We propose a realistic setup for electro-optic sampling with heralded quantum gates stemming from bright entangled twin beams. The signal-to-noise ratio is increased 7-fold without breaking the weak-measurement condition.
Employing electro-optic sampling (EOS) with ultrashort probe pulses, recent experiments showed direct measurements of quantum vacuum fields and their correlations on subcycle timescales. Here, we propose a quantum-enhanced EOS where bright photon-number entangled twin beams are used to derive conditioned nonclassical probes. In the case of the quantum vacuum, this leads to a sixfold improvement in the signal-to-noise ratio over the classically probed EOS. In addition, engineering of the conditioning protocol yields a reliable way to extract higher-order moments of the quantum noise distribution and robust discrimination of the input quantum states, for instance, a vacuum and a few-photon cat state. These improvements open a viable route toward robust tomography of quantum fields in space-time, an equivalent of homodyne detection in energy-momentum space, and the possibility of precise experiments in real-space quantum electrodynamics.
In conventional balanced-detection measurements, sensitivity can generally be improved by increasing the optical power. However, in many systems this approach reaches an upper bound beyond which increasing the probe power can damage the sample or degrade the measurement. In THz time-domain spectroscopy, this bound is set by nonlinear noise and measurement back-action. When this ceiling is reached by classical techniques, a potential avenue to improve the signal-to-noise ratio is by using quantum metrological resources. This work reports the development of a bright, ultrafast twin-beam source, exhibiting more than 6 dB of noise suppression up to microwatt optical power levels, together with an ultra-low-noise balanced photoreceiver with sub-shot-noise performance. Integrating these technologies into a THz-TDS system developed in collaboration with colleagues we demonstrate, for the first time to our knowledge, a measurable enhancement in sensitivity by harnessing quantum correlations when compared with a classical source under identical conditions. Although the implemented THz-TDS setup deviates from conventional polarimetric schemes, which typically achieve higher sensitivity, these results show that quantum methods can provide a sensitivity enhancement in field-resolved THz spectroscopy. Beyond this demonstration, the developed twin-beam source has clear potential for wider use as its brightness, stability and correlation strength make it an attractive candidate for ultrafast, differential transient absorption measurements.
We report an experimental demonstration of quantum-enhanced displacement measurement of a membrane in an optical cavity by using bright quantum correlated light to surpass the shot-noise limit (SNL). The optical cavity enhances the displacement signal and the twin beams suppress the noise background 3 dB below the SNL at an equivalent optical power. Thus, the signal-to-noise ratio is significantly improved by simultaneously using optical cavities and twin beams, and a sensitivity of 200 am/Hz is achieved. The frequency fluctuations of twin beams are analyzed and reduced by using optical twin cavities. These results provide a useful method for precision measurements as well as to control cavity optomechanical systems with nonclassical light.