Control over the atomic spins within certain molecules by NMR technique

Linked project

• This project is conducted by the same people as 👉 Optical control of TMDCs valley pseudospin qubits

Group members

• XU ZIZHOU (Bensen)
  • Matric Number: A0229645W
  • Email: zizhou_xu@u.nus.edu
• CHU WENHAO
  • Matric Number: A0228747R
  • Email: e0675585@u.nus.edu

Description

• NMR has been the workhorse for the experimental implementation of quantum protocols, allowing exquisite control of systems up to seven qubits in size. However, there exists some experimental limitations in terms of the cross-talk, coupled evolution, instrumental errors and so on. Thanks to the current advanced pulse techniques, we can reduce these influences and extend this technique to a new stage that the experimental limits can be neglected. In this experiment, we try to use composite pulses to compensate RF field strength variations and frequency offsets.
• Introduction of NMR:The phenomenon of nuclear magnetic resonance is based on the fact that nuclei of certain elements possess a spin angular momentum and an associated magnetic moment.some well-known magnetic nuclei, such as $^{1}H$ and $^{31}P$ were able to absorb radio frequency energy when placed in a magnetic field of a strength that was specific to the nucleus. Upon absorption, the nuclei begin to resonate and different atoms within a molecule resonated at different frequencies. For example, when such neclei are placed in a magnetic field, they can adopt one of a number of quantized orientations,each orientation corresponding to a particular energy level.Especially, The orientation with the lowest energy is the one in which the nuclear magnetic moment is most closely aligned with the external magnetic field while the orientation with the highest energy is the one in which the nuclear magnetic moment is least closely aligned with the magnetic field.
• Quantized orientations:It is a fundamental fact of physics that a spinning charged body produces a magnetic moment. Since a nucleus is positively charged, if it then has a spin angular momentum, $P$, its spinning will result in the rotation of the positive charge which may be compared to a current flowing in a circle. This would produce a magnetic field parallel to the spin axis, and the nucleus would have a magnetic moment,In quantum mechanics, the angular momentum P is given by the relationship

Method

Principle

We aim to use the C-shaped electromagnet that we build, to send magnetic pulse to the solid sample, which is between the poles of the magnet. Hopefully we could observe the reaction of the sample to see if we can control the electron spin direction within it.

C-shaped Electromagnet

Experimental setup

Fabrication of C-shaped electromagnet

• The steel core of the electromagnet we built is made of non-grain oriented electrical steel, by using laser beam, we cut the U-shaped and rectangular pieces out of the steel plate (to form the C-shaped core later). After that, we collected the U-shaped pieces and used screws to cramp all the pieces together, forming a 40mm×40mm U-shaped core.

non-grain oriented electrical steel

Steel cutter

Laser cutter

• Next, we built an acrylic plastic holder to hold the copper wire, we used the manual wrapping machine to wrap the copper coils around the pillar of the plastic holder. The number of coils required for generating 0.5 Tesla is based on the formula below:

1. • $B=I\cdot \mu _{0}{\frac {\mu _{r}}{L+d\cdot \mu _{r}}}$
     • $\mu _{0}$ is the permeability of free space
     • $\mu _{r}$ is the relative permeability
     • $L$ is the length of the magnet
     • $d$ is the gap between two poles
     • $B$ is the magnetic flux density (Tesla or $Wb/m^{2}$)
   • According to the calculation, the number of coils of on each side of the magnet should be 250 round.

Copper coil wrapping

• Then we built another plastic holder to stack the rectangular piece on the U-shaped steel core, creating the C-shaped core, by drilling holes on the top of the acrylic holder, we can adjust the distance between poles of the magnet. (Note that the gap between two poles should be small, as long as suitable enough to put sample in)

Final form of the magnet

• Lastly, we measured the resistance of the coils, and sent in voltages around 20 Volts to generate 0.05 Tesla out of the magnet (measured by Gauss Metre)

Resistance measured

Procedure

1. Put our sample within the gap of the C-shaped electromagnet
2. Change the input radio frequency pulse to control the spin of the electrons within sample
   
   
3. Use hall probe connected to sample to readout the result

Results

• Just finished the fabrication of electromagnet, will continue using it to control the spins within solids.