⊆From tunnelling through impenetrable barriers to being in two places at the same time, the quantum world of atoms and particles is famously bizarre. Yet the strange properties of quantum mechanics are not mathematical quirks – they are real effects that have been seen in laboratories over and over.
One of the most iconic features of quantum mechanics is “entanglement” – describing particles that are mysteriously linked regardless of how far away from each other they are. Now three independent European research groups have managed to entangle not just a pair of particles, but separated clouds of thousands of atoms. They’ve also found a way to harness their technological potential.
When particles are entangled they share properties in a way that makes them dependent on each other, even when they are separated by large distances. Einstein famously called entanglement “spooky action at a distance”, as altering one particle in an entangled pair affects its twin instantaneously – no matter how far away it is.
While entanglement may sound wacky, experiments have been able to show that it exists for many years now. It also has the potential to be exceptionally useful – particles linked in this way can be used to transfer a particle’s quantum state, such as spin, from one location to another immediately (teleportation). They can also help store a huge amount of information in a given volume (super-dense coding).
Along with this storage capacity, entanglement can also help link and combine the computing power of systems in different parts of the globe. It is easy to see how that makes it a crucial aspect of quantum computation. Another promising avenue is truly secure communications. That’s because any attempt to interfere with systems involving entangled particles immediately disrupts the entanglement, making it obvious that a message has been tampered with.
It is also possible to use entangled photons to enhance the resolution of imaging techniques. Researchers at the University of Waterloo are currently hoping to develop a quantum radar that may be capable of detecting stealth aircraft.
Delivering on the promises of entanglement-based technologies, however, is proving to be difficult. That’s because entanglement is a very fragile phenomenon. Experiments on entanglement typically produce individual pairs of particles. However, single particles are difficult to detect accurately and they are often lost or obscured by background noise. So the task of producing them in entangled states, manipulating them in the ways required for useful operations, and finally using them, is often daunting.
Instead of taking single particles and entangling them one at a time, the researchers begin with an ultra-cold gas – a collection of thousands of atoms. These are cooled to within a hair’s breadth of absolute zero, the lowest temperature possible.
When confined in a small volume, atoms in such a cloud become indistinguishable from each other, forming a new state of matter known as a Bose-Einstein condensate. The atoms in the cloud now behave collectively – they are entangled. Scientists first discovered this state of matter in 1995, earning them the Nobel Prize in Physics in 2001. Although it has been known for some time that this method entangles thousands of atoms simultaneously, no one had demonstrated a technique to actually make use of it – until now.
The researchers behind the new study showed that you can split these clouds into groups and still preserve the quantum connection between the atoms inside. They did this by releasing the atoms from their confined space and using a laser to split it and measure the properties of distant parts of the expanded cloud.
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The researchers speculate that the methods developed could be expanded to allow every atom from the cloud to be used independently – if this were achieved, then there would be huge benefits for quantum computing. In digital computing, information is processed as ones and zeros, binary digits (or bits). The analogue to these in quantum computing are known as qubits. The current record for producing qubits one-by-one in entangled states for ions (charged atoms) is just 20, so producing thousands of qubits simultaneously in a cloud like this would represent a huge advancement.
Another field that will benefit from this breakthrough is metrology, the science of ultra-precise measurements. When entanglement is established between two particles or systems, measurements made on one half reveal information about the other. This allows parameters to be measured with greater sensitivity than would otherwise be possible. Using entanglement in this way could improve the accuracy of atomic clocks and with it the global position system
(GPS), or make more sensitive detectors for MRI machines, for example.
请参照译文：科学家们在量子物理“幽灵般”的理论中取得突破
从隧道穿越到两个地方的不可逾越的障碍，原子和粒子的量子世界是出了名的怪异。然而量子力学的奇异特性并不是数学上的怪僻——它们是在实验室中反复出现的真实效应。
量子力学最具标志性的特征之一就是“纠缠”——描述那些神秘地连在一起的粒子，不管它们相距多远。现在，三个独立的欧洲研究小组已经成功地将数千个原子的云分离开来，而不仅仅是一对粒子。他们也找到了一种利用他们的技术潜力的方法。
当粒子纠缠在一起时，它们会以一种相互依赖的方式分享属性，即使它们被远距离分离。爱因斯坦著名的称为纠缠“幽灵般的行动在远处”，因为改变一个粒子在一个纠缠的一对影响它的孪生瞬间-不管它有多远。
“量子随机性”机器产生了有史以来最不可预测的数字。
虽然纠缠可能听起来很古怪，但实验已经证明它已经存在多年了。它也有可能成为特别有用的粒子——以这种方式连接的粒子可以用来转移粒子的量子态，比如自旋，从一个位置到另一个位置(瞬间移动)。它们还可以在给定的卷(超密集编码)中存储大量信息。
除了这种存储容量之外，缠绕还可以帮助连接和结合全球不同地区的系统的计算能力。很容易看出这是如何使它成为量子计算的一个关键方面。另一个有前途的途径是真正的安全通讯。这是因为任何试图干扰包含纠缠粒子的系统的尝试都会立即扰乱这种纠缠，使信息明显被篡改。
还可以使用纠缠光子来提高成像技术的分辨率。滑铁卢大学的研究人员目前希望研制一种能够探测隐形飞机的量子雷达。
然而，事实证明，实现基于纠缠的技术的承诺是困难的。这是因为缠结是一种非常脆弱的现象。关于纠缠的实验通常产生单个的粒子对。然而，单粒子很难准确地探测到，而且常常由于背景噪声而丢失或模糊。因此，在纠缠态中产生它们的任务，以有用的操作所需要的方式来操纵它们，最后使用它们，常常是令人生畏的。
这是发表在《科学》三篇论文中的新研究(你可以在这里和这里看到)，取得了重大突破。研究人员开始使用一种超冷气体——一种由数千个原子组成的集合，而不是单一的粒子，一次纠缠一个粒子。这些冷却到头发的绝对零度，最低的温度可能。
当被限制在一个小体积内，这样的云中的原子变得难以区分，形成了一种新的物质状态，称为玻色-爱因斯坦凝聚态。云中的原子现在是集体行为——它们被纠缠在一起。科学家们于1995年首次发现了这种物质状态，并于2001年获得了诺贝尔物理学奖。尽管这一方法在一段时间内就已经被人们所知，它同时使成千上万的原子同时存在，但至今还没有人演示一种真正利用它的技术。
这项新研究的研究人员表明，你可以将这些云分成不同的组，并且仍然可以保持内部原子之间的量子联系。他们这样做的方法是将原子从封闭空间中释放出来，并利用激光将其分离，并测量膨胀云的远端部分的性质。
研究人员推测，开发的方法可以被扩展，使每一个原子都能独立使用——如果实现了，那么量子计算将带来巨大的好处。在数字计算中，信息被处理为1和0，二进制数字(或比特)。量子计算中的类似物被称为量子位。目前在纠缠态的离子(带电原子)中一个一个的产生量子比特的记录只有20个，因此在这样的云中同时产生成千上万的量子位将会是一个巨大的进步。
另一个受益于这项突破的领域是计量学，即超精确测量的科学。当在两个粒子或系统之间建立纠缠时，对其中一半的测量会显示另一个粒子的信息。这使得可以用更大的灵敏度来测量参数。以这种方式使用纠错可以提高原子钟的精确度，并能提高全球定位系统(GPS)的精确度，或者为核磁共振机器制造更灵敏的探测器。
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