravitational waves? What is this dark energy that is said to be accelerating the expansion of the universe?
Experiments at Imperial are probing some of the most fundamental mysteries of the universe, such as gravitational waves, dark matter and energy, and time itself
“There's a chance that the LHC will not find anything else at all,” says Michael Tarbutt, professor of experimental physics at Imperial’s Centre for Cold Matter. “They will learn a lot of physics but they may not discover any new physics.”
Unlike the multibillion-pound experiments with thousands of collaborators, a tabletop experiment fits into a small lab. The team might include a principal investigator, a research fellow and a handful of research assistants and PhD students.
There is a thriving cluster of such experiments tucked away in Imperial’s Blackett Laboratory. Pop your head around a door and you find scientists adjusting red and green lasers, mirrors and lenses on optical tables that look like Lego sets for grown-ups. Vacuum pumps hum away while MRI scanner-like electromagnets clunk away in the background.
These experiments are looking at the quantum behaviour of atoms, molecules, ions and light using relatively modest “tabletop” experimental set-ups to test and push the boundaries of fundamental physics.
“It is a passion and curiosity about nature at the fundamental level and a desire to make measurements using precision low energy techniques. We have very small teams and you can be very agile in that scenario,” says Professor Tarbutt.
“It's an interesting time in fundamental physics. There are many mysteries and many ideas about how to resolve those mysteries but there is no overwhelming evidence to suggest that there is one direction to go in. It's important that we try and cover as many bases as we possibly can.”
These tabletop experiments typically take place at low energies and cold temperatures, a very different environment to the high-energy world of large particle accelerators.
Temperatures as low as just a few fractions of a degree above absolute zero are achieved using a variety of methods including using very cold helium gas as a buffer gas.
One method that seems counterintuitive is to use lasers to cool the experiments.
“The laser is a stream of photons and every photon carries some energy and momentum. If you tune everything just right, you can then use the pressure of the light to slow molecules down. It’s like trying to stop a juggernaut by throwing peas at it. It sounds futile but you can throw a lot of peas in a short amount of time.”
Fundamental science at the quantum level
Imperial researchers on the AION collaboration
Imperial researchers on the AION collaboration
Gravitational waves
Tiny ripples in space, created by the early universe, could be observed using ultracold atoms by scientists on the AION collaboration
Dr Michael Vanner of the Quantum Measurement Lab
Dr Michael Vanner of the Quantum Measurement Lab
Quantum sound
Imperial’s Quantum Measurement Lab is looking at how sound may hold the key to unifying gravity and quantum mechanics
Scientists at Imperial are investigating how symmetries are broken in the universe
Scientists at Imperial are investigating how symmetries are broken in the universe
The anti-matter mystery
There is an imbalance in the universe and Imperial's scientists are looking at the shape of the electron to find out why
Dr Jonas Rodewald adjusts the CaF experiment, part of QSNET
Dr Jonas Rodewald adjusts the CaF experiment, part of QSNET
Precision time
Molecular clocks are helping test whether fundamental constants really are fixed or whether variations can reveal new physics
Dr Jack Devlin and Jiacheng Shi characterise a radiofrequency circuit for the QuEPA project
Dr Jack Devlin and Jiacheng Shi characterise a radiofrequency circuit for the QuEPA project
The stuff of the universe
Ultracold, tabletop experiments at Imperial College London may be the key to understanding abundant dark energy and dark matter
Fundamental physics research could yield spin-offs in quantum computing technology
Fundamental physics research could yield spin-offs in quantum computing technology
Applications in the real world
The research being undertaken at Imperial's Department of Physics will prove useful in developing new qauntum technologies
Ripples in space
In 2015, gravitational waves were directly detected for the first time by the LIGO collaboration following the merger of two black holes, confirming a key prediction of Einstein’s theory of general relativity. LIGO uses something called a laser interferometer, where light is split in two and sent along long tunnels at right angles to each other before the light is recombined and analysed to see whether it has been stretched by these ripples in spacetime .
Imperial is pioneering a new type of equipment to seek gravitational waves called an atom interferometer as part of the Atom Interferometer Observatory and Network (AION) collaboration.
Professor Oliver Buchmueller (left) and Dr Richard Hobson (right) of the AION collaboration are using atomic clocks to explore gravitational waves
Professor Oliver Buchmueller (left) and Dr Richard Hobson (right) of the AION collaboration are using atomic clocks to explore gravitational waves
“Where LIGO splits light down two arms, we only need one. Since atoms make extremely good clocks you can measure tiny fluctuations in the light propagation time,” says Dr Richard Hobson, joint head of the AION project at Imperial.
AION measures effects in a different range of wavelengths to existing experiments.
Professor Oliver Buchmueller, principal investigator of the AION consortium says, “Strontium atomic clocks underwent a significant improvement over the last ten years. Our idea is to build on that development to transform atom interferometry.”
Such experiments are a proof of concept for larger experiments that may be able to probe the formation of the supermassive black holes at the centres of galaxies and could be sensitive to gravitational waves generated in the very early universe.
London’s other whispering gallery
Sound waves have been analysed for centuries using ideas from classical physics but Imperial’s physicists are now realising that sound offers a promising path to test quantum mechanics at more macroscopic scales.
Drop in to see Dr Michael Vanner of Imperial’s Quantum Measurement Lab (pictured here) and you will see what looks like a glass dumbbell on his desk.
It is a scaled-up artist’s model of a microstructure used in Dr Vanner’s experiments. The real thing is about the width of a human hair and is made from glass or crystalline materials shaped using lasers or a nano-lathe with an extremely sharp diamond tip.
When laser light passes close to the surface of this structure, something wonderful happens.
“The light essentially jumps inside the microstructure and circulates in what we call ‘whispering gallery mode’, named after the same effect that occurs in the dome of St Paul’s,” says Dr Vanner.
When the light circulates, it interacts with a sound wave at a frequency of about 10 gigahertz, way beyond human hearing. Dr Vanner’s team are aiming to generate quantum states of sound in this device that are not describable by Newton's laws but can only be described by quantum mechanics.
One tantalising prospect is that synthesising such quantum states of sound may give insights into the interplay between gravity and quantum mechanics—a highly sought after goal in physics.
It’s about time
Time-keeping has become more accurate over the centuries and we have come a long way since Christiaan Huygens invented the first working pendulum clock in the 17th century.
Today, high precision time-keeping is now achieved using clocks which rely on energy transitions in atoms and molecules. Even though such clocks can be very beautiful thanks to their use of lasers, it is unlikely you would want one in your sitting room.
What a new collaboration called QSNet, involving Imperial, the National Physical Laboratory and the universities of Birmingham and Sussex is doing is to see whether differences between such clocks can shed light on questions in fundamental physics.
“The constants of fundamental physics may not be constants at all,” says Professor Tarbutt. “Is the proton mass divided by the electron mass a constant?”
To test this, the collaboration is comparing one clock which is sensitive to that quantity and another clock which is not.
If such variations are observed, they may provide evidence for some theories of ultralight dark matter and dark energy and may also hint at an overall unification theory that would combine the fundamental forces of nature.
Exploring symmetries
One of the most promising areas of Imperial’s research is on the electron electric dipole moment (EDM), a measure of how electric charge is distributed around the electron. By looking at whether the behaviour of electrons changes in electromagnetic fields, which cause the electron to wobble like a spinning top, researchers can measure the EDM. If the EDM is anything other than zero, there are important consequences for physics.
“It would violate time reversal symmetry,” says Professor Ben Sauer, an expert in time-reversal symmetry who works on an experiment at Imperial called eEDM which aims to measure the electron’s dipole moment.
Professor Ben Sauer of the eEDM experiment
Professor Ben Sauer of the eEDM experiment
Time-reversal symmetry sounds like something you would read in a science fiction novel but Professor Sauer is trying to turn fiction into fact.
In the early days of particle physics, it was believed that interactions followed various conservation laws or symmetries where the laws of physics would remain unchanged if you swapped a particle for its antiparticle, mirrored its spatial co-ordinates or reversed the flow of time. That has been shown not to be the case.
Michael Ziemba controls the ultracold EDM experiment
Michael Ziemba controls the ultracold EDM experiment
The EDM also promises advances in understanding anti-matter and why there is so little in the universe.
“The Standard Model of particle physics says that there should be almost equal amounts of matter and antimatter after the Big Bang,” says Dr Jongseok Lim, an Advanced Research Fellow. “But somehow the balance between matter and antimatter is broken and the universe is made of matter only.”
Shedding light on darkness
All is not quite right with the universe. Observation of the rotation of galaxies reveal that there must be additional material in these galaxies beyond the stars and gas clouds we can see. Scientists believe this dark matter must make up around 24% of the universe, six times more than the material we can actually see.
If dark matter does exist – and not everyone is convinced it does - we don’t know what form it will take. One candidate type of dark matter being explored by Imperial’s Dr Jack Devlin and his team is the axion.
The axion, named after a detergent because it cleans up at least one particle physics mystery, is expected to have a tiny mass - at least one million times lighter than the electron – and interact very weakly with other particles. This is a very different order of magnitude from big particle accelerators.
The Quantum Enhanced Particle Astrophysics (QuEPA) group is constructing a new experiment to detect these axions – if they exist – by converting them into microwave photons using a cavity - a highly reflective container for microwaves - immersed in a strong magnetic field.
Dr. Jack Devlin and Jiacheng Shi characterise a radiofrequency circuit for the QuEPA project
Dr. Jack Devlin and Jiacheng Shi characterise a radiofrequency circuit for the QuEPA project
Dr Devlin says, “We're basically building an antenna for ultralight dark matter and for these kind of masses, it tends to behave more like a wave than a particle. If it exists, it would be everywhere, like the CMB, but barely interacting.”
Scientists have also realised that the expansion of the universe is speeding up, suggesting that the Big Bang model needs an update. The leading candidate proposed for this expansion is known as dark energy and it may make up around 70% of the entire universe. We have few clues as to what this dark energy might be.
Chameleon fields are one proposed form of dark energy. These forces, which are predicted to be even weaker than gravity, would be mediated by a chameleon particle which has a variable mass that depends on the ambient energy density. There is speculation that such particles could be detected in a laboratory sized vacuum system using an atom interferometer, work being undertaken at Imperial by Professor Ed Hinds.
From fundamental physics to the real world
While the work of physicists at Imperial is focused on new discoveries in fundamental physics, the research being carried out here has exciting real-world applications in fledgling quantum technologies.
Funding for many of these Imperial experiments comes from a £40 million EPSRC-STFC initiative whose objective is to sustain the UK as a ‘first rank’ nation in physics and quantum communities globally.
Dr Vanner is excited about the applications of his research on quantum states of sound in quantum computing, for example.
“You could write a quantum state from light to the sound wave and it might remain coherent for, say, up to a millisecond. During that time you can do some other gate operations in your quantum information circuit and then you can later retrieve that. So in essence it's RAM for a future quantum computer.”
Marrying progress in fundamental physics with real-world quantum technologies is an example of what Imperial does best.
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