Photonic Molecule Entangled Photon Source
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Solution Overview
Problem
Current sources of entangled photons are limited in speed, with a low probability of emitting pairs per excitation pulse, making them inefficient for secure quantum cryptography over long distances, as they cannot amplify signals without losing security.
Innovation Solution
A source of polarization-entangled photon pairs is developed using a 'photonic molecule' structure, comprising two optical cavities coupled with a quantum emitter, enhancing photon extraction efficiency while preserving entanglement by tuning cavity modes to match the energy transitions of the quantum box.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If parametric frequency conversion is used in a nonlinear optical medium, then entangled photon pairs can be generated, but the output is limited due to Poisson statistics with only 2-5% probability of emitting a pair per excitation pulse
Solution Approach 1:
The patent changes the fundamental operating parameter from continuous wave parametric conversion to pulsed excitation of quantum dots, where the discrete nature of quantum dot emission allows for deterministic single-pair generation. By tuning the excitation pulse parameters and quantum dot properties, the system achieves high probability of emitting exactly one entangled pair per pulse, resolving the contradiction between throughput and emission reliability.
2Reliability
If radiative cascade in a semiconductor quantum dot is used, then one entangled photon pair can be generated per excitation pulse, but the extraction efficiency is low with only 2% of photons extractable
Solution Approach 1:
The patent introduces an optical cavity as an intermediary system between the quantum dot emitter and the external environment. The cavity modes are specifically designed to match the emission frequencies of the quantum dot, creating strong coupling that enhances photon extraction. This intermediary structure transforms the previously inefficient direct emission into an enhanced extraction process, achieving high photon collection efficiency while maintaining the deterministic single-pair generation capability.
3Loss of energy
If a single optical cavity is used to enhance photon extraction, then extraction efficiency improves, but entanglement between photons may be compromised
Solution Approach 1:
The patent segments the optical cavity system into two separate cavities, each coupled to different emission transitions of the quantum dot. This segmentation allows each cavity to be optimized for its specific transition without interfering with the other, thereby preserving the quantum coherence and entanglement relationships. The first cavity enhances extraction for one transition while the second cavity handles the other transition, maintaining the correlated nature of the emitted photon pairs.
Solution Approach 2:
The patent uses two optical cavities as intermediary systems that mediate the photon extraction process while preserving entanglement. Each cavity acts as a selective mediator for its corresponding transition, with carefully designed coupling strengths and frequency matching that maintain the quantum correlations between photons. This dual-cavity intermediary system resolves the contradiction by providing enhanced extraction without compromising the entanglement fidelity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly increases the probability of emitting entangled photon pairs per excitation pulse, achieving high-speed and efficient extraction of entangled photons, essential for secure quantum cryptography over long distances.
Implementation Method 1
In the radiative cascade that occurs in a quantum emitter such as a semiconductor quantum dot, a pair of entangled photons can be generated for each excitation pulse
Implementation Method 2
A first idea underlying the invention is to place the quantum emitter generator in an optical cavity to facilitate the extraction of photons emitted by the Purcell effect
Implementation Method 3
these photons are emitted isotropically in a medium with a high refractive index, meaning that only a small fraction of them—on the order of 2%—can be extracted
Data Source
Figure 1~3
Figure 4A~8B
Figure 5A~5C
AI summary
An entangled photon pair source including: a quantum emitter having a ground state, two degenerate states that have one elementary excitation and different spins, and a state having two elementary excitations; a first optical cavity, wherein the quantum emitter is inserted; and a second optical cavity coupled with the first cavity. The geometry of the first and second cavities, and force of coupling thereof, are selected such that the whole formed by both coupled cavities has a first pair of polarization-degenerate modes, that are resonant with transitions between the state having two elementary excitations and the two degenerate states having one elementary excitation from the quantum emitter, and a second pair of polarization-degenerate modes that are resonant with transitions between the degenerate states, having one elementary excitation, and the ground state.