A new approach removes magnetic noise for the rapid optical control of a coherent hole in a microcavedity

Quantum technologies, devices that work by taking advantage of quantum mechanical effects, could surpass conventional technologies in certain areas and parameters. The so-called spin (that is, the intrinsic angular moment) transported by quantum particles is at the heart of the functioning of quantum systems, as it can store quantum information.

However, to reliably share quantum information on a network, the towers must be linked to photons (that is to say particles of light). For decades, quantum engineers and physicists have tried to design approaches to interface towers and photons.

A strategy to achieve this is to use quantum points, semiconductive structures on a nanometric scale which can trap electrons or holes in distinct energy levels. When placed in carefully modified optical resonators called microcavities, these structures can generate individual photons. However, ensuring that the consistency of spins is not disturbed by magnetic noise from nearby nuclear spins and thus facilitating the preservation of quantum information over time has proven to be difficult.

Researchers from the University of Basel and Ruhr-Universität Bochum have recently introduced a new approach to suppress magnetic noise and allow the rapid optical control of a coherent hole in a microcavity. Their approach, introduced in an article published in Nature physicsBases on a combination of laser pulses and a technique known as nuclear spin-spin, which removes magnetic noise.

“A quantum point can be used as an effective source of unique photons,” said Mark R. Hogg and Richard J. Warburton, first author and principal author of the newspaper. “There are several ways to proceed. We have been the pioneers of a technique – the quantum point lies in an” open microcavedity “and end -to -end efficiency is currently the highest with this approach. The unique photons are important in quantum communication and quantum IT. However, tangled photons are better.”

A way to transform a single source of photons into a source of tangled photons is to add an electron or a hole to a quantum point, then take advantage of its rotation. To achieve this, you must first carry out the tangle between a rotation and a photon, then rotate the rotation and repeat this process, finally reaching a tangle with spin-photon photons.

“This process can be repeated to create so -called cluster states,” said Hogg and Warburton. “To implement this idea, a crucial step is to add the control of the spin to our unique source of photons. This is what we decided to try. We did not know if it would be possible. There seemed to be serious problems.

“Is spin control compatible with open microcavity by designing a device with a very specific resonance frequency? In addition, spin coherence in quantum points is seriously limited by magnetic noise in the semiconductor matrix. Would a spectacle?”

While Hogg, Warburton and their colleagues had not yet had clear answers to these questions, they have in any case decided to carry out their experience and found that their configuration worked. First, they trapped a single hole in a quantum point based on a phenomenon known as the Coulomb blockage, which guarantees that a load of load is locked at each biases tension.

“In practice, all we have to do is choose the right bias,” said Hogg and Warburton. “The second step is to initialize the spin in one of its basic states,” up “or” downwards “. We do it with an old technique in atomic physics called” optical pumping “. Rotation now points either to the North Pole, or the South Pole in the Bloch sphere.

To cause a so-called Raman transition, the researchers used two lasers with a frequency difference which corresponded to the fractionation of the spin (that is to say the Zeeman frequency). This allowed their system to create a “bridge” from one spin state to another.

In addition, lasers were granted to a frequency slightly lower than the resonance of the system, a strategy known as the red detaining. This approach, which is well established, guarantees that lasers do not directly excite the system, but rather, they stimulate the so-called Raman process.

“However, it was not clear at first if this process would work in our case,” said Hogg and Warburton. “The quantum point lies in a narrow strip optical cavity to ensure that the quantum -point photons go where we want them: in the cavity of the place where they escape and go to our detector. Raman lasers are far in frequency of this resonance. But it turns out that everything works well, still in frequency with a different scale with the change of frequency of Raman lasers.”

When the researchers turned the rotation around the Bloch sphere, they found that the magnetic noise resulting from nuclear spins in their system was considerably reduced. Although there are other strategies known for achieving it in systems with an electron spin, so far, it is not clear if it could also be carried out when using a hole rotation.

“We have discovered that this” environmental engineering “works very well for a hole rotation,” said Hogg and Warburton. “Our study has two main achievements: first, we add spin control to a single source of photons at the cutting edge of technology. Second, we extend the consistency of the spin by preparing the environment in a low noise state. “

These recent works could soon open up new possibilities for the realization of quantum cluster states with high efficiency and high fidelity in tangle. In addition, the methods they have used could be used by other research teams to obtain rapid optical control of individual holes towers, as opposed to electron towers, allowing the use of these towers to store quantum information.

“The physical questions we plan to answer now include: how was the hole exactly in the quantum point reduces noise in nuclear towers? And how far can we go with this approach?” Adding Hogg and Warburton.

“In addition, we plan to use everything we have achieved so far – an effective collection of photons, spin control, improved spin consistency – to create tangled photons, that is to say cluster states.

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