Silicon quantum computer research- Pathway to ultimate semiconductor devices
Silicon semiconductor technology plays a pivotal role in todayfs IT society. Itfs advancement based on miniaturization and integration has lead to development of ever faster and smaller IT devices, e. g., cell phones, PADs, IC tags, etc. Figure 1 shows the roadmap of silicon technology based on the Moors Law, the law that has successfully guided the silicon technology in the past 30 years. According to this figure, which shows a characteristic length scale needed to store a bit (0 or 1) of information, the size of one-bit will reach the size of an atom by year 2030 !
We have shown theoretically that such gmission impossibleh is possible by utilizing stable isotopes of silicon. Naturally available silicon is made of Si-28, Si-29, and Si-30 stable isotopes of which only Si-29 is a magnet possessing a nuclear spin. We can also introduce to silicon magnetic nuclei such as phosphorus (P-31), which will accompany an electron that is also a magnet. Letfs define the states g0h and g1h of the bit information by the direction of the magnet pointing up and down, respectively. If one can manipulate the pointing direction of these nuclear and electron magnet at his will, there goes a computer based on the individual nucleus and electron ! Figure 2 shows the concept of such a computer.
It is easy to say this but actual realization in the lab is very challenging since all the phenomena at the atomic level obeys the law of quantum mechanics rather than classical mechanics. This means, unlike the standard bit in todayfs computer which takes either g0 or 1h, the single nuclear or electron bit (hereafter refers to as a gquantum bit (or qubit)h) takes a strange state g0 and 1.h A computer based on quantum bits is known as a quantum computer.
The quantum computer research is truly exciting because it promotes the state-of-the-art interdisciplinary research. At first, we think about the devices design (architecture) using individual atoms based on the combination of the advanced computer science and solid-state physics knowledge. Then we work in the labs to think about how to put each atom in the way we designed by the bottom-up approach. Figure 3 shows a snapshot of silicon atoms taken by scanning-tunneling-microscope (STM) taken in our laboratory. Dots representing individual atoms are placed in the specific patterns to control the position of the nuclear magnet qubits. You can see that this is nanotechnology at its best. Then we manipulate these magnetic qubits to perform quantum computation using nuclear and electron spin resonance spectrometers. This part of the research requires the advanced knowledge and experimental skills in solid-state and quantum physics. At the end, we have to read-out the state of each qubit 0 or 1. The magnetic moment of each qubit is so weak that it is very challenging to measure it directly. One path we are taking is to measure the polarization of individual photon, a particle of light, which carriers the quantum information of the P-31 nuclear magnet. You can see that we have to be as imaginative as possible to do whatever it takes to realize each element of quantum computing.
At the end of the road, an important question remains; what good is the quantum computer for our society? A legendary physicist Richard Feynman pointed out in 1982 that it should be useful for quantum simulation. After all everything we see is made of atoms whose behavior is governed by quantum mechanics and, therefore, computer simulations of everything from revealing the origin of the universe to making of new drugs eventually should employ quantum computers rather than todayfs classical computers operating of classical physics 0 or 1. A quantum software (algorithm) allowing for the quantum simulations are yet to be developed but quantum computer scientists all over the world is working for it. Our research teamfs wildest dream is to develop quantum-classical hybrid silicon quantum computer, which works at the room temperature (Fig. 4). It may take another 30 years but someone has to do it.