Strong Coupling Through Spin Trio

spin qubit
A spin trio of electrons trapped in quantum dots (red). Quantum-mechanical tunneling between the quantum dots produces a dipole moment that strongly couples to the electromagnetic wave of a resonator (yellow). (Graphic: ETH Zurich / Andreas Landig)

In order to make qubits less susceptible to interference for quantum computers, it is preferable to use the spin of an electron, for example. ETH researchers have now developed a method by which such a spin qubit can be strongly coupled to microwave photons.

Researchers from several professorships at ETH Zurich have now shown, with the help of theoretical physicists at the University of Sherbrooke in Canada, how to circumvent this problem. They found a way to couple a microwave photon to a spin qubit in a quantum dot.

Qubits with charge or spin

In quantum dots, electrons are first captured in semiconductor structures only a few nanometers in size, which are cooled to less than one degree above absolute zero. The logical values 0 and 1 can now be realized in two ways. Either one defines a qubit by having the electron on the left or right side of a double quantum dot, or by the spin of the electron pointing up or down.

In the first case we speak of a charge qubit, which strongly couples to electromagnetic waves due to the electrical charge transfer. A spin qubit, on the other hand, can be thought of as a tiny compass needle pointing up or down. Like a compass needle, the spin is magnetic and therefore does not couple to electrical fields but to magnetic fields. The coupling of the spin qubit to the magnetic component of electromagnetic waves is much weaker than that of a charge qubit to the electrical component.

Three spins for stronger coupling

As a result, a spin qubit is on the one hand less susceptible to interference and retains its coherence (on which the mode of operation of the quantum computer is based) over a longer period of time. On the other hand, it is also much more difficult to couple spin qubits to each other over long distances by means of photons. To make this possible anyway, the working group uses a trick, as Jonne Koski, postdoctoral fellow in the group of ETH Professor Klaus Ensslin, explains: “By using not one, but three spins for the realization of the qubit, we can Advantages of a spin qubit combine with those of a charge qubit. »

In practice, three quantum dots are produced on a semiconductor chip, which are close to each other and can be controlled by means of tiny wires by applied voltages. In each of the quantum dots, electrons can be trapped with the spin oriented up or down. Through one of the wires, the spin trio is also connected to a microwave resonator. The voltages at the quantum dots are now set so that each of the quantum dots contains one electron and the spins of two of the electrons are in the same direction, the third in the opposite direction.

Charge shift by tunneling

In addition, according to the rules of quantum mechanics, the electrons can, with a certain probability, tunnel back and forth between the quantum dots. As a result, two of the three electrons may temporarily be in the same quantum dot, whereas one remains empty. In this constellation, the electric charge is now unevenly distributed. This charge shift, in turn, creates an electrical dipole that can strongly couple to the electric field of a microwave photon.

The ETH scientists were able to clearly demonstrate this strong coupling by measuring the resonant frequency of the microwave resonator. They observed how the resonance of the resonator was split by the coupling to the spin trio. From the data, they were able to deduce that the coherence of the spin qubit was retained over more than 10 nanoseconds.

Spin trios for quantum bus

The researchers are confident that with this technique, a transmission path for quantum information between two spin qubits (a so-called quantum bus) can be realized soon. “For this we need to place two spin trios at both ends of the microwave resonator and show that the qubits are then coupled together via a microwave photon,” says lead author Andreas Landig, a doctoral student in Ensslin’s group. This would be an important step towards a network of spatially distributed spin qubits. In addition, the researchers emphasize that their method can be easily transferred to other materials such as graphene and is therefore very versatile.

This work was part of the National Research Program Quantum Science and Technology ( NCCR QSIT) carried out. Scientists from the professorships of Klaus Ensslin, Thomas Ihn, Werner Wegscheider and Andreas Wallraff were involved by ETH Zurich.

Source : ETH Zurich