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Quantum technologies, devices that work by leveraging quantum mechanical effects, could outperform classical technologies in some fields and settings. The so-called spin (i.e., intrinsic angular momentum) carried by quantum particles is central to the functioning of quantum systems, as it can store quantum information.

To reliably share quantum information across a network, however, spins need to be linked to photons (i.e., particles of light). For decades, engineers and quantum physicists have thus been trying to devise approaches to interface spins and photons.

One strategy to achieve this entails the use of quantum dots, nanoscale semiconductor structures that can trap electrons or holes in distinct energy levels. When placed in carefully engineered optical resonators known as microcavities, these structures can generate individual photons. Nonetheless, ensuring that the coherence of spins is not disrupted by magnetic noise originating from nearby nuclear spins and thus facilitating the preservation of quantum information over time has so far proved challenging.

Researchers at the University of Basel and Ruhr-Universität Bochum recently introduced a new approach to suppress magnetic noise and enable the fast optical control of a coherent hole spin in a microcavity. Their approach, introduced in a paper in Nature Â鶹ÒùÔºics, relies on a combination of laser pulses and a technique known as nuclear spin cooling, which suppresses magnetic noise.

"A quantum dot can be used as an efficient source of single photons," Mark R. Hogg and Richard J. Warburton, first author and senior author of the paper, respectively, told Â鶹ÒùÔº. "There are several ways to go about this. We have pioneered one technique—the quantum dot resides in an 'open microcavity,' and the end-to-end efficiency is currently the highest with this approach. Single photons are important in quantum communication and photonic quantum computing. Yet entangled photons are better."

One way to turn a single photon source into a source of entangled photons is to add an electron or hole to a quantum dot and then leverage its spin. To achieve this, one first needs to realize the entanglement between a spin and a photon, then rotate the spin and repeat this process, ultimately achieving spin-photon-photon entanglement.

"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 spin control to our single photon source. This is what we decided to try. We weren't sure if it would be possible. There seemed to be some serious problems.

"Is spin control compatible with the open microcavity by designing a device with a very specific resonance frequency? Further, the spin coherence in quantum dots is severely limited by the magnetic noise in the semiconductor matrix. Would this be a showstopper?"

While Hogg, Warburton and their colleagues did not yet have clear answers to these questions, they set out to perform their experiment anyway and found that their setup worked. Firstly, they trapped a single hole in a quantum dot relying on a phenomenon known as Coulomb blockade, which ensures that a charge state is locked at every bias voltage.

"In practice, all we have to do is to choose the right bias," explained Hogg and Warburton. "The second step is to initialize the spin into one of its basis states, either 'up' or 'down.' We do this with an old technique in atomic physics called 'optical pumping.' The spin now points either to the north pole or the south pole in the Bloch sphere. We then rotate the spin to lie along any point we like on the Bloch sphere. To do this, we use another old technique from atomic physics, the Raman process."

To prompt a so-called Raman transition, the researchers used two lasers with a difference in frequency that matched the spin splitting (i.e., Zeeman frequency). This allowed their system to create a "bridge" from one spin state to another.

In addition, the lasers were tuned at a frequency slightly lower than the system's resonance, a strategy known as red detuning. This approach, which is well-established, ensures that the lasers do not directly excite the system, but rather, they drive the so-called Raman process.

"However, it was unclear at the start if this process would work in our case," said Hogg and Warburton. "The quantum dot resides in a narrowband optical cavity to make sure that the quantum dot photons go where we want them to: into the cavity from where they leak out and head to our detector. The Raman lasers are far away in frequency from this resonance. But it turns out that it all works fine nonetheless, albeit with a different scaling with the frequency shift of the Raman lasers."

When the researchers rotated the spin around the Bloch sphere, they found that the magnetic noise arising from nuclear spins in their system was significantly decreased. While there are other known strategies to achieve this in systems with an electron spin, so far, it is not clear whether it could also be achieved when utilizing a hole spin.

"We found out that this 'environment engineering' works really well for a hole spin," said Hogg and Warburton. "Our study has two main achievements: First, we add spin control to a state-of-the-art single photon source. Second, we extend the spin coherence by preparing the environment in a low-noise state."

This recent work could soon open new possibilities for the realization of cluster quantum states with high efficiency and attaining a high entanglement fidelity. In addition, the methods they employed could be used by other research teams to achieve the fast optical control of individual hole spins, as opposed to electron spins, thus enabling the use of these spins to store quantum information.

"The physics questions we plan to answer now include: how exactly does the hole spin in the quantum dot reduce the noise in the nuclear spins? And how far can we go with this approach?" added Hogg and Warburton.

"In addition, we plan to use everything we've achieved so far—efficient collection of the photons, spin control, enhanced spin coherence—to create entangled photons, i.e., cluster states. First steps have already been taken in this direction by other groups working on quantum dots. Our opportunity is to improve the performance metrics towards a level where the cluster states are good enough to use as so-called resource states in photonic quantum computing, which will be a big and complex task."