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In order to make qubits less susceptible to interference for quantum computers, it is best to use the spin of an electron, for example. Researchers at ETH Zurich have now developed a method by which such a spin qubit can be strongly coupled to microwave photons
Quantum computers calculate quantum quantum states with quantum or qubit bits. atoms or electrons, for example. logical values "0" and "1" can assume. To merge many of these qubits into a powerful quantum computer, you have to couple them together over distances of a few millimeters or even several meters. This can be achieved, for example, quite similar to a radio antenna, via the charge transfer by an electromagnetic wave. However, this coupling also exposes the qubit to disturbing influences coming from undesirable electric fields, which seriously degrades the quality of the logical operations qubit.
Researchers from several Chairs at ETH Zurich have shown with the support of theoretical physicists from the University of Sherbrooke in Canada how to handle this problem. They found a way to couple a microwave photon to a spin qubit in a quantum dot
Qubits with charge or spin
The first case is called a charge qubit, which strongly couples with the electromagnetic waves due to the transfer of electric charge. A spin qubit, on the other hand, can be thought of as a small compass needle pointing up or down. Like a compass needle, the spin is magnetic and therefore does not couple to electric fields but to magnetic fields. The coupling of the spin qubit to the magnetic part of the electromagnetic waves is much lower than that of a charge qubit to the electric fraction
Three spins for a stronger coupling
This makes a spin qubit less sensitive to interference and, on the other hand retains its coherence (on which rests the operation of the quantum computer) over a longer period. On the other hand, it is also much more difficult to couple the spin qubits to each other over long distances by means of photons. To make this possible, the workgroup is using a trick, as Jonne Koski, a postdoctoral fellow at ETH's Klaus Ensslin group, explains: "Not using one, but three spins for the realization of the qubit, we can The advantages of a spin qubit combine with those of a qubit load. "
In practice, three quantum dots are produced on a semiconductor chip, close to each other and controllable by tiny wires by applied voltages. In each of the quantum dots, the electrons can be trapped with the spin up or down. Through one of the wires, the spin trio is also connected to a microwave resonator. The quantum dot voltages are now adjusted so that each of the quantum dots contains an electron, and the spins of two of the electrons are in the same direction, the third in the opposite direction.
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