[Paper Review] Entangling extreme ultraviolet photons through strong field pair generation

Jamison Sloan, Alexey Gorlach+5 more|arXiv (Cornell University)|Sep 28, 2023

Laser-Matter Interactions and Applications6 citations

TL;DR

This paper proposes strong field pair generation (SFPG), a non-perturbative nonlinear optical process in which intense infrared laser fields drive noble gases to emit entangled extreme ultraviolet (XUV) photon pairs via quantum vacuum fluctuations during electron recombination. The method produces thousands of entangled pairs per shot across a broad XUV bandwidth, with spatially and spectrally distinct emission from background high harmonic generation (HHG), enabling background suppression of 4–5 orders of magnitude and enabling attosecond Hong-Ou-Mandel correlations.

ABSTRACT

Entangled photon pairs are a vital resource for quantum information, computation, and metrology. Although these states are routinely generated at optical frequencies, sources of quantum of light are notably lacking at extreme ultraviolet (XUV) and soft X-ray frequencies. Here, we show that strongly driven systems used for high harmonic generation (HHG) can become versatile sources of entangled photon pairs at these high frequencies. We present a general theory of photon pair emission from non-perturbatively driven systems, which we refer to as "strong field pair generation" (SFPG). We show that strongly driven noble gases can generate thousands of entangled pairs per shot over a large XUV bandwidth. The emitted pairs have distinctive properties in angle and frequency, which can be exploited to discriminate them from the background HHG signal. We connect SFPG theory to the three-step-model of HHG, showing that this pair emission originates from the impact of high frequency vacuum fluctuations on electron recombination. The light produced by SFPG exhibits attosecond Hong-Ou-Mandel correlations, and can be leveraged as a source of heralded single photon attosecond pulses. Our findings aid ongoing efforts to propel quantum optics into the XUV and beyond.

Motivation & Objective

• To develop a compact, non-perturbative source of entangled photons in the extreme ultraviolet (XUV) and soft X-ray regimes, where such sources are currently scarce.
• To address the lack of quantum light sources at high frequencies, especially in the XUV and 'water window' regions critical for biological imaging.
• To leverage existing high harmonic generation (HHG) platforms to generate entangled pairs without requiring pre-existing high-frequency sources.
• To enable new applications in quantum metrology, imaging, and attosecond science through XUV entangled photon pairs.
• To establish a theoretical framework for strong field pair generation (SFPG) that connects to the three-step model of HHG and quantum vacuum effects.

Proposed method

• Develops a general quantum field theory of non-perturbative photon pair emission from strongly driven systems, termed strong field pair generation (SFPG).
• Models the process as the conversion of q pump photons (at frequency ω₀) into an entangled pair of photons at frequencies ω and ω′, satisfying ω + ω′ = qω₀.
• Applies the theory to noble gases driven by intense infrared lasers, showing that electron recombination under strong-field conditions leads to entangled XUV pair emission via vacuum fluctuations.
• Uses the three-step model of HHG to identify the origin of SFPG in the recombination phase, where vacuum fluctuations induce correlated electron dynamics.
• Predicts that SFPG pairs are emitted in wide-angle cones (tens to hundreds of milliradians), spatially separating them from the narrow, forward-peaked HHG signal.
• Demonstrates that SFPG can produce non-harmonic frequencies and both even and odd harmonic pairs, enabling spectral discrimination from conventional HHG.

Figure 1: Concept of strong field pair generation (SFPG). (a) A strong infrared laser pulse of frequency $\omega_{0}$ is incident on a sample. When SFPG takes place, entangled photon pairs of frequencies $\omega$ and $\omega^{\prime}$ are produced at angles away from the incident axis. (b) Feynman d

Experimental results

Research questions

• RQ1 Can entangled photon pairs be generated in the extreme ultraviolet (XUV) regime using only strong-field driving without pre-existing high-frequency sources?
• RQ2 How does strong field pair generation (SFPG) differ from conventional high harmonic generation (HHG) in terms of angular and spectral emission patterns?
• RQ3 What is the quantum origin of SFPG, and how is it connected to vacuum fluctuations during electron recombination in the three-step model?
• RQ4 Can SFPG be engineered to achieve significant background suppression relative to HHG, enabling experimental detection of entangled pairs?
• RQ5 What are the implications of SFPG for generating heralded single-photon attosecond pulses and quantum interfaces between optical and XUV frequencies?

Key findings

• Strong field pair generation (SFPG) produces up to thousands of entangled photon pairs per laser shot in the extreme ultraviolet (XUV) range using noble gas targets.
• SFPG pairs are emitted in wide-angle cones (tens to hundreds of milliradians), enabling spatial separation from the narrow, forward-peaked HHG background.
• The method achieves up to 4–5 orders of magnitude suppression of background HHG through optimal matching, enabling clean detection of entangled pairs.
• SFPG can generate non-degenerate pairs with frequencies not present in standard HHG, including both even and odd harmonics and non-harmonic frequencies.
• The emitted XUV photons exhibit attosecond Hong-Ou-Mandel correlations, confirming their quantum nature and enabling use as heralded single-photon attosecond pulses.
• The theory connects SFPG to the three-step model of HHG, identifying vacuum fluctuations during electron recombination as the quantum origin of the entangled pair emission.

Figure 2: Strong field pair generation from single atoms. (a) HHG spectrum for a 1D model of Neon driven by $800$ nm radiation with intensity $I=200$ TW/cm 2 . The system exhibits a plateau over many harmonics, before reaching a cutoff at $q_{c}\approx 39$ . (b) Differential emission probability of