たとたん

Expresses that the moment one action or change occurs, something else happens almost at once. The second event is usually sudden or unexpected.

Neutral in register and common in both speech and writing, especially when narrating past events where something happened without warning. The second clause describes an event outside the speaker's control, not a planned action. Compared with the N1 patterns や否や and が早いか, which are more literary, たとたん is the everyday choice.

• や否やN1「や否や」 has a similar meaning but attaches to the 「辞書形」 of verbs (「入るや否や」), and its strong written flavor gives a stiffer, more elegant tone, so it appears mostly in novels and news. When narrating events that have already happened, the two are often interchangeable, but it is generally not used in everyday conversation.
• が早いかN1「が早いか」 is similar in meaning but is a written-language expression used mostly in novels and narrative prose, emphasizing speed to the point of almost 'getting in first'; it attaches to either 「辞書形」 or 「た形」, as in 「鳴るが早いか」. In everyday conversation, 「たとたん」 is more natural.
• た拍子にN1「た拍子に」 emphasizes that the impact of the first action directly triggers something unexpected, so it cannot be used when the two actions are merely consecutive in time — 「部屋に入ったとたん電話が鳴った」 cannot be switched to it. It can also attach to noun + 「の」, as in 「何かの拍子に」.
• なりN1「なり」 attaches to the 「辞書形」, leans literary, and generally narrates third-person actions. The subjects before and after are usually the same, and when they differ the second action must be directly triggered by the first, so 「部屋に入ったとたん、電話が鳴り始めた」 should not be recast with 「なり」.
• そばからN1「そばから」 describes something that happens as soon as an action is done and keeps happening that way, often with a resigned sense that one's efforts come to nothing; it attaches to both 「辞書形」 and 「た形」. It cannot be used for one-time events, so for a one-time 'as soon as...' situation, stick with 「たとたん」.
• かと思ったら / かと思うとN2「かと思ったら／かと思うと」 puts more emphasis on contrast and surprise, and in the 'as soon as...' sense it is not used for one's own actions. 「かと思ったら」 also has the separate meaning 'I thought... but (actually)...' and can attach to nouns and the like — a usage 「たとたん」 does not have.

や否や and が早いか are literary N1 equivalents that also mean 'as soon as', and なり stresses one person's sudden follow-up action, while た拍子に highlights an accidental result caused by the momentum of an action. そばから describes something repeatedly undone right after it is done, and かと思ったら / かと思うと frames the change as the speaker's surprised observation, usually of someone else.