In this review, we discuss the use of continuous variable spectroscopy techniques for investigating quantum coherence and light-matter interactions in semiconductor systems with ultrafast dynamics. We focus on multichannel homodyne detection as a powerful tool to measure the quantum coherence and the full density matrix of a polariton system. By monitoring the temporal decay of quantum coherence in the polariton condensate, we observe coherence times exceeding the nanosecond scale. Our findings, supported by proof-of-concept experiments and numerical simulations, demonstrate the enhanced resourcefulness of the produced system states for modern quantum protocols. The combination of tailored resource quantifiers and ultrafast spectroscopy techniques presented here paves the way for future applications of quantum information technologies.
In this review, we discuss the use of continuous variable spectroscopy techniques for investigating quantum coherence and light-matter interactions in semiconductor systems with ultrafast dynamics. We focus on multichannel homodyne detection as a powerful tool to measure the quantum coherence and the full density matrix of a polariton system. By monitoring the temporal decay of quantum coherence in the polariton condensate, we observe coherence times exceeding the nanosecond scale. Our findings, supported by proof-of-concept experiments and numerical simulations, demonstrate the enhanced resourcefulness of the produced system states for modern quantum protocols. The combination of tailored resource quantifiers and ultrafast spectroscopy techniques presented here paves the way for future applications of quantum information technologies.
This review examines the use of continuous-variable spectroscopy techniques for investigating quantum coherence and light-matter interactions in semiconductor systems with ultrafast dynamics. Special emphasis is placed on multichannel homodyne detection as a powerful tool to measure the quantum coherence and the full density matrix of a polariton system. Observations, such as coherence times that exceed the nanosecond scale obtained by monitoring the temporal decay of quantum coherence in a polariton condensate, are discussed. Proof-of-concept experiments and numerical simulations that demonstrate the enhanced resourcefulness of the produced system states for modern quantum protocols are assessed. The combination of tailored resource quantifiers and ultrafast spectroscopy techniques that have recently been demonstrated paves the way for future applications of quantum information technologies.
The goal of this work is to examine the coherence of classical light fields in order to assess their usefulness for quantum-informational tasks and to draw conclusions about the properties of the light source. To this end, we use and extend several optical homodyne tomography techniques employing four-port, eight-port, and sixteen-port detection schemes. To understand the role of coherence of a light field in solving a quantum informational task such as coherent qubit control, we begin by analyzing the qubit excitation process from a resource-theoretical perspective. By simulating the light-matter interactions, we demonstrate how the quantum coherence of the driving light field emerges as a key ingredient for coherent qubit control. At the same time, we show how the presence of thermal noise or dephasing significantly degrades the achievable quantum coherence of the light field. After studying the coherence properties of conventional lasers, we turn to non-conventional coherent light sources and investigate the emission of a non-resonantly excited exciton-polariton condensate. By employing long-lived polaritons and minimizing interactions with the reservoir, we optimize the condensation process and achieve a pronounced buildup of quantum superpositions in the condensate immediately above the condensation threshold. Finally, we shift our focus entirely to the matter system itself and use the coherence of the emitted light field to investigate its dynamics. For this purpose, we introduce a sixteen-port detection scheme that enables non-stationary and conditional analyses tailored to the requirements of semiconductor spectroscopy. Benchmarking the setup with a thermal light field, then demonstrate how we can resolve the stochastic fluctuations of the thermal state on the femtosecond level. Overall, our results establish connections between quantum optics, quantum information science, and semiconductor physics, providing insights relevant for quantum technologies and future hybrid quantum devices.
Long-term quantum coherence constitutes one of the main challenges when engineering quantum devices. However, easily accessible means to quantify complex decoherence mechanisms are not readily available, nor are sufficiently stable systems. We harness novel phase-space methods-expressed through non-Gaussian convolutions of highly singular Glauber-Sudarshan quasiprobabilities-to dynamically monitor quantum coherence in polariton condensates with significantly enhanced coherence times. Via intensity- and time-resolved reconstructions of such phase-space functions from homodyne detection data, we probe the systems' resourcefulness for quantum information processing up to the nanosecond regime. Our experimental findings are confirmed through numerical simulations, for which we develop an approach that renders established algorithms compatible with our methodology. In contrast to commonly applied phase-space functions, our distributions can be directly sampled from measured data, including uncertainties, and yield a simple operational measure of quantum coherence via the distribution's variance in phase. Therefore, we present a broadly applicable framework and a platform to explore time-dependent quantum phenomena and resources.
Continuous variable quantum state tomography is one of the most powerful techniques to study the properties of light fields in quantum optics. However, the need for a fixed phase reference has so far prevented widespread usage in other fields such as semiconductor spectroscopy. Here, we introduce nonstationary quantum state tomography, which adapts the technique to the special requirements of ultrafast spectroscopy. In detail, we gain access to the amplitude and phase of light fields with a temporal resolution of about 100 fs without the need for a fixed phase reference. Furthermore, we show how our technique allows us to perform conditional studies of stochastic dynamics that are inaccessible experimentally by conventional means and demonstrate the capabilities experimentally by monitoring the stochastic dynamics of a thermal light field on the sub-ps scale. Finally, we discuss differences and similarities to more standard Hanbury Brown--Twiss photon correlation experiments, which may be considered as the discrete variable analogs of our technique.
Ultrafast continuous-variable quantum states offer new opportunities for advanced quantum technologies, but efficient homodyne detection of these states remains challenging. Here, we present a method for efficient ultrafast homodyne detection by exploiting temporal correlations in detector signals. By optimizing the temporal weight used to extract quadrature outcomes, we achieve a substantial increase in the signal-to-noise ratio of ultrafast homodyne detection, thereby improving the detection efficiency. We analyze the autocorrelations of shot noise and electronic noise and determine the optimal weight by solving a generalized Rayleigh quotient problem. The optimal weight enhances the squeezing and anti-squeezing levels observed experimentally. These results highlight the importance of optimized signal processing for efficient quantum measurements.
Classical light such as lasers and thermal radiation has long been utilized for spectroscopy to explore material structures and dynamics. However, quantum states of light open new avenues for ultrafast optics and spectroscopy, by the variation of their unique properties, e.g., entanglement and photon statistics, after interacting with materials. Quantum light possesses unique characteristics that distinguish it from classical light, particularly the phase-space distribution which reflects its fundamental quantum statistics and the quantum control protocol that may take advantage of noise reduction. This Review will focus on spectroscopic applications using entangled photons and squeezed light, in conjunction with quantum coherent control. We provide a brief introduction to the generation of the entangled and squeezed light, discussing their fundamental properties and connections to the control protocol that may enhance spectroscopic techniques. A microscopic theory for the spectroscopy and corresponding signals induced by certain transition processes is developed. Nonlinear optical signals induced by entangled photons for the coherent anti-Stokes Raman spectra (CARS) and the stimulated Raman spectra (SRS) are investigated. We further extend our efforts to the transient absorption process that enables real-time monitoring of the exciton transfer in the transition metal dichalcogenides (TMDs), namely, using the WS 2 monolayer. This series of works on nonlinear optical spectroscopy using quantum light explicitly demonstrates the quantum supremacy in ultrafast spectroscopy through jointly enhancing the time and energy scales beyond the classical bound.