quantum-optics-photon-delay

Unexpected Delay in Quantum Optics Yields Photon Pair Generation

Introduction to Spontaneous Parametric Down-Conversion (SPDC)

What is SPDC?

Spontaneous parametric down-conversion (SPDC), first realized in the 1960, has played a pivotal role in quantum optics, underpinning experiments in quantum mechanics and applications like cryptography, metrology, and simulation.

How SPDC Works

SPDC refers to the spontaneous division of a photon into two photons upon passing through a nonlinear medium, such as specific crystals. This instantaneous, nonlinear process ensures the signal and idler photons conserve the energy and momentum of the original pump photon. Specially designed crystals are often employed in SPDC to generate entangled photon pairs.

The Discovery of Gain-Induced Group Delay in Photon Pair Generation

The Role of the Canadian Research Team

A Canadian research team has identified a delay in detecting the two output photons, influenced by the intensity of incoming light interacting with the crystal, termed as "gain-induced group delay."

Impact on Quantum Technologies

The research, published in Physical Review Letters, integrates theoretical analysis, simulation studies, and experimental observations, revealing a delay that may disrupt photon-dependent technologies like quantum sensors and computers.

Theoretical Background: Group Delay and Perturbation Theory

Understanding Group Delay in SPDC

The concept of time delay, or group delay, was theoretically discovered by analyzing SPDC through perturbation theory. This standard physics method simplifies complex energy operators by retaining only the leading terms of their expansion, similar to a Taylor series, making calculations more manageable. Perturbation expansions are crucial for Feynman diagram calculations.

Perturbation Expansions in Photon Scattering

In this context, each term in the expansion signifies a progressively complex form of SPDC photon scattering. The lowest-order term describes the basic process where a pump photon scatters into two lower-energy photon. The subsequent term involves the scattering of the two photon from this pair, resulting in the production of three photons.

It is assumed that both the signal and idler fields start in a vacuum state, which makes this term equal to zero. The third-order term describes a process where two pump photons each create pairs of down-converted photon, followed by the up-conversion of two of these photons.

Significance of Interaction Strength

Each term is proportional to the interaction strength's power, which results in progressively smaller successive terms. This leads to the use of complex quantum mechanical equations to determine the expression for the group delay.

Mathematical Modeling of Time Delays

The researchers built a model of the SPDC perturbation processes to enable numerical computation of time delays.

Experimental Validation of Theoretical Analysis 

Details of the Experiment

To validate their theoretical analysis, the team employed interferometry. They used a pulsed laser with variable power ranging from 0 to 60 milliwatts, generating 180-femtosecond  pulses (0.18 trillionths of a second) centered at 779 nanometers, with a pulse width of 5.37 nanometers at full width at half maximum.

The light used in their experiment falls in the near-infrared range, just outside human visual perception. A 2-mm long potassium titanyl phosphate (KTP) crystal, specifically engineered for this purpose, produced a collinear pair of photons, each with a wavelength centered at 1,558 nm (192 terahertz frequency).

The photons polarizations were oriented perpendicularly, and a polarizing beam splitter was used to separate the signal and idler before directing them through an interferometer. The recorded time delay was 150 nanoseconds.

Key Statements from the Research Team

Insights from Lead Author Guillaume Thekkadath

"Our findings show that photon pairs produced through spontaneous parametric down-conversion display a gain-induced group delay," stated the research team, headed by lead author Guillaume Thekkadath from the National Research Council of Canada in Ottawa. (The other six co-authors are also based in Canada.)

Significance for Future Technologies

"The results indicate that the joint amplitude of the output light is not merely a combination of the two overlapping photons. These setups are becoming more significant for applications such as photonic quantum computing, Gaussian boson sampling, interferometry, and quantum frequency conversion," the team writes.

Challenges and Future Considerations

The researchers indicate that compensating for such delays is simple in bulk optical components—such as mirrors, lenses, prisms, windows, and crystals—but proves problematic with thin films. "However, the delay might lead to challenges when designing chip-integrated quantum interference circuits," they said.

Relevance to Spontaneous Four-Wave Mixing

Shared Quantum Mechanical Description

The study, which analyzed SPDC using ultrashort laser pulses as the pump photons, highlights that their results are also relevant to spontaneous four-wave mixing. This process, which involves photons of two or three wavelengths interacting to generate photons of one of two new wavelengths, shares the same quantum mechanical description.

Applications and Impact

Such applications are prevalent in integrated circuits and optical fibers. The study authors also mention that the observed group delay will influence sources driven by longer pulses or even continuous-wave light.

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Twistronics and 2D material technology

Revolutionary Device Optimizes 2D Material Manipulation for Twistronics Platforms

Breakthrough in Condensed-Matter Physics

In 2018, a discovery shook the foundations of condensed-matter physics: Two ultra-thin carbon layers stacked at a slight angle formed a superconductor, with their electrical properties modifiable through twist-angle adjustments. This breakthrough led to the emergence of "Twistronics," a field pioneered in a landmark paper by Yuan Cao, then an MIT graduate student and currently a Harvard Junior Fellow.

Advancements in Twistronics

Collaborative Efforts in Twistronics Research

Together with Harvard's Amir Yacoby, Eric Mazur, and others, Cao and his colleagues have enhanced their foundational research, developing a simplified method for twisting and studying multiple materials, thereby fostering further twistronics advancements.

New Device Simplifies Material Manipulation

In a recent paper published in Nature, the team details a fingernail-sized machine capable of twisting thin materials at will, eliminating the need for fabricating twisted devices individually. These thin, 2D materials, with easily manipulated properties, hold vast potential for advancing high-performance transistors, optical technologies like solar cells, and quantum computing.

Key Insights and Implications

Expert Commentary on the Breakthrough

Yacoby, a Harvard Professor of Physics and Applied Physics, remarked, "This breakthrough simplifies the process of twisting 2D materials to the level of controlling electron density. Previously, adjusting density was the main method for discovering novel phases in low-dimensional matter; now, with control over both density and twist angle, we unlock countless possibilities for new discoveries."

Overcoming Challenges in Twisting Devices

During his time as a graduate student in Pablo Jarillo-Herrero's lab at MIT, Cao successfully created twisted bilayer graphene. Despite the excitement surrounding this accomplishment, it was accompanied by difficulties in consistently replicating the twisting process.

Development of the MEGA2D Platform

According to Cao, the challenge of producing each twisted device was significant, with the need for numerous unique samples complicating the scientific process. The team sought to simplify this by developing a micromachine that could twist two layers of material on demand, resulting in the creation of the MEGA2D, a MEMS (micro-electromechanical system)-based actuation platform for 2D materials.

Collaboration and Future Prospects

Collaborative Design of the MEGA2D Device

The development of this new tool kit was a collaborative effort between the Yacoby and Mazur labs, and it is adaptable for use with graphene as well as other materials.

Future Applications and Discoveries

As an assistant professor at the University of California Berkeley, Cao stated, "Our MEGA2D technology offers a new "Knob" that we expect will quickly unravel many of the complex issues in twisted graphene and other materials. Moreover, it will likely lead to further groundbreaking discoveries."

Research Findings and Potential

Demonstrating the Device's Utility

In their publication, the researchers showcased the effectiveness of their device with two sheets of hexagonal boron nitride, a material akin to graphene. They investigated the optical properties of the bilayer device and discovered quasiparticles exhibiting sought-after topological properties.

Scientific and Technological Implications

Their new system's ease of use unlocks multiple scientific avenues, such as leveraging hexagonal boron nitride twistronics to engineer light sources that enhance low-loss optical communication.

Conclusion

Optimism for Broader Adoption

Cao expressed optimism that their methodology will gain traction among other researchers in this thriving field, enabling widespread benefits from these new capabilities.

Insights from the Primary Author

The primary author of the paper, Haoning Tang, a distinguished nanoscience and optics specialist and postdoctoral researcher in Mazur's lab as well as a Harvard Quantum Initiative fellow, remarked that creating the MEGA2D technology was a prolonged journey of experimentation and refinement.

"We initially lacked comprehensive knowledge on real-time control of 2D material interfaces, and the conventional techniques proved inadequate," she noted. "Through extensive hours in the clean room and numerous iterations of the MEMS design--despite several setbacks--we eventually Nanofabrication was conducted at Harvard's Center for Nanoscale Systems, where Tang acknowledged the critical technical support provided by the staff.

Recognition of the Nanofabrication Achievement

Mazur, the Balkanski Professor of Physics and Applied Physics, described the integration of MEMS technology with a bilayer structure in nanofabrication as an impressive achievement. He noted that the ability to adjust the nonlinear response of the device paves the way for novel innovations in optics photonics.

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