Nanosized Box to Confine Electrons – Quantum Dot -

Among the atoms that make up the material around us, electrons with a negative charge are captured around a nucleus with a positive charge. Quantum Dot (QD) artificially creates this condition. When the QD, which is a semiconductor crystal in nanometer (one billionth of meter) size, is irradiated with a laser light, electrons get captured, showing discrete energy structure similar to that of an atom (Fig. 1). Each electron confined in the QD and the nucleus constituting the QD has a spin freedom, which is one of the characteristics of a small magnet. The goal of my research is to freely manipulate the spins using light.

Fig. 1 Sample of Semiconductor Quantum Dot (a) and Photo-luminescence Spectrum (b). A discrete Energy Structure Similar to an atom is shown.

 

Creating Nanosized Magnets Using Light

A spin of a nucleus has been studied for a long time, and one of its application technologies, magnetic resonance imaging (MRI), is used today as a useful tool in medical practice. The quantum dot, because of its structural uniqueness, shows interesting behavior in its nuclear spin, one of which is the “formation of nanosized magnets” which I describe below.

Fig. 2 Nuclear Magnetic Field (B_N) vs. Light Intensity. The intensity of light can change the strength of the B_N.

A single QD contains tens of thousands of nuclear spins. Usually, the nuclear spins point in random directions, and therefore on average, they don’t show any spin polarization which induces the characteristics of a magnet. However, when the electrons that have spins oriented in one direction are injected into the QD, a magnetic interaction occurs between the electronic spins and nuclear spins, which then orients the nuclear spins in the same direction as the injected electrons. Repeating the polarization process of electron-nucleus spins aligns several thousands nuclear spins into one direction, leading to the formation of a strong magnet (nuclear magnetic field). In addition, the spin direction of the electrons injected into the QD can be controlled with light, and the strength of the magnetic field can be regulated with the intensity of that light (Fig. 2). The magnetic field affects only the electrons confined in the QD, which means it can be considered a nanosized magnet (nano-magnet). This magnet is expected to function like memory in the quantum information processing field because of its long holding time.

 

What Kind of Devices are You Using in Your Experiments?

We use a spectrometer and a CCD camera to measure the spectrum emitted from a QD sample which is irradiated with laser light. The sample was prepared using a crystal growth apparatus. In addition to lasers, spectrometers and other large devices such as superconductive magnet (Fig. 3 (a)), we use many regular, small devices such as mirrors, lenses, and prisms (Fig. 3 (b)) in our experiments. In addition, we use a positional control system to compare a current image of the sample surface with that at the original position (Fig. 3 (c)) in order to correct any positional misalignment due to vibration and to help us obtain stable signals.

Fig. 3 (a) (b) Devices Used in our Experiments. Precise adjustment of mirrors and lenses is essential to detect faint signals. (c) Sample Surface Image. Positional misalignment due to vibrations can be automatically corrected to the original position.

 

What Interests You in This Field?

Recently, there has been a flurry of research on quantum cryptographic communication and quantum computer which use photons and electrons. And, semiconductor QD is being given special attention as a medium for connecting those photons and electrons. I already mentioned that a nano-magnet could take on a memory function role. Furthermore, by focusing on the QD, we could manipulate the results calculated by the quantum computer that uses electrons and transform that to photon information and then send it to remote areas. We hope that our research can help in the application of quantum information communication and processing, which continues to be pursued all over the world.