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Quantum Aperture: Atomic Radio Technology

  Aperture, that is, a radio receiving device. In classical radio science, aperture refers to the effective receiving area of ​​a radio receiving system, which is used to characterize its ability to receive radio waves. In the “quantum apertures” program proposed by the US Defense Advanced Research Projects Agency, aperture is used to refer to a radio receiving device, and this use is continued in this article. Apertures have been widely used in wireless communication, remote sensing, and detection systems such as mobile phones, radars, and radio telescopes, and are one of the indispensable tools for human survival and development. Since the German physicist HR Hertz completed a series of experiments to prove Maxwell’s electromagnetic theory in 1888 and confirmed the existence of radio waves, after two years of rapid development, the classical aperture based on the century-old Hertz system has infinitely approached its size. The physical performance limit is gradually difficult to meet the needs of national defense, military industry, scientific research and other fields. The radio measurement technology based on the quantum system, atomic radio technology, came into being. Under the development trend of contemporary quantum science, it has given a new connotation to “the radio wave that never dies”. With the US $45 million (about 300 million RMB) in October 2020, the four-year “Quantum Aperture” program was launched to develop quantum apertures based on atomic radio technology and demonstrate their practicality in defense applications , the prelude to the development of atomic radio technology to the application was officially opened.
waves that never die

  The survival and development of human beings depend on the observation of nature. For most of human evolutionary history, however, observation of visible light with the naked eye was the only means by which humans could improve their understanding of nature. It was not until 1800 that “the father of stellar astronomy” Herschel (FW Herschel) measured the temperature of different colors by measuring the light split by the prism through a thermometer, and noticed that the thermometer reading was invisible outside the red light. The highest value, from which it is found that there is also infrared radiation in addition to visible light. Then, people discovered ultraviolet radiation, radio waves, and X-rays one after another. In 1914, PU Villard discovered gamma rays, which finally constituted the electromagnetic spectrum known to mankind. In just over 100 years, human understanding of the electromagnetic spectrum has expanded by about 20 orders of magnitude (from one thousandth of a nanometer to hundreds of thousands of kilometers) from the early visible light band (wavelengths between 380 and 700 nanometers). By detecting these electromagnetic radiations, human beings have greatly increased the means of observing nature and greatly improved their understanding of the natural world. The most famous example is the observational experiment on the blackbody radiation spectrum, which showed that the law of equal distribution of energy in classical statistical mechanics fails (ultraviolet catastrophe) when analyzing the blackbody radiation spectrum, and the Michelson-Morley experiment by measuring The speed of light in the vertical direction proves that the static ether wind does not exist. These two problems were called by Kelvin “the 19th century empty dark cloud of the kinetic theory of heat and light” [1]. The former led M. Planck to make the assumption, which seemed incredible at the time, that electromagnetic radiation can only be emitted or absorbed in the form of discrete energy packets (quantum), which in turn promoted the establishment and development of quantum mechanics. The latter sparked a flurry of research that culminated in the birth of special relativity.
  All in all, the measurement of electromagnetic waves has been a hot topic in science for nearly 200 years. Among them, radio waves are favored by humans because of their unique characteristics: they are in the absorption transparent window of the atmosphere, and have excellent propagation characteristics that pass through leaves and most building materials, are reflected by metal objects, and bypass obstacles by diffraction. As early as before and during World War II, the Soviet Union, the United States and other countries independently and secretly developed detection systems that use radio waves to determine the distance, angle or speed of objects, that is, radar systems. Today, 5G radio communication equipment has penetrated into various fields and has become a key new infrastructure supporting the digital, networked, and intelligent transformation of the economy and society. In 2021, the world’s largest and most sensitive single-aperture radio telescope “China Sky Eye” (FAST) will be officially opened to astronomers around the world, helping countless astronomers “observe” the universe by receiving invisible radio waves in the universe. The radio measurement system will be a necessary tool for human survival and development today and in the future, and it will be widely used in people’s daily life, scientific research and national defense and military industries. Radio can be described as a veritable radio wave that never dies, and ultra-sensitive measurement of it has become an eternal topic.
Aperture: From Classical to Quantum

  Between 1886 and 1888, Hertz carried out a series of landmark experiments that laid a solid foundation for the entire classical radio science [2]. The receiver used by Hertz looks rather rudimentary today, breaking at one point in a wire loop, creating a spark gap where tiny sparks can be observed when the wire loop receives radio waves from the transmitter. Based on a series of experiments on the receiving device, he proved the existence of radio waves without a doubt, and proved that they have properties similar to light waves: they travel at close to the speed of light, have polarization properties, and can be reflected, refracted, etc.
  The device used by Hertz, although rudimentary, is a good illustration of the basic principles of the classical aperture: First, a metal sensor, such as the loop of wire used by Hertz, is now called an “antenna”, which is transmitted through its internal free electrons. Under the action of the radio wave to be measured, the macroscopic movement (ie the formation of current) is carried out, and the energy is converted from the form of the space electromagnetic field to the form of the current inside the conductor to realize the collection of the radio wave; secondly, a device that processes the current signal (such as Hertz The part of the experiment that produced the electrical spark, now called the “receiver”) processed the electrical current signal converted by the antenna into a signal that humans could recognize. Driven by the major strategic needs of military detection in the two world wars and the huge market for civilian wireless communications after the war, the classic aperture has undergone rapid development for more than 100 years. Antennas have developed dipole antennas, horn antennas, parabolic antennas, microstrip antennas and other forms from the original metal ring. The well-designed antenna can achieve almost 100% energy conversion efficiency; electrical signal processing devices have experienced electrical sparks. Age, vacuum tube age, and semiconductor age, today’s integrated circuit-based electrical signal processing devices can extract vast amounts of information from electrical currents that are less than the size of a coin. Fully developed, the classical aperture has achieved great success, and at the same time, its capabilities have reached the physical limit.

  During the 130 years of rapid development, continuous improvement and application of classical radio technology along the road laid by JC Maxwell in his electromagnetic theory research as early as 1864, the progress of basic science has turned mankind’s understanding of nature upside down. The change. Driven by a generation of scientific giants such as Planck, Einstein, L. de Broglie, WK Heisenberg, E. Schrödinger and P. Dirac Quantum mechanics was established and developed rapidly: theoretically, the mechanism by which electromagnetic radiation (photons) interacts with matter (atoms and molecules) has been precisely explained at the microscopic level [3]; technically, the invention of lasers and materials Advances in science have made it possible to precisely manipulate and measure matter at the level of individual molecules or even atoms. Based on new principles and new technologies, people have begun to manipulate matter at the microscopic level to create targeted ideal quantum states that do not exist in nature, so as to achieve what classical means cannot. The concepts of quantum communication, quantum computing and quantum precision measurement have been proposed one after another, setting off a wave of quantum technology revolution. Riding this wave, based on the principle of the physical idea of ​​quantum precision measurement, radio technology has also begun to transform from classical to quantum, giving birth to atomic radio technology and the birth of quantum aperture.

  Quantum precision measurement includes three processes, namely the preparation of the initial quantum state, the evolution of the wave function under the action of the physical quantity to be measured, and the reading of the final state wave function [4]. First, the quantum system is prepared under the precise manipulation of a controllable electromagnetic field to a preferred initial quantum state with a known initial wave function, which usually has a strong interaction with the physical quantity to be measured; The evolution begins under the action of the measured physical quantity. After the evolution, its initial wave function becomes the final state wave function; by measuring the final state wave function and comparing its change relative to the initial state wave function, the physical quantity to be measured can be inferred, Realize the measurement of the physical quantity to be measured. In atomic radio technology, people use optical means to prepare alkali metal atoms into a Rydberg state that is sensitive to radio waves, so that the wave function evolves under the action of the radio wave to be measured, and finally uses optical means to read The final state wave function, thus realizing the measurement of radio waves. The preparation of Rydberg atoms and the optical non-destructive reading of the final state wave function are two important links in the realization of atomic radio technology.

  Rydberg atom and its preparation
  Rydberg atom is a highly excited atom in the Rydberg state, the outermost electron (valence electron) outside the nucleus is in the electron orbit of high principal quantum number, and the average radius of the electron from the nucleus is large [5] ]. For example, the valence electrons of a cesium Rydberg atom with a principal quantum number of 30 are mainly distributed in the range of 100 nanometers away from the nucleus, while the ground state cesium valence electrons are mainly distributed in the range of about 0.1 nanometers away from the nucleus. The size of the primary atom is thousands of times larger than that of the ground state atom, and it is a well-deserved “Big Mac” atom. The larger radius from the nucleus makes its valence electrons less bound by the nucleus, and the wave function is easily changed under the disturbance of the external electromagnetic field, so the Rydberg atom can be used for sensitive measurement of radio waves. Alkali metal atoms such as rubidium and cesium are usually used as quantum platforms in atomic radio technology. These alkali metal atoms are filled into a closed high-vacuum transparent glass cell in the form of dilute metal vapor for convenient use. Thanks to the high light transmittance of the glass material, the laser can interact with the ground state alkali metal atoms through the glass cell wall, and prepare them into Rydberg atoms through single-step or multi-step excitation.
  Final state wave function reading
  There are usually two ways to read the final state wave function of a Rydberg atom with a radio wave. One is the field ionization measurement. The Rydberg atoms are ionized into anions and cations by applying a slowly increasing ionization electric field, so that the ion detector detects the ion signal, and the final state wave function information can be obtained through the ionization field strength. In this measurement method, the process of ionization causes the atoms to be destroyed, and the atoms are consumed as a one-time resource, so it is a destructive measurement. Based on this measurement mechanism, Professor S. Haroche of the Collège de France has achieved the measurement of a single radio wave photon, which is far superior to the sensitivity achieved by the classical system, proving that the quantum system has a measurement far superior to that of the classical system Potential, Arrosh also won the 2012 Nobel Prize in Physics for related series of work. However, destructive measurements lead to continuous consumption of atomic samples, making them unable to work continuously, and the measurement setup is very complex, and the corresponding technology is difficult to apply outside the laboratory environment. Another method is based on all-optical measurement. A beam of probe laser interacts with the final state Rydberg atom, and the information of the final state is recorded in the amplitude and phase of the probe light, and the final state information is obtained by measuring the probe light [6] . All-optical measurement only triggers atoms to transition between their internal states, without destroying the structure of atoms, and atoms can be reused, so this method is a non-destructive measurement of atoms. The invention of optical non-destructive measurement methods has made sustainable measurement in a portable atomic gas chamber a reality.
Application of Atomic Radio Technology

  Quantum science is not a panacea “magic medicine”, but a highly customized “special medicine” realized by precise manipulation of molecules and atoms on a microscopic scale. Therefore, in atomic radio technology, Rydberg atoms will be further It can precisely prepare special quantum states with different characteristics, so as to realize quantum apertures with different functions, which can not be achieved by classical radio technology in specific application scenarios. For example, the preparation of Rydberg atoms into a sharp state (a quantum state with zero orbital angular momentum) enables more accurate electric field meters, and the preparation of Rydberg atoms into a decorated state enables more sensitive superheterodyne receivers, And the Rydberg state itself can realize a more efficient electrode small communication receiver.
  More accurate electric field meter
  Metrology is called the mother of science and is the cornerstone of all sciences. The electric field measurement of radio waves is also the basis of radio science. A classic electric field meter consists of a metal antenna and a receiver. Due to the lack of isotropy of metal antennas, each antenna responds differently to radio waves in terms of efficiency and directivity. Imagine using a mobile phone, and sometimes the orientation of the mobile phone will greatly affect the signal. Good or bad, this is caused by the directivity of metal antennas, which cannot be avoided in metal antennas. Therefore, the characteristics and placement direction of the antenna will bring great uncertainty to the final measurement result. Metal antennas are opaque to radio waves, and have strong disturbances to the radio waves to be measured, affecting the final measurement accuracy. In addition, metal antenna measurement characteristics can change over time due to surface oxidation. These flaws make it impossible for classical electric field meters to achieve accurate measurements even after extremely complex and frequent periodic calibrations. The atomic electric field meter is quite different. The isotropy and space-time invariance of atoms make it possible to obtain consistent results when using atoms to measure the same radio wave signal at any time and any place. The atomic electric field meter has a clear and concise traceability chain. The atomic gas chamber as the sensing device is made of glass material, and there is almost no disturbance to the radio wave to be measured. Precise preparation of atoms to a sharp state allows perfect isotropic responses of atoms, and sensor placement no longer introduces errors. The above advantages make it possible to realize an ideal high-precision electric field meter based on atomic radio technology, and form an electric field standard with high precision and stability [7]. Atomic radio technology has already shown clear advantages over classical systems in the field of metrology. At present, the National Institute of Standards and Technology of the United States and the Chinese Academy of Metrology are promoting the establishment of electric field standards based on atomic radio technology.

  More sensitive superheterodyne receivers
  Classical radio technology absorbs radio waves in space through metal antennas, converting energy from the form of an electromagnetic field to the form of free electron kinetic energy (current), a destructive measurement of radio waves. At the same time, just as pollen does random Brownian motion in hot water, free electrons in circuits with temperatures greater than absolute zero (such as at room temperature) also undergo random thermal motion, resulting in noise in the current. The information of the radio wave to be measured and the noise caused by thermal motion are all encoded in the current, which cannot be physically distinguished, so that the minimum measurable signal in the classical system has a physical limit, which is called thermal noise. The measurement sensitivity of atomic radio technology is limited by quantum noise, and its theoretical sensitivity is much better than the classical thermal noise limit [8]. In recent years, various international research teams have aimed at the quantum noise limit to promote the measurement sensitivity of atomic radio technology. Among them, the new atomic superheterodyne measurement method [9] has achieved the highest sensitivity record in the world so far. In the structure of the atomic superheterodyne receiver, the atoms are prepared into the decorated Rydberg state by introducing a controllable local oscillator field, which increases the intrinsic gain of the optical readout of the final state wave function and realizes the amplitude of the radio wave to be measured. The linear response of atomic radio technology has taken a critical step towards quantum noise limit sensitivity. At the same time, as the reference of frequency and phase, the local oscillator field realizes the measurement of the frequency and phase information of radio waves, which enables atomic radio technology to have the ability to fully characterize radio waves, opening its applications in radar remote sensing, communications and other fields door. The decorated Rydberg state can realize quantum non-destructive measurement of radio waves. Combined with the cavity enhancement method, atomic radio technology will eventually realize a superheterodyne receiver that is more sensitive than classical, greatly improving the measurement sensitivity of existing radar systems or The data capacity of the communication receiver.

  More Efficient Electrode Small Communication Receiver
  Maxwell’s electromagnetic theory shows that the metal antenna has the effect of secondary radiation, and the energy absorbed by it will be released again in the form of radio waves, and vector superposition with the original radio wave to be measured. The superposition effect of the two varies depending on the electrical size of the antenna (the electrical size is the ratio of the physical size of the antenna to the wavelength of the radio wave to be measured), which can be resonance-enhanced, thereby increasing the efficiency of the antenna, such as a half-wavelength dipole antenna ( The electrical dimension is 1/2); it can also cancel each other out, thereby reducing the efficiency of the antenna, such as the electrode small dipole antenna (electrical dimension is much smaller than 1). Take the retractable metal whip antenna (a typical dipole antenna) on an FM radio (usually it receives electromagnetic waves at frequencies around 100 MHz with a wavelength of about 3 meters), when we try to adjust the whip antenna The length of the dipole antenna to make the radio signal clearer is the process of adjusting the length of the dipole antenna to the most efficient half wavelength (about 1.5 meters); however, when we use this antenna to receive a radio frequency of 1 MHz At wave time (its wavelength is about 300 meters), it becomes an inefficient electrode small dipole antenna. For radio waves with frequencies less than megahertz (wavelengths greater than 300 meters), almost all physically achievable metal antennas can be regarded as small electrode antennas, and their efficiency is inversely proportional to the square of the wavelength of the radio wave to be measured, and increases rapidly as the wavelength increases This physical limit is called the Juran Cheng limit and is the physical limit that can be reached by an ideal classical communication receiver. Therefore, in classical radio technology, the measurement of long-wave radio waves has always faced great challenges. In atomic radio technology, the response of atoms to long-wave radio waves is based on the Stark effect, and its response does not change with the wavelength of radio waves, so as the wavelength of radio waves gradually increases, atomic radio technology will reflect obvious advantages over classical radio technology . In 2018, the research team of the U.S. Army Research Laboratory used atomic radio technology to develop a high-efficiency electrode small quantum communication receiver that broke through the Julan Cheng limit. The data capacity of this quantum receiver is at least 4 orders of magnitude better than that of an ideal classical receiver [10]. This research and the “Quantum Aperture” program all show that atomic radio technology has attracted the attention of the US defense department, and is strongly supporting the application of this technology in the military field.

The future can be expected

  With the advent of the quantum technological revolution, atomic radio technology has pushed radio technology from the classical system to the quantum system, and has achieved vigorous development. As one of the quantum technologies that are relatively close to practical use, in less than 10 years, atomic radio technology has attracted attention in many fields and has entered the process of engineering application. Nonetheless, atomic radio technology is still a young discipline and faces many of its own technical challenges. It is expected that with the development of quantum technology, in the near future, radio telescopes will be able to see deeper into the universe, invisible targets will no longer be transparent to radar, and communication problems in deep space and deep sea will no longer prevent us from exploring nature.

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