The world of quantum physics is experiencing a second revolution, which will drive an exponential leap in the progress of computing, the internet, telecommunications, cybersecurity and biomedicine. Quantum technologies are attracting more and more students who want to learn about concepts from the subatomic world — such as quantum entanglement or quantum superposition — to explore the innovative potential of quantum science. In fact, understanding the non-intuitive nature of quantum technology concepts and recognizing their relevance to technological progress is one of the challenges of 2025, declared the
International Year of Quantum Science and Technology by UNESCO.
Now, a team from the Faculty of Physics of the University of Barcelona has designed new experimental equipment that makes it possible for students to familiarize themselves with the more complex concepts of quantum physics. The configuration they present —versatile, cost-effective and with multiple ways of application in the classroom — is already operational in the Advanced Quantum Laboratory of the UB’s Faculty of Physics and could also be accessible in less specialized centres.
This innovation is presented in an article in the journal
EPJ Quantum Technology, which results from a collaboration between professors Bruno Juliá, from the Department of Quantum Physics and Astrophysics and the UB Institute of Cosmos Sciences (ICCUB); Martí Duocastella, from the Department of Applied Physics and the UB Institute of Nanoscience and Nanotechnology (IN2UB), and José M. Gómez, from the Department of Electronic and Biomedical Engineering. It is based on the result of Raúl Lahoz’s master’s final project, with the participation of experts Lidia Lozano and Adrià Brú.
Study of phenomena unique to quantum mechanics
Quantum mechanics makes it possible to create so-called entangled systems — for example, with two particles or two photons — that behave in a non-intuitive way. In 1964, the physicist John S. Bell experimentally proved that the predictions of quantum mechanics were totally incompatible with a classical description of physics — a hypothesis that had been advocated by Albert Einstein — and consolidated the probabilistic nature of quantum mechanics. In 2022, scientists Alain Aspect, John F. Clauser and Anton Zeilinger were awarded the Nobel Prize in Physics for pioneering experiments in quantum information on entangled photons and the experimental demonstration of the violation of Bell’s inequalities.
Quantum entanglement is today one of the fundamental resources to drive the development of quantum technologies (quantum computers, data encryption, etc.). “The study of Bell inequalities — in particular, observing violations of the inequalities — is fundamental to characterizing quantum entangled systems. It is important to be able to perform these experiments in a teaching laboratory to understand Bell’s inequalities, quantum entanglement and the probabilistic nature of quantum mechanics”, says Bruno Juliá.
Martí Duocastella explains in the article that they have designed “new experimental equipment capable of providing students with direct measurements of quantum entanglement”. “From our perspective, — says the researcher — we believe that allowing students to make these measurements will greatly facilitate their understanding of this unintuitive phenomenon”.
Introducing students to advanced tools
The system designed by the UB team makes it possible to study Bell inequalities and also to perform full two-photon state tomography. With a simple operation, it can prepare different quantum entangled states. Compared to previous proposals, “the new equipment has improved the photon-capture process: it uses detectors assembled to optical fibres, one of the key innovations to simplify the experiment, which facilitates the alignment of the system and increases the efficiency of the detection. Thus, a complete measurement of the Bell inequalities can be performed during a practical laboratory session (between one and two hours)”, say Juliá and Duocastella.
The results reveal successful manipulation of the quantum state of photons and the achievement of high-fidelity entangled states and significant violations of Bell inequalities. Also, the elements of the system are widely used in current quantum technologies, facilitating students’ contact with advanced instrumentation.
This innovation, which has already been applied in bachelor’s and master’s degree courses, has received very positive feedback from all students. In the bachelor’s degree in Physics, it allows experimental demonstrations to be carried out to complement the subject of Classical and Quantum Information Theory and Quantum Mechanics. In the master’s degree course, it is one of the four experiments in the Advanced Quantum Laboratory of the Master’s degree in Quantum Science and Technologies.
This study has received funding from both the Spanish Ministry of Science, Innovation and Universities and the European Union’s Next Generation EU funds.