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Quantum and RF microwave measurement

Views:3     Author:Site Editor     Publish Time: 2021-07-23      Origin:Internet

As a person in the industry related to radio frequency and microwave, when they intersect with the quantum field, they are at a loss. For RF engineers, quantum theory belongs to the category of almost metaphysics. RF measurement engineers usually obtain accurate and definite values to describe RF signals, while the "superposition state" of quantum seems to be impossible for RF engineers and RF instruments. Completed measurement tasks. The radio frequency technology and radio frequency instruments are sure to be applied to quantum communication and quantum computing.


Quantum is not a specific particle, but refers to an indivisible basic unit. Quantum is the smallest basic unit that can exhibit the characteristics of a substance or physical quantity. Planck discovered that for radiation of a certain frequency ν, an object can only absorb or emit it with hν as the energy unit, and h is called the Planck constant. In other words, the absorption or emission of electromagnetic radiation by an object can only be carried out in a quantum manner, and the energy of each quantum is E=hν, which is called an action quantum. Since quantum is the smallest unit, it has a fixed minimum corresponding to its existence, and other values can only be taken as integer multiples of it. Therefore, the value of the physical quantity is discrete, rather than taking arbitrary values continuously. This is the physical quantity. Quantization".


The paradox of the quantum measurement problem: the measurement result of the instrument is a certain value indicated by the instrument, not the quantum superposition state. The measured value of the instrument can only represent the eigenstate of the quantum, and cannot represent the quantum superposition state.


In the current wireless communication field, based on radio frequency electromagnetic field and transmission line theory, various generations of wireless communication systems have been developed, from 2G, 3G, 4G, to the current 5G and 6G in research and development; quantum communication mainly includes quantum key distribution (quantum key distribution). distribution, QKD) and quantum teleportation.


Quantum key distribution can establish a secure communication password, through a one-time encryption method, a point-to-point secure classic communication can be realized. The security here is the security that has been strictly proven in mathematics, which is what traditional communication technology has done so far. Less than. The existing quantum key distribution technology can realize a quantum key distribution network on the order of hundreds of kilometers. The following figure shows the established experimental network.

Quantum and RF microwave measurement 1


The teleportation of quantum states is a physical carrier based on the distribution of quantum entangled states and quantum joint measurement, which realizes the space transfer of quantum states (quantum information) without moving the quantum states. Existing quantum communication experiments generally use photons as a quantum state carrier, and its manifestation is photon state transmission, and the encoding of quantum information is mainly realized by light polarization.


Improving the coherence time of quantum systems has always been a challenge in the field of quantum technology. As one of the most promising platforms for realizing general-purpose quantum computers, one of the advantages of the ion trap system is its long coherence time. Among them, the coherence time directly limits the maximum number of quantum operations that a quantum computer can continuously perform, and it is also a prerequisite for achieving high-fidelity metric operations. In addition to quantum computing, coherence time is also of great significance to the fields of quantum communication and quantum precision measurement.


The Jin Qihuan research group of the Quantum Information Center of Tsinghua University's Interdisciplinary Information Institute has made important progress in the field of quantum computing. For the first time, it achieved a single-qubit coherence time of more than one hour in an ion trap system, breaking the previous record of 660 seconds.

Quantum and RF microwave measurement 2


Based on the above research results, the main limiting factors are identified: microwave signal phase noise, residual magnetic field fluctuations and microwave signal leakage. During the experiment, the research group used a variety of radio frequency and microwave instruments, placed the experimental system in a double-layer magnetic shielding system to reduce the coupling between ambient magnetic field noise and the quantum system, and reduced the interference of signal leakage to the system by increasing the isolation of laser and microwave signals. Reduce the phase noise of the microwave signal by optimizing the reference of the local microwave oscillator. In order to further suppress noise, the research team adopted dynamic decoupling technology and optimized pulse parameters according to the noise environment to further reduce the impact of various noise sources. In the end, the coherence time of the qubit is increased to 5500 seconds, which is an order of magnitude improvement over the previous one.


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