What happened Shares of Arqit Quantum (NASDAQ: ARQQ) stock were slumping on Thursday. The stock started the day up as much as 16.6% but has since fallen sharply, down as much as 21.9% on the day. As of 12:48 p.m. EDT, Arqit Quantum stock is down 16.7% today. So what Arqit Quantum completed its special purpose acquisition company (SPAC) merger that helped it enter the public markets less than a month ago. Like a lot of SPAC stocks that have a low float (which means there are a limited amount of shares being publicly traded) early in their trading history, small changes in demand from traders can cause huge changes in the share price. This is likely what is happening with Arqit Quantum. Image source: Getty Images. Why are investors/traders so excited about this company? Because in its SPAC presentation, Arqit claims it can disrupt the encryption/cybersecurity market by bringing something called symmetric encryption to the cloud. According to the company, this can help solve encryption issues even if/when quantum computers greatly increase the capabilities of hackers (hence the company's name). The company is still in the very early phases but has large customer deals with companies like Verizon (NYSE: VZ), Northrup Grumman (NYSE: NOC), and a major payments provider. These large deals, along with other customers, have built a $1.1 billion backlog for Arqit Quantum at the time of its SPAC merger presentation. Now what While Arqit Quantum is making big claims and has a lot of potential future revenue coming, it currently is a tiny business. Management is only guiding for $14 million in revenue this year, which is small compared to the stock's market cap of $3 billion to $4 billion. The company is expecting $660 million in revenue in 2025, which would make the current market cap more palatable. But investors should understand that there is a ton of execution risk with Arqit Quantum stock -- no matter how promising the technology is. 10 stocks we like better than Northrop GrummanWhen our award-winning analyst team has a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* They just revealed what they believe are the ten best stocks for investors to buy right now... and Northrop Grumman wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of September 17, 2021 Brett Schafer has no position in any of the stocks mentioned. The Motley Fool recommends Verizon Communications. The Motley Fool has a disclosure policy.Source