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Atomic tweezers — levitated optomechanics

John Healy

[“Nanotech” series]

I’ve been following articles for awhile about micron, nanometer, and atomic level confinement and manipulation.

The development of “optical tweezers” facilitated exploration of biological particles with sizes in the micrometer and nanometer range such as viruses and bacteria and subcellular components.

Optical traps also facilitated exploring properties of trapped individual molecules and atoms.

Related techniques have been used to better understand the physics of exotic condensed matter such as the quantum properties of 1d and 2d collections of atoms. Research in manipulating quantum properties of individual atoms is particularly fascinating.

So, to start this topic, here’s an APS article on cooling nanoparticles: “Viewpoint: Nanoparticles Get Cool by Light Scattering” (March 27, 2019).

Researchers performed 3D cavity cooling of levitated nanoparticles, reaching record low temperatures by utilizing light that scatters off the particles.

Arthur Ashkin pioneered the optical manipulation of small particles with the development of optical tweezers, for which he was awarded the 2018 Nobel Prize in Physics. (See 4 October 2018 Focus story.) The ability to control small particles with tweezers and other optical tools has enabled many breakthroughs in biology, physical chemistry, and atomic, molecular, and optical physics. As part of this trend, researchers have developed ways to “cool” trapped nanoparticles by reducing the amplitude of their motion within the trap. However, effort is still needed to reach the quantum limit where the motion is dominated by quantum fluctuations. A new method … is promising to reduce the motion of a levitated nanoparticle to its quantum-mechanical ground state.

Key factors in levitated optomechanics:

• Isolation from the thermal environment (air molecules, vibration)
• Position stability of optical tweezer
• Cavity cooling vs. feedback cooling
• Coherent light scattering
• Optical cavity tuning
• Particle position monitoring

Notes

I’ve already encountered articles which discuss sorting and assembly of individual atoms and fabrication of macroscopic layers only a single atom in thickness.

Ultracold atom [https://en.wikipedia.org/wiki/Ultracold_atom]

Ultracold atoms are atoms that are maintained at temperatures close to 0 kelvins (absolute zero), typically below temperatures of some tenths of microkelvins (µK). At these temperatures the atom’s quantum-mechanical properties become important.

To reach such low temperatures, a combination of several techniques has to be used. First, atoms are usually trapped and pre-cooled via laser cooling in a magneto-optical trap. To reach the lowest possible temperature, further cooling is performed using evaporative cooling in a magnetic or optical trap.

Fermi–Dirac statistics[https://en.wikipedia.org/wiki/Fermi–Dirac_statistics]

In quantum statistics, a branch of physics, Fermi–Dirac statistics describe a distribution of particles over energy states in systems consisting of many identical particles that obey the Pauli exclusion principle.

Bose–Einstein statistics [https://en.wikipedia.org/wiki/Bose–Einstein_statistics]

In quantum statistics, Bose–Einstein statistics (or colloquially B–E statistics) is one of two possible ways in which a collection of non-interacting indistinguishable particles may occupy a set of available discrete energy states, at thermodynamic equilibrium. The aggregation of particles in the same state, … a collection of identical and indistinguishable particles …

Pauli exclusion principle [https://en.wikipedia.org/wiki/Pauli_exclusion_principle]

The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state within a quantum system simultaneously.

A more rigorous statement is that with respect to exchange of two identical particles the total wave function is antisymmetric for fermions, and symmetric for bosons. This means that if the space and spin co-ordinates of two identical particles are interchanged, then the wave function changes its sign for fermions and does not change for bosons.

5 thoughts on “Atomic tweezers — levitated optomechanics”

  1. Here’s a Phys.org article about putting “one atom inside each of two laser beams before moving them together” — which I assume uses optomechanics: “Atom interaction discovery valuable for future quantum technologies” by University of Otago published on April 24, 2019.

    In a study, just published in Nature Communications, researchers put one atom inside each of two laser beams before moving them together until they started to interact with each other.

    Co-author Associate Professor Mikkel F. Andersen, of the Department of Physics, says this allows the atoms to exchange properties in a way which could be “very useful” for future quantum technologies.

    As atoms are like magnets, when the pair start interacting, they start changing each other’s direction, counterbalancing each other.

    “Assembling small physical systems atom by atom, in a controlled way, opens up a wealth of research directions and opportunities that are not otherwise possible. It also leads to the atoms displaying different behaviours than if they were one of many in the system,” Dr. Andersen says.

    This includes a finite-temperature quantum entanglement resource. This is significant because entangled particles remain connected, even over great distances, and actions performed on one affect the other.

  2. Here’s another article on research based on use of optical cavities.

    Caltech > News > “Tiny Optical Cavity Could Make Quantum Networks Possible” by Robert Perkins (March 30, 2020) – Engineers at Caltech have shown that atoms in optical cavities – tiny boxes for light – could be foundational to the creation of a quantum internet. Their work was published on March 30 by the journal Nature [“Control and single-shot readout of an ion embedded in a nanophotonic cavity”].

    In order to work, a quantum network needs to be able to transmit information between two points without altering the quantum properties of the information being transmitted. One current model works like this: a single atom or ion acts as a quantum bit (or “qubit”) storing information via one if its quantum properties, such as spin. To read that information and transmit it elsewhere, the atom is excited with a pulse of light, causing it to emit a photon whose spin is entangled with the spin of the atom. The photon can then transmit the information entangled with the atom over a long distance via fiber optic cable.

    … researchers led by Caltech’s Andrei Faraon, professor of applied physics and electrical engineering, constructed a nanophotonic cavity, a beam that is about 10 microns in length with periodic nano-patterning, sculpted from a piece of crystal. They then identified a rare-earth ytterbium ion in the center of the beam. The optical cavity allows them to bounce light back and forth down the beam multiple times until it is finally absorbed by the ion.

    … the ytterbium ions are able to store information in their spin for 30 milliseconds. In this time, light could transmit information to travel across the continental United States.

  3. Here’s an interesting article on experiments regarding quantum computing, but particularly about manipulating ultra-cold rubidium atoms (a favorite for physics research) in a coherent state using so-called optical tweezers.

    • SciTechDaily > “Harvard-MIT Quantum Computing Breakthrough – ‘We Are Entering a Completely New Part of the Quantum World’” by Harvard University (July 9, 2021)

    (quote) This new system allows the atoms to be assembled in two-dimensional arrays of optical tweezers. This increases the achievable system size from 51 to 256 qubits. Using the tweezers, researchers can arrange the atoms in defect-free patterns and create programmable shapes like square, honeycomb, or triangular lattices to engineer different interactions between the qubits.

    (caption) Dolev Bluvstein looks at 420 mm laser that allows them to control and entangle Rydberg atoms. Credit: Harvard University

    “The workhorse of this new platform is a device called the spatial light modulator, which is used to shape an optical wavefront to produce hundreds of individually focused optical tweezer beams,” said Ebadi [physics student in the Graduate School of Arts and Sciences].

    Lasers give the researchers complete control over the positioning of the atomic qubits and their coherent quantum manipulation.

    This work was supported by the Center for Ultracold Atoms, the National Science Foundation, the Vannevar Bush Faculty Fellowship, the U.S. Department of Energy, the Office of Naval Research, the Army Research Office MURI, and the DARPA ONISQ program.

    References

    As referenced elsewhere (in comments for How to Create Entangled Photon Pairs), Arthur Ashkin was awarded the 2018 Nobel Prize in Physics (with two others) for “for groundbreaking inventions in the field of laser physics”, in particular “for the optical tweezers and their application to biological systems.”

    • Wiki

    Rubidium metal is easily vaporized and has a convenient spectral absorption range, making it a frequent target for laser manipulation of atoms.

    • Physics World > “The rise of Rydberg physics” by Keith Cooper (April 7, 2016)

    (quote) For physicists chasing the holy grail of quantum computing, one tasty recipe is becoming increasingly widespread. Sprinkle a handful of atoms – rubidium is a popular ingredient – into a vacuum chamber. Treat with laser beams to cool the atoms to mere fractions of a degree above absolute zero. Then add a couple of photons and hey presto – you’ve created one of the basic building blocks of a quantum computer.

    At least, “that’s the basic idea”, says Mark Saffman, an atomic physicist at the University of Wisconsin–Madison in the US. Central to it all are Rydberg atoms, which have a single outer valence electron that can be excited to higher quantum states. They’re the big daddies of the atomic world. Typically an atomic nucleus is femtometres in size, but in a Rydberg atom the excited valence electron can travel microns from the nucleus while still remaining bound to it, ballooning the atomic radius a billion-fold in size. With such a great reach, a Rydberg atom can interact with other nearby atoms via a powerful electric dipole moment a million times better than “ordinary” atoms. It’s this interactive power – and the ability to control it with a single, carefully chosen photon – that makes Rydberg atoms such a potent force in the world of quantum information systems.

  4. Another role for rubidium in research, in this case rubidium-87 and evolution of the solar system and our planet. And nanometer-sized grains.

    • Caltech Weekly > Caltech > “New Type of Stellar Grain Discovered” (July 9, 2021)

    (quote) A team led by cosmochemists from Caltech and Victoria University of Wellington in New Zealand studied ancient minerals aggregates within the Allende meteorite (which fell to Earth in 1969) and found that many of them had unusually high amounts of strontium-84, a relatively rare light isotope of the element strontium …

    Rubidium-87 has a very long half-life, 49 billion years, which is more than three times the age of the universe.

    What is particularly attractive about using the Rb–Sr pair for dating is that rubidium is a volatile element—that is, it tends to evaporate to form a gas phase at even relatively low temperatures—while strontium is not volatile.

  5. How to control individual atoms?

    • NIST > News > “NIST Physicist Adam Kaufman Wins Breakthrough New Horizons in Physics Prize” (September 22, 2022)

    Boulder, Colo. — Physicist Adam Kaufman of the U.S. Department of Commerce’s National Institute of Standards and Technology (NIST) has been awarded the 2023 New Horizons in Physics Prize from the Breakthrough Prize Foundation for his work in advancing the control of atoms and molecules to improve atomic clocks and quantum information processing.

    Kaufman shares the $100,000 prize with selected winners, all from different institutions (Hannes Bernien, Manuel Endres [1], Kang-Kuen Ni, Hannes Pichler and Jeff Thompson). The prize committee selected the winners “for the development of optical tweezer arrays to realize control of individual atoms for applications in quantum information science, metrology, and molecular physics.”

    Physicists are continually looking for better ways to isolate and control the molecules, ions and atoms that make up the quantum world. Optical tweezers are very tightly focused beams of light that can quickly trap and manipulate atoms and molecules. Arthur Ashkin, who developed optical tweezers while at Bell Laboratories, received a Nobel Prize in Physics in 2018 for the invention.

    Notes

    [1] Manuel Endres, Caltech professor of physics and Rosenberg scholar, won “for the development of optical tweezer arrays to realize control of individual atoms.”

    Optical tweezers cartoon
    Credit: Composited from Pixabay/CC0 Public Domain

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