A light pulse can be compared to a musical chord: several notes sound simultaneously, each with its own loudness and phase. In quantum optics, such 'chords' are called multimode squeezed states. They are important for secure communication and ultra-precise measurements, but to use them, you need to know the shape of each 'note'.

Conventional measurement methods are either too complex or assume that the pulse shape is known in advance. The new approach uses an optical parametric amplifier, a device that amplifies a weak signal without destroying its quantum properties, and the FROG method (frequency-resolved optical gating). FROG works like a stroboscope: a short reference pulse 'illuminates' a slice of the signal, and a spectrometer captures the instantaneous picture. This allows simultaneous reconstruction of both the pulse shape and the degree of squeezing — a quantity showing how strongly quantum noise is suppressed in one quadrature and amplified in another.

Simulations showed that the method works for ultrashort pulses and complex shapes. This paves the way for practical analysis of large quantum systems where many modes are intertwined. Instead of bulky setups — a single compact device that reads the quantum 'chord' in its entirety without drowning out the quiet notes.