# Fast Reset Protocol for Superconducting Transmon Qubits
## 1 Introduction  
As quantum computers can solve some hard problems for classical computing [1; 2; 3; 4; 5; 6], it has attracted much attention in the last few decades. However, when a quantum computer is running complex and/or large number of operations, it needs the ability to reset qubits to their ground states quickly with high fidelity [7; 8; 9; 10; 11]. A simple reset scheme is to passively wait for the qubit naturally dissipate to its ground state [12], and the reset time is longer than the qubit relaxation times (\(T_{1}\)) [13], and thus is not preferable. Subsequently, active qubit reset has been proposed to break through the limitation from \(T_{1}\) and improve the operational efficiency for qubits [14; 15; 16; 17; 18; 19; 20; 21; 22]. Recently, some active qubit reset protocols have been demonstrated on many quantum computing platforms.  
There are two types of active qubit reset protocols for superconducting circuits; one is the measurement-based reset [14; 15; 16] and the other one is the non-measurement reset [17; 18; 19; 20; 21; 22]. In the measurement-based scheme, a qubit can be reset based on measurement results. For instance, if the result is that the qubit is in excited state, a \(\pi\) pulse is added to bring it back to the ground state [14]. However, this method relies heavily on qubit measurement fidelity and low-latency feedback techniques [19]. For the non-measurement scheme, a qubit is coupled to a low-temperature dissipative environment in a controlled way. For superconducting qubits, the dissipation can be naturally provided by a low-quality readout from a superconducting transmission-line resonator.  
There are two mainly technical routes for the non-measurement reset of superconducting qubits [17; 18; 19; 20; 21; 22]. One route is for the frequency-tunable qubit, which can be rapidly tuned into resonance with the low-quality resonator [17; 21; 22]. However, it requires complex calibration techniques to avoid the influence from the crosstalk on neighboring qubits. Another route is to apply a microwave pulse to induce exchange interaction between the qubit and a low-quality resonator [18; 19; 20; 21]. However, this requires multiple microwave pulses, which complicates its corresponding experiment realizations.  
Here, we propose a fast and simple reset scheme for a superconducting qubit based on one of the popular chip designs, i.e., the coupler-coupled-qubits architecture. We first apply a mixing pulse to transfer the excited state of a superconducting qubit to the doubly excited state of a coupler and a readout resonator. Then, the doubly excited state will quickly decay to the ground states of the coupler and a readout resonator, due to their short coherence times compared with the superconducting qubit. That is, we realize fast reset for a superconducting qubit, which is used in a mainstream superconducting qubit chip structure [23; 24; 25; 26; 27; 28; 29; 30]. We use the black-box superconducting circuit quantization method [31; 32] to analyze the Hamiltonian of the setup, which can transfer the excited state of a superconducting qubit to the doubly excited states of a coupler and a readout resonator. The numerical simulation results show that population of the qubit's excited state can be suppressed to 0.04% within 28 ns. Meanwhile, if photon depletion of the readout resonator is required, the reset time will be 283 ns, which is still far shorter than the qubit decay time. Therefore, our scheme provides a promising way for the high-efficiency reset of superconducting qubits, which is necessary for larger scale quantum information processing.