Quantum Information

https://phys.org/news/2022-03-quantum-bit-fora-record-breaking-milliseconds.html

Computers, smartphones, GPS: quantum physics has enabled many technological advances. It is now opening up new fields of research in cryptography (the art of coding messages) with the aim of developing ultra-secure telecommunications networks. There is one obstacle, however: after a few hundred kilometers within an optical fiber, the photons that carry the qubits or "quantum bits" (the information) disappear. They therefore need "repeaters," a kind of "relay," which are partly based on a quantum memory. By managing to store a qubit in a crystal (a "memory") for 20 milliseconds, a team from the University of Geneva (UNIGE) has set a world record and taken a major step towards the development of long-distance quantum telecommunications networks. This research can be found in the journal npj Quantum Information. Developed during the 20th century, quantum physics has enabled scientists to describe the behavior of atoms and particles as well as certain properties of electromagnetic radiation. By breaking with classical physics, these theories generated a real revolution and introduced notions without equivalent in the macroscopic world such as superposition, which describes the possibility for a particle to be in several places at once, or entanglement, which describes the ability of two particles to affect each other instantaneously even at a distance ("spooky action at a distance"). Quantum theories are now at the heart of much research in cryptography, a discipline that brings together techniques for encoding a message. Quantum theories make it possible to guarantee perfect authenticity and confidentiality for information (a qubit) when it is transmitted between two interlocutors by a particle of light (a photon) within an optical fiber. The phenomenon of superposition let the sender know immediately whether the photon conveying the message has been intercepted. Memorizing the signal However, there is a major obstacle to the development of long-distance quantum telecommunication systems: beyond a few hundred kilometers, the photons are lost and the signal disappears. Since the signal cannot be copied or amplified—it would lose the quantum state that guarantees its confidentiality—the challenge is to find a way of repeating it without altering it by creating "repeaters" based, in particular, on a quantum memory. In 2015, the team led by Mikael Afzelius, a senior lecturer in the Department of Applied Physics at the Faculty of Science of the University of Geneva (UNIGE), succeeded in storing a qubit carried by a photon for 0.5 milliseconds in a crystal (a "memory"). This process allowed the photon to transfer its quantum state to the atoms of the crystal before disappearing. However, the phenomenon did not last long enough to allow the construction of a larger network of memories, a prerequisite for the development of long-distance quantum telecommunications.

量子信息的中繼器

https://phys.org/news/2022-03-quantum-bit-fora-record-breaking-milliseconds.html

計算機、智能手機、GPS：量子物理學促成了許多技術進步。它現在正在開闢密碼學（信息編碼藝術）的新研究領域，旨在開發超安全的電信網絡。然而，有一個障礙：在光纖中幾百公里之後，攜帶量子比特或“量子比特”（信息）的光子消失了。因此，他們需要“中繼器”，一種部分基於量子存儲器的“中繼器”。日內瓦大學 (UNIGE) 的一個團隊通過設法將一個量子比特在晶體（“內存”）中存儲 20 毫秒，創造了一項世界紀錄，並朝著長距離量子電信網絡的發展邁出了重要一步。這項研究可以在 npj Quantum Information 雜誌上找到。

在 20 世紀發展起來的量子物理學使科學家們能夠描述原子和粒子的行為以及電磁輻射的某些特性。通過打破經典物理學，這些理論產生了一場真正的革命，並引入了宏觀世界中沒有等價物的概念，例如疊加，描述了一個粒子同時在多個地方的可能性，或者糾纏，描述了兩個粒子的能力即使在遠處也會立即相互影響（“遠處的幽靈行動”）。

量子理論現在是密碼學研究的核心，密碼學是一門將信息編碼技術結合在一起的學科。當信息（量子比特）通過光纖內的光粒子（光子）在兩個對話者之間傳輸時，量子理論可以保證信息（量子比特）的完美真實性和機密性。疊加現象讓發送者立即知道傳遞信息的光子是否被截獲。

記憶信號

然而，長距離量子通信系統的發展存在一個主要障礙：超過幾百公里，光子丟失，信號消失。由於信號不能被複製或放大——它會失去保證其機密性的量子狀態——挑戰在於找到一種在不改變信號的情況下重複信號的方法，特別是基於量子存儲器創建“中繼器”。

2015 年，由日內瓦大學理學院（UNIGE）應用物理系高級講師 Mikael Afzelius 領導的團隊，成功地將光子攜帶的量子比特在晶體中存儲了 0.5 毫秒（a “記憶”）。這個過程允許光子在消失之前將其量子態轉移到晶體的原子上。然而，這種現象並沒有持續足夠長的時間來構建更大的記憶網絡，而這是發展長距離量子電信的先決條件。

存儲記錄

今天，在歐洲量子旗艦計劃的框架內，Mikael Afzelius 的團隊通過將一個量子比特存儲 20 毫秒，成功地顯著增加了這一持續時間。 “這是基於固態系統的量子存儲器的世界紀錄，在這種情況下是晶體。我們甚至設法達到了 100 毫秒的標記，保真度略有下降，”研究人員興奮地說道。與他們之前的工作一樣，UNIGE 的科學家們使用摻雜了某些稱為“稀土”（在本例中為銪）的金屬的晶體，能夠吸收光然後重新發射。這些晶體保持在 -273,15°C（絕對零），因為超過該溫度 10°C，晶體的熱攪動會破壞原子的糾纏。

“我們向晶體施加了千分之一特斯拉的小磁場，並使用了動態去耦方法，其中包括向晶體發送強烈的無線電頻率。這些技術的效果是將稀土離子從擾動中分離出來。環境並將我們迄今為止已知的存儲性能提高了近 40 倍，”UNIGE 應用物理系的博士後研究員 Antonio Ortu 解釋說。這項研究的結果構成了長距離量子電信網絡發展的重大進步。它們還將光子攜帶的量子態的存儲帶到了人類可以估計的時間尺度。

10年的高效系統

然而，仍有一些挑戰需要解決。 “現在的挑戰是進一步延長存儲時間。理論上，增加晶體暴露於無線電頻率的持續時間就足夠了，但目前，在較長時間內實施它們的技術障礙阻止了我們不會超過 100 毫秒。但是，可以肯定的是，這些技術難題是可以解決的，”Mikael Afzelius 說。

科學家們還必須找到設計存儲器的方法，該存儲器一次能夠存儲多個光子，因此具有“糾纏”的光子，這將保證機密性。 “我們的目標是開發一個在所有這些方面都表現良好並且可以在十年內上市的系統，”研究人員總結道。

The Bohr model: The famous but flawed depiction of an atom

https://www.space.com/bohr-model-atom-structure

The Bohr model: The famous but flawed depiction of an atom By Tereza Pultarova Contributions from Tereza Pultarova published about 7 hours ago The Bohr model is neat, but imperfect, depiction of atom structure. The Bohr model, introduced by Danish physicist Niels Bohr in 1913, was a key step on the journey to understand atoms. Ancient Greek thinkers already believed that matter was composed of tiny basic particles that couldn't be divided further. It took more than 2,000 years for science to advance enough to prove this theory right. The journey to understanding atoms and their inner workings was long and complicated. It was British chemist John Dalton who in the early 19th century revived the ideas of ancient Greeks that matter was composed of tiny indivisible particles called atoms. Dalton believed that every chemical element consisted of atoms of distinct properties that could be combined into various compounds, according to Britannica. Dalton's theories were correct in many aspects, apart from that basic premise that atoms were the smallest component of matter that couldn't be broken down into anything smaller. About a hundred years after Dalton, physicists started discovering that the atom was, in fact, really quite complex inside. Related: There's a giant mystery hiding inside every atom in the universe British physicist Joseph John Thomson made the first major breakthrough in the understanding of atoms in 1987 when he discovered that atoms contained tiny negatively charged particles that he called electrons. Thomson thought that electrons floated in a positively charged "soup" inside the atomic sphere, according to Khan Academy. 14 years later, New Zealand-born Ernest Rutherford, Thomson's former student, challenged this depiction of the atom when he found in experiments that the atom must have a small positively charged nucleus sitting at its center. Based on this finding, Rutherford then developed a new atom model, the Rutherford model. According to this model, the atom no longer consisted of just electrons floating in a soup but had a tiny central nucleus, which contained most of the atom's mass. Around this nucleus, the electrons revolved similarly to planets orbiting the sun in our solar system, according to Britannica. Some questions, however, remained unanswered. For example, how was it possible that the electrons didn't collapse onto the nucleus, since their opposite charge would mean they should be attracted to it? Several physicists tried to answer this question including Rutherford's student Niels Bohr. NIELS BOHR AND QUANTUM THEORY Bohr was the first physicist to look to the then-emerging quantum theory to try to explain the behavior of the particles inside the simplest of all atoms; the atom of hydrogen. Hydrogen atoms consist of a heavy nucleus with one positively-charged proton around which a single, much smaller and lighter, negatively charged electron orbits. The whole system looks a little bit like the sun with only one planet orbiting it. Bohr tried to explain the connection between the distance of the electron from the nucleus, the electron's energy and the light absorbed by the hydrogen atom, using one great novelty of physics of that era: the Planck constant. The Planck constant was a result of the investigation of German physicist Max Planck into the properties of electromagnetic radiation of a hypothetical perfect object called the black body. Strangely, Planck discovered that this radiation, including light, is emitted not in a continuum but rather in discrete packets of energy that can only be multiples of a certain fixed value, according to Physics World.That fixed value became the Planck constant. Max Planck called these packets of energy quanta, providing a name to the completely new type of physics that was set to turn the scientists' understanding of our world upside down. What role does the Planck constant play in the hydrogen atom? Despite the nice comparison, the hydrogen atom is not exactly like the solar system. The electron doesn't orbit its sun —the nucleus — at a fixed distance, but can skip between different orbits based on how much energy it carries, Bohr postulated. It may orbit at the distance of Mercury, then jump to Earth, then to Mars. The electron doesn't slide between the orbits gradually, but makes discrete jumps when it reaches the correct energy level, quite in line with Planck's theory, physicist Ali Hayek explains on his YouTube channel. Bohr believed that there was a fixed number of orbits that the electron could travel in. When the electron absorbs energy, it jumps to a higher orbital shell. When it loses energy by radiating it out, it drops to a lower orbit. If the electron reaches the highest orbital shell and continues absorbing energy, it will fly out of the atom altogether. The ratio between the energy of the electron and the frequency of the radiation it emits is equal to the Planck constant. The energy of the light emitted or absorbed is exactly equal to the difference between the energies of the orbits and is inversely proportional to the wavelength of the light absorbed by the electron, according to Ali Hayek. Using his model, Bohr was able to calculate the spectral lines — the lines in the continuous spectrum of light — that the hydrogen atoms would absorb. The Bohr model seemed to work pretty well for atoms with only one electron. But apart from hydrogen, all other atoms in the periodic table have more, some many more, electrons orbiting their nuclei. For example, the oxygen atom has eight electrons, the atom of iron has 24 electrons. Once Bohr tried to use his model to predict the spectral lines of more complex atoms, the results became progressively skewed. There are two reasons why Bohr's model doesn't work for atoms with more than one electron, according to the Chemistry Channel. First, the interaction of multiple atoms makes their energy structure more difficult to predict. Bohr's model also didn't take into account some of the key quantum physics principles, most importantly the odd and mind-boggling fact that particles are also waves, according to the educational website Khan Academy. As a result of quantum mechanics, the motion of the electrons around the nucleus cannot be exactly predicted. It is impossible to pinpoint the velocity and position of an electron at any point in time. The shells in which these electrons orbit are therefore not simple lines but rather diffuse, less defined clouds. Only a few years after the model's publication, physicists started improving Bohr's work based on the newly discovered principles of particle behavior. Eventually, the much more complicated quantum mechanical model emerged, superseding the Bohr model. But because things get far less neat when all the quantum principles are in place, the Bohr model is probably still the first thing most physics students discover in their quest to understand what governs matter in the microworld. 玻爾模型：著名但有缺陷的原子描述

作者：Tereza Pultarova 來自 Tereza Pultarova 的貢獻大約 7 小時前發表

玻爾模型是對原子結構的簡潔但不完美的描述。

丹麥物理學家尼爾斯·玻爾於 1913 年提出的玻爾模型是了解原子之旅的關鍵一步。

古希臘思想家已經相信物質是由無法進一步分割的微小基本粒子組成的。科學花了 2000 多年的時間才發展到足以證明這一理論的正確性。了解原子及其內部運作的旅程漫長而復雜。

是英國化學家約翰道爾頓在 19 世紀初復興了古希臘人的觀點，即物質是由稱為原子的微小不可分割的粒子組成的。根據大英百科全書的說法，道爾頓相信每一種化學元素都由具有不同性質的原子組成，這些原子可以組合成各種化合物。道爾頓的理論在許多方面都是正確的，除了原子是不能分解成更小的物質的最小成分的基本前提。道爾頓之後大約一百年，物理學家開始發現原子內部實際上非常複雜。

相關：宇宙中的每個原子內部都隱藏著一個巨大的謎團

1987 年，英國物理學家約瑟夫·約翰·湯姆森在對原子的理解上取得了第一個重大突破，當時他發現原子中含有微小的帶負電粒子，他稱之為電子。根據可汗學院的說法，湯姆森認為電子漂浮在原子球內的帶正電的“湯”中。

14 年後，湯姆森以前的學生、出生於新西蘭的歐內斯特·盧瑟福在實驗中發現原子中心必須有一個帶正電荷的小原子核時，對這種對原子的描述提出了挑戰。基於這一發現，盧瑟福隨後開發了一種新的原子模型，即盧瑟福模型。根據這個模型，原子不再只是由漂浮在湯中的電子組成，而是有一個微小的中心核，它包含了原子的大部分質量。根據大英百科全書的說法，圍繞這個原子核，電子的旋轉類似於太陽系中圍繞太陽運行的行星。

然而，有些問題仍未得到解答。例如，電子怎麼可能沒有坍縮到原子核上，因為它們的相反電荷意味著它們應該被它吸引？幾位物理學家試圖回答這個問題，包括盧瑟福的學生尼爾斯·玻爾。

尼爾斯·玻爾和量子理論

玻爾是第一位利用當時新興的量子理論來試圖解釋所有原子中最簡單的粒子行為的物理學家。氫原子。氫原子由一個重原子核和一個帶正電的質子組成，一個帶負電的、更小、更輕的單個電子圍繞該質子運行。整個系統看起來有點像太陽，只有一顆行星圍繞它運行。玻爾試圖解釋電子與原子核的距離、電子的能量和氫原子吸收的光之間的聯繫，使用了那個時代物理學的一大新奇：普朗克常數。普朗克常數是德國物理學家馬克斯·普朗克對被稱為黑體的假設完美物體的電磁輻射特性進行研究的結果。

奇怪的是，普朗克發現，包括光在內的這種輻射不是以連續體的形式發出的，而是以離散的能量包的形式發出的，這些能量包只能是某個固定值的倍數。這個固定值變成了普朗克常數。馬克斯·普朗克稱這些能量子包為全新類型的物理學提供了一個名稱，這種物理學旨在顛覆科學家對我們世界的理解。普朗克常數在氫原子中起什麼作用？儘管進行了很好的比較，但氫原子並不完全像太陽系。玻爾假設，電子不會以固定的距離圍繞它的太陽——原子核——運行，但可以根據它攜帶的能量在不同的軌道之間跳躍。它可能在水星的距離上運行，然後跳到地球，然後到火星。物理學家阿里哈耶克在他的 YouTube 頻道上解釋說，電子不會逐漸在軌道之間滑動，而是在達到正確的能級時進行離散的跳躍，這與普朗克的理論非常吻合。

玻爾認為，電子可以進入的軌道數量是固定的。當電子吸收能量時，它會跳到更高的軌道殼。當它通過輻射而失去能量時，它會下降到較低的軌道。如果電子到達最高軌道殼層並繼續吸收能量，它將完全飛出原子。電子的能量與其發射的輻射頻率之比等於普朗克常數。根據阿里哈耶克的說法，發射或吸收的光的能量正好等於軌道能量之間的差異，並且與電子吸收的光的波長成反比。

使用他的模型，玻爾能夠計算出氫原子會吸收的光譜線——連續光譜中的線。玻爾模型似乎對只有一個電子的原子很有效。但除了氫之外，元素週期表中的所有其他原子都有更多、更多的電子圍繞其原子核運行。例如，氧原子有8個電子，鐵原子有24個電子。一旦玻爾試圖使用他的模型來預測更複雜原子的譜線，結果就會逐漸出現偏差。

根據化學頻道的說法，玻爾模型不適用於具有多個電子的原子有兩個原因。首先，多個原子的相互作用使它們的能量結構更難預測。根據教育網站可汗學院的說法，玻爾的模型也沒有考慮到一些關鍵的量子物理原理，最重要的是粒子也是波這一奇怪而令人難以置信的事實。

由於量子力學，無法準確預測電子圍繞原子核的運動。不可能在任何時間點確定電子的速度和位置。因此，這些電子在其中運行的殼不是簡單的線，而是擴散的、不太明確的雲。該模型發表僅幾年後，物理學家就開始根據新發現的粒子行為原理改進玻爾的工作。最終，出現了更複雜的量子力學模型，取代了玻爾模型。但是因為當所有的量子原理都到位時，事情就變得不那麼整潔了，玻爾模型可能仍然是大多數物理學生在探索微觀世界中支配物質的過程中發現的第一件事。如果你讓光通過大量均勻分佈的平行狹縫，稱為衍射光柵，就會發生一件有趣的事情。創建的干涉圖案與雙縫形成的干涉圖案非常相似（見圖 27.16）。衍射光柵可以通過用鋒利的工具在許多精確定位的平行線上劃傷玻璃來製造，未觸及的區域就像狹縫一樣。這些可以相當便宜地以照相方式大量生產。衍射光柵既可用於透射光，如圖 27.16 所示，也可用於反射光，如圖 27.17 中的蝴蝶翅膀和澳大利亞蛋白石或本章開篇照片中所示的 CD，圖 27.1。除了用作新奇物品外，衍射光柵通常用於光譜色散和光分析。使它們特別有用的是它們形成比雙縫更清晰的圖案。也就是說，它們的亮區更窄更亮，而它們的暗區更暗。圖 27.18 顯示了展示更清晰模式的理想化圖表。自然衍射光柵出現在某些鳥類的羽毛中。規則圖案中的細小指狀結構充當反射光柵，產生相長干涉，使羽毛顏色不僅僅是因為它們的色素沉著。這稱為虹彩。

授課教師
陳永忠 ycchen@thu.edu.tw