A 30-year-old woman is brought by her partner 10 days after giving birth. For four nights she has slept for less than two hours, says the infant has been exchanged for a dangerous impostor and reports hearing a voice telling her to ‘make things safe’; she tried to leave the house with the baby during the consultation. She has no previous psychiatric diagnosis and does not recognise that she is unwell. Which one of the following is the most appropriate next step?