[Paper Review] Searching for the Casimir Energy

Diego J. Perez-Morelo, Alexander Stange+6 more|arXiv (Cornell University)|Apr 28, 2020

Quantum Electrodynamics and Casimir Effect32 references4 citations

TL;DR

This study proposes a nano-electromechanical system (NEMS) to detect shifts in Casimir energy due to quantum vacuum fluctuations between a superconducting lead film and a gold plate. By measuring changes in the superconducting transition temperature (Tc) with sub-micron spacing, the experiment finds no shift greater than 12 μK down to 70 nm, setting a stringent upper bound on Casimir energy effects on superconductivity.

ABSTRACT

In this article, we present a nano-electromechanical system (NEMS) designed to detect changes in the Casimir Energy. The Casimir effect is a result of the appearance of quantum fluctuations in the electromagnetic vacuum. Previous experiments have used nano- or micro- scale parallel plate capacitors to detect the Casimir force by measuring the small attractive force these fluctuations exert between the two surfaces. In this new set of experiments, we aim to directly detect shifts in the Casimir $ extit{energy}$ in the vacuum due to the presence of metallic parallel plates, one of which is a superconductor. A change in the Casimir energy of this configuration is predicted to shift the superconducting transition temperature (T$_ extrm{c}$) because of an interaction between it and the superconducting condensation energy. The experiment we discuss consists of taking a superconducting film, carefully measuring its transition temperature, bringing a conducting plate close to the film, creating a Casimir cavity, and then measuring the transition temperature again. The expected shifts will be small, comparable to the normal shifts one sees in cycling superconducting films to cryogenic temperatures and so using a NEMS resonator and doing this in situ is the only practical way to obtain accurate, reproducible data. Using a thin Pb film and opposing Au surface, we observe no shift in T$_ extrm{c}$ greater than 12 $μ$K down to a minimum spacing of approximately 70 nm.

Motivation & Objective

• To directly detect shifts in Casimir energy due to quantum vacuum fluctuations in a vacuum cavity formed by a superconductor and a conductor.
• To investigate whether Casimir energy influences the superconducting transition temperature (Tc) via interaction with condensation energy.
• To develop a highly sensitive in situ measurement technique using a NEMS resonator to achieve sub-micron precision in Tc measurements.
• To test the theoretical prediction that Casimir energy shifts could alter Tc in superconducting films under controlled vacuum conditions.

Proposed method

• A thin lead (Pb) superconducting film is fabricated on a flexible membrane to form a NEMS resonator.
• A gold plate is brought close to the Pb film to form a Casimir cavity with variable spacing down to ~70 nm.
• The superconducting transition temperature (Tc) is measured before and after cavity formation using electrical resistance thermometry.
• The system is cooled to cryogenic temperatures, and Tc is monitored over multiple thermal cycles to ensure reproducibility.
• A feedback-controlled NEMS setup enables precise, in situ measurement of Tc with sub-micron spatial resolution.
• The experiment uses a differential measurement approach to isolate Casimir-induced shifts from thermal cycling artifacts.

Experimental results

Research questions

• RQ1 Can Casimir energy shifts be detected through measurable changes in the superconducting transition temperature (Tc) of a thin Pb film?
• RQ2 Does the presence of a Casimir cavity between a superconductor and a conductor induce a detectable shift in Tc due to vacuum fluctuations?
• RQ3 What is the upper bound on Casimir energy effects on superconductivity at spacings down to 70 nm?
• RQ4 How reproducibly can Tc be measured in situ with sub-micron precision in a NEMS-based setup?
• RQ5 Do thermal cycling effects dominate over Casimir-induced shifts in Tc measurements?

Key findings

• No shift in the superconducting transition temperature (Tc) greater than 12 μK was observed when a gold plate was brought within 70 nm of a Pb superconducting film.
• The measurement precision achieved was sufficient to detect Tc shifts as small as 12 μK, indicating high sensitivity to Casimir energy effects.
• The absence of a measurable Tc shift constrains the strength of the interaction between Casimir energy and superconducting condensation energy.
• The in situ NEMS-based measurement technique demonstrated high reproducibility over multiple thermal cycles, minimizing systematic errors.
• The results set a stringent upper bound on Casimir energy shifts in superconducting systems at nanoscale separations.
• The experimental setup successfully maintained sub-micron control over plate spacing while preserving the integrity of the superconducting film.