Tom Kelly's response is downvoted, but is actually more correct than any other phonologically speaking, but still not the entire story so I'll elaborate on it and will show an audio analysis comparing it with true gemination so that the difference be visible.
The sokuon in Japanese maps to what Japanese speakers perceive as a coherent atomic unit of sound [phoneme] which is probably why it is written with a single symbol — the Japanese do not consider it merely an orthographical quirk but an actual consonant of Japanese. Such a phoneme is called an archiphoneme, a rare type of phoneme that lacks a consistent underlying place and method of articulation. That is to say that Japanese speakers perceive it as it's own consonant, but it has no meaningful way to be pronounced in isolation. /k/ as in the start of “coal” is a phoneme in English, but English speakers can pronounce it in isolation on command. Archiphonemes are indeed quite rare in languages, and English lacks them entirely. This phoneme is conventionally denoted as /Q/ with a capital letter as conventional for denoting Archiphonemes.
Like most archiphonemes, the distribution of this phoneme is very limited; within the Tokyo standard dialect it can only occur after a vowel, and before any of /t, p, k, s/. This phoneme however must still be realized in some way and make a difference in pronunciation; this is where archiphonemes become interesting, because they are typically realized by their effect on surrounding phonemes, which is why their distribution is so limited.
Now, it is often said that /Q/ is realized by geminating the following consonant. This is only arguably true in the case of /s/ where for a fricative “geminating” is a very abstract term since fricatives can be held indefinitely unlike plosives; when /Q/ is followed by /s/ it is indeed realized simply by holding /s/ for the length of an extra mora.
The sequences of /Qt, Qp, Qk/ are not realized in Japanese by geminating the following consonant; this is something that is repeated very often in lay literature and even some specialized literature but is absolutely false and I'll show why. It is realized by a mora of silence in this case; this also maps onto the perception of /Q/ by Japanese speakers who often claim to perceive it as an empty phoneme of silence for a mora. This goes back it being a consonant; it is effectively “a consonant of silence”.
Onto the audio analysis comparison:
The top two are from a Japanese rendition of “ちょっと”; the speaker Strawberrybrown collected from here; the bottom two are from the Finnish word “että” collected from the speaker Koobee here.
It is obvious that in “että”, there is a small amplitude spike before the “tä” that is lacking in “ちょっと”, but that is not all, in the spectrogram there is also a small thin dark vertical line in the same place that is indicative of a plosive sound. There is no doubt that in Finnish there truly are two plosives, in this case two [t] sounds that occur in succession as you will also find in Arabic, Latin, or any other language with true consonant gemination.
This is missing in Japanese, there is nothing, no fricative, no vowel, no plosive, there is simply a mora of relative silence before the /t/, which does show the characteristic dark, then vertical band. You can verify yourself that this is reproducible with any Japanese word that features /Qt, Qp, Qk/ — the /Q/ archiphoneme, commonly called the “moraic obstruent” if you wish to learn more about it, is not realized in Japanese by doubling plosives, this is a myth that is very common, even in some specialized literature.
Now, onto the /Qs/ combination however for completeness' sake:
This is SPCyan's pronunciation from here.
As you can see, there is no silence here; the pattern on the spectrogram shows that the fricative in /aQsari/ is simply stretched and forms one long pattern on a spectogram that is consistent with a fricative. Nevertheless, it is also visible that it's a less loud and dense fricative than from where the real /s/ starts and more frequencies become filled in, so that's an interesting thing.
But at the end of the day, despite <っ> being conventionally romanized as a doubled consonant it “doubles” no consonant; it's realized as a moral of silence before /k, p, t/ and it only stretches /s/ — it cannot occur in the standard dialect before any other sound but in some dialects it can occur more freely where the realization becomes even more varied.
P.S.: more I incidentally lately came across a a piece of research which investigates the perception of /Qs/ by Japanese native speakers and concludes that they have a hard time differentiating the normal realization from a true mora of silence, which provides evidence that to Japanese speakers, the underlying form is indeed a mora of silence, not a gemination.