Why ?A watched pot never boils?

Peter W. Milonni

Robert Pool

Jörg Schmiedmayer

Mikhail Lemeshko, Břetislav Friedrich

We present the physics of the quantum Zeno effect, whose gist is often expressed by invoking the adage "a watched pot never boils". We review aspects of the theoretical and experimental work done on the effect since its inception in 1977, and mention some applications. We dedicate the article - with our very best wishes - to Rudolf Zahradnik at the occasion of his great jubilee. Perhaps Rudolf's lasting youthfulness and freshness are due to that he himself had been frequently observed throughout his life: until the political turn-around in 1989 by those who wished, by their surveillance, to prevent Rudolf from spoiling the youth by his personal culture and his passion for science and things beautiful and useful in general. This attempt had failed. Out of gratitude, the youth has infected Rudolf with its youthfulness. Chronically. Since 1989, Rudolf has been closely watched by the public at large. For the same traits of his as before, but with the opposite goal and for the benefit of all generations. We relish keeping him in sight ...

James Norman Ibbotson

Observe an unstable quantum system frequently enough and its decay slows, and in the limit of continuous observation stops altogether. This is measurement-induced state freezing — the quantum Zeno effect, the physicist's "a watched pot never boils." It was named by Misra and Sudarshan (1977) and measured by Itano, Heinzen, Bollinger and Wineland in a ion trap (1990). It is standard, established quantum mechanics and is not in dispute. This paper does three things. (i) It restates, from first principles and with a numerical runner, why the effect exists at all: it is a statement about the exponent of the short-time survival law, and about nothing else. Frequent projective measurement can suppress a loss that grows quadratically in time (a coherent two-state amplitude, survival ), and cannot suppress a loss that grows linearly in time (a memoryless classical rate, survival ). (ii) It reports the two cases side by side, matched to agree when unwatched and then watched, together with a fitted short-time exponent — 1.9999 for the coherent case, 0.9967 for the classical case — and a "look but do not reset" control. (iii) In a clearly labelled interpretive section it examines the Temporal Dynamics Framework's account of the effect and shows that this account is a fork: the framework's fundamental object [the flip — in standard vocabulary a two-state transition of the underlying oscillation, which may be read either as a coherent amplitude or as a classical rate] must be committed to one reading, and the two readings give opposite answers. Only the coherent reading reproduces Zeno. This record holds one paper on its own and is refreshed every couple of months. The complete deposit, 10.5281/zenodo.21788037, holds every paper with its code and data and is updated first; for the newest version of this paper, go there.