Entangled Light Unlocks Faster Measurements of Atomic and Material Dynamics

Zijian Lyu and colleagues have extended quantum correlations into the extreme ultraviolet (XUV) domain, enabling the probing of ultrafast material dynamics. High-harmonic generation, driven by high-harmonic generation from entangled photon pairs in solids, facilitated this achievement. Their work addresses key challenges in extending quantum features from infrared to ultraviolet frequencies and utilising these quantum properties as a new spectroscopic tool. Observation of photon bunching behaviour and wavelength-resolved correlations in harmonic emissions reveals insights into the microscopic mechanisms governing harmonic generation and establishes a pathway towards quantum-enhanced attosecond spectroscopy and control of ultrafast processes.

Non-monotonic photon bunching tracks current transitions in high-harmonic generation

Harmonic generation up to the 10th order has been achieved, representing a strong improvement over previous methods limited by semiclassical descriptions. Traditionally, high-harmonic generation (HHG) is understood through a semiclassical three-step model involving ionisation, acceleration of electrons in a strong field, and subsequent recombination, emitting high-frequency photons. However, this model fails to fully capture the quantum nature of the process, particularly when driven by non-classical light sources. This breakthrough enables the transfer of quantum photon correlations into the extreme ultraviolet (XUV) domain, a range previously inaccessible with conventional techniques. The XUV region, spanning approximately 10 to 124 electronvolts, is crucial for probing core-level electronic structures and ultrafast dynamics in materials with attosecond temporal resolution. Single-shot measurements reveal that photon bunching, a measure of photon clustering, initially increases then decreases with harmonic order, directly tracking the microscopic mechanisms responsible for harmonic emission. Photon bunching is quantified by the second-order correlation function, g⁽²⁾, which describes the probability of detecting two photons simultaneously. A value of g⁽²⁾ greater than one indicates photon bunching, signifying that photons are emitted in clusters rather than randomly.

Numerical simulations utilising semiconductor Bloch equations confirm the non-monotonic trend arises from a transition between intraband and interband current contributions to harmonic generation. The semiconductor Bloch equations are a set of quantum mechanical equations of motion that describe the dynamics of electrons in a periodic potential, such as that found in a solid. These simulations allow researchers to model the complex interactions between the driving field and the material’s electronic band structure. Intraband currents involve transitions within the same energy band, while interband currents involve transitions between different energy bands. The relative contribution of these currents to harmonic generation depends on the driving field’s frequency and intensity, as well as the material’s band structure. This offers a new pathway for attosecond quantum optical spectroscopy, allowing for more detailed analysis of ultrafast processes. Attosecond spectroscopy, utilising pulses of light lasting only a few attoseconds (10^-15 seconds), allows for the observation of electron dynamics in real-time. Lithium niobate crystals, operating in the nonperturbative regime, demonstrate how broadened distributions induced by the squeezed light driving field map onto fluctuations in the microscopic current and polarization, ultimately influencing the photon number statistics of the generated harmonics. The nonperturbative regime ensures that the driving field does not significantly alter the material’s properties, allowing for a more accurate interpretation of the results.

Understanding the interplay between macroscopic photon statistics and the underlying microscopic processes driving harmonic emission is now possible, although establishing precise control over these quantum states and scaling the process to higher harmonic orders remains a key challenge. The ability to correlate macroscopic observables, such as photon statistics, with microscopic current dynamics represents a significant advance in the field. A transition between intraband and interband current contributions near the material’s bandgap explains the observed non-monotonic trend in photon statistics. The bandgap is the energy difference between the valence band and the conduction band in a semiconductor. This detailed understanding of current dynamics provides a more complete picture of harmonic generation than previously possible with traditional methods. Further research is needed to explore the potential of this technique for characterising a wider range of materials and optimising the efficiency of harmonic generation.

Quantum correlations extended to the extreme ultraviolet via high-harmonic generation

Attosecond quantum optics has been unlocked, successfully transferring delicate quantum links between photons into the extreme ultraviolet range via high-harmonic generation in solid materials. This builds upon the recent advances in generating squeezed light in the infrared (IR) regime, which involves reducing the quantum noise in one quadrature of the electromagnetic field. Squeezed light, possessing non-classical correlations, serves as an ideal driving source for exploring quantum effects in HHG. While this achievement currently demonstrates potential for quantum enhancement, it does not yet demonstrate actual quantum-enhanced spectroscopy or control of ultrafast dynamics. The current work focuses on demonstrating the feasibility of transferring quantum correlations to the XUV range; future research will aim to exploit these correlations to improve the sensitivity and resolution of spectroscopic measurements. Microscopic mechanisms within the harmonic emission process are revealed, moving beyond traditional, less detailed descriptions. The ability to resolve these mechanisms is crucial for developing a more complete theoretical understanding of HHG and for optimising the process for specific applications.

High-harmonic generation in solid lithium niobate confirms the transfer of quantum links into the XUV range. Lithium niobate was chosen for its relatively large bandgap and high nonlinear optical susceptibility, making it an efficient medium for HHG. Intense light driving materials to emit frequencies multiplied from the original light source now incorporates quantum properties via entangled photon pairs. Entangled photon pairs exhibit strong correlations, meaning that the measurement of one photon instantaneously influences the state of the other, regardless of the distance separating them. Variations in photon clustering across different harmonic orders reveal these microscopic mechanisms, providing insights into the fundamental physics of harmonic generation. The observed variations in photon clustering provide a direct probe of the quantum dynamics occurring within the material during the HHG process. This allows researchers to gain a deeper understanding of the role of quantum coherence and entanglement in shaping the emitted XUV radiation.

The research successfully transferred quantum correlations from infrared light to extreme ultraviolet (XUV) light using high-harmonic generation in lithium niobate. This is significant because it demonstrates a method for extending quantum properties to higher frequencies, which is challenging in attosecond quantum optics. Measurements of harmonic emission revealed variations in photon clustering, tracking microscopic mechanisms responsible for harmonic generation and offering a new way to study ultrafast dynamics. The authors intend to build on this work to improve the sensitivity and resolution of spectroscopic measurements using these quantum correlations.