Quantum computing

[prime]

Quantum communication follows a similar idea, but individual photons act as the information carriers. A zero or one is encoded through the direction of the photon's polarization (i.e., their orientation in the horizontal and vertical directions or in a superposition of both states). Because photons behave according to quantum mechanics, their polarization cannot be measured without leaving detectable traces. Any attempt to intercept the message would be exposed.

Teleportation requires the photons to be nearly identical in properties such as timing and color. Producing such photons is hard because they come from separate sources.

At the University of Stuttgart, the researchers successfully teleported the polarization state of a photon from one quantum dot to a photon produced by a second quantum dot. One dot emits a single photon and the other generates an entangled photon pair. "Entangled" means the two photons share a single quantum state even when physically apart. One photon from the pair travels to the second quantum dot and interacts with its photon. When the two overlap, their superposition transfers the information from the original photon to the far-away partner of the entangled pair.

A key element of this achievement was the use of "quantum frequency converters," devices that adjust small frequency mismatches between photons.

https://www.sciencedaily.com/releases/2025/11/251129044516.htm

[Vishnus_Strongest_Abo]

New paper shows coherence improvements of superconducting 2-D transmon qubits of up to 1.68 milliseconds via changes in the substrate to that of silicon; applicable to current architectures and gate schemes.

"Materials improvements are a powerful approach to reducing loss and decoherence in superconducting qubits because
such improvements can be readily translated to large scale processors. Recent work improved transmon coherence by
utilizing tantalum (Ta) as a base layer and sapphire as a substrate [1]. The losses in these devices are dominated by
two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics [2], indicating that
both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate
with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged
quality factors (𝑄) exceeding 1.5 × 107, reaching a maximum 𝑄 of 2.5 × 107, corresponding to a lifetime (𝑇1) of up to
1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved,
low-contamination junction deposition to achieve Hahn echo coherence times (𝑇2E) exceeding 𝑇1. We achieve these material
improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum
control gates. We demonstrate single qubit gates with 99.994% fidelity. The Ta-on-Si platform comprises a simple
material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale
quantum processors."

https://arxiv.org/pdf/2503.14798

Curious if this can scale to multi-qubit CNOT operations in the future. It would make Steane Codes applicable for toy programs.