Quantum Light Speeds Atomic Ionization

Scientists have demonstrated a new method using quantum light that sharply enhances nonlinear tunneling ionization, a phenomenon crucial to attosecond science and the control of electron motion. Zhejun Jiang and colleagues at the East China Normal University, alongside collaborators, report the first experimental observation of this effect in a single sodium atom, achieved through the utilisation of a bright squeezed vacuum. A beam delivering just 300 nJ of squeezed vacuum energy yielded equivalent tunneling ionization to that produced by a 7.1 J coherent light source, indicating a dramatic improvement in nonlinear efficiency. This ability to control the effective intensity via the degree of phase squeezing offers new avenues for efficient frequency conversion and precise control over molecular reactions with tailored quantum light.

Enhanced tunnelling ionization via bright squeezed vacuum light demonstrates increased nonlinear

A 300 nanojoule beam of bright squeezed vacuum (BSV) light now achieves equivalent tunneling ionization to a 7.1 microjoule coherent light source, representing a more than 20-fold boost in nonlinear efficiency. This breakthrough surpasses the long-held limitation of requiring ever-increasing light intensity to drive tunneling ionization, an important process in attosecond science and the study of electron movement. Traditionally, powerful lasers were necessary to manipulate this process. BSV light, a specially-engineered quantum state of light with reduced quantum fluctuations, offers a new pathway to control light-matter interactions. Tunneling ionization is a fundamental process where an electron overcomes a potential barrier, even when it lacks the classical energy to do so, and is central to understanding how light interacts with matter at extremely short timescales. The significance of this enhancement lies in its potential to reduce the energy requirements for experiments probing these ultrafast phenomena, making them more accessible and efficient.

Photoelectron statistics confirmed this effect, with BSV light generating a non-Poissonian distribution, a clear signature of its quantum nature, in contrast to the expected Poissonian profile from classical light. Classical light sources exhibit Poissonian statistics due to the random nature of photon emission, whereas the squeezed vacuum exhibits correlated photons, leading to deviations from this classical behaviour. Analysis of electron kinetic energy spectra revealed extended high-energy tails when using BSV light, directly indicating amplitude stretching originating from quantum fluctuations and confirming a boosted nonlinear response. This stretching of the electron energy distribution is a direct consequence of the altered quantum statistics of the light field, effectively increasing the probability of high-energy electron emission. The experiment utilised a cold-target recoil ion momentum spectrometer to detect both electrons and ions in coincidence, ensuring accurate momentum resolution and statistical validation. This sophisticated detection system allows for precise measurement of the momentum of both the emitted electron and the remaining ion, providing detailed information about the ionization process. However, current results rely on a multimode source and do not yet demonstrate efficient scaling towards practical, high-repetition-rate applications. The development of a high-repetition-rate source is crucial for enabling time-resolved studies and applications requiring continuous operation. The success of this work hinges on the precise control of quantum properties, opening avenues for more sensitive and efficient devices ranging from advanced microscopy to controlling chemical reactions with unprecedented precision.

High-gain parametric down-conversion and sodium atom interaction for quantum enhancement

Bright squeezed vacuum (BSV) light, reducing natural quantum fluctuations in one aspect, was central to this work. This reduction concentrates the light’s energy, akin to sharpening a blurry image. Quantum fluctuations are inherent uncertainties in the electromagnetic field, arising from the wave-particle duality of light. By squeezing the vacuum state, these fluctuations are reduced in one quadrature (phase) of the electromagnetic field, at the expense of increased fluctuations in the other. Generating it involved high-gain parametric down-conversion, utilising barium borate crystals pumped by a powerful laser to create light with tailored quantum properties. Parametric down-conversion is a nonlinear optical process where a high-energy photon is split into two lower-energy photons, conserving energy and momentum. The barium borate crystals act as a medium for this process, efficiently converting the pump laser light into the desired squeezed vacuum state. This carefully crafted light then interacted with a beam of sodium atoms within a high-vacuum chamber, allowing observation of the subtle effects of quantum enhancement on a fundamental atomic process. Maintaining a high vacuum is essential to prevent collisions between the sodium atoms and background gas molecules, which would disrupt the experiment and reduce the signal. A 790nm laser pumped barium borate crystals, generating light with a broad spectral bandwidth of 1400, 1800nm centred on 1580nm, before focusing it onto a sodium vapour jet within an ultrahigh-vacuum chamber. The spectral bandwidth of the generated light is crucial for achieving efficient ionization, as it allows for the excitation of a wider range of electronic states within the sodium atom.

Quantum light enhances electron removal from single sodium atoms

This experiment confirms bright squeezed vacuum light can dramatically enhance nonlinear tunneling ionization in a single sodium atom. Equivalent ionization to a conventional light source with over twenty times less energy represents a fundamental advance in strong-field physics. Tunneling ionization, where electrons escape an atom’s grasp, is central to attosecond science and understanding electron movement. The ability to observe this effect in a single atom demonstrates the sensitivity of the process to quantum fluctuations and validates the theoretical predictions. Precise control over this ionization process was demonstrated by manipulating quantum fluctuations within the light itself, opening new possibilities for efficient frequency conversion. Frequency conversion, the process of shifting the frequency of light, is essential for many applications, including spectroscopy and optical communication, and can be significantly enhanced by utilising squeezed light.

The ability to manipulate light at the quantum level promises substantial gains in fields like attosecond science, where understanding and controlling electron movement is the goal. Attosecond science aims to study and control the dynamics of electrons on the attosecond timescale (10^-18 seconds), which is the natural timescale of electron motion. However, this experiment focused solely on a single sodium atom, and scaling these results to more complex atoms or molecular systems presents a significant hurdle. The complexity of multi-atom or molecular systems introduces additional factors that can affect the efficiency of the quantum enhancement. Maintaining the precise control of phase-squeezing, essential for this quantum boost, becomes increasingly difficult as system complexity grows. Factors such as decoherence and the presence of multiple interacting pathways can degrade the squeezed state and reduce the observed enhancement. Future research will need to address these challenges to realise the full potential of quantum light for controlling light-matter interactions in complex systems.

The research demonstrated that nonlinear tunneling ionization in a single sodium atom could be achieved with a bright squeezed vacuum beam containing 300 nanojoules of energy, matching the ionization produced by a conventional coherent light source delivering 7.1 microjoules. This represents a substantial increase in efficiency through the use of phase-squeezed quantum light, and suggests that controlling the quantum fluctuations within the light allows for precise control over the ionization process. The findings offer fundamental insights into quantum-boosted nonlinear effects and may contribute to more efficient frequency conversion techniques. Researchers intend to address the challenges of scaling these results to more complex atomic and molecular systems.