Background: Atomic Force Microscopy

Figure 1. Q-plus force sensor equipped in QNS. Two wires read AFM signal and metal tip is connected separately for STM.

The AFM is a cousin to the STM which uses the force between the surface and the tip rather than a tunneling current. This has the huge advantage that AFM can be performed on insulating materials since no current has to flow. Thus AFM has much broader applicability in nanoscience than STM. However, nobody has achieved a quantum-coherent measurement with AFM yet, QNS wants to change that!

STM needs only atomically sharp metal needle to sense the tunneling current but AFM needs more complicated sensor which measures atomics force. QNS uses a single pronged tuning fork as a force sensor, which vibrates at a given frequency naturally all the time without any interaction.

Figure 1 shows the force sensor equipped in QNS. Once the atomic force is applied to the sensor, the natural frequency is changed proportional to the magnitude of the force.

Figure 2 shows the frequency change increases in negative value as more force is applied when the tip gets closer to the Fe atom on MgO. When metal needle is used as an AFM tip, we can operate AFM and STM simultaneously. Refined scanning conditions, AFM can achieved similar atomic resolution as STM.

There exist several kinds of forces depending on the nature of the tip-sample interaction. Long range attractive force is widely used for common non-contact AFM and repulsion force by contact is used for contact AFM. Moreover, forces given by electric and magnetic interaction etc. can also be detected by AFM. QNS is focusing on force come from spin-spin interaction using spin-polarized tip, which enables to measure the spin state of single atoms or molecules as the similar way to spin-polarized STM.

Combined with insulator accessibility and spin sensitivity using AFM, QNS will explore single atom or molecule qubits on insulators to understand the decoherence by the interactions with surrounding electrons more precisely. It will open a new opportunity to implement full quantum coherent manipulation of qubits with longer quantum coherence time when combined with electron spin resonance (ESR) technique.

Figure 2. Frequency shift measured as a function of distance from a tip to an Fe atom on MgO. As the tip gets closer to the Fe atom, more force applied to the tip gives more frequency shift from natural frequency of the force sensor.