A major breakthrough has been achieved in the development of diamond-based quantum computers.
Quantum computers and quantum communication are groundbreaking technologies that enable faster and more secure data processing and transmission compared to traditional computers. In quantum computers, qubits serve as the fundamental units of information, functioning as the quantum mechanical equivalent of bits in classical computing.
Where, for example, laser pulses in a glass fiber transport information from A to B in classical digital communication, quantum mechanics uses individual photons. In principle, this makes it impossible to intercept the transmitted data. Qubits that are optically addressable (can be controlled or read out with light) are suitable for storing the photons’ information and processing it in quantum computers. The qubits can store and process quantum states, and absorb and emit them in the form of photons.
Qubit Stability Is Key
A major challenge in qubit development is extending the coherence time, i.e. the time in which qubits can store information in a stable manner. Being able to control qubits and keep them stable enough to exploit their characteristics in practical applications will be crucial to the feasibility of developing efficient and scalable quantum computers.
At KIT’s Physikalisches Institut, doctoral researchers Ioannis Karapatzakis and Jeremias Resch have investigated how to precisely control a special defect in diamonds known as a tin-vacancy (SnV) center. Their work was part of two projects funded by Germany’s Federal Ministry of Education and Research: QuantumRepeater.Link (QR.X) for secure fiber-based quantum communication and SPINNING, which aims to develop a diamond spin-photon-based quantum computer.
“A defect in the lattice structure of a diamond’s carbon atoms occurs when atoms are missing or are replaced by other atoms such as tin,” said Karapatzakis. Such defects can be used as qubits for quantum communication because they have special optical and magnetic properties that enable states such as their electron spin to be manipulated using light or microwaves. The defects can then be used as stable qubits that can store and process information and couple it with photons.
Considerable Improvement in Coherence Times
Diamond qubits have the advantage of existing in the solid phase, making them easier to work with than other quantum materials, e.g. atoms in a vacuum. Karapatzakis and Resch were able to precisely and observably control the electron spins of tin-vacancy center qubits using microwaves. “We were able to increase the coherence times of the diamond SnV centers to as long as ten milliseconds – a major improvement,” says Resch.
They did so with dynamical decoupling, which largely suppresses interference. A further special aspect of the researchers’ results is their success in demonstrating for the first time that this type of diamond defect can be very efficiently controlled with superconducting waveguides, which efficiently direct microwave radiation to the defects without generating heat.
“That’s very important because these defects are generally investigated at very low temperatures near absolute zero. Higher temperatures would make the qubits useless,” says Karapatzakis.
“To establish communication between two users or (later) between two quantum computers, we need to transfer the qubit quantum states to photons,” notes Resch. “With optical readout of qubits and by reaching stable spectral properties, we’ve taken an important step in that direction. So our results on controlling tin-vacancy centers in diamonds offer the potential for an important breakthrough in the future development of secure and efficient quantum communication.”
Reference: “Microwave Control of the Tin-Vacancy Spin Qubit in Diamond with a Superconducting Waveguide” by Ioannis Karapatzakis, Jeremias Resch, Marcel Schrodin, Philipp Fuchs, Michael Kieschnick, Julia Heupel, Luis Kussi, Christoph Sürgers, Cyril Popov, Jan Meijer, Christoph Becher, Wolfgang Wernsdorfer and David Hunger, 27 August 2024, Physical Review X.
DOI: 10.1103/PhysRevX.14.031036