Researchers Create Stable Quantum Gate as Basic Element for Quantum Computer

A team of researchers from the University of Konstanz, Princeton University, and the University of Maryland have developed a stable quantum gate for two-quantum bit systems made of silicon, a milestone on the path to the quantum computer.

The quantum gate can perform all essential standard operations of the quantum computer. The electron spin of individual electrons in silicon is used as the standard storage unit ("quantum bits"). The research results were reported ahead of print in the online edition First Release of Science on 7 December 2017.

Few more years will be required before the first quantum computers arrive at department stores. Even today, however, it has become obvious that the quantum computer will be a huge leap in computer technology. The quantum computer will be more efficient and will be able to solve issues where existing computers hit a wall.

However, the quantum computer reacts a lot more sensitively to outside disturbances than a conventional machine. Therefore, a principal goal is to develop stable "quantum gates" - the standard "building block" of the quantum computer.

Researchers from the University of Konstanz, Princeton University and the University of Maryland have succeeded in developing stable quantum gates for two-quantum bit systems. Their quantum gate uses separate electrons in silicon to store the data ("quantum bit") and they can precisely manipulate and read out the interaction of two quantum bits. This way the experiment includes all essential standard processes of the quantum computer.

From Electron to Quantum Bit

Just as a conventional digital computer utilizes the "bit" with the values of either zero or one as the standard unit for all the calculation processes, a quantum computer, too, requires a standard storage unit: the quantum bit. The difference is that the quantum bit is not restricted to two states (zero and one), but can exist in numerous states at once and is thus a lot more complex in its implementation than a simple digital system.

Scientists have derived several ideas for technically realizing a quantum bit, for instance using superconducting systems or ions. The researchers from Konstanz, Princeton, and Maryland, however, use the electron spin, the basic angular momentum of a single electron, in the semiconductor material silicon as the foundation of quantum bits. The electron's direction of rotation matches the zero and one values of the digital bit, but in its exact quantum state, the electron is able to hold more data than just a simple zero or one.

The team’s first accomplishment was thus to extract a single electron from numerous atoms of a silicon piece. "That was an extraordinary achievement by our colleagues from Princeton", says physicist Professor Guido Burkard, who coordinated the theoretical study in Konstanz. The researchers use a mixture of electromagnetic attraction and repulsion to isolate a single electron from the electron bunch. The separated electrons are then aligned precisely and each is embedded in a kind of "hollow", where they are kept in a floating state.

The subsequent challenge was to create a system to manipulate the angular momentum of separate electrons. Konstanz physicists Guido Burkard and Maximilian Russ have created the following technique: a nano-electrode is applied to each electron. Using a magnetic field gradient, the physicists can develop a position-dependent magnetic field the electrons can be independently accessed with, thus enabling the team to regulate the angular momentum of the electrons. This way they have built stable one-quantum bit systems to store and read out data in the form of electron spins.

The Step Towards the Two-Quantum Bit System

One quantum bit, however, is not sufficient to produce the standard switching system of a quantum computer. To achieve that, two quantum bits are needed. The important step the Konstanz researchers took towards the two-quantum bit system was to connect the states of two electrons. Such a link makes it possible to build basal switching systems with which all standard operations of the quantum computer can be done. For instance, the system can be programmed in such a way that an electron rotates only when its neighboring electron has a spin in a prearranged direction.

This meant that the Konstanz team had to develop a stable system to connect the spins of two separate electrons. "That was the most important and difficult part of our work", says Guido Burkard, who designed and planned the technique along with team member Maximilian Russ. They designed a switching system that manages the angular momentum of two electrons in inter-dependence. An extra nano-electrode is positioned between the two "hollows" in which the silicon electrons are floating. This electrode manipulates the coupling between the two electron spins. With this technique, the physicists have realized a stable and functional standard processing unit of a quantum computer. Fidelities for single quantum bits are over 99%, and about 80% for two interacting quantum bits - considerably more stable and more accurate than in earlier attempts.

Silicon - A "Silent Material"

The quantum gate’s base material is silicon. "A magnetically very silent material with only a low number of own nuclear spins", Guido Burkard recaps the benefits of silicon. It is imperative that the atomic nuclei of the selected material do not have too many spins, that is, intrinsic angular momentum, which could hinder with the quantum bits. Silicon, with a share of around 5%, has a very low spin activity of its atomic nuclei and is thus a particularly ideal material. Another benefit: silicon is the basic material of semiconductor technology and therefore well researched. The researchers can therefore gain from many years of experience with the material.