To repeat what the other answers have already said, yes, your example sentence is perfectly grammatical and idiomatic. For that matter, so are all of these sentences as well:
- How can one single photon be detected?
- How can the single photon [emitted by some specific source] be detected?
- How can single photons be detected?
- How can two single photons be detected [by the same detector]?
"A" in English is an indefinite article — basically an obligatory feature of English grammar that carries very little meaning, other than signifying that the following noun phrase refers to some single but indeterminate thing whose identity is not assumed to be known by the reader. In English, noun phrases involving singular countable nouns must (almost) always start with a determiner. While there are many such determiners (such as numerals, demonstratives, possessives, interrogatives, etc.) with different meanings, the definite article "the" and the indefinite article "a" / "an" serve as generic neutral determiners that can (and usually must!) be used when no other determiner is appropriate.
"Single", meanwhile, is an adjective that refers to something being or occurring alone, apart from other similar things or events. It is not a determiner, so e.g. *"single photon" is not a complete English noun phrase — it needs a determiner like "a single photon", "the single photon", "this single photon" or even "no single photon". The adjective "single" can also be applied to plural nouns, as in "single photons", to signify that all of the many things being referred to are distinct and separated from each other. Thus, for example,
A single-photon detector can detect single photons and send a signal when each one is detected, allowing the number of photons received by the detector to be counted.
is a perfectly idiomatic (and factual!) English sentence.
Basically, the words "a" and "single" in the phrase "a single photon" have completely different grammatical roles, and also quite little semantic overlap. The article "a" cannot be removed from the phrase at all without making it ungrammatical, unless it is replaced by some other determiner (or unless the phrase is made plural instead). Meanwhile, the adjective "single" serves to emphasize that the photon is to be detected in isolation, separately from other photons, and removing it would change the meaning of the sentence. In particular, a valid answer to the question:
How can a photon be detected?
would simply be "with any camera, or with the human eye; they all detect light, which is made up of photons." But that's clearly not the question you wanted to ask.
How can just/even a single photon be detected?