Universal quantum control of an atomic qubit on a surface

Our paper demonstrates universal quantum control of an atomic spin qubit on surface using vector microwave control technique. We were able to control and measure an arbitrary superposition state of the spin, a significant step towards quantum applications of spin architectures at the atomic scale.
Published in Physics
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Atomic spins have emerged as fundamental building blocks for creating nanoscale devices with spin functionalities. In spintronics and quantum nanoscience, it is crucial to comprehend and regulate their magnetic properties. Quantum coherent control of a single spin requests extremely high spatial resolution as well as energy resolution. Scanning tunneling microscopy (STM) has been an exceptional method for producing and analyzing spin systems on a surface at the atomic scale, however, the energy resolution is not sufficient. A technique with higher energy resolution is electron spin resonance (ESR), but a conventional ESR requires billions of identical spins. The idea of coupling both techniques and so to combine the advantages of STM's extreme spatial resolution and ESR's energy resolution was born at the Aspen Winter Conference as Heinrich and Arzhang met in 2008.

Figure 1: Conceptual schematics of spin qubit control. (a) superposition state (|y) of a spin 2-level system. (b) electron spin resonance (ESR) in STM with radio frequency bias (VRF) of phase (d) control.   (c) 2-axis control of a spin Bloch vector.

Eight years ago (2015), Heinrich’s team at the IBM Almaden Research Center was able to perform so-called a single atom ESR on surface, demonstrating the possibility of coupling STM with ESR to achieve the desired resolution [Fig. 1(b)]. In 2019, we at Center for Quantum Nanoscience (QNS) were able to take this further by demonstrating coherent control of a single atomic/molecular spin on a surface in collaboration with IBM. To utilize an on-surface spin as quantum bit (qubit), however, it is necessary to control arbitrary superposition state of the spin, so called 'single qubit universal quantum gating'. Conceptually, the step was relatively straightforward, but the devil, as always, is in the detail.

These experiments presented us with several tough challenges. One of the trickiest parts was figuring out how to properly apply such a cumbersome quantum technique to a single atomic spin in ESR-STM. Running experiments with multiple radio-frequency pulses of different frequencies and phases required a lot of effort to develop, test, and implement a brand-new home-built control method into our STM, which took us around half a year. When we were able to do this successfully for the first time, we were extremely happy. Every research is planned with clean goals, but who knows what will actually come as results in the end or even in the next few days. Surprisingly, we got the results even faster than expected and celebrated the success from our fatigueless efforts.

Compared to a classical bit, a qubit is much more complex and has much more potential for using as information. A classical bit is always either in State |0˃ or in State |1˃, but a quantum bit can be in superposition states [Fig. 1(a)]. To control and measure the exact state of a qubit, you have to perform more elaborate operations. If you think of the qubit as a vector that’s pointing in a certain spatial direction, you should be able to set an arbitrary superposition state of the qubit vector and measure its spatial (x,y,z) components in a fully quantitative manner. In this work, we demonstrated 'single qubit universal quantum gating' of an on-surface spin using two consecutive radio-frequency pulses with a precise phase difference ('2-axis control' of a spin state [Fig. 1(c)]; see animations of its corresponding time-evolution [Fig. 2]). Our 2-axis control enabled us to access and address any quantum state of a qubit at the atomic scale.


Figure 2: Time evolution of a Bloch vector under 2-axis control in the lab frame (left) and a frame rotating at resonance (right)

Our work leverages atoms and molecules on surfaces to advance quantum information science. This offers to the ability to tune and adjust the qubits precisely. We can manipulate the distance between qubits and even change one qubit from an atom to a molecule or a different type of atom. This level of flexibility sets our qubit platform unique from other material systems, where you're restricted to using a specific material like a superconductor or a particular energy center.

This project was a collaborative effort, and over the course of the last six months, we worked closely with our co-authors from the University of Tokyo (Japan), University Paris-Saclay (France), and the University of Oxford (UK) to bring this project to completion. In total, we were a team of eight individuals dedicated to these experiments. Working in such a relatively large group presented its challenges at times, but each person played a unique role and made valuable contributions to the project. It was truly rewarding to collaborate with these international researchers, and the experience enriched our work.

The next step for us is to move towards quantum coherent control of multiple spin qubits, such as entanglement of two coupled qubits, which is a key step to further brightening a way to raise the on-surface spins on a stage of the quantum-coherent applications. At the Center for Quantum Nanoscience, we are now advancing on this challenge by using STM combined with ESR to individual atomic and molecular spins on surfaces.

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