Atoms queue up for quantum computer networks

The experiment is carried out in a glass cell with very low pressure. In here is an ultra-thin glass fiber and a gas of cesium atoms. Using lasers and a magnetic field, the atoms are cooled down to almost absolute zero (minus 273 degrees Celsius) and the atoms gather as a cloud around the glass fiber. Then two laser beams with very different frequencies are transmitted into the fiber, thereby capturing atoms above the fiber surface. By measuring the difference in the speed of light for two other light beams on each side of the atoms' absorption line, you can measure the number of atoms.
Credit: Ola Jakup Joensen, Niels Bohr Institute

In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute have developed a method with a trap that captures the atoms along an ultra thin glass fiber, where the atoms can be controlled. The results are published in the scientific journal, Physical Review Letters.

The research is carried out in the quantum optics laboratory in the basement of the Niels Bohr Institute in Copenhagen. The underground laboratory is set back from the road so there are no vibrations from traffic. Here, the researchers have designed experiments in which they can perform ultrasensitive trials with quantum optics.

"We have an ultra-thin glass fiber with a diameter of half a micrometer (a hundred times smaller than a strand of hair). Along this glass fiber we capture cesium atoms. They are cooled down to 100 micro Kelvin using a laser -- this is almost absolute zero, which is equivalent to minus 273 degrees Celsius. This system acts like a trap that holds the atoms on the side of the glass fiber," explains Jürgen Appel, Associate Professor in the research group Quantop at the Niels Bohr Institute, University of Copenhagen.

Atoms and light linked together

When light is transmitted through the glass fiber thread, the light will also move along the surface because the fiber is thinner than wavelength of the light. This creates strong interaction between the light and the atoms sitting securely above the surface of the fiber.

"We have developed a method where we can measure the number of atoms. We send two laser beams with different frequencies through the glass fiber. If there were no atoms on the fiber, the speed of light would be the same for both light beams. However, the atoms affect the two frequencies differently and by measuring the difference in the speed of light for the two light beams on each side of the atoms' absorption lines, you can measure the number of atoms along the fiber. We have shown that we can hold 2,500 atoms with an uncertainty of just eight atoms," says Jürgen Appel.

These are fantastic results. Without this method, you would have to use resonant light (light that the atoms absorb) and then you would scatter photons, which would kick the atoms out of the trap, says Jürgen Appel and explains that with this new method they can measure and control the atoms so that only 14 percent are kicked out of the trap and are lost.

"Our resolution is only limited by the natural quantum noise (the laser light's own minimal fluctuations) so our method could be used for so-called entangled states of atoms along the fiber. Such an entangled system with strongly interacting atoms and light is of great interest for future quantum computer networks," notes Jürgen Appel.

Source: University of Copenhagen - Niels Bohr Institute

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Quantum physics just got less complicated: Wave-particle duality and quantum uncertainty are same thing

Quantum physics says that particles can behave like waves, and vice versa. Researchers have now shown that this 'wave-particle duality' is simply the quantum uncertainty principle in disguise. Credit: Timothy Yeo / CQT, National University of Singapore; CC-BY

Here's a nice surprise: quantum physics is less complicated than we thought. An international team of researchers has proved that two peculiar features of the quantum world previously considered distinct are different manifestations of the same thing. The result is published 19 December in Nature Communications.

Patrick Coles, Jedrzej Kaniewski, and Stephanie Wehner made the breakthrough while at the Centre for Quantum Technologies at the National University of Singapore. They found that 'wave-particle duality' is simply the quantum 'uncertainty principle' in disguise, reducing two mysteries to one.

"The connection between uncertainty and wave-particle duality comes out very naturally when you consider them as questions about what information you can gain about a system. Our result highlights the power of thinking about physics from the perspective of information," says Wehner, who is now an Associate Professor at QuTech at the Delft University of Technology in the Netherlands.

The discovery deepens our understanding of quantum physics and could prompt ideas for new applications of wave-particle duality.

Wave-particle duality is the idea that a quantum object can behave like a wave, but that the wave behaviour disappears if you try to locate the object. It's most simply seen in a double slit experiment, where single particles, electrons, say, are fired one by one at a screen containing two narrow slits. The particles pile up behind the slits not in two heaps as classical objects would, but in a stripy pattern like you'd expect for waves interfering. At least this is what happens until you sneak a look at which slit a particle goes through -- do that and the interference pattern vanishes.

The quantum uncertainty principle is the idea that it's impossible to know certain pairs of things about a quantum particle at once. For example, the more precisely you know the position of an atom, the less precisely you can know the speed with which it's moving. It's a limit on the fundamental knowability of nature, not a statement on measurement skill. The new work shows that how much you can learn about the wave versus the particle behaviour of a system is constrained in exactly the same way.

Wave-particle duality and uncertainty have been fundamental concepts in quantum physics since the early 1900s. "We were guided by a gut feeling, and only a gut feeling, that there should be a connection," says Coles, who is now a Postdoctoral Fellow at the Institute for Quantum Computing in Waterloo, Canada.

It's possible to write equations that capture how much can be learned about pairs of properties that are affected by the uncertainty principle. Coles, Kaniewski and Wehner are experts in a form of such equations known as 'entropic uncertainty relations', and they discovered that all the maths previously used to describe wave-particle duality could be reformulated in terms of these relations.

"It was like we had discovered the 'Rosetta Stone' that connected two different languages," says Coles. "The literature on wave-particle duality was like hieroglyphics that we could now translate into our native tongue. We had several eureka moments when we finally understood what people had done," he says.

Because the entropic uncertainty relations used in their translation have also been used in proving the security of quantum cryptography -- schemes for secure communication using quantum particles -- the researchers suggest the work could help inspire new cryptography protocols.

In earlier papers, Wehner and collaborators found connections between the uncertainty principle and other physics, namely quantum 'non-locality' and the second law of thermodynamics. The tantalising next goal for the researchers is to think about how these pieces fit together and what bigger picture that paints of how nature is constructed.

Source: Centre for Quantum Technologies at the National University of Singapore

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