Spaced repetition is one of the most reliable findings in the study of memory, and also one of the most mis-sold. Somewhere along the way it became a synonym for flashcard apps, and for a lot of people that turned a simple idea into a chore they quit. You do not need a deck to get the benefit. You need spacing, and spacing is free.
The spacing effect
The effect is straightforward: if you are going to review something more than once, spreading those reviews out over time beats doing them back to back. The same total effort, spaced instead of massed, produces memory that lasts longer. A 2006 meta-analysis by Nicholas Cepeda and colleagues, pooling more than a century of studies, found the advantage of distributed practice to be large and consistent. Cramming works for tomorrow. Spacing works for next year.
Why flashcards are the wrong tool
Flashcard systems are built for atomic facts: a word and its translation, a term and its definition. Research papers are not atomic facts. Their value is in an argument, a method, and a result you might lean on later, and none of that survives being chopped into question-and-answer cards. Worse, the work of building the cards is friction, and friction is what kills a reading habit. You end up maintaining a deck instead of understanding a field.
Get spacing without decks
The trick is to notice that a re-encounter is a repetition. Every time you meet a paper again, and try to recall what it said before you look, you are spacing your practice, whether or not an app is involved. So build re-encounters into work you already do.
Keep a short note for each paper that matters, a few sentences of the claim, the evidence, and why you cared. When the topic comes up again, in a draft you are writing, a talk you are giving, a problem you are stuck on, pull that note up and recall it first, then read it. Cite the paper, argue about it with a colleague, apply its method. Each of those is a spaced review that costs you nothing extra, because you were going to do the work anyway.
A minimal schedule
If you want something more deliberate, keep it light. Right after reading, write the note from memory. About a week later, glance at the title and try to recall the note before you open it. After that, let use do the spacing: any time the paper is relevant again, recall before you reread. Three touches, spread out, will hold a paper far better than reading it three times in one sitting.
Spacing is the timing half of remembering. The effort half is pulling the memory out yourself, which is why you forget most of what you read and what to do about it. Together they turn reading from something that evaporates into something you keep.