Countries advance Quantum Microwave Technology to enable future Quantum computer Network and Quantum radar that would render Stealth Aircrafts Obsolete

Currently, the most advanced and promising (actively developed by big companies such as Google, IBM, Intel, etc.) platform for future quantum computing and simulations is superconducting circuits. These typically consist of thin film layers of superconducting materials and Josephson junctions comprising altogether a network of resonators and qubits. They interact precisely by means of microwave photons, and the quantum state of the qubits is read out by measuring these photons. Quantum information processing occurs inside a fridge at milikelvin temperatures, since higher temperatures imply additional thermal photons which destroy coherence.

Proposals for novel quantum information processing techniques often rely on a quantum network, linking together multiple qubits or groups of qubits to enable quantum‐secure communication, novel metrology techniques, or distributed quantum computing. However, microwave frequency photons are difficult to transmit over long distances—typical attenuation in low‐loss microwave cables at 10 GHz is more than 1 dB m−1, which compares very poorly with optical fibres with losses below 0.2 dB km−1 at telecom wavelengths ( 𝜆≈1550nm, 𝑓≈193THz). The advantages of transmitting quantum information over fibers is immediately apparent.

By transmitting information in the optical telecom band, fiber-based quantum networks over tens or even hundreds of kilometers can be envisaged. In the optical regime, impressive technological advances have been reached in the last years, such as the first quantum communication between ground and satellites, as well as the first proof-of-principle experiments in quantum sensing. The experimental progress in this area with optical photons has been astonishing, including a 143 km quantum communication between the Spanish islands of Tenerife and La Palma, a 96 km connection between Sicily and Malta through a submarine cable, or the recent quantum communication and quantum key distribution using satellites, among others.

“In order to connect several quantum computing nodes over large distances into a quantum internet, it is therefore vital to be able to convert quantum information from the microwave to the optical domain, and back,” says Prof. Simon Groeblacher of Delft University of Technology. “This will not only be extremely interesting for quantum applications, but also for highly efficient, low-noise conversion between classical optical and electrical signals.”

However, the advances in the use of microwaves in the quantum regime for technological applications were more gradual than with optical photons. There are several technological challenges which make the control of microwave photons much subtler than optical photons. Conversion between signals in the microwave and optical domains is of great interest, particularly for connecting future superconducting quantum computers into a global quantum network. Many leading efforts in quantum technologies, including superconducting qubits and quantum dots, share quantum information through photons in the microwave regime. While this allows for an impressive degree of quantum control, it also limits the distance the information can realistically travel before being lost to a mere few centimeters.