Nello and Colleagues Present Frequency-Resolved Squeezing Maps for Quantum Communication

Daniele Nello and colleagues report frequency-dependent quadrature squeezing in a quantum well laser, achieved using a fully quantum Langevin approach. The study reveals, for the first time, both frequency-dependent squeezing and complex squeezing within a semiconductor laser. A thorough analysis of the laser field’s squeezing map identifies optimal squeezing curves and examines the role of the linewidth enhancement factor. These results confirm semiconductor lasers as a key platform for generating non-classical light, potentially advancing applications in quantum communication and sensing.

Revealing quadrature-noise structure unlocks frequency-dependent squeezing in semiconductor lasers

Previously unobserved, a hidden squeezing phenomenon was revealed within a semiconductor laser. Frequency-dependent squeezing extends the sub-shot-noise bandwidth beyond predictions based solely on amplitude squeezing. Conventional methods focused on amplitude spectrum analysis, limiting access to the complete quantum-noise structure of semiconductor lasers. This new framework accesses the full quadrature-noise structure of the emitted field, providing a more thorough understanding. The significance of this lies in the fact that traditional analysis often assumes a static noise profile, neglecting the frequency-dependent variations inherent in quantum systems. By examining the quadrature components, analogous to the real and imaginary parts of the electric field, the researchers were able to map the noise characteristics across a range of frequencies, uncovering behaviours previously masked by averaging techniques. This detailed mapping is crucial for optimising laser performance in applications where specific frequency components are critical.

The quantum Langevin approach retains the full dynamics of the active medium, avoiding simplifications used in prior studies, and unlocks potential for improved quantum communication and sensing technologies. Unlike classical rate equation models, the Langevin approach incorporates the quantum fluctuations of the laser field and the active medium, providing a more accurate description of the noise properties. These fluctuations are essential for understanding and manipulating squeezing. The alpha factor, also known as the linewidth enhancement factor, plays an important role in generating these newly observed quantum effects within the semiconductor laser. The alpha factor represents the coupling between the amplitude and phase of the laser field; a larger alpha factor generally leads to stronger squeezing but also increased technical noise. The researchers carefully analysed the interplay between the alpha factor and the squeezing characteristics to identify optimal operating conditions. Analysis of the spectral covariance matrix uncovered instances of ‘hidden’ squeezing, correlations between optical sidebands undetectable by standard measurement techniques like homodyne detection. These sidebands arise from the interaction between the laser field and the quantum fluctuations of the gain medium.

Observing these correlations requires more advanced methods such as synodyne detection or interferometers with memory effects. Synodyne detection, unlike homodyne detection, is sensitive to the correlations between different frequency components of the light field, allowing it to reveal the hidden squeezing. Interferometers with memory effects can exploit the temporal correlations in the light field to enhance the detection of weak signals. The fully quantum Langevin approach revealed that optimal squeezing, the reduction of quantum noise, is not static but rotates with increasing frequency, extending the range where noise is below the standard quantum limit. This rotation offers a significant advantage over static squeezing, potentially broadening the bandwidth for applications requiring low noise across a range of frequencies. The standard quantum limit represents the minimum noise level achievable with a classical light source. By surpassing this limit, squeezed light enables more precise measurements and more secure communication. Computational modelling provides insights into the underlying mechanisms driving this behaviour, and the team investigated specific conditions where light can be manipulated to exhibit non-classical properties, key for advancing quantum technologies. The simulations involved solving the quantum Langevin equations numerically, a computationally intensive task requiring significant resources.

Revealing frequency-dependent and hidden squeezing in semiconductor laser behaviour

A new level of control over light emission from semiconductor lasers was achieved, revealing both frequency-dependent squeezing and a more subtle ‘hidden’ squeezing effect. This addresses the vital need for brighter, more reliable sources of non-classical light, essential components in emerging quantum technologies like secure communication networks and highly sensitive sensors. The development of practical quantum technologies relies heavily on the availability of efficient and robust sources of non-classical light. Semiconductor lasers offer a promising platform due to their compact size, low cost, and ease of integration. Modelling the laser’s behaviour using a complex mathematical approach forms the basis of the current analysis, allowing for detailed exploration of the quantum noise characteristics. The model incorporates the quantum mechanical properties of the semiconductor material, the laser cavity, and the interaction between light and matter. Squeezing enhances the sensitivity of sensors and the security of communication by reducing the uncertainty of light’s properties, and this demonstrates a pathway to achieving it more effectively in a readily available device. Quantum sensors exploit the reduced noise of squeezed light to improve their ability to detect weak signals, while quantum communication systems use it to enhance the security of data transmission. A detailed understanding of how semiconductor lasers generate squeezed light, a non-classical state where quantum noise is reduced in one direction at the expense of increased noise in another, is now available, with significant implications for improving the performance of quantum devices. The trade-off between noise reduction in one quadrature and increased noise in the other is a fundamental characteristic of squeezed light, and careful control of this trade-off is essential for optimising performance. The researchers’ findings pave the way for the development of new quantum technologies that exploit the unique properties of squeezed light, potentially revolutionising fields such as metrology, imaging, and cryptography. Further research will focus on exploring the limits of squeezing achievable in semiconductor lasers and developing practical devices based on these principles. The observed squeezing levels are significant, potentially enabling a 3dB improvement in signal-to-noise ratio in certain applications.

This research demonstrated both frequency-dependent and complex squeezing in a quantum well laser, a key achievement in the generation of non-classical light. It establishes semiconductor lasers as a viable platform for creating this light, which is crucial for enhancing the sensitivity of quantum sensors and the security of quantum communication systems. The study used a fully quantum Langevin approach to analyse the laser’s behaviour and identify optimal squeezing curves. The authors intend to continue exploring the limits of squeezing achievable in these lasers and developing practical devices based on these principles.

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