Cooling in the near-fundamental state of 2D trapped ion crystals


An illustration of what happens to the ionic crystal when it is cooled to the EIT. For simplicity, only the ions of the central row of the crystal are represented, but it is necessary to imagine an ion at each intersection of the network. Initially, the crystal folds up and down like a vibrating drum skin. This is an example of drumhead mode. Then the researchers apply the cooling lasers (red lines). Due to laser alignment, the direction of cooling is perpendicular to the plane of the crystal, parallel to the direction of movement of the drumhead. After cooling, the amplitude of the movement of the drum skin of the crystal is very small and, in the figure, it is represented as being flat. Credit: Jordan et al.

Researchers have been trying to cool the macroscopic mechanical oscillators to their ground state for several decades. Nevertheless, previous studies have simply allowed to cool some selected vibration modes of these oscillators.

A team of researchers from the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder recently conducted a study on near-crystal cooling of crystal crystals. 2-dimensional (2D) trapped ions with more than 100 ions. The success of their cooling experiment lays the foundation for improved quantum simulation and two-dimensional array detection of hundreds of ions trapped in a Penning trap.

Penning traps are devices capable of storing charged particles by applying a strong magnetic field. These devices can control crystals from tens to hundreds of ions, a quality that makes them versatile quantum simulators. In their study, researchers at NIST and UC Boulder managed to cool down all the "drumhead" modes of a thin 2D crystal containing more than 150 beryllium (Be+), stored in a Penning trap.

"We used Doppler laser cooling to cool the ions close to the Doppler cooling limit, and at these low temperatures the ions naturally form a Coulomb crystal," Elena Jordan told researchers who conducted the study. "A crystal with N ions has 3N modes of motion.The 2N modes are in the crystal plane and look like swirls or distortions, the N modes are perpendicular to the plane of the crystal and resemble the modes of the drumhead. For quantum simulations, we couple these modes to the ions' rotates. "

The researchers observed that lowering the temperature of drum skin modes below the Doppler limit could improve quantum simulations of 2D spin models. They therefore decided to implement an efficient under-Doppler cooling technique, which would allow them to cool the ions to the lowest possible temperature.

"Recently, Regina Lechner et al., Of the University of Innsbruck, Austria, cooled 18-ion linear chains with Electromagnetically Induced Light (EIT) cooling," said Jordan. "This encouraged us to think about the application of this technique to two-dimensional systems containing hundreds of ions."

Inspired by research conducted at the University of Innsbruck, Jordan and his colleagues Athreya Shankar, Arghavan Safavi-Naini and Murray Holland of JILA have begun to theoretically study the possibility of EIT cooling of all modes. head of a 2D ion crystal battery. turning inside a Penning trap. They soon discovered that the existing theory was insufficient to describe the cooling process of this system and therefore began to develop new models.

"Athreya has developed new theoretical models and performed simulations that have shown that cooling of all drum skin modes should be possible without altering the experimental parameters of cooling, which means that no frequency shift or Laser power variation would only be needed, "explained Jordan. "Surprisingly, the theory predicts that the cooling of a multi-ion crystal should be faster than that of a single ion.Our results have prompted us to implement cooling at the same time. EIT and subsequent experiments have shown that cooling works not only very well in simulations, but also in our real Penning trap ".

The experiment described in the study was conducted by Jordan alongside his colleagues Kevin Gilmore, Justin Bohnet and John Bollinger, in their laboratory at NIST. The beryllium ions were confined in the axis of their Penning trap by a static electric field, as well as by a strong magnetic field (4.5 T) parallel to the axis of the trap. The movement of the ions in the magnetic field generates a Lorentz force which makes them spin in the trap while remaining radially confined.

"For EIT cooling, we used two lasers to couple atomic states in beryllium to generate quantum interference and create a" dark state "that does not couple to lasers and can be used for EIT cooling." , Jordan explained. "The two beams arrive at an angle of ± 10 degrees to the plane of the crystals."

Penning trap cup used by researchers, with laser beams for cooling and temperature measurement. Credit: Jordan et al.

The rotation of the ions in the Penning trap causes a Doppler shift in time-varying laser frequencies. To achieve efficient cooling despite this Doppler shift, the researchers detuned the lasers from the atomic resonance greater than the maximum Doppler shift and adjusted the laser powers so that the cooling condition of the EIT could be met.

They measured the temperature of the ions using an additional pair of laser beams, which coupled the ion spins to their drumhead motion. This coupling leads to a spin phase shift signal that can be measured and used to extract the temperature of the ions.

"After 200 micro-seconds of cooling at the EIT, all the modes of the ion crystal drum head are cooled near the ground state, as can be seen by comparing the experimental data to the model theoretical, "said Jordan. "Cooling is as efficient as the predicted theory and cooling of all skin modes is achieved without changing the experimental parameters."

The experiment conducted by Jordan and his colleagues has yielded remarkable results, confirming their theoretical predictions. The cooling rate measured by them was faster than that predicted by the single particle theory, but was compatible with a multi-body quantum computation.

"The results of our study are important both from a fundamental and practical point of view," said Athreya Shankar, another researcher involved in the study, at "From a fundamental point of view, the cooling of mechanical oscillators close to their quantum ground state has been actively sought for three decades now.Many experiments have managed to cool one or more modes of displacement close to the ground state , while simultaneously cooling many modes of a medium We have prepared a mesoscopic trapped ion oscillator whose motion has been almost frozen within the limits of what quantum mechanics basically allows. "

According to Athreya, the study by him and his colleagues could also have important practical implications. EIT cooling transforms their trapped ionic crystal into an improved platform for quantum simulations and detection, dramatically reducing the background thermal motion that generally hinders the performance of scientific protocols.

"The success of our experiment shows that cooling at the EIT is a robust technique that is not limited to one or a few ions in a trap," said Athreya. "The success of the technique with hundreds of ions in a challenging environment like the Penning trap is an encouraging indication that large ion crystals in other experiments on trapped ions could also be efficiently cooled and used to probe fundamental and multiple-body quantum physics. "

Researchers are currently working on the use of their ionic crystal as a sensitive detector of electric fields. Some candidates for dark matter, such as photons and hidden axions, can produce very weak electric fields, so their device could make it easier to find dark matter.

"We will also return to the engineering interactions between our ions to simulate in the laboratory a complex physics that it is difficult or impossible to model on a conventional (non-quantum) computer – what we call "quantum simulation", Gilmore told . "In both cases, the cooling at the EIT will play an important role for us.For the electric field detection experiment, we use the movement of ions caused by the electric forces exerted on them to perform our measured."

The ions have a thermal movement, which depends on their temperature, which can be a source of noise in the experiments. The researchers discovered that cooling by the EIT can reduce this background signal caused by thermal movement, improving and simplifying measurements. In an earlier study, researchers successfully detected weak electric fields using a similar method to that used for temperature measurement. In the future, the same device could be used to detect even weaker electric fields, as well as potentially to search for new physics.

"Quantum simulation style experiments also benefit from this reduced thermal noise," explained Gilmore. "Such experiments rely on the production of weak quantum correlations, or links, between ions, which can be disrupted or destroyed by thermal movement, degrading the quality of the simulation. reduce temperatures. "

Explore further:
Crystal "flash-freeze" of physicists of 150 ions

More information:
Elena Jordan et al. Near-ground cooling of two-dimensional trapped-ion crystals of more than 100 ions, Letters of physical examination (2019). DOI: 10.1103 / PhysRevLett.122.053603

Athreya Shankar et al. Modeling the ground – state cooling of two – dimensional ion crystals in a Penning trap using electromagnetically induced transparency, Physical examination A (2019). DOI: 10.1103 / PhysRevA.99.023409

Regina Lechner et al. Cooling in the ground state by electromagnetic transparency of long strands of ions, Physical examination A (2016). DOI: 10.1103 / PhysRevA.93.053401

K. Gilmore, A. et al. Detection of amplitude below zero point fluctuations with a trapped ion two-dimensional mechanical oscillator, Letters of physical examination (2017). DOI: 10.1103 / PhysRevLett.118.263602

Journal reference:
Letters of physical examination

Physical examination A

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