Rare-earth Spins on a Surface

Introduction

Spin qubits in QNS have to date primarily consisted of transition metal atoms of the 3d electronic configuration, such as iron, titanium and copper. These spins have the unpaired electrons in the outermost shell, which has advantages and disadvantages. On the pro-side, it offers the electrons that flow in the tunnel current of an STM a good chance to interact with the magnetic atoms. On the con-side, this same interaction with the environment also decreases the spin coherence times.

In this research effort, we are focused on the magnetic properties of rare earth spins on surfaces. Rare earth atoms contain a partially filled 4f electron shell and crucially, this 4f shell is hidden deeper inside the atom. Therefore their interaction with the environment is generally weaker. Also, because they consist of heavy atoms, the spin degree of freedom is locked to the angular momentum degree of freedom, which generally makes the interpretation of the magnetic behavior easier.

In this group, we rely on a combination of experimental and theoretical techniques. On the experimental side, we study the ensemble properties of the rare earth spins by using x-ray absorption spectroscopy, especially the magnetic technique, called X-ray Magnetic Circular Dichroism (XMCD). Our team regularly travels to the world’s best synchrotron radiation facilities to perform these experiments, both in Korea and in Europe. We utilize STM to understand the absorption of the rare earth spins on surfaces and ESR-STM to unravel the fundamental mechanisms to coherently drive and detect their 4f electrons (Figure 1). On the theory side we use multiplet calculations and density functional theory, which are described in more detail in the theory team’s pages.

Figure 1: coherent driving of the 4f electrons of an erbium atom (depicted in yellow) in a coupled structure with a Ti atom (purple), probed by an ESR-STM tip. Credit: Stefano Reale

Our goal is to identify surface-adsorbed rare earth atoms with quantum levels that are suitable for coherent manipulation and with long coherence time. In addition, we aim at engineering the occupation of their individual orbitals in order to optimize the detection of their magnetic signal using STM and ESR-STM. These atoms could be an excellent platform to test advanced pulse sequences for quantum logic and error corrections.

Longer-term Goals

  • Utilize rare earth atoms to test quantum logic operations and error correction
  • Obtain entanglement in multiple qubits consisting of rare-earth atoms

Near-term Goals

  • Understand the impact of the environment (temperature, local electric and magnetic fields) on the stability of the quantum states
  • Unravel the mechanisms for coherent drive of single rare-earth atoms on a surface
  • Find rare-earth atoms with long coherence time

Further Reading