Electron Spin Resonance Enhances Precision in Quantum Sensors and Computing

Electron Spin Resonance (ESR) is a technique used to study materials with unpaired electrons, with applications in quantum computing and condensed matter physics. Recent advancements have been made in the use of ESR in quantum sensors, allowing for precise measurements of electric and magnetic fields at the atomic scale. One system that displays ESR at the single ion level is the NV center in diamond, which can measure local magnetic fields. However, technical challenges exist, such as the need to determine the exchange field produced by Fe atoms on the tip. The future of ESR in quantum sensors is promising, with potential for detecting both magnetic and electric fields at the atomic scale.

What is Electron Spin Resonance (ESR) and its Role in Quantum Sensors?

Electron Spin Resonance (ESR) is a technique used to study materials with unpaired electrons. The basic concepts of ESR are similar to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei. ESR has a wide range of applications in various fields, including quantum computing and condensed matter physics.

In 2015, the observation of an ESR signal from a single atom using a Scanning Tunneling Microscope (STM) tip was realized. Since then, significant progress has been made in this field. Two recent papers, authored by Taner Esat, Dmitriy Borodin, Jeongmin Oh, Andreas J Heinrich, F Stefan Tautz, Yujeong Bae, Ruslan Temirov, Yu Wang, Yi Chen, Hong T Bui, Christoph Wolf, Masahiro Haze, Cristina Mier, Jinkyung Kim, DeungJang Choi, Christopher P Lutz, Soohyon Phark, and Andreas J Heinrich, report particularly striking advances in this area.

How Does ESR Work in Quantum Sensors?

In the first paper, a molecule carrying S=1/2 is attached to an STM tip, and a sharp ESR is observed. The shift of this resonance can be used for the sensing of very small magnetic fields and electric fields with angstrom scale spatial resolution. This is a significant advancement in the field of quantum sensors, as it allows for highly precise measurements of electric and magnetic fields at the atomic scale.

The second paper reports the use of the ESR signal of a sensor atom located on a surface to interrogate two other S=1/2 atoms, which serve as qubits. Remarkable coherence properties and two-qubit operations are demonstrated using pulsed fields techniques. This is a significant step forward in the field of quantum computing, as it allows for the manipulation and interrogation of qubits at the atomic scale.

What is the Role of NV Centers in ESR?

A well-known system that displays ESR at the single ion level is the NV center in diamond. The very narrow resonance of the NV center can be used to measure local magnetic fields down to microTesla/Hz^1/2. By placing the diamond on an Atomic Force Microscope (AFM) tip, scanning is also possible. However, since the NV centers are located on the scale of tens of nanometers from the surface, this limits the distance of the NV center from its target and therefore the spatial resolution to tens of nanometers.

How is ESR Detected in Quantum Sensors?

For NV centers, the spin state is excited by a microwave field which drives the resonance and is detected optically by the luminescence from an optically pumped level. In contrast, for ESR-STM, the drive is accomplished by modulating the voltage between the tip and substrate. With appropriate spin-orbit coupling of the magnetic atom, the AC electric field has a magnetic component which drives the resonance. Detection is accomplished by using a spin-polarized tip – the magnetic state of the spin is detected by measuring the tunneling current, which depends on the magnetization of the sensor atom or molecule via a magneto-resistance effect.

What are the Technical Challenges in ESR?

There are several technical challenges in ESR. For instance, the Fe atoms on the tip produce an exchange field which needs to be determined because the ESR frequency is sensitive to the vector sum of the exchange field and the external field. This can be done by fitting the spectra for two orientations of the applied fields. Once the exchange field is known, the molecule is now a mobile scanning sensor of magnetic field with the angstrom resolution of the STM tip.

What is the Future of ESR in Quantum Sensors?

The future of ESR in quantum sensors is promising. The ESR line is also sensitive to a local electric field, presumably because it shifts the position of the bonding electrons and affects the environment of the spin state. This opens up new possibilities for the use of ESR in quantum sensors, as it allows for the detection of both magnetic and electric fields at the atomic scale. However, more research is needed to fully understand and exploit this property.