For any employee you may have in another state, that state's employment laws apply to both you and your employee---including the obligation to pay payroll taxes to the state. When you pay the employee, you will have to pay unemployment taxes and state disability taxes---and in CA you will have to provide workers' compensation for your employee. Additionally, you will have to withhold and file state tax withholding for the wages paid to that state employee.
As the employee is not selling anything, there is no business income tax that your company will have to pay to the state of CA, or the county or the city where the employee works.
Your federal filings will not change either, fortunately.
CA is not a particularly employer-friendly state and the burdens placed on having just one employee in the state may not justify the trouble and expense. Here are links to a couple of state websites that may provide you with a good idea of the employer obligations when having employees working in the state:
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I wish you the best in your future,