Interview: Wolfgang Ketterle, Nobel laureate and ultracold atom expert
Image credit: Schaffhausen Institute of Technology
German-born Wolfgang Ketterle is a leading expert on the weird world of ultracold atoms. E&T spoke to Kettlerle about the phenomena and technologies only possible near absolute zero, and his fears for a fractured, anti-scientific world in which inconvenient facts are discarded.
The world is full of noise; the most serene scenes are abuzz with moving atoms. Under certain extraordinary conditions, however, these atoms can be frozen in their tracks.
Nearly a century ago, Calcutta-born polymath Satyendra Nath Bose and Albert Einstein laid out a framework to describe the behaviour of bosons (one of two fundamental types of particle). Bose-Einstein statistics predicted that when a gas is cooled towards absolute zero, its atoms lose energy and fall to the lowest quantum state. This predicted the existence of a new state of matter: a Bose-Einstein Condensate (BEC).
Scientists doubted the feasibility of creating a BEC, which would require a system to keep the gas in gaseous form as it cooled, and to cool and trap it during the process. In 1995, however, US-based scientists made this novel state of matter a reality by combining two methods: first precooling the gas through laser cooling (carrying away energy with scattered light) and then caging the atoms in a magnetic field for evaporative cooling (ejecting the warmest atoms from their magnetic trap). This allowed them to reach nanokelvin depths: just billionths of a degree above absolute zero.
The 2001 Nobel Prize in Physics was awarded to three men involved with the creation of the first BECs: Carl Wieman and Eric Allin Cornell, who produced the first BEC from rubidium-87 atoms at the University of Colorado Boulder, and Wolfgang Kettlerle, who created a larger BEC from sodium-23 atoms soon afterwards at MIT.
When a cloud of atoms nears absolute zero, it leaves the behaviour of ordinary matter behind and enters a single quantum state, embracing wavelike quantum characteristics which had previously only been observed at a microscopic scale. This gives rise to the most curious behaviour: wavelike interference between condensates; propagation of a beam of coherent atoms (an “atom laser”); and the creation of a “supersolid” with molecules sliding friction-free beneath a fixed shape. All these phenomena were first demonstrated in Kettlerle’s lab: the Center for Ultracold Atoms at MIT.
E&T spoke to Ketterle pre-pandemic, at an event hosted by the newly founded Schaffhausen Institute of Technology in Switzerland, which has ambitions to become something akin to a European MIT. Kettlerle is softly spoken, thoughtful, and well-practiced at communicating his work to non-experts.
There have been several milestones in ultracold physics since BECs were first created, he explains. Most notably, the range of ultracold systems has expanded to include objects like fermions – the other type of fundamental particle – and whole molecules constructed with “ultracold chemistry”.
“Bose-Einstein condensation meant certain atoms at ultra-low temperatures,” Kettlerle said. “Since then the field has expanded by development of techniques for the other kind of atoms [fermions] and we have obtained ultracold molecules, and then we have obtained ultracold atoms which are very magnetic. In that sense, the range of materials and atoms and systems we can study in the ultracold has expanded.”
BECs and other ultracold systems are fragile compared with other states of matter. The slightest disturbance can cause them to hurtle back to higher temperatures and out of coherence. This fragility – as well as the intense effort necessary to create and maintain them – make many practical applications unfeasible. However, ultracold atoms are a natural fit for applications which already require minimal thermal noise, such as precision measurement.
“If atoms stand still you can do much more precise measurements on them, and the important application is atomic clocks,” Ketterle explains. “We now have optical clocks [a very precise type of atomic clock] which have a precision which is more than a thousand times better than of clocks just ten years ago, so there’s really been a revolution in atomic clocks.
“The fact that the atoms cannot be held forever doesn’t really play a role for those applications because every second you can cool atoms you can look at them for a second then look at the next batch of atoms, so there is no need to hold them for longer.”
In atomic clocks, electrons are observed as they leap between energy levels of atoms with known frequency. The colder the atoms, the more precisely they can be localised and the longer they can be observed, allowing for great precision. They are vital components in satellite navigation systems: atomic clocks are carried on satellites to measure minute time delays between transmission and reception of radio signals, so distances and locations can be calculated.
The extreme control over atoms possible at nanokelvin temperatures has led to speculation that some ultracold system could form a platform for quantum computing, which will require systems of atoms to remain coherent. Already, Ketterle and his lab have been using ultracold atoms as “quantum simulators”. If an equation cannot be solved on a conventional computer, scientists can assemble ultracold atoms into a system which obeys the equation, forming an analogue computer. These simulators have been used to simulate certain properties of a black hole’s event horizon.
Despite considerable excitement about these potential applications of ultracold atoms, Kettlerle refuses to be drawn into speculation about new technologies. His creation of mindbending ultracold systems such as supersolids seems to be driven by curiosity alone.
“This doesn’t mean that we can do anything useful with [supersolids]. Now you know that those materials exist and maybe in 10, 20 or 30 years someone will find some way to synthesise such a material at a higher temperature [which could be used in practical applications],” he said. “The big impact of our research is to get deeper inside into properties of materials, and what is possible in nature, and these principles in general. Hopefully we can stimulate other research and hopefully lead to other materials which can be used.”
“With the supersolid I don’t even know what it would be good for even in fundamental research. You try to get the deepest possible insight, show the most general principle you can show because the more general your research is, the broader the impact. But we are looking for materials which sort of put nature to the test. This is our criteria, not what could possibly lead to an application.”
As a scientist free to pursue fundamental research, Ketterle is in an enviable position. He receives a Department of Defence grant to do the science he thinks is best without having to promise it will plant the seeds of lucrative new technologies.
“In the area of pure fundamental research, the field of merit is the impact on knowledge,” he said. “This still exists, and we have to defend that because there are so many examples of fundamental research making discoveries which later were solutions to problems. Sometimes people find solutions first, and then find the problem for the solution.”
Despite his own comfortable position, Ketterle has concerns about the future of science. He characterises the move away from free movement in the US, UK, and elsewhere as a threat to science, which is inherently collaborative. He also raises concern about the irrational anti-expert rhetoric brewing in these countries amid a resurgence of populism.
“In the US with the [Trump administration] there is definitely an anti-intellectual climate. We feel that a lot of important decisions should be fact based, science based, and if there are people who are leading the country who feel they can ignore facts or even worse talk about fake facts and almost mingle things which contradict each other, it becomes more political statement if you call this a fact or call that a fact,” he said. “This is anti-science, this undermines the scientific principles of proven facts, experimental investigation, and truth.”
Ketterle says that while scientists do not make political decisions, scientific knowledge must inform political decisions. For some politicians to ignore the science is “terrible”, he says. His pre-pandemic comments are prescient, given the strain placed on the relationship between the White House and public health experts like Dr Anthony Fauci during the Covid-19 pandemic.
When asked if there is anything to be done about the rejection of science by many politicians and swathes of the public, Ketterle sighs and pauses to think for a while.
“I think it becomes a question of culture and climate and general population; what do people read, what do people believe?” he said. “The internet is a wonderful source of information but some people also use it to read one-sided news and actually misinform themselves. We’re really talking here about a phenomenon in politics but also a cultural phenomenon throughout probably all of society.”
Ketterle feels he has a responsibility to communicate to the public, including by explaining the process of science (he finds that error bars and levels of certainty can be confusing). However, while people like him can share a little of their work through public lectures, at the end of the day, the public needs to trust scientists.
“Science is complicated. I try my best to explain science in laypeople’s terms, to bring science to general audiences, but in the end I do more complicated things and […] ultimately, people have to trust experts,” he said. “The experts have to earn the trust but to say sort of “I don’t like what experts say therefore I ignore it” or “I don’t trust experts because I don’t want to make an effort to understand what they’re saying or what they’re saying is inconvenient”. This is just wrong.”
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