量子榨能機－藍色情懷

量子榨能機
- 量子蒸汽機從熱力學第二定律殺出另一條活路

作者╱明克爾 ( JR Minkel )
譯者╱胡崇德

Law and Disorder
- A QUANTUM STEAM ENGINE GETS AROUND THE SECOND LAW

By JR Minkel

　　熱力學領域的前輩結合了冷熱庫與氣體操作引擎，通過這個想像中的實驗，在理論上獲得了長足的進展。現在，一些浸淫在量子力學世界中的物理學家，希望藉由量子蒸汽機，找出產生和控制能量流動的方法。他們認為，也許可以利用量子的妙計來推翻這項神聖不可侵犯的熱力學第二定律。

The fathers of thermodynamics got a lot of mileage from thought experiments about gas-filled engines conjoined to reservoirs of hot and cold. Today a few physicists are playing with quantum mechanics in hopes of finding new methods to control and create energy flow in quantum versions of the steam engine. Their research suggests that it is possible to “beat” the inviolable second law of thermodynamics with some quantum sleight-of-hand.

　　任何物理循環過程的效率都會受到熱力學第二定律的規範，亦即能量必須在兩個溫度不同的熱庫間流動才能產生功，而能量的流動往往會使系統的無序程度增加。這兩個熱庫溫度的比值，決定出熱機運轉的最高效率，而這就是有名的卡諾效率（Carnot efficiency），以此紀念19世紀法國物理學家卡諾（Sadi Carnot）。

　　The second law limits the efficiency of any physical process. In essence, it states that, to perform work, energy must flow between two reservoirs set at different temperatures. The flow introduces disorder into the system. The temperature difference between the two baths determines the engine's maximum, or Carnot, efficiency, named after 19th-century French physicist Sadi Carnot.

　　美國德州農工大學的史卡利（Marlan O. Scully）的量子光學研究群，想到一個可以從單一個熱庫裡榨取出功的法子，如此一來，似乎便可以跨越卡諾極限而打破熱力學第二定律了。這套實驗的構想是讓光子在空腔的兩面鏡子間來回反射，其中一面鏡子可以像活塞一般地活動。氣體原子循環通過空腔構成熱庫，它們會以光子的形式釋放出熱能，而且這些原子很特別，它們的狀態由三種電子態組成：一個受激態，以及兩個幾乎相同且疊加在一起的鬆弛態。這就是所謂的同調性，它會影響光子吸收，卻對光子放射沒有影響。通常溫度會決定原子所能釋放的熱量，但如此一來原子就能釋出更多的熱。

　　 Marlan O. Scully's quantum optics group at Texas A&M University has calculated a way to extract work from a single heat bath, thereby surmounting the Carnot limit and giving the appearance of breaking the second law. This setup would rely on photons rebounding in a small cavity between two mirrors, one of which would act as a piston. The bath is a circulating gas of atoms that emits heat in the form of photons as it passes through the mirrored cavity. The atoms are prepared in a special fashion. Each atom has three electron states: an excited state and two nearly identical relaxed states that are quantum-mechanically mixed. This so-called coherence interferes with the absorption of photons but permits the emission of photons to proceed unfettered, causing an excess of heat beyond what the temperature of the atoms alone would dictate.

　　同調性是量子力學的特性之一，它能使光子步調一致，讓原子同時處於兩種狀態。從熱力學的觀點來看，同調性讓系統具有更多的秩序，能使熱機跳脫熱力學第二定律的平衡狀態；反過來說，美國喬治亞大學物理學家李（M. Howard Lee）的研究證實，沒有同調性的光子氣體便會以一般的卡諾效率推動活塞。

　　Coherence is a unique property of quantum mechanics that allows laser photons to march in lockstep and atoms to be in two states at once. Thermodynamically speaking, coherence is an extra dose of order, which pushes the engine out of the uniform state of equilibrium, where the second law applies. An incoherent photon gas, in contrast, pumps a piston with the usual Carnot efficiency, as shown in a study by physicist M. Howard Lee of the University of Georgia.

　　史卡利聲稱他的團隊已經展現出古典的熱機無法產生的效應，一點點同調性就能使輸出的功大為增加。但是同調性非常脆弱，就像用撲克牌搭成的房子，為了製造同調性，所輸入的微波能量是輸出功的三到四倍，因此這套裝置要實際派上用場，可沒那麼容易。不過，美國麻省理工學院的羅伊德（Seth Lloyd）卻認為，史卡利的研究能為同調性的利用指引一條明路，例如如何讓雷射或熱電式冷凍機達到理想的效率極限，他說︰「如果量子同調性能做到這點就太好了。」

　　Scully states that his team's analysis demonstrates effects that classical heat engines cannot produce—a tiny bit of coherence can cause a significant boost in work output. Coherence is as fragile as a house of cards, however, and building it up in this case costs three or four times as much energy, in the form of microwaves, as the engine puts out. These handicaps may mean that the effect probably will not readily find applications. Nevertheless, Seth Lloyd of the Massachusetts Institute of Technology comments that Scully's investigation may help point the way toward using coherence to bring lasers or thermoelectric refrigerators closer to their ideal efficiency limits. “If quantum coherence could do that for you, that would be great,” Lloyd notes.

　　史卡利也想建造一台雷射和熱機的混合體。2002年，他提出量子後燃器的構想：如果基態的原子受到激發，使它們產生雷射，就可以從效率低於卡諾極限的熱機，壓榨出更多的功來。
　　大多數物理學家不認為量子力學會動搖卡諾理論的地位，羅伊德認為，在這個過程中，量子轉變保持了系統無序的狀態，所以熱力學第二定律一開始就引入了。他提到，找尋改善熱機效率的方法，是「量子工程一項值得肯定的工作，但是不要認為熱力學第二定律會被推翻。」

　　Scully is also attempting to construct and test a laser-engine hybrid. In 2002 he proposed that a quantum “afterburner” could squeeze extra work from some ideal engines that operate below the Carnot limit if the exhaust atoms were stimulated to produce laser light.
　　Most physicists see no reason why quantum mechanics should damage Carnot's result. Quantum changes preserve disorder, so the second law is built in from the beginning, Lloyd observes. Looking for ways to improve heat engines is “a praiseworthy branch of quantum engineering,” he remarks, “but don't expect violations of the second law—it's not going to happen.”

不可不知：隨機作功

　　有些研究量子熱機的科學家，正想盡辦法要駕馭粒子的隨機擾動：布朗運動。兩個熱庫的電子接觸時，電子會流向較冷的一方；在大部份的裝置中，電子流會產生功，但是電子的隨機來回運動會降低這個效率。2002年，美國俄勒岡大學的林克及其同事論證，利用半導體篩子選出特定波長的電子，能防止能量的逸失，使布朗運動的熱機效率能夠接近卡諾極限。澳洲雪梨第三代光電能中心的葛林宣稱，他的研究群正在探討類似的效應能否增加太陽能電池的效率。

NEED TO KNOW: RANDOM WORK

　　Some researchers have focused on quantum heat engines that harness the random twitchings of particles—that is, their Brownian motion. When two reservoirs of electrons at different energies make contact, electrons flow toward the colder bath. But random back-and-forth electron motions spoil the efficiency of most devices designed to turn that flow into work. In 2002 Heiner Linke of the University of Oregon and his colleagues reasoned that a semiconductor filter for electrons of specific wavelengths could prevent wasteful energy flow and allow a Brownian heat engine to operate at or near the Carnot limit. Martin A. Green of the Center for Third Generation Photovoltaics in Sydney, Australia, says that his group is currently exploring whether a similar effect could enhance the efficiency of solar cells.