Post Date
Jan 3 2023

A new path in the advancement of quantum information technology

Be it artificial intelligence, development of pharmaceutical drugs or understanding climate change processes, quantum computers have the potential to dominate the future technological landscape when solving sophisticated tasks in a fraction of a second. This is enabled through Quantum Information Technology – a discipline which uses the counterintuitive principles of quantum mechanics to develop information processing. Just like any paradigm shifting technology, full implementation of quantum computing is currently facing challenges, solving which could be the key to advancement in the field. One of the key challenge has been addressed by Dr Evgeny Chekhovich from the University of Sheffield , Dr Ata Ul Haq from the Department of Physics at LUMS and their fellow researchers in their research paper entitled “Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constant in GaAs/AlGaAs quantum dots”.
We would have to take a closer look at the core/building blocks of quantum information technology to understand the significance of this research paper. The secret to quantum computer’s power lies in its ability to generate and manipulate quantum bits, or qubits as they are called. The speed up in quantum computers is due to the ability of qubits to form quantum superposition of its own and entanglement with other qubits. These features arise from a property called quantum coherence. Quantum coherence is destroyed by the tiniest of fluctuations caused by thermal energy at high temperatures. Preparation of physical systems in which qubit exists at ultralow temperature is one of the key challenges in quantum information technology today. This is where the importance of this study comes in. This study uses quantum dots (QDs) which are nanoscale semiconducting structures also called as artificial atoms. Nuclear spins confined within gallium arsenide (GaAs) quantum dots (QDs) can act as a qubit. However, these spins fluctuate even at temperatures as low as a few kelvin. The main challenge is to align all the nuclear spins in a QD so that a quantum memory can be formed and, in the process, cool it down to a few milli kelvin temperature. The researchers of this paper have for the first time achieved the lowest temperature of a qubit system within a nanostructure. Its is also the first time that the temperature of a single nanoparticle has been measured experimentally. All this is achieved using a combination of sophisticated radiofrequency and optical pulses applied to a GaAs structure.   

Schematic of a nanohole in-filled GaAs/AlGaAs quantum dot (QD). An electron (blue) with spin S=1/2 trapped in the dot interacts with >104 nuclei (green), each processing a nuclear spin I =3/2 for gallium and arsenic, or I=5/2 for aluminium. 


The proposed methodology is a new approach in which the radiofrequency depolarization is performed on quantum dots. The role of the short optical readout pulse is to excite photoluminescence, a process in which a molecule absorbs a photon, excites an electron to a higher electronic state, and then radiates a photon as the electron returns to a lower state. This spectrum is then analysed to calculate the nuclear spin alignment (also called spin polarization) within a QD. Getting close to 100% polarization is ideal and the researchers of this paper observed up to 80% polarization in GaAs – the highest reported so far for optical cooling in QD. Previous research on polarization in diamond and Silicon Carbide (SiC) has been limited to 50-60% for the nuclei spins in quantum dots. The quantum dots spins are cooled down to 1.3 milli kelvin in the process, which is the lowest recorded temperature in nanostructures. The observations made by Dr. Ata Ul Haq and fellow researcher have not only unveiled the capabilities of GaAs, but also unlocked a route for further progress in achieving long qubit coherence through deep cooling of the mesoscopic nuclear spin ensemble.