Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2021	Feb. 23, 2022	Jun. 30, 2021
Entity Listings [Line Items]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
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	530 Old Whitfield Street
Entity Address, City or Town	Guilford
Entity Address, State or Province	CT
Entity Address, Postal Zip Code	06437
City Area Code	203
Local Phone Number	458-7100
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 1.1
Auditor Name	Deloitte & Touche LLP
Auditor Location	New York, New York
Auditor Firm ID	34
Class A Common Stock [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		118,728,140
Class B Common Stock [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