https://www.nature.com/articles/d41586-021-02267-2 Skip to main content Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Advertisement Advertisement Nature * View all journals * Search * My Account Login * Explore content * Journal information * Publish with us Subscribe * Sign up for alerts * RSS feed 1. nature 2. news & views 3. article * NEWS AND VIEWS * 25 August 2021 Single proton cooled by distant ions Laser-cooled ions have been used to substantially lower the temperature of a proton located several centimetres away. This technique could be useful in ultraprecise measurements of the properties of antimatter particles. * Manas Mukherjee ^0 1. Manas Mukherjee 1. Manas Mukherjee is at the Centre for Quantum Technologies, National University of Singapore, 117543 Singapore, and at the Center for Quantum Engineering Research and Education, Kolkata, India. View author publications You can also search for this author in PubMed Google Scholar * Share on Twitter * Share on Facebook * Share via E-Mail You have full access to this article via your institution. Download PDF Download PDF In a paper in Nature, Bohman et al.^1 (the BASE Collaboration) report the cooling of a single proton by a cloud of laser-cooled beryllium ions. Remarkably, the ions were separated from the proton by a distance of about 9 centimetres -- which is too far apart for the charges on the ions to have interacted with that of the proton. This means that the ions could not have exerted a direct cooling effect on the proton. Instead, the researchers used an indirect cooling process, mediated by an electric circuit that established an effective interaction. This approach has potential applications in studies of antimatter particles and in the field of quantum information. [d41586-021] Read the paper: Sympathetic cooling of a trapped proton mediated by an LC circuit A wealth of knowledge about nature's inner workings comes from studies of fundamental particles, such as electrons and protons. Currently, the most accurate theoretical model of the forces of nature is the standard model of particle physics, which describes how fundamental particles interact with each other and thereby build up the macroscopic world. The standard model has passed many stringent tests using various experimental tools, at particle energies that range from 10^11 electronvolts in particle accelerators^2 to only about 0.0001 electronvolts in ion traps^3. However, it is widely accepted that the standard model does not explain some natural phenomena, such as the fact that the Universe is made up of only matter. It also does not account for the existence of dark matter -- the invisible and largely unaccounted for mass of the Universe. High-precision measurements of fundamental particles and their corresponding antiparticles provide opportunities to verify the standard model, and maybe even to find evidence of new physics that goes beyond the currently accepted model^4^,^5. Two conditions must be met to perform such measurements: the particles must be spatially confined; and they should be very nearly at rest (that is, the particles must be cooled to almost zero kelvin, to minimize their kinetic energy). The first of these requirements can be solved using a combination of static electric and magnetic fields in a device called a Penning trap. Laser-cooling methods were first reported^6^,^7 in 1975, and have since been widely developed to reduce the motion of particles. This approach works well for atoms, but not for particles that do not absorb light, such as protons. Scientists have therefore invented other cooling methods, such as resistive cooling^8 (in which ions dissipate their energy by inducing a current in a cold electric circuit) and synchrotron cooling^9 (in which fast-rotating particles with low mass radiate energy by emitting electromagnetic radiation). However, the lowest particle temperatures achieved using those approaches are roughly 1,000 to one million times higher than those of laser-cooled atoms. [d41586-021] Measurement-based system provides quantum control of nanoparticles An interesting alternative is to cool a charged particle by bringing it close to another, colder charged particle^10 -- an approach commonly known as sympathetic cooling. For example, consider a positively charged atomic ion that is being continuously laser-cooled to one-thousandth of a kelvin, and which is then brought close to a proton that is initially at 4 K in an ion trap. The proton and ion will repel each other within the confinement of the ion trap, effectively transferring kinetic energy from the proton to the ion. Because the ion is constantly being laser-cooled, the repulsive interactions will eventually chill the proton to the same temperature as the ion, even though the proton is not being cooled directly. Sympathetic cooling works well, but the nearby presence of an ion would be undesirable when making ultraprecise measurements of a proton's properties. Furthermore, the method requires that the particle and the ion have charges of the same polarity, to provide the necessary repulsive interactions. Bohman and colleagues' work provides a potential solution to these issues. The authors used separate Penning traps to confine a cloud of beryllium ions and a proton in an ultrahigh vacuum, and continuously laser-cooled the ions (Fig. 1). The proton and the ions were then set up to 'talk' to an electrical resonator circuit, which enables the two trapped-particle systems to interact only when the natural oscillation frequencies (the resonance frequencies) of the two systems match exactly. Bohman et al. demonstrated the influence of the ions on the proton using an established technique, in which electrical 'noise' in the resonator circuit is analysed to directly determine the temperatures of the two systems. figure 1 Figure 1 | Sympathetic cooling at a distance. Bohman et al.^1 used devices called Penning traps to capture a proton at 17 kelvin and a cloud of beryllium ions, which was continuously cooled by a laser to a much lower temperature. The traps were connected by a wire to a circuit known as a resonator. Oscillations (coloured arrows) of the proton and of the ions generate electrical currents (not shown) in the electrodes of their respective traps; these currents oscillate at the same frequency as the particles that generate them. If the natural oscillation frequency of the proton is the same as that of the ions and of the electrical current in the resonator circuit, a phenomenon called resonance allows the currents in the system to interact. The ions therefore cool the resonator, which, in turn, cools the proton. Such indirect cooling of the proton by the ions is called sympathetic cooling through the resonator. To further ensure that the proton cooling is indeed caused by the ions, the authors fixed the oscillation frequency of the proton, and then varied the oscillation frequency of the ions. They observed that cooling interactions occurred only when the ions' natural oscillation frequency matched that of both the proton and the resonator circuit, as expected. Furthermore, the researchers found that numerical simulations of the cooling set-up matched the observed experimental result, confirming the ions' proton-cooling influence. Impressively, Bohman et al. show that the proton temperature can be reduced by 85%, which would be a substantial amount in an ultraprecise measurement of a fundamental particle. The authors' technique opens up the possibility of being able to cool any charged particle by 'wiring it up' to laser-cooled ions, with any distance between the particle and the ions. The results also have implications for research in quantum information. A goal for this field is to exchange single bits of quantum information between spatially separated quantum systems. However, it is challenging to do this using a conducting wire. Bohman and colleagues' findings suggest a possible solution to this problem, but it will first be necessary to broaden our understanding of how single quanta of energy are exchanged over large distances, and to greatly improve the rate of energy exchange between the separated systems. Nature 596, 490-491 (2021) doi: https://doi.org/10.1038/d41586-021-02267-2 References 1. 1. Bohman, M. et al. Nature 596, 514-518 (2021). Article Google Scholar 2. 2. CMS Collaboration. Phys. Lett. B 716, 30-61 (2012). Article Google Scholar 3. 3. Neuhauser, W., Hohenstatt, M., Toschek P. E. & Dehmelt, H. Phys. Rev. A 22, 1137-1140 (1980). Article Google Scholar 4. 4. Ulmer, S. et al. Nature 524, 196-199 (2015). PubMed Article Google Scholar 5. 5. Schneider, G. et al. Science 358, 1081-1084 (2017). PubMed Article Google Scholar 6. 6. Hansch, T. W. & Schawlow, A. L. Opt. Commun. 13, 68-69 (1975). Article Google Scholar 7. 7. Wineland, D. J. & Dehmelt, H. Bull. Am. Phys. Soc. 20, 637 (1975). Google Scholar 8. 8. Cornell, E. A. et al. Phys. Rev. Lett. 63, 1674 (1989). PubMed Article Google Scholar 9. 9. Wineland, D. J. & Dehmelt, H. G. J. Appl. Phys. 46, 919 (1975). Article Google Scholar 10. 10. Drullinger, R. E., Wineland, D. J. & Bergquist, J. C. Appl. Phys. 22, 365-368 (1980). Article Google Scholar Download references Competing Interests The author declares no competing interests. 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