Nature | News

Quantum gas goes below absolute zero

Ultracold atoms pave way for negative-Kelvin materials.

Temperature in a gas can reach below absolute zero thanks to a quirk of quantum physics.

PHOTOCREO Michal Bednarek/Thinkstock

It may sound less likely than hell freezing over, but physicists have created an atomic gas with a sub-absolute-zero temperature for the first time1. Their technique opens the door to generating negative-Kelvin materials and new quantum devices, and it could even help to solve a cosmological mystery.

Lord Kelvin defined the absolute temperature scale in the mid-1800s in such a way that nothing could be colder than absolute zero. Physicists later realized that the absolute temperature of a gas is related to the average energy of its particles. Absolute zero corresponds to the theoretical state in which particles have no energy at all, and higher temperatures correspond to higher average energies.

However, by the 1950s, physicists working with more exotic systems began to realise that this isn't always true: Technically, you read off the temperature of a system from a graph that plots the probabilities of its particles being found with certain energies. Normally, most particles have average or near-average energies, with only a few particles zipping around at higher energies. In theory, if the situation is reversed, with more particles having higher, rather than lower, energies, the plot would flip over and the sign of the temperature would change from a positive to a negative absolute temperature, explains Ulrich Schneider, a physicist at the Ludwig Maximilian University in Munich, Germany.

Peaks and valleys

Schneider and his colleagues reached such sub-absolute-zero temperatures with an ultracold quantum gas made up of potassium atoms. Using lasers and magnetic fields, they kept the individual atoms in a lattice arrangement. At positive temperatures, the atoms repel, making the configuration stable. The team then quickly adjusted the magnetic fields, causing the atoms to attract rather than repel each other. “This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” says Schneider. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.”

At positive temperatures, such a reversal would be unstable and the atoms would collapse inwards. But the team also adjusted the trapping laser field to make it more energetically favourable for the atoms to stick in their positions. This result, described today in Science1, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero.

Wolfgang Ketterle, a physicist and Nobel laureate at the Massachusetts Institute of Technology in Cambridge, who has previously demonstrated negative absolute temperatures in a magnetic system2, calls the latest work an “experimental tour de force”. Exotic high-energy states that are hard to generate in the laboratory at positive temperatures become stable at negative absolute temperatures — “as though you can stand a pyramid on its head and not worry about it toppling over,” he notes — and so such techniques can allow these states to be studied in detail. “This may be a way to create new forms of matter in the laboratory,” Ketterle adds.

If built, such systems would behave in strange ways, says Achim Rosch, a theoretical physicist at the University of Cologne in Germany, who proposed the technique used by Schneider and his team3. For instance, Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity4.

Another peculiarity of the sub-absolute-zero gas is that it mimics 'dark energy', the mysterious force that pushes the Universe to expand at an ever-faster rate against the inward pull of gravity. Schneider notes that the attractive atoms in the gas produced by the team also want to collapse inwards, but do not because the negative absolute temperature stabilises them. “It’s interesting that this weird feature pops up in the Universe and also in the lab,” he says. “This may be something that cosmologists should look at more closely.”

Journal name:
Nature
DOI:
doi:10.1038/nature.2013.12146

References

  1. Braun, S. et al. Science 339, 52–55 (2013).

  2. Medley, P., Weld, D. M., Miyake, H., Pritchard, D. E. & Ketterle, W. Phys. Rev. Lett. 106, 195301 (2011).

  3. Rapp, A., Mandt, S. & Rosch, A. Phys. Rev. Lett. 105, 220405 (2010).

  4. Mandt, S., Rapp, A. & Rosch, A. Phys. Rev. Lett. 106, 250602 (2011).

Comments

  1. Report this comment | #53525

    Anand Ramanathan said:

    As a former quantum physicist, I find this News and Views as well as the Science article quite misleading. Although the work itself is very impressive, the authors misuse the definition of temperature to claim to achieve negative temperatures. Using their definition, anyone who creates a 2-level (or 3-level) quantum energy system with higher particle occupation at the higher energy states can claim negative temperature. This can easily be done in any cold atom laboratory. I would like someone to correct me if I am wrong.

  2. Report this comment | #53529

    Matthew Nelson said:

    I don't believe that the claim was that an 2-level quantum energy system produces negative temperatures, but rather that within the Probability Density Function describing the temperature of the system there exists a non-zero probability of particles with lower than 0 K temperature, thus you can create particles with less than 0 K. As a college physics student, please don't hesitate to correct me if I'm wrong!

  3. Report this comment | #53540

    Eugine Kang said:

    As a former physics class student 6 years ago, I have no clue what you two are saying. However, I highly respect both of your last sentence. "I would like someone to correct me if I am wrong," and "please don't hesitate to correct me if I'm wrong." Thanks

  4. Report this comment | #53542

    Uwe Emmrich-Kießling said:

    As a non physic at all, I do believe that the error lies in the interpretation of "temparature". Like mentioned in the text above, it is the equivalent of the medium moving energy of a large amount of atoms, thus being a statistical measure. The scientists however arranged some atoms in a latice structure and observed some local energetic states. This is far from being the statistical "medium moving energy of a large amount of atoms" and cannot be called "temperature". Whatever they measured, it was something other than "temperature below 0 Kelvin". Again, correct me if I am wrong...

  5. Report this comment | #53543

    Chip L. said:

    What I get from the article is the original definition of absolute zero assumed that all atoms achieve the zero energy state at the same temperature but this study is questioning that hypothesis. This study suggests that certain other atoms may achieve a zero energy state at an even lower temperature (fractionally lower to be sure) and that this extreme low temperature appears to have even more profound atomic effects. That's just how I'm reading the article.

  6. Report this comment | #53545

    Ian Lynch said:

    Seems to me it's all down to definition. If absolute zero is the zero energy state of the particles in an object, temperatures below that represent a negative energy state. That seems consistent with concepts of "dark energy". We only know it from its effects not exactly what it is. The explanation in the article is not very clear to me. I think it means that if some particles are abnormally energetic in something at absolute zero others must be negatively energetic so taking them as a group they would have to be below absolute zero. Again correct me if I'm wrong!

  7. Report this comment | #53546

    Zhou Fang said:

    Correcting most previous authors (because they are wrong), the issue here is fairly well explained in the wikipedia article on the subject .

    Essentially negative temperature here refers to a thermodynamic property of the system that is perhaps divergent from the ordinary definition in this case. Temperature here defines the rate at which entropy increases as thermal energy is added to the system. Ergo, negative temperature relates to a state where adding energy at least incrementally might decrease the entropy of the system.

    Negative temperature is not exactly new – the originality of this research relates to the relatively macroscopic nature of the negative temperature system. The lede of this article needs to be read with this emphasis in mind.

    Correct me if I'm... oh you get the idea.

  8. Report this comment | #53551

    diana sampson said:

    Okay all you smarty pants. I'm a social scientist but very interested in new developments. Problem is, I have no clue what all of this means. I don't even know what Quantum Gas is to tell the truth. So can someone please explain to me in plain English the significance of this scientific development? And why I should be really excited about it? TVM!

  10. Report this comment | #53568

    Departmentov Corrections said:

    "Many respectful physicists oppose the Infinite Improbability Drive, partly because it is a debasement of science, but mainly because they didn't come up with it." -Hitchhiker's Guide to the Galaxy

    Billy White, walk this way...

  11. Report this comment | #53571

    Krishna Psaffy said:

    The response to this research study is so cold!! ;)

    If I wrong, please correct me. Because I am so polite.

  12. Report this comment | #53572

    B.T. See said:

    I just discover something that i wish to share with you Nature. Before that, i have to say i m a long-term learner and enthusiast on optics, chemistry, mathematics and electricity. I just discover the formula for OneWave. OneWave is related to the Big Bang, i believe the empty space is not empty and there are plenty extremely weak electromagnetic waves exist in it. Before explaine OneWave, let look at an atom, an atom could be divided into proton, neutron and electron. These sub-particle could be further divided into quarks like bosons and Fermions, but how if we further and further divide the Fermions and bosons? According to classical physics and pratical physics, fermions and bosons are too hard to be further divided again, but i believe fermions and bosons can be further and further divided into the same electromagnetic waveforms, which energy level is extremely weak and just not yet to equal to zero and i call it OneWave.

    At the present, physicists try their best to combine the theory of general relativity with theory of quantum mechanics to complete the uncompleted quantum physics but they face one major problem i.e. they failed to unify the electromagnetic force with gravity force. i believe OneWave will be the vital key to solve the problem. Hawking had explaine the universe was born after a Big Bang and assumed and calculate a great matter in a very small space but nobody had explaine where is the great matter came from. Hawking had said that there is a rule underlying all the laws of physics and governs the universe, the rule is very simple but not yet known by human beings.....

    Now i try to introduce the Theory Of OneWave, before the event of Big Bang, the empty space had no any particles, however, the empty space is fully filled with a kind of extremely weak electromagnetic waveforms which enegy level is just not yet zero. To create the universe, God have to lead a great number of OneWaves to a single pointed space (He just behave like a super great laser). Then, let the below formula to explaine the creation:

    NWo

                • = HsM

    Sp

    where N ------------> infinity
    Wo -----------------------------------> 0
    Sp ----------------> 0

    N is the number of waveforms which is approach to infinity
    Wo is the energy level of OneWave which is extremely close to zero and just not yet zero
    Sp is the pointed space which is approach to zero
    M is the matter produced and is depend on the number of OneWaves. M = f (NWo)
    Hs is a variable and is depend on 1 / Sp, i.e. the smaller the value of Sp, the greater the value of Hs. Hs = g (1/Sp)

  13. Report this comment | #53573

    B.T. See said:

    Hi Bella,

    I just discover something that i wish to share with you. Before that, i have to say i m a long-term learner and enthusiast on optics, chemistry, mathematics and electricity. I just discover the formula for OneWave. OneWave is related to the Big Bang, i believe the empty space is not empty and there are plenty extremely weak electromagnetic waves exist in it. Before explaine OneWave, let look at an atom, an atom could be divided into proton, neutron and electron. These sub-particle could be further divided into quarks like bosons and Fermions, but how if we further and further divide the Fermions and bosons? According to classical physics and pratical physics, fermions and bosons are too hard to be further divided again, but i believe fermions and bosons can be further and further divided into the same electromagnetic waveforms, which energy level is extremely weak and just not yet to equal to zero and i call it OneWave.

    At the present, physicists try their best to combine the theory of general relativity with theory of quantum mechanics to complete the uncompleted quantum physics but they face one major problem i.e. they failed to unify the electromagnetic force with gravity force. i believe OneWave will be the vital key to solve the problem. Hawking had explaine the universe was born after a Big Bang and assumed and calculate a great matter in a very small space but nobody had explaine where is the great matter came from. Hawking had said that there is a rule underlying all the laws of physics and governs the universe, the rule is very simple but not yet known by human beings.....

    Now i try to introduce the Theory Of OneWave, before the event of Big Bang, the empty space had no any particles, however, the empty space is fully filled with a kind of extremely weak electromagnetic waveforms which enegy level is just not yet zero. To create the universe, God have to lead a great number of OneWaves to a single pointed space (He just behave like a super great laser). Then, let the below formula to explaine the creation:

    NWo / Sp = HsM

    where N ------------> infinity
    Wo -----------------------------------> 0
    Sp ----------------> 0

    N is the number of waveforms which is approach to infinity
    Wo is the energy level of OneWave which is extremely close to zero and just not yet zero
    Sp is the pointed space which is approach to zero
    M is the matter produced and is depend on the number of OneWaves. M = f (NWo)
    Hs is a variable and is depend on 1 / Sp, i.e. the smaller the value of Sp, the greater the value of Hs. Hs = g (1/Sp)

  14. Report this comment | #53574

    Henry Higgins said:

    This article makes no sense to me. I thought absolute zero was defined as the point at which the system had no energy (of any kind) and no entropy. If you put energy into the system, by definition you cannot be below absolute zero. If the atoms are suddenly switched to a "higher energy state", again, by definition, the particles are not at absolute zero. The system is not closed, as the change in external magnetic field changed the system.

  15. Report this comment | #53578

    Bing Zong said:

    What confused me is why they said it was the first time? Doesn't a laser system have the negative temperature?

  16. Report this comment | #53581

    Xiao XU said:

    I agree with Anand Ramanathan.The title is rather misleading.Negtive temprature is an old and popular conception since LASER has been found. If the stable state at negtive temprature of the atom gas must be sticked by the Laser light outside the gas system,then the system is not such that stable.The system will collapse inwards while the Laser stop working.I really this is an interesting experiment,but I do not think it is a piece of big news.

  17. Report this comment | #53582

    Greg Mann said:

    Zhou Fang's answer is correct as I understand it. I love the discussion that this article provokes because it reveals the delightful fact that although we all understand temperature quite well based on our everyday experience, within that concept is concealed a beautifully complex bit of physics!

  18. Report this comment | #53593

    Xiao XU said:

    逗你玩的“负绝对温度”
    已有 1782 次阅读 2013-1-5 11:21 |个人分类:生活点滴|系统分类:科研笔记|关键词:的

    看到报道：《科学家造出低于绝对零度的量子气体》，“据《自然》杂志网站1月3日报道，德国物理学家用钾原子首次造出一种低于绝对零度的量子气体。科学家称这一成果为“实验的绝技” ” http://news.sciencenet.cn/htmlnews/2013/1/273628.shtm 何为低于绝对零度？ 文中说到：“慕尼黑路德维格·马克西米利安大学物理学家乌尔里奇·施奈德解释说，从技术上讲，人们能从一条温度曲线上读出一系列温度数，但这些数字表示的只是它所含的粒子处于某个能量状态的概率。通常，大部分粒子的能态处于平均或接近平均水平，只有少数粒子在更高能态上下。理论上，如果这种位置倒转，使多数粒子处于高能态而少数粒子在低能态，温度曲线也会反过来，温度将从正到负，低于绝对零度。2001年诺贝尔物理学奖获得者沃尔夫冈·克特勒也曾证明，在磁场系统中存在负绝对温度。” 文中还说到：“施奈德和同事用钾原子超冷量子气体实现了这种负绝对零度。他们用激光和磁场将单个原子保持晶格排列。在正温度下，原子之间的斥力使晶格结构保持稳定。然后他们迅速改变磁场，使原子变成相互吸引而不是排斥。施奈德说：“这种突然的转换，使原子还来不及反应，就从它们最稳定的状态，也就是最低能态突然跳到可能达到的最高能态。就像你正在过山谷，突然发现已在山峰。” 至此，玩弄概念的本质就表露无遗！！！ 开氏绝对温度是以绝对运动能量定义的，而文中的能量是以相对于平均能量的低（或负）能量定义绝对温度的，从而，玩了一个“大挪移”！！ 而且，该研究者还把位能的相对性（差一个任意的常数）搞混，这是搞的故作惊人之语。此风表明：对熵、温度、运动、能量等的概念出现学科性混乱。想混水摸鱼者大有人在。 这项研究工作的学术价值是较大的，但是，以这种“新闻”的方式去糊弄大众则是违反科学研究工作基本原则的！！ 我认为，“科学家称这一成果为“实验的绝技” ”这句话的讽刺味是显然的被作者淡化了。 真是笔杆子，有了这句话，可以进退自如。

    Nature 和Science 玩的这种游戏也在腐蚀西方的科学界。

    本文引用地址：http://bbs.sciencenet.cn/blog-39419-649741.html

    分享到：收藏 分享 举报
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    发表评论 评论 (19 个评论)
    19徐晓 2013-1-6 10:04
    To soifaint:兄长，物理起于实验，而不是理论。“below absolute zero”就是误导，否则，讲negtive temperature就好。18soifaint 2013-1-6 08:40
    to 博主：负绝对温度，乃至一般来说非稳态系统的温度定义，总是从统计物理出发的。热力学温标只是个历史遗留概念，它在某些情况下可以使用，但具体到稍微现实一点的复杂体系，大家就总是使用统计法来处理。17soifaint 2013-1-6 08:35
    热力学温度对于系统有内部自由度的情况是没有正确定义的，因为它依赖于第零定律，而第零定律在涉及到演化的时候并不是一个有意义的定律。本质上热力学只是唯像学科，统计物理才是基础。16徐晓 2013-1-5 18:33
    至15楼：肖教授在13楼的答复，非常清楚，虽然态度偏激，但道理没错。Nature上的标题至少是误导读者，其实那篇文章下面已经又好多议论。15physicism 2013-1-5 18:26
    负温度本质上是布居反转的等效描叙，是一种亚稳态的代名词，博主非要理解成为一种热力学的平衡态的温度，实在有必要到附近大学重修本科统计物理课程。
    博主回复(2013-1-5 22:53)：你最好是能看懂8楼给出的那篇博文，祝你好运！
    博主回复(2013-1-5 22:52)：我们读的是不同的书！ 14徐晓 2013-1-5 15:38
    建华友，你说得不错，玻尔兹曼温度不能代替热力学之温度，这正是我说奶车为赚眼球而搞的鬼。我之所以被那文章吸引，也是被”绝对零度以下“骗进去的。13肖建华 2013-1-5 14:50
    补充一句：用玻尔兹曼分布函数来定义的温度，正确的说法是：玻尔兹曼温度。
    开氏绝对温度有自身的热力学定义。
    统计物理无权把自身小学科的定义作为普遍定义或唯一定义。
    该文玩的文字游戏的隐瞒在于：把统计学的温度“转换为”开氏的绝对温度。
    统计物理学对温度的解释是不完备的，理性论据也不足。
    看到那么多评论对负绝对温度持“热情”的支持态度，特此补充。12ewq 2013-1-5 14:13
    都市玩文字游戏而已，什么绝对不绝对，等宇宙演化一切归于静止就是绝对零度11徐晓 2013-1-5 14:00
    建华友，见http://bbs.sciencenet.cn/blog-39419-649741.html我的说明10seaocean 2013-1-5 14:00
    粒子数反转和负绝对温度是激光的最主要的理论基础，具有很强的实用性。9seaocean 2013-1-5 13:53
    博主讲得不对。
    负绝对温度是统计物理学中的一个基本概念，不是谁今天才提出的。
    在量子物理学中也是一个基本内容。
    博主回复(2013-1-5 22:50)：请看8楼给出的链接 8汶伟强 2013-1-5 13:37
    http://physicsworld.com/blog/2013/01/have_physicists_ventured_below.html
    said: "So have Schneider and colleagues ventured below absolute zero? No, but they have done a nifty experiment!"7秦鸿翼 2013-1-5 13:26
    负热力学温标而已，统计物理有讲的。6王春艳 2013-1-5 13:23
    人的劣根性都一样的5张信 2013-1-5 13:15
    是楼主没有理解对吧。4徐晓 2013-1-5 13:12
    我在那篇报道下留了言，然后到Nature上留言，但是居然不给我Comment，我只好Moderate其中一条。不过如{2}所说，你搞错了3t13340033021 2013-1-5 12:37
    布居反转的即是2soifaint 2013-1-5 11:41
    温度用的是分布函数定义，虽然记者写得不是很清楚，但是你的理解是明显弄错了。1吕喆 2013-1-5 11:29
    概念游戏而已。

  19. Report this comment | #53594

    Xiao XU said:

    sorry,the comment I posted just now is in Chinese,which can't be displayed correctly.Delete it,please.If you are interested in the topic on BBS of chinese science website (in Chinese),please link to
    http://bbs.sciencenet.cn/home.php?mod=space&uid=39419&do=blog&id=649741
    ,and my opinion is put on
    http://bbs.sciencenet.cn/home.php?mod=space&uid=731678&do=blog&id=649789
    And I don't agree with Zhou Fang(comment 53546),because we can't measure the change of entropy and energy of the gas system directly,so we can't use the formular T=dU/dS to calculate the temperature directly.

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