Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2024	May 03, 2024
Entity Listings [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2024
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Document Transition Report	false
Entity File Number	001-39486
Entity Registrant Name	QUANTUM-SI INCORPORATED
Entity Central Index Key	0001816431
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	85-1388175
Entity Address, Address Line One	29 Business Park Drive
Entity Address, City or Town	Branford
Entity Address, State or Province	CT
Entity Address, Postal Zip Code	06405
City Area Code	866
Local Phone Number	688-7374
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Common Class A [Member]
Entity Listings [Line Items]
Title of 12(b) Security	Class A common stock, $0.0001 per share
Trading Symbol	QSI
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		121,878,989
Common Class B [Member]
Entity Listings [Line Items]
Entity Common Stock, Shares Outstanding		19,937,500
Redeemable Warrants [Member]
Entity Listings [Line Items]
Title of 12(b) Security	Redeemable warrants, each whole warrant exercisable for one share of Class A common stock, each at an exercise price of $11.50 per share
Trading Symbol	QSIAW
Security Exchange Name	NASDAQ