"Leave light" for 1 hour! Chinese scientists set new world record

Recently, the team of Chinese scientist Academician Guo Guangcan increased the optical storage time to one hour, breaking the world record of one minute set by a team of German scientists in 2013, and taking an important step towards realizing a quantum USB flash drive. The optical storage used in quantum USB disk technology is completely different from the existing optical storage technology. Their principles and implementation difficulties are more than a million miles apart. So, how do common optical discs store data? How do quantum U-disks "store" light? Why do we want to achieve this kind of storage? What is the difficulty and significance of "keeping" light for one hour? This article will use the most popular metaphors to reveal the answers to these questions, allowing you to reach the boundaries of human technology.

1. Light that can both illuminate and transmit information. Light can not only be used as a lighting tool, but also an important information medium. Traffic lights as important indicator signals and optical fibers used for daily Internet access are the most common examples of using light to transmit information. The reason why light can act as such a changeable information medium is essentially because light is an electromagnetic wave. Just like the microwave signals emitted by our mobile phones and the radio waves from our radios, light as electromagnetic waves can also carry a lot of information. But why can we see light despite being unable to perceive the existence of electromagnetic waves? This is because microwaves and other electromagnetic waves have different wavelengths (or frequencies) from visible light. The wavelength range of visible light around us is only within a narrow range of 380nm to 750nm, and all electromagnetic waves outside this range cannot be perceived by the naked eye. In addition to the transmission of information, our entire communication network and the various terminals in the hands of users are also inseparable from the storage of information. When you send selfies to relatives and friends via WeChat, the photos are first transmitted through WiFi or the operator's wireless network. When they arrive at the other party's mobile phone, even though they may not have It has been downloaded to the photo album, but it has actually been stored in the WeChat cache of the phone. This simple example well reflects the importance of information transmission and storage. In the future, when quantum communications and quantum computers truly become practical, today's computers and entire communication networks will undergo a major reshuffle, and we will have to develop corresponding transmission and storage technologies from scratch. Today’s article does not talk about transmission, but mainly talks about the various entanglements between light and information storage from now to the future. Let's first take a look at how traditional optical storage media, such as CDs, DVDs, and Blu-ray DVDs, store information. 

2. How do optical discs store information? The common CD-ROMs and other optical discs around us are a typical example of using light to store information. First, the special material on the back of the disc is burned with a laser, leaving "pits" on the disc. In this way, when the optical drive reads the disc information, the laser spot will scan the specified position on the surface of the disc, and the areas without "pits" will obviously reflect light. This state corresponds to the "on" in the circuit, recorded as "1"; the reflection in the areas with "pits" is not obvious, corresponding to the "off" in the circuit, recorded as "0". In this way, a series of information strings containing "0" and "1" can be obtained during the scanning process. Through this principle, information can be written and read using light. So have you discovered that the optical storage we call in our daily life does not actually store light itself, but stores a series of patterns (information) that can be read with light. So, what is the optical storage technology in quantum communication?
The reason why the surface of the optical disc appears iridescent is the diffraction caused by the tiny structure. (Image source: TWENTY 20.com) 3. This optical storage is not that optical storage: Quantum U disk that "freezes" light Obtaining information through the method of "0" and "1" only uses the on and off of the optical path, and other dimensional information contained in the light (such as the polarization, amplitude, frequency and phase of the light, etc.) is almost completely ignored. This is like buying a Ferrari but only using it to buy groceries. It's overkill and underutilized. Therefore, scientists continue to innovate other methods in order to utilize the multi-information dimensions of light as much as possible to achieve novel and interesting applications. Quantum USB flash drives in quantum computer technology can be implemented using optical storage. However, the optical storage here is completely different from the optical disk mentioned above. We can call it quantum optical storage. Speaking of quantum, it is difficult to explain it clearly in a few words. Here you only need to know two basic knowledge points: the quantum world and the macroscopic world are two completely different worlds; the principles that can be used in the macroscopic world may be completely ineffective in the quantum world. For example, many people may say that since we can use light to read information on optical discs, wouldn’t it be great to apply this technology to quantum computers? In fact, using the opening and closing of light paths to Storing and reading information is not impossible in the quantum world. After all, the opening and closing of light paths is the most basic property of light. But relying solely on this property is not enough. After all, if humans want to truly break into the quantum world, they will have to use all kinds of martial arts. Simply controlling the on/off of a light path is like using a flashlight to signal bacteria. The other party will definitely be a little confused. 4. To challenge cutting-edge quantum technology, humans must use all possible means. Some people may ask, since optical storage may not work in the quantum world, why should we work hard to develop related technologies? This is actually a good question. After all, we now have more than just optical storage as a means of information storage. Magnetic storage (such as traditional hard drives) and electrical storage (such as USB flash drives and SSDs) are also ubiquitous in daily life. In fact, light, electricity, and magnetism have many similarities in nature, and they often appear at the same time in actual quantum applications. We have not given up on the technical route of using electricity and magnetism as quantum storage methods. In fact, the current various quantum storages are basically the comprehensive application of light and electromagnetism. As mentioned before, entering the world of quantum computers and quantum communications requires mankind to go all out. The technologies that can be used are currently in the hot development stage. At present, quantum optical storage and quantum computing have a good match and have outstanding development prospects. So, how is quantum optical storage realized? Speaking of information storage, there must be a medium. Tapes, disks, flash memories, and even our brains all have media to store information (in fact, the medium is a certain form of matter). We cannot save information in a vacuum. The slippery ground will leave footprints, and the sunburned skin will turn red and black. All forms of information must leave their own traces through the medium. So, what are the interactions between light as a kind of information and the medium? The simplest interaction is of course the blocking of the light path by the medium. In addition, there are also the reflection, refraction, interference and diffraction of light by the medium. However, in the world of quantum computing, there are many magical ways for light and media to interact.

Shadow is the most common phenomenon when light and matter interact

5. Wheat waves blown by the wind: When light enters the medium, something magical happens. First of all, there may be a transfer of states between light and atoms of the medium. The specific mode of this transfer is extremely complicated, so we will not describe it here. However, we can imagine this state transfer as the wind blowing through the wheat field, and the wheat dancing with the wind. There is a state transfer relationship between the wind and the wheat fields. If the wind is strong, the wheat head will be crooked. On the contrary, if the wheat head is not too crooked, it means the wind is not too strong. When light passes through atoms, similar connections will occur between them, and the state of the light (actually the information carried by the light) will be transmitted to the atoms.

Secondly, atoms can also reduce the "speed of light". Note that the speed of light here is in quotation marks. It is not the real speed of light, but a concept called "the group velocity of light." Group velocity is a phenomenon produced when light interacts with media. We are still not prepared to discuss what group velocity is here. But you can imagine the following scene. The speedboat flies across the water, causing ripples that spread slowly from the stern to both sides. Light is like a speedboat, ripples are like group speed, and the medium is the water surface. Although the speedboat Yiqi Juechen disappeared in the blink of an eye, the ripples on the water told us that it had been there. Although the examples of wind-blown wheat waves and speeding boats may not be completely accurate, they well depict the physical picture of the interaction between light and medium - light can transfer its state (information) to the medium (stored in the medium for a long time) in a relatively long period of time (compared to the speed of light).

We just said that the medium is actually matter, and matter is essentially composed of atoms. Theoretically speaking, the process described above will occur when light passes through any substance. This is the basic principle on which quantum light storage relies. It should be noted that what is "stored" is not the light itself, but certain states (or properties) of the light, which is a bit like "a wild goose plucking its hair". However, the properties of substances vary widely, and not all of them can produce very obvious quantum interaction effects after interacting with light. Therefore, the substances that quantum optical storage relies on are very special. This time, Academician Guo’s team used europium-doped yttrium silicate ensemble. I believe that the name alone has already made readers’ heads spin, but it doesn’t matter, we just need to understand the ensemble as a collection of matter. So, what is the awesome ability of the europium-doped yttrium silicate ensemble that can increase the quantum light storage time to the level of 1 hour?

6. How to improve the storage life of quantum light? In the past, although light was "stored" by scientists using various special substances and various special means, the storage life was still very short. Therefore, trying to improve the lifetime of optical storage has become a new goal that scientists need to overcome. It is understood that the optical memory that was closest to actual use was the cold atomic memory based on the rubidium atomic ensemble of Professor Pan Jianwei's research group. This memory achieved a storage life of 0.22 seconds and a storage efficiency of 76%. But it was found that solid-state-based systems, such as ion systems doped with rare earths, could provide longer optical storage lifetimes. Recently, Academician Guo Guangcan’s team has made important breakthroughs in this regard. They have increased the quantum optical information storage time to one hour. This research was also published in the journal Nature Communications. The europium-doped yttrium silicate europium ion system mentioned above can well resist magnetic field disturbances in the environment, thus greatly improving the stability of quantum light storage. Although the lifespan of quantum optical storage has only been increased to 1 hour, this short 1 hour is a big step in the development of quantum communication and quantum computer technology.