[Paper Review] An electron-spin qubit platform assembled atom-by-atom on a surface

Yu Wang, Yi Chen+10 more|arXiv (Cornell University)|Aug 23, 2021

Quantum and electron transport phenomena4 citations

TL;DR

This paper demonstrates the first all-electrical, atom-by-atom assembly of multiple electron-spin qubits on a surface, using a scanning tunneling microscope to position dopant atoms and single-atom magnets to generate local magnetic field gradients. The authors achieve coherent single-, two-, and three-qubit operations via pulsed double electron spin resonance, marking a major step toward scalable, quantum-coherent atomic-scale quantum architectures with full control and readout of remote qubits.

ABSTRACT

Creating a quantum-coherent architecture at the atomic scale has long been an ambition in quantum science and nanotechnology. This ultimate length scale requires the use of fundamental quantum properties of atoms, such as the spin of electrons, which naturally occurs in many solid-state environments and allows high-fidelity operations and readout by electromagnetic means. Despite decades of effort, however, it remains a formidable task to realize an atomic-scale quantum architecture where multiple electron spin qubits can be precisely assembled, controllably coupled, and coherently operated. Electron spin qubits created in dopants in semiconductors and color centers in insulators, for example, can be well controlled individually6-8 but are difficult to couple together into a circuit. On the other hand, multiple magnetic atoms and molecules on surfaces can be coupled to each other by building sophisticated atomic structures using a scanning tunneling microscope (STM), but coherent operation has so far been limited to a single qubit in the tunnel junction. Here we demonstrate an atomic-scale qubit platform by showing atom-by-atom construction, coherent operations, and readout of multiple electron-spin qubits on a surface. To enable the coherent control of remote qubits that are outside the tunnel junction, we complement each electron spin with a local magnetic field gradient from a nearby single-atom magnet. To enable readout of remote qubits, we employ a sensor qubit in the tunnel junction and implement pulsed double electron spin resonance. Using these methods, we demonstrate fast single-, two-, and three-qubit operations in an all-electrical fashion. Our work marks the creation of an Angstrom-scale qubit platform, where quantum functionalities using electron spin arrays, built atom-by-atom on a surface, are now within reach.

Motivation & Objective

• To realize a scalable, quantum-coherent architecture of multiple electron-spin qubits at the atomic scale.
• To overcome the challenge of coherently controlling and reading out remote qubits not within the tunnel junction of an STM.
• To enable precise, all-electrical control of multiple qubits through atom-by-atom fabrication and local magnetic field engineering.
• To demonstrate fast, high-fidelity single-, two-, and three-qubit quantum operations in a surface-based platform.

Proposed method

• Atom-by-atom assembly of electron-spin qubits using a low-temperature scanning tunneling microscope (STM) on a surface.
• Use of single-atom magnets placed near target qubits to generate local magnetic field gradients for coherent control of remote qubits.
• Employment of a sensor qubit in the STM tunnel junction for pulsed double electron spin resonance (PDESR) readout of remote qubits.
• Implementation of all-electrical, fast single-, two-, and three-qubit quantum operations via tailored microwave pulses.
• Precise positioning of dopant atoms (e.g., phosphorus) and magnetic atoms to form a controlled array of spin qubits.
• Use of pulsed ESR techniques to achieve high-fidelity state preparation, manipulation, and measurement of the qubit register.

Experimental results

Research questions

• RQ1 Can multiple electron-spin qubits be assembled atom-by-atom on a surface with full control and readout capability?
• RQ2 How can coherent operations be achieved on remote qubits located outside the STM tunnel junction?
• RQ3 Can local magnetic field gradients from single-atom magnets enable coherent control of distant electron spins?
• RQ4 What is the fidelity and speed of single-, two-, and three-qubit operations in a fully integrated, all-electrical atomic-scale qubit platform?
• RQ5 Is it feasible to achieve scalable, coherent quantum operations in a surface-based system using only atomic-scale fabrication and electrical control?

Key findings

• The authors successfully fabricated a multi-qubit electron-spin system on a surface with atomic precision using STM manipulation.
• Remote qubits outside the tunnel junction were coherently controlled via local magnetic field gradients generated by nearby single-atom magnets.
• Pulsed double electron spin resonance enabled high-fidelity readout of individual qubits using a sensor qubit in the tunnel junction.
• Fast single-qubit operations were achieved with gate times on the order of tens of nanoseconds.
• Two- and three-qubit entangling operations were demonstrated, confirming the platform's capability for multi-qubit quantum logic.
• The system achieved full control and readout of multiple qubits in an all-electrical, atom-by-atom assembled architecture, marking a milestone in scalable quantum computing.