Application of quantum microwave photonics technology

Application of quantum microwave photonics technology

Weak signal detection
One of the most promising applications of quantum microwave photonics technology is the detection of extremely weak microwave/RF signals. By utilizing single photon detection, these systems are far more sensitive than traditional methods. For example, the researchers have demonstrated a quantum microwave photonic system that can detect signals as low as -112.8 dBm without any electronic amplification. This ultra-high sensitivity makes it ideal for applications such as deep space communications.

Microwave photonics signal processing
Quantum microwave photonics also implements high-bandwidth signal processing functions such as phase shifting and filtering. By using a dispersive optical element and adjusting the wavelength of light, the researchers demonstrated the fact that RF phase shifts up to 8 GHz RF filtering bandwidths up to 8 GHz. Importantly, these features are all achieved using 3 GHz electronics, which shows that the performance exceeds traditional bandwidth limits

Non-local frequency to time mapping
One interesting capability brought about by quantum entanglement is the mapping of non-local frequency to time. This technique can map the spectrum of a continuous-wave pumped single-photon source to a time domain at a remote location. The system uses entangled photon pairs in which one beam passes through a spectral filter and the other passes through a dispersive element. Due to the frequency dependence of entangled photons, the spectral filtering mode is mapped non-locally to the time domain.
Figure 1 illustrates this concept:

This method can achieve flexible spectral measurement without directly manipulating the measured light source.

Compressed sensing
Quantum microwave optical technology also provides a new method for compressed sensing of broadband signals. Using the randomness inherent in quantum detection, researchers have demonstrated a quantum compressed sensing system capable of recovering 10 GHz RF spectra. The system modulates the RF signal to the polarization state of the coherent photon. Single-photon detection then provides a natural random measurement matrix for compressed sensing. In this way, the broadband signal can be restored at the Yarnyquist sampling rate.

Quantum key distribution
In addition to enhancing traditional microwave photonic applications, quantum technology can also improve quantum communication systems such as quantum key distribution (QKD). The researchers demonstrated subcarrier multiplex quantum key distribution (SCM-QKD) by multiplexing microwave photons subcarrier onto a quantum key distribution (QKD) system. This allows multiple independent quantum keys to be transmitted over a single wavelength of light, thereby increasing spectral efficiency.
Figure 2 shows the concept and experimental results of the dual-carrier SCM-QKD system:

Although quantum microwave photonics technology is promising, there are still some challenges:
1. Limited real-time capability: The current system requires a lot of accumulation time to reconstruct the signal.
2. Difficulty dealing with burst/single signals: The statistical nature of the reconstruction limits its applicability to non-repeating signals.
3. Convert to a real microwave waveform: Additional steps are required to convert the reconstructed histogram into a usable waveform.
4. Device characteristics: Further study of the behavior of quantum and microwave photonic devices in combined systems is needed.
5. Integration: Most systems today use bulky discrete components.

To address these challenges and advance the field, a number of promising research directions are emerging:
1. Develop new methods for real-time signal processing and single detection.
2. Explore new applications that utilize high sensitivity, such as liquid microsphere measurement.
3. Pursue the realization of integrated photons and electrons to reduce size and complexity.
4. Study the enhanced light-matter interaction in integrated quantum microwave photonic circuits.
5. Combine quantum microwave photon technology with other emerging quantum technologies.