Tunable passive squeezing of squeezed light through unbalanced double homodyne detection

The full characterization of quantum states of light is a central task in quantum optics and information science. Double homodyne detection provides a powerful method for the direct measurement of the Husimi Q quasi-probability distribution, offering a complete state representation in a simple experimental setting and a limited time frame. Here, we demonstrate that double homodyne detection can serve as more than a passive characterization tool. By intentionally unbalancing the input beamsplitter that splits the quantum signal, we show that the detection scheme itself performs an effective squeezing or anti-squeezing transformation on the state being measured. The resulting measurement directly samples the Q function of the input state as if it were acted upon by a squeezing operator whose strength is a tunable experimental parameter: the beamsplitter's reflectivity. We experimentally realize this technique using a robust polarization-encoded double homodyne detection to characterize a squeezed vacuum state. Our results demonstrate the controlled deformation of the measured Q function's phase-space distribution, confirming that unbalanced double homodyne detection is a versatile tool for simultaneous quantum state manipulation and characterization.