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One might think that the optical clamp – a focused laser beam capable of trapping small particles – is now out of date. After all, the tweezers were invented by Arthur Ashkin in 1970. He was awarded the Nobel Prize this year, probably after his main implications were taken into account during the last half-century.
Surprisingly, it is far from true. The optical forceps reveal new capabilities while helping scientists understand quantum mechanics, a theory that explains nature in terms of subatomic particles.
This theory led to strange and counter-intuitive conclusions. One of them is that quantum mechanics allows a single object to exist in two different states of reality at the same time. For example, quantum physics allows a body to be simultaneously in two different places in space – or both dead and alive, as in the famous Schrödinger cat thought experiment
The technical name of this phenomenon is superposition. Overlays have been observed for tiny objects such as simple atoms. But clearly, we never see a superposition in our daily lives. For example, we do not see a cup of coffee in two places at the same time.
To explain this observation, theoretical physicists have suggested that for bulky objects, even for nanoparticles containing about a billion atoms, overpositions collapse rapidly in one or the other. One of two possibilities, due to the failure of standard quantum mechanics. For larger objects, the rate of collapse is faster. For Schrodinger's cat, this collapse – "alive" or "dead" – would be virtually instantaneous, which would explain why we never see the overlay of a cat lying in two states at once.
Until recently, these "theories of collapse". which would require modifications to the quantum mechanics of textbooks, could not be tested, as it is difficult to prepare a large object in an overlay. In fact, large objects interact more with their environment than atoms or subatomic particles, which leads to heat leaks that destroy quantum states.
As physicists, we are interested in the theories of collapse, because we would like to better understand quantum physics. specifically because there are theoretical indications that the collapse could be due to gravitational effects It would be interesting to find a link between quantum physics and gravitation, since all physics is based on these two theories and that their unified description – the so-called theory of everything – is one of the great aims of modern science.
Enter the optical clamp
The optical clamp exploits the fact that light can exert pressure on the material. Although the radiation pressure, even of an intense laser beam, is quite low, Ashkin was the first person to show that she was big enough to support a nanoparticle, thus counteracting gravity and making it levitate .
In 2010, a group of researchers realized that such a nanoparticle contained in an optical clamp was well insulated from its environment since it was not in contact with any medium equipment. Following these ideas, several groups suggested different ways to create and observe overlays of a nanoparticle at two distinct spatial locations
. An intriguing scheme proposed by the groups of Tongcang Li and Lu Ming Duan in 2013 involved a nanodiamond crystal in a tweezers. The nanoparticle does not remain motionless in the tweezers. Instead, it oscillates like a pendulum between two places, the restoring force coming from the radiation pressure due to the laser. In addition, this diamond nanocrystal contains a contaminating nitrogen atom, which can be considered a tiny magnet, with a north pole (N) and a south pole (S).
Li-Duan's strategy consisted of three stages. First, they proposed to cool the movement of the nanoparticle to its quantum ground state. It is the lowest energy state that can have this type of particle. We could expect that in this state, the particle stops moving and does not oscillate at all. However, if that happens, we would know where the particle was (in the center of the tweezers) and how fast it was moving (not at all). But the famous Heisenberg uncertainty principle of quantum physics does not allow a perfect simultaneous knowledge of position and velocity. Thus, even in its lowest energy state, the particle moves a little, just enough to satisfy the laws of quantum mechanics.
Second, Li and Duan's scheme required that the magnetic nitrogen atom be prepared in a superposition of its north pole, oriented upward and downward.
Finally, a magnetic field was needed to connect the nitrogen atom to the movement of the diamond crystal levity. This would transfer the magnetic superposition of the atom to the superposition of the nanocrystal. This transfer is made possible by the fact that the atom and the nanoparticle are entangled in the magnetic field. It occurs in the same way that the superposition of the decomposed and undecomposed radioactive sample is converted into an overlay of the Schrodinger's cat in dead and alive states.
Demonstration of the Theory of Collapse
What gave this theoretical work of teeth are two exciting experimental developments. Already in 2012, the groups of Lukas Novotny and Romain Quidant had shown that it was possible to cool a nanoparticle with optical levitation up to one hundredth of a degree above absolute zero – the lowest temperature theoretically possible – by modulating the intensity of the optical clamp. The effect was the same as slowing a child on a swing by pushing him at the right time.
In 2016, the same researchers managed to cool down a ten thousandth of a degree above absolute zero. Around this time, our groups released a paper establishing that the temperature required to reach the quantum ground state of a pinched nanoparticle was about a millionth of a degree above absolute zero. This requirement is difficult, but within the scope of current experiments.
The second interesting development is the experimental levitation of a nanodiamond bearing nitrogen defects in 2014 in Nick Vamivakas' group. Using a magnetic field, they were also able to perform the physical coupling of the nitrogen atom and the crystal motion required by the third step of the Li-Duan scheme. according to Li-Duan's plan – an object in two places can be observed falling apart into a single entity. If the superimpositions are destroyed at the rate predicted by the theories of collapse, quantum mechanics as we know it will have to be revised.
Explore Further:
How Einstein's principle of equivalence extends to the quantum world
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