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Quantum Dots

An atom has a nucleus and electrons in orbitals around the nucleus. We can create artificial atoms known as quantum dots using our ability to manipulate electrical fields and create material barriers to the movement of electrons.

Real atoms

The nucleus of an atom creates an electrical potential centered around itself that attracts the electrons, with the lowest energy at the center. One of the central problems with classical mechanics, which drove researchers to create quantum mechanics, is the fact that the electron doesn’t fall into that center well, but instead maintains its distance from the nucleus.

It turns out that the electrons can only take certain discrete energy levels that correspond to standing waves around the nucleus. The electron’s position is a quantum probability amplitude, rather than a single fixed point, and is known as an orbital. The shape and size of these orbitals depend on the strength of the electrical potential created by the nucleus.

Artificial atoms

Of course, with real atoms, we have only a very limited ability to influence the electrical field. But if we can confine one or a few electrons in ways that are similar to the electrons in ordinary atomic orbitals, we can create artificial atoms known as quantum dots. We confine the electrons using a combination of the material structure and electrical fields created using voltages. An electron won’t normally move past the physical edge of the device. With a junction made of certain types of materials, it takes substantial energy to cross the boundary. When a positive voltage is applied to a wire, it attracts electrons, and conversely a negative charge repels electrons. Using these characteristics carefully, we can confine an electron tightly in either a disk-shaped area or a small volume.

Quantum dots can be either self-assembled or gate-defined. Self-assembled quantum dots are made of materials that clump together. In one approach, small amounts of a certain type of semiconductor are grown on top of a substrate (chip surface) of another type of semiconductor, and differences in the spacing between the atoms cause the new material to bead up, like water on a good rain jacket. In the vertical direction, our electron will then be confined to the boundary where the materials meet, and horizontally, it will be limited to the disk-shaped footprint of the dot.

Gate-defined quantum dots are made like computer chips, with microscopic wires laid on top of a material such as silicon using a process known as photolithography. But there is an additional twist: different layers of material are deposited before the wires are created on top. The electrons are then confined in the vertical dimension by the material boundary, and in the horizontal directions by electrical fields created by voltage on the wires.

Qubits

So how do we use this ability to confine individual electrons to create a qubit? With a single electron, we can use the spin of the electron as our qubit, as we discussed in the video on electron spin as our state variable. Because spin is a magnetic effect, we need to use magnetic fields to define “up” and “down” for the electron, and to control its spin. For some designs, we can also control the spin optically, using laser pulses.

Changing the spin of an electron requires careful dynamic control of magnetic fields, which tends to be slow and is difficult to target to a single quantum dot. Quantum dot designs have evolved over the last two decades to work around this problem. Several proposals actually use more than one quantum dot to hold a qubit, sometimes with more than one electron. If the magnetic field differs slightly from place to place, this difference can be used to control the spin. With two electrons, we can also define our qubit to be the difference in their states. With a careful choice of qubit representation, we can eliminate the need to modify the magnetic field, instead controlling the exchange of electrons between two dots to execute single-qubit and two-qubit gates.

Strengths

Quantum dots rely on fabrication techniques that have been developed over a half century of silicon photolithography. The ability to couple some designs to photons makes them attractive for communications, as well. Especially for the exchange-oriented designs, the time to execute a gate is very fast, meaning that performance will be excellent if the system can be scaled up.

The biggest weakness in quantum dots has been memory lifetime, which is hurt by stray electrons, stray magnetic fields, and interaction with the nuclei of nearby atoms. Recent work has made tremendous progress in this area. Also, like some other technologies, quantum dots require temperatures near absolute zero, so the experimental setup is complex, if well understood. Finally, while in theory it is possible to put many quantum dots on a single chip, the wires necessary for controlling them clutter the arrangement, and finding an architecture for many qubits has been challenging.

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This article is from the free online course:

Understanding Quantum Computers

Keio University

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