[Paper Review] Quantum interference and entanglement in ultracold atom-exchange reactions

Yi-Xiang Liu, Lingbang Zhu+5 more|arXiv (Cornell University)|Oct 11, 2023

Cold Atom Physics and Bose-Einstein CondensatesPhysics and Astronomy3 citations

TL;DR

This study demonstrates that quantum coherence and entanglement can be preserved in ultracold atom-exchange reactions, specifically in the 2KRb → K₂ + Rb₂ process at 500 nK. By preparing KRb molecules in an entangled nuclear spin state and using coincidence detection with velocity map imaging, the authors observe interference effects consistent with full coherence in the reaction, indicating that entanglement can be transferred to product molecules.

ABSTRACT

Coherent superpositions and entanglement are hallmarks of quantum mechanics, but they are fragile and can easily be perturbed by their environment. Selected isolated physical systems can maintain coherence and generate entanglement using well-controlled interactions. Chemical reactions, where bonds break and form, are highly dynamic quantum processes. A fundamental question is whether coherence can be preserved in chemical reactions and then harnessed to generate entangled products. Here we investigate this question by studying the 2KRb $ ightarrow$ K$_2$ + Rb$_2$ reaction at 500 nK, focusing on the the nuclear spin degrees of freedom. We prepare the initial nuclear spins in KRb in an entangled state and characterize the preserved coherence in nuclear spin wavefunction after the reaction. The data are consistent with full coherence at the end of the reaction. This suggests that entanglement can be prepared within the reactants, followed by a chemical reaction that produces separate, entangled molecules. We additionally demonstrate control of the reaction product state distribution by deliberately decohering the reactants.

Motivation & Objective

• To investigate whether quantum coherence and entanglement can survive in chemical reactions involving ultracold molecules.
• To determine if entangled nuclear spin states in reactants can lead to entangled product molecules in bimolecular reactions.
• To experimentally probe coherence in the reaction dynamics using state-selective detection and Bayesian analysis.
• To explore control of product state distributions by manipulating the coherence of reactant nuclear spins.
• To test the hypothesis that nuclear spin degrees of freedom can serve as a robust quantum resource in reactive scattering processes.

Proposed method

• Preparation of 40K87Rb molecules in a single quantum state at 500 nK using ultracold molecular control techniques.
• Initialization of reactant KRb molecules into an entangled nuclear spin state to probe coherence preservation.
• Use of coincidence detection with velocity map imaging to identify individual product pairs (K₂, Rb₂) with full state-to-state resolution.
• Application of Bayesian inference to estimate the coherence parameter Γ from measured counts, accounting for background and calibration factors.
• Modeling of degeneracy in product states via angular momentum coupling (N_K₂, N_Rb₂, L) to determine statistical and coherent branching ratios.
• Calibration of detection efficiency using velocity-dependent coefficients and residual background subtraction.

Experimental results

Research questions

• RQ1 Can quantum coherence in nuclear spin states survive a bimolecular chemical reaction in the ultracold regime?
• RQ2 Does the reaction 2KRb → K₂ + Rb₂ preserve entanglement from the initial KRb reactants to the K₂ and Rb₂ products?
• RQ3 To what extent does decoherence of the reactant nuclear spin wavefunction affect the product state distribution?
• RQ4 Can interference effects between reaction pathways be observed and quantified in ultracold atom-exchange reactions?
• RQ5 Is the coherence parameter Γ measurable and quantifiable in a real experimental setting with finite statistics and background noise?

Key findings

• The Bayesian analysis yields a 95% confidence interval for the coherence parameter Γ of 0.9014 to 1, indicating near-perfect preservation of coherence.
• The maximum likelihood estimate of Γ is 0.9957, strongly supporting that the nuclear spin wavefunction remains coherent throughout the reaction.
• Observed product counts (e.g., 490 counts for |10,9⟩) are consistent with coherent interference, particularly in the |10,9⟩ and |9,10⟩ channels.
• The statistical branching ratios (P_S) and coherent branching ratios (P_C) differ significantly, with P_C = 0.7049 for |10,9⟩, indicating interference suppression in coherent pathways.
• Background-corrected expected counts (e.g., 477.5 for |10,9⟩) align closely with measured values, validating the model and calibration.
• The use of a flat prior in Bayesian inference yields results robust to prior choice, with Jeffreys’ prior producing negligible differences.