표면에서 개별 스핀의 양자일관 제어

소개

The combination of ESR with STM merges some of the most powerful advantages of both techniques: The atomic-scale spatial resolution of STM with the much higher energy resolution of ESR. This integration of two techniques allows us to achieve the single spin sensitivity at atomic precision. Ensemble ESR uses oscillating magnetic fields at RF frequencies (10 – 100 GHz) to drive and sense the spin resonance. This is usually achieved by using a special cavity to enhance those fields. However, incorporating such an approach into an STM is very difficult and has not been achieved to date. On the other hand, an STM tunnel junction always contains strong electric fields. This led to the key idea, conceived in 2008 by QNS’s director and his colleague, Arzhang Ardavan from Oxford University, to realize that RF frequency oscillating electric fields in the tunnel junction could be used to drive ESR in an STM, illustrated in Figure 1.

Figure 1: ESR STM. A quantum spin (yellow) is put on a thin insulating film (purple) that is supported on a metal substrate (grey). The STM tip contains a magnetic apex (grey arrow) which results in a spin-polarized tunnel current. We add a GHz frequency RF voltage to the normal DC tunnel voltage that is applied between tip and sample.

There are several key ingredients highlighted in Figure 2. First, the spin needs to be decoupled from a metallic substrate via a thin insulating film in order to maintain its quantum-ness. Second, the tip needs to be spin-polarized in order to measure the states of the spin on the surface. Third, an external magnetic field is used to adjust the Zeeman energy of the quantum spin on the surface to be in resonance with the RF electric field. And fourth, a high-frequency oscillating electric field is applied between tip and sample. With this set of ingredients, we are able to electrically drive the spin resonance of atoms and molecules on a surface. A typical ESR-STM spectrum is obtained by measuring the change of the tunnel current while sweeping the frequency of the RF voltage at a fixed external magnetic field, see Figure 2. We see the ESR signals as current changes of several hundreds of femto-ampere (10-12 A), when the RF voltage is resonant with the Zeeman energy. One can also clearly see that the Zeeman energy, and hence the frequency of the ESR resonance, changes in proportion to the magnitude of magnetic field. The exquisite energy resolution of ESR STM can be appreciated here by pointing out that magnetic fields of 0.65 T and 0.70 T result in well-separated ESR spectra. For the experts, the minimum linewidth of ESR-STM is on order of MHz which corresponds to about 10 neV.

Figure 2: ESR spectra of Ti spin on MgO. Here the x-axis is the frequency of the RF voltage and the y-axis is the tunnel current. According to the basic ESR equation, a higher field requires a higher frequency. Measured with our “Bob” system at 0.9 K.

Longer-term Goals

  • Measure and coherently control the quantum states of atoms on surfaces
  • Use precise atom manipulation to build engineered nanostructures
  • Use atoms as qubits for quantum information protocols
  • Demonstrate quantum sensing with improved sensitivity
  • Extend ESR-STM beyond 3d transition metal atoms on MgO

Near-term Goals

  • Implement pulsed spin resonance
  • Broaden the investigation in double-resonance spectroscopy
  • Demonstrate switchable entanglement between two qubits
  • Demonstrate pulsed electron-nuclear spin double resonance