Fundamental bounds on qubit reset

Abstract

Qubit reset is a key task in the operation of quantum devices which, for many quantum 
hardware platforms, presently limits device clock speed. While it is known that coupling 
the qubit to an ancilla on demand allows for the fastest qubit reset, the limits on reset
accuracy and speed due to the choice of ancilla have not yet been identified - despite 
the great flexibility in device design for most quantum hardware platforms. Here, we 
derive bounds on qubit reset in terms of maximum fidelity and minimum time, assuming 
control over the qubit and no control over the ancilla. For two-level ancillas, we find 
a provably time-optimal protocol which consists of purity exchange between qubit and 
ancilla brought into resonance. The globally minimal time can only be realized for 
specific choices of coupling and control which we identify. When increasing the size of 
the ancilla Hilbert space, the maximally achievable fidelity increases, whereas the reset
time remains constant. Our results translate into device design principles for realizing,
in a given quantum architecture, the fastest and most accurate protocol for qubit reset.