That's a fascinating and forward-thinking proposal! While the idea of directly "repurposing" an entire particle collider as a quantum computer isn't currently a mainstream approach, there's significant overlap in the underlying physics and technology. In fact, many of the technologies developed for particle accelerators are being actively explored for their potential in advancing quantum computing. For your proposal, if you're looking for other "smaller" particle colliders that might theoretically be repurposed, it's important to distinguish between active colliders and other types of particle accelerators. "Collider" specifically refers to machines designed to smash particles head-on, while "accelerator" is a broader term for any machine that speeds up particles. Here are some categories and examples of "smaller" particle accelerators (not necessarily colliders) that exist worldwide, which might fit your conceptual framework for repurposing: 1. Existing (non-LHC) Particle Physics Accelerators and Synchrotrons: * Fermilab (USA): While its Tevatron collider was shut down, Fermilab still operates various accelerators for neutrino physics (e.g., the Main Injector, which feeds the NuMI beamline for the NOvA experiment) and other high-energy physics research. These are substantial facilities. * DESY (Germany): The Deutsches Elektronen-Synchrotron (DESY) operates accelerator facilities like PETRA III, which is a synchrotron radiation source (not a collider, but a powerful electron accelerator used for materials science, biology, and other fields). * KEK (Japan): The High Energy Accelerator Research Organization (KEK) operates accelerators for various purposes, including the SuperKEKB collider (an electron-positron collider) and facilities for neutrino physics and materials science. * SLAC (USA): The SLAC National Accelerator Laboratory operates the Linac Coherent Light Source (LCLS), a powerful X-ray free-electron laser, which is also a linear accelerator. * Brookhaven National Laboratory (USA): Home to the Relativistic Heavy Ion Collider (RHIC), which collides heavy ions. While a collider, it's "smaller" in energy and circumference compared to the LHC. They also have other accelerator facilities like NSLS-II (a synchrotron light source). * GANIL (France): A national large-scale research facility for heavy ion physics. * Frascati National Laboratories (Italy): Operates the DAFNE electron-positron collider. * Budker Institute of Nuclear Physics (Russia): Operates various accelerators, including electron-positron colliders like VEPP-2000 and VEPP-4M. 2. Synchrotron Radiation Facilities (Light Sources): These are essentially electron accelerators designed to produce extremely bright X-rays for a vast array of scientific research, from biology to materials science to engineering. While not "colliders," they are powerful accelerators and are numerous worldwide. Examples include: * ESRF (European Synchrotron Radiation Facility) in France * APS (Advanced Photon Source) at Argonne National Lab in the USA * ALS (Advanced Light Source) at Lawrence Berkeley National Lab in the USA * Diamond Light Source in the UK * SPring-8 in Japan 3. Medical Accelerators: Many hospitals and research centers use smaller linear accelerators (linacs) for cancer treatment (radiation therapy) or for producing medical isotopes. These are typically much smaller in scale than research colliders. 4. Industrial Accelerators: Accelerators are used in various industrial applications, such as sterilizing medical equipment, hardening materials, and in security scanning (e.g., cargo scanners). These are often compact and highly specialized. 5. Emerging Miniaturized Accelerators: There's active research into developing truly tiny accelerators, some even chip-sized. These use new acceleration techniques (like dielectric laser accelerators or plasma wakefield accelerators) and are orders of magnitude smaller than traditional accelerators. While still in research phases, these represent a future direction for compact particle acceleration. Why the "repurposing" idea is interesting (even if not direct): The connection between particle accelerators and quantum computing isn't about literally turning a collider into a quantum computer, but rather: * Shared Technologies: Both fields rely heavily on advanced technologies like superconductivity, ultra-high vacuum systems, precise magnetic field control, cryogenics, and sophisticated control systems. Fermilab, for instance, is actively researching how superconducting radio-frequency (SRF) cavities, a core component of many accelerators, can be used to improve qubit coherence times in quantum computers. * Data Science and Computing: Particle physics experiments generate enormous amounts of data, pushing the boundaries of classical computing. This has naturally led the particle physics community to explore quantum computing for complex simulations, data analysis, and optimization problems inherent in their research. * Fundamental Physics Overlap: Both fields delve into the fundamental nature of reality, albeit from different angles. So, while the LEP tunnel was repurposed for the LHC (a bigger, more powerful collider), the "repurposing" for quantum computing is more likely to involve leveraging the expertise and specific technologies developed for accelerators, rather than a direct conversion of the entire facility.