Making quantum leaps in a new lab
The CarboQuant project at Empa is set to enter the next phase—thanks to a new, highly specialised laboratory funded in large part by the Werner Siemens Foundation. Researchers now have the capacity to study and manipulate the quantum magnetic states of their carbon materials with even greater precision.
State-of-the-art equipment is the sine qua non of cutting-edge research—a statement that holds particularly true for disciplines like quantum physics. This, because the quantum mechanical properties of materials and molecules are not only extremely complex and fragile, but often also confined to atomic structures such that they appear only at minuscule scales. That’s why the two ultra-modern scanning tunnelling microscopes installed in the new CarboQuant Lab at Empa in Dübendorf, Switzerland, represent a milestone in the CarboQuant project, which receives funding from the Werner Siemens Foundation.
In the lab, which opened at the end of January, Yujeong Bae and Stepan Kovarik are discussing final adjustments in front of one of the microscopes. Bae leads the group for quantum magnetism at the nanotech@surfaces Laboratory at Empa, and Kovarik is a postdoc in her group. Over the past few months, the two have set up the scanning tunnelling microscope, preparing it for use. Now they can begin their actual work: a detailed investigation of the quantum physical properties of nanographenes.
Nanographenes are nanometre-sized pieces of the two-dimensional carbon material graphene, which has several interesting properties. Not only is it harder than diamond, extremely tear-resistant and gas-impermeable, it’s also an excellent electrical and thermal conductor. “Additionally, several years ago we observed that certain nanographenes also have magnetic properties, so-called spin effects that enable quantum applications,” says Roman Fasel, head of the nanotech@surfaces Laboratory and co-leader of CarboQuant.
Simultaneously in two states
Spin is one of the fundamental quantum mechanical properties of charged particles like electrons and protons. Put simply, a spin is a type of magnetic moment resulting from the particles’ charge and angular momentum that, in the simplest cases, indicates “up” (state 1) or “down” (state 0). In the case of nanographene molecules, these magnetic moments occur when unpaired electrons are present.
When two or more spins are coupled, they exercise a mutual influence on each other and what’s known as “quantum superposition” arises: a mind-bending phenomenon in which the quantum system can take on several states in any conceivable combination between 0 and 1—at the exact same time. This ambiguity is what makes quantum-based technologies so intriguing, and it’s what theoretically makes it possible for quantum computers to perform several arithmetic operations simultaneously, rather than one after the other.
Roman Fasel and his team have perfected the synthesis of nanographene materials over the past few years and today are at the forefront of research into quantum physical phenomena using nanographenes. The researchers have already developed a type of modular system enabling them, in a first step, to predict the required configuration of a molecule based on the desired spin properties—and then, in a second step, to synthesise the structure. Another major accomplishment is the ability to turn the spins off and on with precision.
Liquid Helium and high vacuum chamber
In their new CarboQuant Lab, which was financed in part by the Werner Siemens Foundation, the researchers can now study the quantum magnetic properties of their nanographenes in yet greater detail. The two scanning tunnelling microscopes appear quite similar: both have an ultra-high vacuum chamber, powerful magnetic fields and liquid helium tanks for cooling samples to temperatures approaching absolute zero, at minus 273.15 degrees Celsius. “But the microscopes are different in performance and precision,” Yujeong Bae explains. One of the microscopes is more compact and can be cooled to 1.3 degrees Celsius above absolute zero. The other, even more precise device can attain a temperature of just 0.4 degrees Celsius above absolute zero.
Unlike ordinary microscopes, scanning tunnelling microscopes function without lenses and other optical components. Instead, they sense the electronic states of a sample’s surface with a sharp tip positioned less than a nanometer away—without ever making contact. Both the tip and the sample are conducting; when a voltage is applied between the tip and the conducting surface, the ensuing electric current—which varies according to the topography and conductance of the sample—is measured. This enables the individual atoms and molecules on surfaces to be studied.
Manipulation through microwaves
But that’s not all: the new devices also allow the researchers to control and describe quantum magnetic moments. This is Yujeong Bae’s area of expertise. Before joining Empa at the start of 2024, Bae conducted research at the Center for Quantum Nanoscience in Seoul, South Korea, where she and her research group studied methods for realising the desired quantum control and analysis. The trick is combining scanning tunnelling microscopy with so-called electron spin resonance, a technique based on radiating a sample with microwaves—and similar in its functioning to magnetic resonance imaging (MRI) common in the field of medicine.
The scanning tunnelling microscope is fitted with two high-frequency cables, one for the tip and one for an antenna. The researchers transmit microwaves through the cables into the gap between the scanning tunnelling microscope’s tip and the sample’s surface. “With this technology, we can manipulate the electron spins while also controlling and studying their superpositions,” Bae explains. “Our goal is to be the first to demonstrate this quantum control on nanographenes.”
In perfect order
CarboQuant co-leader Oliver Gröning adds, “It’s only been in the last ten years that scanning tunnelling microscopy with electron spin resonance has been used to manipulate spins, generally with single atoms.” Indeed, most methods for combining several atoms use the tip of the scanning tunnelling microscope to place them in the right order, behind or next to one another. However, Gröning continues, “exact placement using physical methods is incredibly difficult”.
The Empa researchers take a different approach by using the tools of chemistry to fabricate their nanographene molecules. “The chemical bonds guarantee that the distance between the individual atoms is—and remains—exactly the same,” Gröning explains. Inside the measurement chamber of the scanning tunnelling microscope, the microscope tip “only” has to remove a hydrogen atom and its electron at the right place. After this procedure, the molecule is left with an unpaired electron and a magnetic moment arises.
“With the new CarboQuant Lab, we can gain insights into the quantum effects that occur—and then learn how to control them,” Oliver Gröning says. This new understanding would be a major step towards demonstrating the quantum potential hidden in the Empa team’s nanographenes.
