Laser Detector in Two Places: How to Hear a Quantum Field express
Physicists have proposed an experiment: a laser beam acts as a quantum detector and splits into two beam-ears. Each ear is directed through a tiny cloud of ultracold atoms — in this state, atoms merge into a single wave. The clouds are separated, so the detector is in superposition — it’s as if it’s listening to the field from two positions at once. Then the ears merge, and the light is directed to a photodetector and mixed with a reference beam (like tuning a radio) — analyzing the current beats reveals the response of the Unruh–DeWitt detector, a simple receiver model.
To prevent the field’s sound from drowning in noise, the physicists use squeezed light — here, noise in one quantum property is sacrificed for silence in another. It’s like muting the hum to make out a whisper. That’s exactly how gravitational waves were caught — the tremor of spacetime from black hole collisions. In the new experiment, the signal will be clearly heard: it will exceed the noise by 10 times. This will be the first direct test of how quantum fields sense curved spacetime.
Original abstract
as written by the authors arXiv:2605.21595
We propose a realizable experiment to test the response of a particle detector prepared in a superposition of locations interacting with a relativistic quantum field. Using a beamsplitter to prepare two superposed branches of a modulated laser probe, these branches are directed to intersect a pancake-shaped Bose-Einstein condensate at two separate locations. The branches are then recombined with another beamsplitter. Heterodyning one of the outputs, the response function corresponding to an Unruh-deWitt detector in a superposition of locations interacting with a (2+1)-dimensional massless scalar field is shown to appear in the difference photocurrent power spectrum. Operating beyond the standard quantum limit using squeezed light, we estimate the signal-to-noise ratio $$SNR\gtrsim 10$$ for extracting the response function over a broad set of baseband frequencies.
Mixing the signal with a reference beam, like in a radio receiver, helps pick out weak quantum beats from the noise.
The idea of an object in two places at once inspires science fiction writers: in Hannu Rajaniemi’s novel 'The Quantum Thief,' the protagonist’s consciousness can be distributed across different points.