Is there any way to add security to one field in JotForm?

I would like my potential employees to be able to enter one piece of sensitive information to my form, but I do not want anyone to be able to see except the person filling out the form.  

Pardon me but I'm not sure I understand why your workflow would require this. By default, only you (as the account owner) have access to your Form Submission Data. Your users/visitors/respondents can only see the info they filled out themselves on the form. No one else does.

I checked your account and confirmed you have no Subusers added, this means you're not sharing your submissions with anyone either (not unless you intentionally share the Submission URL to someone else).

To better explain it, only the following people will get the Submission Data of a respondent:

1. The respondent himself (in your case, the employee who submitted the form)

2. You, as the account owner

3. Any subuser you added that has "View Submission" privilege

Related guide: How-to-share-forms-with-a-sub-account-user 

4. Anyone included as the "recipient" for your Email Notifications

Related guide: Setting-Up-Email-Notifications

If you could get back to us with further details on what you're trying to achieve, please do, so we can better assist you.