CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Class A common stock	Class B common stock	Common Stock	Common Stock Class A common stock	Common Stock Class B common stock	Additional paid-in capital	Accumulated other comprehensive gain (loss)	Accumulated deficit
Balance (in shares) at Dec. 31, 2023					121,832,417	19,937,500
Balance at Dec. 31, 2023	$ 271,619				$ 12	$ 2	$ 767,239	$ 0	$ (495,634)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued upon vesting of restricted stock units (in shares)					46,572
Stock-based compensation	1,645						1,645
Net unrealized gain (loss) on marketable securities, net of tax	(28)							(28)
Refund of issuance costs	14						14
Foreign currency translation	(5)							(5)
Net loss	(19,474)								(19,474)
Balance (in shares) at Mar. 31, 2024					121,878,989	19,937,500
Balance at Mar. 31, 2024	253,771				$ 12	$ 2	768,898	(33)	(515,108)
Balance (in shares) at Dec. 31, 2023					121,832,417	19,937,500
Balance at Dec. 31, 2023	271,619				$ 12	$ 2	767,239	0	(495,634)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net unrealized gain (loss) on marketable securities, net of tax	163
Foreign currency translation	(10)
Net loss	(67,886)
Balance (in shares) at Sep. 30, 2024					122,775,352	19,937,500
Balance at Sep. 30, 2024	210,520				$ 12	$ 2	773,873	153	(563,520)
Balance (in shares) at Mar. 31, 2024					121,878,989	19,937,500
Balance at Mar. 31, 2024	253,771				$ 12	$ 2	768,898	(33)	(515,108)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued upon exercise of stock options (in shares)					96,069
Common stock issued upon exercise of stock options	136						136
Common stock issued upon vesting of restricted stock units (in shares)					407,274
Stock-based compensation	2,426						2,426
Net unrealized gain (loss) on marketable securities, net of tax	28							28
Foreign currency translation	(2)							(2)
Net loss	(23,099)								(23,099)
Balance (in shares) at Jun. 30, 2024					122,382,332	19,937,500
Balance at Jun. 30, 2024	233,260				$ 12	$ 2	771,460	(7)	(538,207)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued upon exercise of stock options (in shares)					41,470
Common stock issued upon exercise of stock options	2						2
Common stock issued upon vesting of restricted stock units (in shares)					351,550
Stock-based compensation	2,411						2,411
Net unrealized gain (loss) on marketable securities, net of tax	163							163
Foreign currency translation	(3)							(3)
Net loss	(25,313)								(25,313)
Balance (in shares) at Sep. 30, 2024					122,775,352	19,937,500
Balance at Sep. 30, 2024	210,520				$ 12	$ 2	773,873	153	(563,520)
Balance (in shares) at Dec. 31, 2024		146,953,271	19,937,500		146,953,271	19,937,500
Balance at Dec. 31, 2024	215,420				$ 16	$ 2	811,998	45	(596,641)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued upon exercise of stock options (in shares)					623,834
Common stock issued upon exercise of stock options	1,600						1,600
Common stock issued upon vesting of restricted stock units (in shares)					358,511
Common stock issued from direct equity offering and upon exercise of pre-funded warrants, net of fees and issuance costs (in shares)					15,625,000
Common stock issued from direct equity offering and upon exercise of pre-funded warrants, net of fees and issuance costs	46,774						46,774
Stock-based compensation	2,362						2,362
Net unrealized gain (loss) on marketable securities, net of tax	(53)							(53)
Foreign currency translation	6							6
Net loss	(19,189)								(19,189)
Balance (in shares) at Mar. 31, 2025					163,560,616	19,937,500
Balance at Mar. 31, 2025	246,920				$ 16	$ 2	862,734	(2)	(615,830)
Balance (in shares) at Dec. 31, 2024		146,953,271	19,937,500		146,953,271	19,937,500
Balance at Dec. 31, 2024	215,420				$ 16	$ 2	811,998	45	(596,641)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net unrealized gain (loss) on marketable securities, net of tax	(5)
Foreign currency translation	14
Net loss	(83,728)
Balance (in shares) at Sep. 30, 2025		195,520,747	19,937,500		195,520,747	19,937,500
Balance at Sep. 30, 2025	235,159				$ 20	$ 2	915,452	54	(680,369)
Balance (in shares) at Mar. 31, 2025					163,560,616	19,937,500
Balance at Mar. 31, 2025	246,920				$ 16	$ 2	862,734	(2)	(615,830)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued upon exercise of stock options (in shares)					24,583
Common stock issued upon exercise of stock options	41						41
Common stock issued upon vesting of restricted stock units (in shares)					772,335
Stock-based compensation	2,896						2,896
Net unrealized gain (loss) on marketable securities, net of tax	(54)							(54)
Foreign currency translation	16							16
Net loss	(28,836)								(28,836)
Balance (in shares) at Jun. 30, 2025					164,357,534	19,937,500
Balance at Jun. 30, 2025	220,983				$ 16	$ 2	865,671	(40)	(644,666)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued upon exercise of stock options (in shares)					143,393
Common stock issued upon exercise of stock options	221						221
Common stock issued upon vesting of restricted stock units (in shares)					1,079,701
Common stock issued upon vesting of restricted stock units	1			$ 1
Common stock issued from direct equity offering and upon exercise of pre-funded warrants, net of fees and issuance costs (in shares)					29,940,119
Common stock issued from direct equity offering and upon exercise of pre-funded warrants, net of fees and issuance costs	46,736			$ 3			46,733
Stock-based compensation	2,827						2,827
Net unrealized gain (loss) on marketable securities, net of tax	102							102
Foreign currency translation	(8)							(8)
Net loss	(35,703)								(35,703)
Balance (in shares) at Sep. 30, 2025		195,520,747	19,937,500		195,520,747	19,937,500
Balance at Sep. 30, 2025	$ 235,159				$ 20	$ 2	$ 915,452	$ 54	$ (680,369)

Quantum-Si Inc 10-Q