In June 2021, the eyes of the science community turned to Ehningen, a small town in the Baden-Württemberg region in south-west Germany. There, in a specialist centre called Quantum Computing Baden-Württemberg by the Fraunhofer Gesellschaft, IBM unveiled one of the most powerful quantum computers ever produced—a gleaming, 3-metre assembly of superconducting circuits capable of handling 27 quantum bits, or qubits, simultaneously.
Unlike digital bits, qubits store information in two possible states at the same time using the properties of electron spin. But to carry out mathematical operations, qubits must be arranged to prevent spin arrangements being disturbed by outside influences.
To do this, the new IBM computer cools its device to temperatures close to absolute zero. This energy-intensive approach, however, may prove difficult to scale to the million-qubit levels probably needed to perform meaningful quantum computations.
At the nearby Centre for Integrated Quantum Science and Technology (IQST) in Stuttgart and Ulm, researchers are working on a different approach by developing quantum computers that work at room temperature and so are inherently scalable.
“We are setting up a rival platform, a quantum computer demonstrator that hopefully outperforms what’s currently available,” says Florian Meinert, a group leader at IQST, which is funded by the Carl Zeiss Foundation, Ulm University, the University of Stuttgart and the State of Baden-Württemberg. “It works with neutral atoms trapped in tightly focused laser beams: what we call optical tweezers.”
Meinert and his colleagues use their tweezers to study Rydberg atoms that have electrons excited to high energy quantum states, up to 10,000 times farther away from atomic cores than normal. When manipulated into two-dimensional arrays, the exaggerated dipole moments found in Rydberg atoms can block neighbouring atoms from responding to laser excitation: actions that can form the basis of logic gates involving qubit arrays ten times larger than the IBM- Fraunhofer computer.
“You can apply radio frequencies to split the optical beams, and then you have hundreds of traps from a single initial laser,” explains Meinert. “If you have strong enough interaction between the Rydberg states, you can make this blockade almost perfect.”
IQST takes an interdisciplinary approach to tackling quantum technology problems. Priyadharshini Balasubramanian, an IQST researcher at Ulm University, exemplifies this. Trained as a physicist in India, Balasubramanian works alongside biomedical scientists and engineers as she develops diamond-based quantum sensors that run at ambient temperatures.
“We study diamonds where two carbon atoms are replaced by nitrogen and a vacancy site,” says Balasubramanian. “They have localized electrons with magnetic properties that we use for high resolution sensing.”
The team uses their diamonds as nanoscale thermometers to gauge how effectively photothermal therapies heat up and kill cancer cells. In addition, attaching specific DNA strands to the diamond surface enables the device to identify cancers from an early stage.
“The advantage of this sensor is you can bring it right to the target — you don't need a lot of these mutations to detect if something’s wrong,” says Balasubramanian.
IQST’s approach is attracting attention: in recent years, it has had five research fellows among the most cited researchers world-wide. And with Baden-Württemberg pledging up to €40 million in quantum technology investments by 2024, the region’s appeal is hard to miss. “I used the grant I got from IQST to start a second project on quantum simulations,” says Meinert. “It’s actually cool to see that many of our quantum computer ideas started with the questions I was asking the simulator.”
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