Displacement of entanglement back and forth between the micro and macro domains

Abstract

Does quantum theory still apply at macroscopic scales? Looking for new insights into this open problem, an experiment in the spirit of Schroedinger’s cat gedanken experiment investigates the entanglement between microscopic and macroscopic domains. Quantum theory is often presented as the theory describing the microscopic world, and admittedly, it has done this extremely well for decades. Nonetheless, the question of whether it applies to macroscopic scales remains open, despite many efforts1,2,3. Here, we report on entanglement exhibiting strong analogies with the Schrödinger cat state as it involves two macroscopically distinct states- two states that can be efficiently distinguished using detectors with no microscopic resolution4. Specifically, we start by generating entanglement between two spatial optical modes at the single-photon level and subsequently displace one of these modes up to almost a thousand photons5. To reliably check whether entanglement is preserved, the state is redisplaced back to the single-photon level and a well-established entanglement measure6, based on single-photon detection, is applied. Our results provide a tool to address fundamental questions about quantum theory and hold potential for more applied problems, for instance in quantum sensing.