100 incredible years of physics – quantum physics
Quantum physicist Kai Bongs has grappled with the big questions in physics throughout his career, whether it was attempting to detect gravitational waves for the first time with the GEO600 detector in Germany or creating one of the first Bose–Einstein condensates – an exotic state of matter first predicted to exist in the 1920s. But it is his current work for which he is most proud.
Bongs – an IOP fellow and winner of IOP’s 2019 Dennis Gabor Medal and Prize – leads the Quantum Technology Hub in Sensors and Metrology. It is one of four quantum technology hubs within the National Quantum Technologies Programme.
This Programme is a UK-wide effort to move quantum physics out of the lab and into the real world, driven by quantum optics and information pioneer – and former IOP President – Peter Knight. “Sir Peter is, of course, my big hero,” explains Bongs. “I think without him, we would not have the UK National Quantum Technologies Programme – and that’s what the world is envying us for.”
The Quantum Technology Hub in Sensors and Metrology focuses on cooling atoms to near absolute zero to unveil their quantum mechanical nature. Researchers at the Hub then employ the quantum property of superposition to try to construct extremely sensitive sensors that Bongs says “will essentially add new senses to what humanity can perceive”.
One application the team is pursuing is gravity sensors for detecting objects underground. “Some people say that a metre under London is less known than Antarctica,” Bongs reveals. “We hope to change that to help infrastructure projects around the world.”
Other lines of research include ultra-precise quantum clocks for high-speed internet, banking and space-based navigation, as well as quantum magnetic sensors for various purposes, including medical research. According to Bongs: “There’s some hope of contributing to the big challenges of dementia and child concentration deficit and many other brain disorders, as well as understanding the magnetic fields of the body and what that means for our health.”
Bongs sees his research as one of the latest entries in the rich history of translating our fascination with quantum physics into practical applications that can change the world.
For instance, one of the key early breakthroughs in quantum physics was made by a young French nobleman by the name of Louis de Broglie in 1923. At the same time as Tutankhamun was being discovered in Egypt, and Adolf Hitler was trying and failing to seize power in Germany for the first time in the Beer Hall Putsch, de Broglie was extending the quantum idea that light can be described as both a wave and particle to all matter.
“I’m just fascinated by de Broglie coming up with the idea that particles also have a wave nature,” says Bongs. “For me, it was a pivotal step towards describing everything in the way that quantum mechanics now does.”
It was this quantum property of wave–particle duality that unlocked almost all modern technology. Being able to describe electrons as waves allowed physicists to understand the electrical properties of silicon. This then offered engineers the opportunity to shrink transistors down to today’s size; about 70 silicon atoms wide. Millions of these transistors packed together make the computer chips powering today’s phones and laptops.
Wave–particle duality was the foundation for describing the interactions between radio waves and nuclei as well, leading Peter Mansfield (IOP Honorary Fellow and Dudley Medal and Prize winner) to pioneer magnetic resonance imaging, commonly known as MRI, in the 1970s.
Quantum physics also underpins another ground breaking medical technology: positron emission tomography, or PET. When English theoretical physicist and IOP Honorary Fellow Paul Dirac managed to integrate the disparate theories of quantum mechanics and special relativity – which says nothing travels faster than light – in 1928 for the first time, his equations predicted antimatter.
Discovered experimentally four years later, antimatter particles are doppelgangers of their matter counterparts except that they carry the opposite charge and spin. One of these antimatter particles – the positron (i.e. positive electron) – is the key ingredient allowing PET to produce high-resolution images of the body.
Teleportation of the tiny
In recent decades, some of the more exotic properties of the quantum world have been probed and put to use in dazzling experiments. For instance, Austrian physicist Anton Zeilinger – winner of the inaugural IOP Isaac Newton Medal in 2008 – has made enormous progress in understanding the property of quantum entanglement, and also applying his insights in the real world.
Quantum entanglement is the tendency of particles that interact with each other to become entwined to the point where they cease to have a separate existence, no matter how far apart they are pulled from one another. It was this last aspect of entanglement that Albert Einstein (Honorary Fellow of the Physical Society) took umbrage to in a 1935 paper, describing it as “spooky action at a distance” because a change in one particle will instantly change the other; exceeding the speed of light and thereby violating the Universe’s speed limit.
Starting as early as the late 1940s, numerous experiments have shown that there is no paradox – an entangled particle responds instantly to another entangled particle’s state. Of all of these experiments, Zeilinger’s stand out. “Zeilinger has been driving the quantum communication topic very much forward,” says Bongs. “He has developed thinking about the deep foundations and the boundaries of our quantum understanding, but also asked what we can do with entanglement, how we can put it to use.”
Using photons, Zeilinger was the first to show that multiple particles could be entangled. He also pioneered quantum teleportation, in which the characteristics of one particle are transferred to another. Zeilinger’s teleportation experiments started between laboratories on opposite sides of the river Danube, then across Vienna and in 2012 involved teleporting quantum information 144 km between two Canary Islands.
Most recently, in 2017 Zeilinger was involved in the world’s longest quantum communication experiment. The international team used China’s Micius satellite to beam two photons to Vienna and China. Information on the state of the photons was taken by researchers at each location. This allowed them to construct an unhackable password, which they used to conduct a secure video call across 7600 km.
New foundations needed
Looking towards the next 100 years of quantum physics, Bongs sees quantum communications, quantum imaging and, of course, quantum sensors, as the fields that are likely to have the fastest impact on society, with quantum computing taking longer, but still fascinating people and driving public understanding of quantum technologies.
Though developing these technologies will require huge progress in various physics disciplines and other sciences, an even bigger challenge will be solving the conundrum that has plagued quantum physics from its inception: “How do we unite quantum theory with general relativity?,” Bongs asks. “I think it’s the biggest problem physics has.”
Dirac and later quantum physicists managed to reconcile quantum mechanics with Einstein’s special relativity – eventually giving us the foundations of elementary particle physics, quantum field theory. But no one has yet been able to quantise gravity in a grand unified theory.
Bongs suggests that to get there, it might be worth taking a leaf out of the quantum history book. The seeds of the quantum revolution were sown in 1900 by Max Planck when he proposed that electromagnetic energy could be emitted only in discrete steps, or in other words in ‘quanta’.
“I think Planck’s work was partly inspired by, or at least related to, some issues the lamp industry had at the time in terms of making lightbulbs more efficient,” says Bongs. “It wouldn’t have excited many fundamental physicists of our time, but it created a whole range of quantum mechanics in its wake.”
He continues: “I’m fascinated by the interplay between technology and fundamental new discoveries because you always have this mutual seeding. New discoveries create new technology, but then technology poses questions that can open up the most fundamental new discoveries.”
“We shouldn’t make a very hard boundary between blue-sky and applied research. We should be allowed to be inspired by the needs of industries or the technology at the time, because actually that can lead to some of the most fundamental insights.”
|Louis de Broglie proposes that matter has wave properties||1923||Howard Carter opens Tutankhamun's burial chamber|
|Erwin Schrödinger develops wave mechanics, the bedrock of quantum physics||1926||John Logie Baird demonstrates the television to the public for the first time|
|Werner Heisenberg formulates the uncertainty principle||1927||Joseph Stalin takes control of the USSR|
|Paul Dirac combines quantum mechanics and special relativity to describe the electron||1928||Cartoon character Mickey Mouse makes his first appearance in ‘Steamboat Willie’|
|The EPR paradox and Schrödinger’s cat thought experiments highlight problems with the interpretation of quantum mechanics||1935||Nazi Germany passes the Nuremburg laws to strip Jews of their civil rights|
|Chien-Shiung Wu observes parity violation in the weak interaction, meaning it cannot transform into its mirror image; unlike all other fundamental interactions||1956||Fidel Castro lands in Cuba at the start of the Cuban Revolution|
|Bell’s theorem offers tests to disprove the EPR paradox and study quantum entanglement||1964||The Civil Rights Act is signed into US law|
|Bose–Einstein condensates realised 70 years after quantum mechanics predicted their existence||1995||Over 170 countries extend the Nuclear Nonproliferation Treaty indefinitely and without conditions|
|Error-free quantum teleportation of a qubit demonstrated over a distance of 3 metres||2014||Psy's ‘Gangnam Style’ becomes the first video to reach 2 billion views on YouTube|
|Google’s Sycamore quantum computer appears to achieve quantum supremacy||2019||July declared as the hottest month on record globally|