Reciprocity and Non Reciprocity

• 16 Sep 2023

For Prelims: Non-Reciprocity Methods, Reciprocity Phenomenon, Radar Systems, Magnetic resonance imaging.

For Mains: Non-Reciprocity Methods for tackling the challenges related to Reciprocity.

Source: TH

Why in News?

Scientists have developed devices which break the Principles of Reciprocity tackling the challenges that arise out of the Reciprocity Phenomenon.

What is Reciprocity?

• About:
  • Reciprocity means that if a signal is sent from one point to another, it is sent back from the second point to the first.
    • For Example: It's like when you shine a flashlight at a friend, they can shine it back at you because the light can go both ways through the air.
  • However, there are situations where reciprocity doesn't work as expected.
    • For example, in some movies, a person being questioned can't see the police officers through a window, but the officers can see them.
    • Also, in the dark, one can see someone under a streetlight, but they can't see that person.

Note: Non-reciprocity: The physics of letting waves go one way but not the other.

• Applications:
  • Antenna Testing: Reciprocity simplifies antenna testing. Instead of using multiple signal sources in various directions, one can send one signal into the antenna and observe how it transmits it back.
    • This helps determine the antenna's ability to receive signals from different directions, known as its far-field pattern.
  • Radar Systems: Engineers use reciprocity to test and operate radar systems. By studying how radar antennas send and receive signals, they can improve the system's performance and accuracy.
    • Radar is an electromagnetic sensor used for detecting, locating, tracking, and recognizing objects of various kinds at considerable distances.
  • Sonar Systems: In sonar technology, which is used for underwater detection and navigation, reciprocity aids in testing and optimizing the performance of sonar devices.
  • Seismic Surveys: Reciprocity simplifies the testing and operation of seismic survey equipment used in geology and oil exploration to study subsurface structures.
  • Medical Imaging (MRI): MRI scanners utilize reciprocity principles to send and receive signals for creating detailed medical images of the human body.

What are the Challenges of Reciprocity?

• Spying and Information Security:
  • Reciprocity means that while one can receive signals from the target, his own equipment may unintentionally transmit signals, potentially exposing his location or intentions.
• Back Reflections:
  • When designing high-power lasers for signal transmission, imperfections in the transmission line can lead to harmful backreflections. Reciprocity dictates that these backreflections could re-enter the laser, potentially causing damage or interference.
  • In communication systems, strong back-reflections can occur due to reciprocity, leading to interference and signal degradation.
    • Managing these back-reflections is essential for maintaining the quality and reliability of communication networks.
• Signal Amplification for Quantum Computing:
  • Quantum computers use extremely sensitive qubits that need to be maintained at very low temperatures.
  • To sense their quantum states, the signals must be amplified significantly.
  • However, reciprocity can introduce challenges in achieving efficient and controlled signal amplification without introducing noise or unwanted interactions.
• Miniaturization:
  • As technology moves toward miniaturization at nanometer and micrometer scales, ensuring signal efficiency and control becomes increasingly challenging. In self-driving cars, where monitoring various signals is crucial for safety, managing the complexities of reciprocal signal interactions presents a significant challenge.

What are the Methods Devised to Overcome Challenges Related to Reciprocity?

• Magnet-Based Non-Reciprocity:
  • Scientists have developed magnet-based Non-Reciprocal Devices, consisting of components like wave plates and Faraday rotators.
    • The Faraday rotator, using a magnetic material, allows waves to pass in one direction but blocks them in the reverse direction, breaking the principle of reciprocity.
• Modulation:
  • Modulation involves continuously changing some parameter of the medium, either in time or in space.
  • By altering the properties of the medium, scientists can control wave transmission and address challenges related to signal routing, communication, and interference.
  • This method provides flexibility in managing signals under different conditions.
• Nonlinearity:
  • Nonlinearity involves making the properties of the medium depend on the strength of the incoming signal, which, in turn, depends on the signal's propagation direction.
  • This approach allows scientists to control signal transmission by manipulating the nonlinear response of the medium. It offers a way to achieve non-reciprocity and control signal interactions.