Quantum Communication: Efficient Microwave Control of Diamond Qubits

Researchers at the Karlsruhe Institute of Technology (KIT) have shown for the first time in Germany how so-called tin defects in diamonds can be controlled very precisely using microwaves. These defects have special optical and magnetic properties and can be used as qubits: the smallest computing units for quantum computers and quantum communication. The results are an important step towards the development of powerful quantum computers and secure quantum communication networks. The researchers report in the journal Physical Review X (DOI: 10.1103/PhysRevX.14.031036).

Quantum computers and quantum communication are considered to be pioneering technologies for much faster and more secure data processing and transmission compared to classical computers. The basic information units in quantum computers are qubits, the quantum mechanical equivalent of bits in conventional data processing.

While in classical digital communication, for example, laser pulses in a fiber optic cable transport information from A to B, quantum mechanics uses individual photons, which in principle makes the exchange of information secure against eavesdropping. Optically addressable qubits, i.e. qubits that can be controlled or read out using light, are suitable for storing the information in the photons and processing it in quantum computers. These can store and process the quantum states and absorb and release them in the form of photons.

The key point is the stability of the qubits

One of the biggest challenges in developing qubits is to extend their coherence time, i.e. the time in which they can stably store information. The extent to which efficient and scalable quantum computers can be developed depends crucially on whether it is possible to control qubits and keep them stable in such a way that their properties can be used in practice.

Doctoral students Ioannis Karapatzakis and Jeremias Resch at the KIT Institute of Physics investigated how a special diamond defect, the so-called tin vacancy center (SnV), can be precisely controlled. The work was part of the projects "Quantum Repeater.Link (QR.X)" for secure fiber-based quantum communication and SPINNING, which is working towards a spin-photon-based diamond-based quantum computer, funded by the Federal Ministry of Education and Research. 

"A defect in the lattice structure of the carbon atoms of a diamond occurs when atoms are missing or replaced by other atoms, such as tin," explains Karapatzakis. Such defects can be used as qubits for quantum communication because they have special optical and magnetic properties that allow their states, such as the electron spin, to be specifically manipulated using light or microwaves. This can make the defects usable as stable qubits that can store and process information and couple it to photons. 

Coherence times could be significantly improved

The diamond qubits have the advantage of being in solid form. This makes them easier to handle than other quantum materials such as atoms in a vacuum. By controlling them with microwaves, Karapatzakis and Resch managed to precisely influence the electron spins of the tin vacancy center qubits and make this visible. "We were able to significantly improve the coherence times of the SnV centers in diamond to up to ten milliseconds," says Resch. This was achieved using the method of dynamic decoupling, which largely minimizes interference factors. Another special feature is that the two doctoral students were able to show for the first time that this type of diamond defect can be controlled very efficiently using superconducting waveguides: These guide the microwaves efficiently to the defects without generating heat. "This is of great importance because these defects are generally researched at very low temperatures close to absolute zero. Increased temperatures would render the qubits unusable," says Karapatzakis.

"In order to establish communication between two users or, in the future, two quantum computers, I have to be able to transfer the quantum states of the qubit to photons," explains Resch. "By optically reading out the qubit and achieving stable spectral properties, we were able to take an important step in this direction. Our results on controlling tin vacancy centers in diamonds therefore have the potential for an important breakthrough in the further development of secure and efficient quantum communication." 

Original publication

Ioannis Karapatzakis, Jeremias Resch, Marcel Schrodin, Philipp Fuchs, Michael Kieschnick, Julia Heupel, Luis Kussi, Christoph Sürgers, Cyril Popov, Jan Meijer, Christoph Becher, Wolfgang Wernsdorfer, David Hunger: Phys. Rev. X 14, 27 August 2024, DOI: 10.1103/PhysRevX.14.031036