Hybrid Quantum Science
The microscopic world of atoms and molecules is described by quantum mechanics. Electrons have both particle and wave properties, and each individual electron exhibits interference effects as a wave. This world is distinct from our everyday macroscopic world, which is represented by Newtonian mechanics and electromagnetism. When semiconductors are processed to sizes of about 10 to 100 nanometers, quantum mechanical properties emerge in electrical conduction and other phenomena. This is the "mesoscopic" world, meaning the intermediate between micro and macro.
A device that confines electrons in a mesoscopic box is called a "quantum dot." Inside that box, the energy of the electron is quantized into discrete values (energy levels), and because it exhibits properties similar to microscopic atoms, it is called an "artificial atom" [1]. Electrons have an attribute called spin, which has two states: up |↑⟩ and down |↓⟩. The spin state in an artificial hydrogen atom, where a single electron is confined in a quantum dot, is a superposition of up and down waves
, and quantum computers utilize this. The smallest unit of a conventional computer is a "bit," which represents all numbers in binary as either "0" or "1." On the other hand, the smallest unit of a quantum computer is a "quantum bit (qubit)," which is a superposition of "0 (|↑⟩)" and "1 (|↓⟩)" as shown in Equation (1). By using N qubits, it is possible to create a superposition of 2^N numbers [1, 2]. What happens in an artificial hydrogen molecule? In addition to up and up (|↑⟩|↑⟩), up and down (|↑⟩|↓⟩), etc.,
states like this are possible. This is an "entangled state" of the spins of two electrons. If we focus on only one electron, its spin is either up or down (each observed with a probability of 1/2). However, there is a strong correlation between their states, such that when the spin of one electron is up, the spin of the other is always down. The creation of this "entanglement" is essential for performing quantum computation.
In recent years, a new field called "Hybrid Quantum Science" has been attracting attention. This research explores new physical properties and functions by combining different quantum systems, such as electron charge and spin, photons, and phonons (quanta of lattice vibrations or vibration modes of tiny mechanical oscillators). While the realization of large-scale quantum computers still requires a long time, the underlying idea is to look at the technologies accumulated toward that goal and utilize them broadly and effectively. I have theoretically researched basic physical properties such as electrical conduction in quantum dots and the Kondo effect. As for hybrid systems, in addition to research on phonon lasers using quantum dots, I have recently been focusing on photocurrent in quantum dots. The spacing between energy levels in a quantum dot corresponds to the energy of photons in the terahertz (THz) range. Currently, the technological development of THz electromagnetic waves is very active. Since a current is generated when THz light is shone on a quantum dot, it can be applied to detectors (Figure (a)). While arranging two quantum dots might seem to double the detection sensitivity, in fact, even higher sensitivity can be expected by utilizing quantum effects. When the wavelength of light is sufficiently longer than the distance between the quantum dots, the absorption of a single photon creates entanglement as shown in Equation (2), increasing the absorption rate of the next photon; this is the operating principle of a high-sensitivity detector (Figure (b)). Many unknown possibilities remain in quantum mechanics.
[1] Mikio Eto, "Quantum Mechanics I" (Maruzen, Parity Physics Textbook Series, 2013), p. 115, p. 132
[2] In addition to electron spin, various quantum systems such as nuclear spin and photons (superposition of two polarization directions) are being researched as qubits. "IBM Q" uses qubits based on superconducting circuits.
(a) Conceptual diagram of photocurrent generated when THz light is shone on a quantum dot. When an electron absorbs a photon, it is excited from a lower energy level (|g⟩) to an upper energy level (|e⟩). Lead wires are connected to the left and right of the quantum dot, and electrons enter and exit through high potential barriers via the "tunneling effect." When there is asymmetry in the tunneling rates ( ΓLg / ΓRg ≠ ΓLe / ΓRe ), a current is generated. (b) Photon absorption rate when two quantum dots are arranged. Cases without "entanglement" between quantum dots (left figure) and with it (right figure). When the wavelength of light is sufficiently longer than the distance between the quantum dots, as shown in the right figure, the absorption of a photon creates an "entangled state" |B⟩ = ( |e⟩|g⟩ + |g⟩|e⟩ ) / √2 from |g⟩|g⟩. The transition rate to |B⟩ becomes twice that of the left figure, and the transition to another "entangled state" |D⟩ = ( |e⟩|g⟩ - |g⟩|e⟩ ) / √2 is forbidden.