Some of the stations in Antarctica use heat pumps. They have been proven to work effectively at -53°C (-64°F) and do so reliably.

Are they more efficient at more reasonable temperatures? Yes. But they still work even when it's very cold outside.

How well a heat pump works in cold temperatures obviously depends what temperatures it was designed to operate at. Don't waste your money on a model that is designed to operate in a different climate. In fact a lot of heat pumps aren't even capable of heating at all - they can only output cold air (which they can do even if it's stinking hot outside by the way).