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Machines of the future

Qubits – the building blocks of the quantum computer.

Text: Benedikt Vogel

A qubit can store a single bit – the smallest possible unit of digital information – and is the fundamental building block of a future quantum computer. Qubits made of semiconducting materials, such as those being researched in Basel, are among the most promising candidates.

Model of the Basel qubit: Two individual electrons (red) are captured within a quantum dot. Their spin states (arrows) form the information units (qubits). Gold contacts allow the electrons to be held in stable electrical fields. The structure is approximately half a micrometer in size and is embedded in a semiconducting material – in this case, gallium and arsenic atoms (green/violet). An adjoining sensor is used to measure the spin.
Model of the Basel qubit: Two individual electrons (red) are captured within a quantum dot. Their spin states (arrows) form the information units (qubits). Gold contacts allow the electrons to be held in stable electrical fields. The structure is approximately half a micrometer in size and is embedded in a semiconducting material – in this case, gallium and arsenic atoms (green/violet). An adjoining sensor is used to measure the spin.

Like conventional computers, quantum computers also process and store information digitally: information is encoded using a system made up of only two digits (0 and 1). Each of these digits contains one “bit” of information, which can be stored using any technical system that you can switch on and off. In computers, a bit is stored in a switchable circuit that is either on (ON = 1) or off (OFF = 0). Computers break down complex information such as numbers and text into bits, which are then processed using a multitude of very fast on/off switching processes.

ON and OFF at the same time

A future quantum computer would also break complex information down into bits. However, instead of using switchable circuits, it processes and stores this information using a quantum physical system that can adopt the states ON and OFF. Whereas the switchable circuit is either ON or OFF, in quantum physical systems the states ON and OFF exist simultaneously – even if this goes against our everyday experience. A system of this kind, featuring the superposition of two states, has come to be known as a quantum bit (or qubit).

Still, we should not allow ourselves to be dazzled by the trendy name: a qubit on its own also contains just one bit of information – exactly as much as a switchable circuit. The special properties resulting from superposition only come into play after a qubit is coupled with other qubits in a specific manner – namely, using a characteristic phenomenon of quantum physics known as entanglement. It is only when multiple qubits are entangled into groups (quantum registers) that they give rise to highly powerful information- processing systems.

100 million in one square centimeter

Quantum bits could conceivably be produced in several very different ways. “In physics, we are aware of many systems with two precisely defined states that are subject to the rules of quantum physics. Over the last 20 years, therefore, countless methods have been proposed for building quantum bits,” says Daniel Loss, Professor of Physics at the University of Basel. “However, most of these proposals were later abandoned.” This is because physical systems intended to serve as qubits must have a number of special properties in addition to their quantum physical properties: for example, the qubits must be small enough that you can ideally accommodate 100 million of them on a chip with an area of one square centimeter; otherwise, it will not be possible to build a manageable computer. It must also be possible to switch the qubits from ON to OFF and vice versa at high speed – ideally a billion times a second, as is customary with electrical circuits in modern computers.

As these requirements illustrate, designing a functional qubit is a Herculean challenge. Even a quarter of a century after the first experiments, scientists still have a long way to go. Nevertheless, they have made considerable progress since the 1990s – thanks in part to a plan that Loss published in 1998 together with the American physicist David DiVincenzo. In it, the two scientists outlined a concrete method for creating qubits and using them to build a quantum computer.

Electron spin as a circuit component

Today, the two physicists’ publication is the most cited scientific article on quantum computing. It serves as the basis for the study and construction of qubits made of semiconducting materials in the laboratories of top universities and industrial companies around the world. Among the leading research groups are the solid-state specialists led by Loss. Their basic idea is to use an electron spin as a qubit. It is a bold idea – after all, an electron is extremely small, and the magnetic field associated with the spin is extremely weak and thus hard to measure. However, since the spin can only point “up” or “down”, it has exactly two states. Moreover, the electron spin is subject to the laws of quantum physics. This system therefore has the ideal prerequisites to serve as a fundamental building block in a quantum computer.

For an electron spin to be used as a qubit, there must be a reliable way not only to determine but also to switch its direction. The scientists hope to achieve this by using a concept known as the quantum dot. In very simple terms, a quantum dot is a (dedicated) spherical volume, typically with a diameter of one ten-thousandth of a millimeter, and located inside a solid. A free electron (that is, one that is not bound within an atom) is “locked inside” the sphere. The surrounding solid is built up in layers made of two semiconducting materials (such as silicon and germanium) and cooled to a very low temperature – just one tenth of a degree above absolute zero – and the free electron is held in place using electrical fields. In this configuration, the electron spin can be switched “up” and “down” electrically – and can therefore be used to store one of the smallest units of information (0/1).

Phenomenal power

Twenty years ago, creating a qubit in the form of a quantum dot was merely a bold vision. The intervening years have seen it become a reality in a variety of material systems and configurations. Now, it also enjoys the backing of companies such as Intel. To use the electron spin inside a quantum dot for data processing, two conditions must be met: the superposition of the two spin states (“up” and “down”) must hold for as long as possible, and it must be possible to change the direction of the spin very quickly.

Researchers have managed to maintain the superposition over the space of one millisecond and, in this short time, to perform a million switching processes using electric fields. “As a result, qubits operate at a clock frequency in the gigahertz range, as we are used to seeing in modern computers,” says Loss. “With the quantum dots, we’re also able to entangle multiple qubits, which is a prerequisite for combining large numbers of them in the future to create a computer. This computer’s phenomenal performance would stem from its ability to perform arithmetic operations in parallel as a result of the quantum physical properties of the qubits.”

Three further candidates

In addition to quantum dots made of semiconducting materials, scientists are currently discussing and testing three further concepts for the production of qubits: companies such as IBM and Google want to make qubits from superconducting materials – that is, from substances that conduct electricity with zero resistance when cooled to a very low temperature. These qubits allow relatively fast switching but are currently around 1,000 times bigger than quantum dots made of semiconductors.

Mehrere Forschungsgruppen wollen Qubits auf der Grundlage gefangener Ionen (etwa Kalziumionen) herstellen. Diese haben den Vorteil, dass sie nochmals hundertmal kleiner sind als Quantenpunkte. Sie lassen sich allerdings nur träge schalten und können nicht kompakt zu Qubit-Clustern verbaut werden. Dies, weil die Ionen sehr weit voneinander platziert werden müssen, damit sie sich nicht gegenseitig unkontrolliert beeinflussen. Eine vierte Gruppe sind topologische Qubits, eine Kombination aus dem Halbleiter- und dem Supraleiter-Ansatz. Das Konzept steht noch am Anfang, aber Microsoft steckt viel Geld in die entsprechende Forschung.

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