| Document and Entity Information | Sep. 29, 2021 |
| Entity Listings [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 29, 2021 |
| Entity Registrant Name | QUANTUM-SI INCORPORATED |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-39486 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001816431 |
| Common Stock [Member] | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Class A common stock, par value $0.0001 per share |
| Trading Symbol | QSI |
| Security Exchange Name | NASDAQ |
| Redeemable warrants, each whole warrant exercisable for one share of Class A common stock, each at an exercise price of $11.50 per share [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 |
