One of the main figures of merit for quantum memories and quantumcommunication devices is their quantum capacity. It has been studied forarbitrary kinds of quantum channels, but its practical estimation has so farbeen limited to devices that implement independent and identically distributed(i.i.d.) quantum channels, where each qubit is affected by the same noiseprocess. Real devices, however, typically exhibit correlated errors. Here, we overcome this limitation by presenting protocols that estimate achannel's one-shot quantum capacity for the case where the device acts on (anarbitrary number of) qubits. The one-shot quantum capacity quantifies adevice's ability to store or communicate quantum information, even if there arecorrelated errors across the different qubits. We present a protocol which is easy to implement and which comes in twoversions. The first version estimates the one-shot quantum capacity bypreparing and measuring in two different bases, where all involved qubits areused as test qubits. The second version verifies on-the-fly that a channel'sone-shot quantum capacity exceeds a minimal tolerated value while storing orcommunicating data, therefore combining test qubits and data qubits in oneprotocol. We discuss the performance of our method using simple examples, suchas the dephasing channel for which our method is asymptotically optimal.Finally, we apply our method to estimate the one-shot capacity in an experimentusing a transmon qubit.
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