The metrology of terahertz waves is a prerequisite for future applications in communications, sensing or spectroscopy. Electro-optic transduction, the technique by which a terahertz signal is mapped onto an optical or near-infrared signal, has emerged as one of the most sensitive metrology techniques that also provides sub-picosecond temporal resolution. As such, it has been employed to detect quantum terahertz fields for the first time. Recent advances in the miniaturization of these transducers target increased sensitivities, a small footprint and compatibility with large photonic architectures. In this talk, I will outline the basics of electric field metrology at the quantum limit and discuss how integrated photonics could open entirely new avenues in the area of spatially and temporally multiplexed terahertz detection.
The metrology of terahertz waves is a prerequisite for future applications in communications, sensing or spectroscopy. Electro-optic transduction, the technique by which a terahertz signal is mapped onto an optical or near-infrared signal, has emerged as one of the most sensitive metrology techniques that also provides sub-picosecond temporal resolution. As such, it has been employed to detect quantum terahertz fields for the first time. Recent advances in the miniaturization of these transducers target increased sensitivities, a small footprint and compatibility with large photonic architectures. In this talk, I will outline the basics of electric field metrology at the quantum limit and discuss how integrated photonics could open entirely new avenues in the area of spatially and temporally multiplexed terahertz detection.
Electro-optical sampling of Terahertz fields with ultrashort pulsed probes is a well-established approach for directly measuring the electric field of THz radiation. This technique usually relies on balanced detection to record the optical phase shift brought by THz-induced birefringence. The sensitivity of electro-optical sampling is, therefore, limited by the shot noise of the probe pulse, and improvements could be achieved using quantum metrology approaches using, e.g., NOON states for Heisenberg-limited phase estimation. We report on our experiments on THz electro-optical sampling using single-photon detectors and a weak squeezed vacuum field as the optical probe. Our approach achieves field sensitivity limited by the probe state statistical properties using phase-locked single-photon detectors and paves the way for further studies targeting quantum-enhanced THz sensing.
Electro-optic quantum coherent interfaces map the amplitude and phase of a quantum signal directly to the phase or intensity of a probe beam. At terahertz frequencies, a fundamental challenge is not only to sense such weak signals (due to a weak coupling with a probe in the near-infrared) but also to resolve them in the time domain. Cavity confinement of both light fields can increase the interaction and achieve strong coupling. Using this approach, current realizations are limited to low microwave frequencies. Alternatively, in bulk crystals, electro-optic sampling was shown to reach quantum-level sensitivity of terahertz waves. Yet, the coupling strength was extremely weak. Here, we propose an on-chip architecture that concomitantly provides subcycle temporal resolution and an extreme sensitivity to sense terahertz intracavity fields below 20 V/m. We use guided femtosecond pulses in the near-infrared and a confinement of the terahertz wave to a volume of V T H z ∼ 10 − 9 ( λ T H z / 2 ) 3 in combination with ultraperformant organic molecules ( r 33 = 170 p m / V ) and accomplish a record-high single-photon electro-optic coupling rate of g e o = 2 π × 0.043 G H z , 10,000 times higher than in recent reports of sensing vacuum field fluctuations in bulk media. Via homodyne detection implemented directly on chip, the interaction results into an intensity modulation of the femtosecond pulses. The single-photon cooperativity is C 0 = 1.6 × 10 − 8 , and the multiphoton cooperativity is C = 0.002 at room temperature. We show > 70 d B dynamic range in intensity at 500 ms integration under irradiation with a weak coherent terahertz field. Similar devices could be employed in future measurements of quantum states in the terahertz at the standard quantum limit, or for entanglement of subsystems on subcycle temporal scales, such as terahertz and near-infrared quantum bits.
Modern terahertz detectors enable direct measurement of the electric field, providing simultaneous access to amplitude and phase information of the terahertz radiation. However, conventional systems are often based on single-pixel detectors, limiting most imaging applications to slow raster-scanning techniques. This work presents the demonstration of a novel approach for amplitude- and phase-sensitive imaging in the terahertz range using quantum imaging with undetected photons. By decoupling the sample measurement in the terahertz range from the detection of the information in the visible, this technique enables the use of cost-effective, highly developed multi-pixel optical components. Central achievements are the observation of quantum interference with terahertz photons and its application to material characterization and molecular concentration analysis. Furthermore, image information was transferred from the terahertz to the visible spectral range, achieving a spatial resolution close to the terahertz wavelength. This approach thus provides direct access to amplitude, phase, and spectral information for imaging applications in the terahertz range using a multi-pixel camera.
The manipulation of visible and near-infrared light at the single-photon level plays a key role in quantum communication systems where information is encoded into photonic degrees of freedom. In practical implementations, it is important to achieve this manipulation with high speeds, low loss, and low noise. In this Letter, we propose the use of terahertz electric fields as a pump source for electro-optic modulation of single photons in bulk media. We demonstrate spectral modulations of single photons in the form of frequency translation and bandwidth manipulations as the terahertz field imparts linear and quadratic phases on the photons at various time delays within 1 ps. Our results show frequency translations exceeding 500 GHz at multi-THz modulation speeds and loss levels of ≈1 dB, complementing the current state-of-the-art electro-optic modulation techniques limited to speeds up to 100 GHz. The proposed approach leverages recent developments in terahertz generation techniques, introducing new avenues to manipulate nonclassical light in an unexplored regime for quantum photonics.
The time-resolved detection of mid- to far-infrared electric fields absorbed and emitted by molecules is among the most sensitive spectroscopic approaches and has the potential to transform sensing in fields such as security screening, quality control, and medical diagnostics. However, the sensitivity of the standard detection approach, which relies on encoding the far-infrared electric field into amplitude modulation of a visible or near-infrared probe laser pulse, is limited by the shot noise of the latter. This constraint cannot be overcome without using a quantum resource. Here, we show that this constraint can be overcome using a two-mode squeezed state. Quantum-correlated ultrashort pulses, generated by parametric down-conversion, enhance the sensitivity of far-infrared detection beyond the classical limit, achieving a twofold reduction in measured noise. This advancement paves the way for further development of ultrafast quantum metrology, moving toward quantum-enhanced time-resolved electric field spectroscopy with sensitivities beyond the standard quantum limit.
Quantum sensing and measurement devices are reaching a new level of accuracy for high-precision measurements, allowing detection of the most minute changes in magnetic, electric, and strain fields, gravity, and time. In this talk, I will present an overview of how integrated photonic and nanophotonic devices utilize entanglement to improve measurement accuracy and sensitivity. Examples will include the development of ultrabright and ultra-efficient quantum light sources as resources for sensors, integrated photonic magnetometry with exquisite sensitivity that can be further enhanced with squeezing, and strain sensors based on optomechanical modulation of quantum emitters via surface acoustic wave resonators.