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I wrote this sentence:
How can a single photon be detected?
I think it could mean one of the photons while emphasizing its singleness.
But I feel something's wrong with it because a and single are the same meaning and redundant.
Is the sentence idiomatic?
Yes, your sentence is correct.
How can a single photon be detected?
The indefinite article "a" tells me you want to know how to detect a photon, any photon, not a specific one. If you wanted to detect a specific photon, you would have used the definite article "the."
The word "single" tells me you want to know how to detect only one photon, not more than one.
Thus, you can see that both the words "a" and "single" are needed to clearly state your question.
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?
or even:
- 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.
Short answer: Yes.
You COULD say simply, "Can a photon be detected?" But adding the word "single" makes it clear that you are talking about detecting just one proton.
One could argue that this is redundant, as the word "a" already indicates you are talking about only one. But not all redundancy is bad. Sometimes it's very helpful for emphasis or to make the meaning clear.
Your question sentence:
How can a single photon be detected?
looks good.
If you use a singular countable none first time or in general you should use article "a". And it does not interferes with "single".
I totally agree with Jay - your construction "a single photon" makes the meaning clear.
We cannot remove the article a:
*How can single photon be detected?
is not grammatical. The a is required grammatically regardless of the semantic overlap between a and single.
What is not grammatical is using both a and a number, such as one:
*How can a one photon be detected?
You might be thinking that since this is faulty, substituting single for one should still be faulty.
But single is not in the same special lexical category as one.
Furthermore, it doesn't always mean the same thing: it means one in not a single man came forward, but it means unmarried in he remained a single man all his life.
single is not an indicator of plurality; it's an adjective. Note that single photons is grammatical, just like single women. But one photons and one women isn't!
How can single photons be detected?
*How can one photons be detected?
**A single tree gave shade from the sun.---Longman ** They won the game by a single point.----Longman **We ought to be able to complete the work in a single day.--Macmillan
How can just/even a single photon be detected?