Advancements in Quantum Memory Technologies Using Phonons

Quantum memories are essential for storing and retrieving quantum information, which is critical in the field of on-chip and long-distance quantum communications. They have many applications, including aerospace and medical imaging. One of the promising technologies for quantum memory involves using sound waves (known as Phonons) that can behave according to quantum rules.

#The Role of Phonons

Phonons are quantized vibrations in materials that can be used to store quantum information. Specifically, bulk acoustic wave (BAW) phonons, which move within a material, are viewed as excellent candidates for this purpose because they can have long lifetimes.

#Hybrid Magnonic Systems

In this research, we look at a hybrid magnonic system where BAW phonons are created in a thick film of Gadolinium Iron Garnet (GGG) through interaction with quantized electron spin-waves (Magnons) in a thin film of Yttrium Iron Garnet (YIG). Recent tests on devices made with YIG and GGG show that the memories in these devices are limited by the phonon lifetime, which is around 0.2 seconds at room temperature. However, phonon lifetimes may improve significantly at lower temperatures, although diffraction-a phenomenon where waves spread out-can also limit performance.

#The Importance of Understanding Phonon Lifetimes

The full understanding of how diffraction affects the performance of hybrid magnonic devices is lacking. Therefore, this study presents theoretical and numerical analyses to predict phonon lifetimes limited by diffraction, how phonons interact with magnons, and other relevant factors in two types of structures: planar and confocal high-overtone bulk acoustic wave resonator (HBAR) structures.

#Investigating Phonon Lifetimes

We investigate two methods to analyze the BAW phonons. The first method involves Fourier beam propagation, while the second uses Hankel transform techniques. Both methods provide insights into predicting how phonons behave in these structures.

#Method 1: Fourier Beam Propagation (FBPM)

FBPM is a technique borrowed from optics to track how waves travel through a medium. It has been adapted to study phonons in HBAR structures. The primary advantage of this method is its speed and efficiency in predicting wave behavior at different distances from the source.

#Method 2: Hankel Transform (HK) Method

The Hankel transform is another technique used to solve problems by reducing them from three dimensions to two dimensions. This is especially useful for our study because it simplifies the calculations needed for HBAR structures.

#HBAR Configurations

Two representative HBAR structures are considered in this study: planar and confocal. The planar structure consists of a thick GGG film with a thin YIG film beneath it, while the confocal structure has a dome-shaped GGG surface above the YIG film. Both configurations have specific dimensions, and the relationship between YIG film area and performance is critical.

#Magnon Modes

Magnons are related to magnetic excitations in the YIG film, created by applying specific magnetic fields. These magnons interact with the phonons, particularly the Kittel mode, which serves as a foundational mode for the system.

#Phonon Modes

The interaction of magnons leads to the generation of traveling phonons that propagate through the GGG region. The characteristics of these phonons are influenced by the properties of the materials used, including how effectively they can transfer energy.

#Analyzing Phonon Modes

To analyze shear wave phonon modes in the chosen HBAR structures, we utilize both FBPM and HK approaches. These methods provide valuable insights into the behavior and lifetimes of phonons, which directly relate to the effectiveness of quantum memories.

#Diffraction-Limited Phonon Lifetime

Throughout the investigation, we estimate the diffraction-limited phonon lifetime using three methods: eigenvalue estimation, exponential curve fitting, and a clipping method. Each method has its own process for determining phonon lifetimes based on the available data.

#Magnon-Phonon Coupling

Magnon-phonon coupling is essential as it determines how efficiently energy is exchanged between the two types of waves. The strength of this interaction impacts the overall performance of the hybrid systems significantly.

#Results and Discussion

We found that phonon lifetimes decrease as the YIG film radius shrinks due to increased diffraction. The analysis also highlights a performance figure of merit, known as cooperativity, which combines both the lifetimes and coupling strengths.

#Performance of Planar Structures

In planar HBAR structures, we observed that as the radius of the YIG film decreases, the lifetimes recorded were significantly shorter, indicating that diffraction plays a critical role in limiting performance.

#Performance of Confocal Structures

Conversely, the confocal structures were found to enhance phonon lifetimes and measures of performance due to their unique designs. By focusing the acoustic waves, the confocal setup provides a better environment for quantum memory applications.

#Conclusion and Future Directions

This study enhances our understanding of the diffraction-limited performance of YIG/GGG HBAR systems. The findings indicate that better designs can improve scalability and quantum memory performance, making these systems more applicable for future technologies. Further research should involve detailed analysis of material limits and testing different configurations to maximize efficiency. The possibilities for integrating these systems with superconducting qubits also offer exciting prospects for the future of quantum technologies.

#Key Takeaways

• Quantum memories are vital in the development of quantum communication technologies.
• Phonons, particularly BAW phonons, show promise in storing quantum information thanks to their long lifetimes.
• Hybrid magnonic systems utilizing YIG and GGG films may enhance quantum memory performance.
• The study reveals significant insights into how diffraction limits phonon lifetimes in small-scale devices.
• Future work could improve designs to enhance performance metrics further, making hybrid systems more effective in practical applications.