Superconducting Circuits Generate Squeezed Microwave Signals for Better Sensing

Scientists at CNR-SPIN, University of Naples Federico II, Université Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, and University of Cassino and Southern Lazio have demonstrated the generation of vacuum squeezing using a Travelling Wave Parametric Amplifier (TWPA), a development with significant implications for advancements in quantum sensing and continuous-variable quantum information. Isita Chatterjee and colleagues reveal that residual three-wave mixing (3WM) within a Josephson TWPA, constructed from superconducting nonlinear asymmetric inductive elements (SNAILs) with alternated magnetic flux polarity, successfully generates vacuum squeezing when the operating flux point is carefully selected. The study clarifies the complex interplay between competing nonlinearities inherent in TWPA squeezers and offers key insights for broadening the scope of microwave photonics applications, potentially revolutionising areas reliant on sensitive signal detection.

Superconducting amplifier achieves record gain through suppressed four-wave mixing

System gain increased to 62.0 dB, a substantial improvement over previous devices limited to approximately 61.1 dB, enabling the detection of weaker quantum signals and pushing the boundaries of sensitivity in quantum measurements. Achieving sufficient signal amplification without introducing excessive noise has long been a key barrier in the development of practical quantum technologies, and this advance represents a significant step towards overcoming it. Scientists have unlocked a pathway to more sensitive quantum sensors and enhanced quantum information processing by demonstrating vacuum squeezing generation via residual three-wave mixing within a Josephson Travelling Wave Parametric Amplifier, built with superconducting nonlinear asymmetric inductive elements. The SNAILs employed are crucial; their asymmetric inductive properties are designed to enhance the desired nonlinear effects while suppressing unwanted ones. Selecting the amplifier’s operating flux point carefully proved important, suppressing competing four-wave mixing processes that typically degrade performance and limit the achievable level of squeezing. Four-wave mixing arises from the nonlinear response of the superconducting circuit and creates spurious photons that mask the desired signal, thus reducing the effectiveness of the amplifier.

A phase-sensitive gain of 6.0 dB at a pump power of -84 dBm was achieved when operating at a flux point minimising unwanted four-wave mixing, unlike a second flux point where periodic amplification was absent. This demonstrates a clear control over the amplifier’s nonlinear behaviour, allowing for optimisation towards squeezing generation. The 3WM process, exploited in this research, involves the interaction of three microwave photons, resulting in the creation of a correlated photon pair in a squeezed state. Through 3x 10 6 repeated measurements with 10s integration times per acquisition, single-mode squeezing experiments revealed squeezing levels of -6 dB along both the x and p quadratures at the optimised flux. These quadratures represent the amplitude and phase of the microwave field, and squeezing in one quadrature comes at the expense of increased noise in the other, a fundamental characteristic of squeezed states. A squeezing level of -6 dB indicates a reduction in quantum noise by approximately 50% in the squeezed quadrature, significantly enhancing the signal-to-noise ratio. The system gain was calibrated using a shot noise tunnel junction as a reference, with analysis of the covariance matrix confirming these results and highlighting the impact of noise reduction. The covariance matrix provides a complete characterisation of the quantum state, allowing for precise quantification of the squeezing and verification of its non-classical nature.

Microwave amplifiers are vital for detecting faint quantum signals, and researchers are steadily improving these tools for quantum technologies. The development of highly sensitive microwave amplifiers is crucial for a wide range of applications, including quantum computing, materials science, and medical imaging. A technique for generating squeezed microwave photons, a vital component for enhancing sensitivity in applications like materials science and medical imaging, has been developed. Squeezing reduces noise in one aspect of a signal, improving the precision of measurements crucial for detecting subtle changes in materials or biological samples. In materials science, this could enable the detection of minute defects or changes in material properties. In medical imaging, it could lead to improved resolution and reduced radiation exposure. This advance moves beyond previous bandwidth limitations of approximately 10-100MHz in similar devices, opening possibilities for improved continuous-variable quantum information processing and more sensitive quantum sensors. The increased bandwidth allows for the processing of more complex quantum signals and the implementation of more sophisticated quantum algorithms. Continuous-variable quantum information processing utilises the continuous degrees of freedom of electromagnetic fields, such as amplitude and phase, to encode and manipulate quantum information. The ability to generate and manipulate squeezed microwave photons is essential for building practical continuous-variable quantum computers and communication systems. Furthermore, the enhanced sensitivity offered by this technology will be invaluable for developing next-generation quantum sensors capable of detecting extremely weak signals, potentially leading to breakthroughs in fields such as gravitational wave detection and dark matter searches.

Researchers successfully generated vacuum squeezing using a Josephson Traveling Wave Parametric Amplifier incorporating superconducting nonlinear asymmetric inductive elements. This achievement is significant because squeezing reduces noise in microwave signals, enhancing the precision of measurements. By carefully controlling the operating conditions, the study demonstrated squeezing generation via three-wave mixing, offering improved performance beyond previous bandwidths of approximately 10-100MHz. The authors suggest this work provides insights into competing nonlinearities within these amplifiers, potentially broadening their use in microwave photonics.