Qubit leakage suppression by ultrafast composite pulses.

Area of Science:

• Quantum Information Science
• Atomic Physics
• Quantum Control

Background:

• Quantum systems, particularly qubits, are susceptible to leakage into unwanted states, hindering computational accuracy.
• Three-level ladder-type systems are common architectures where leakage to a third level is a significant challenge.
• Precise control of quantum states is essential for developing reliable quantum technologies.

Purpose of the Study:

• To investigate leakage suppression techniques for a three-level ladder-type quantum system.
• To determine the minimal number of control pulses required for arbitrary qubit control with suppressed leakage.
• To experimentally validate the proposed control strategy.

Main Methods:

• Theoretical analysis of a three-level system with a weakly coupled leakage state.
• Development of phase- and amplitude-controlled pulse sequences for qubit manipulation.
• First-order perturbation theory applied to quantify leakage suppression.
• Experimental implementation using shaped ultrafast optical pulses and cold rubidium atoms.

Main Results:

• Two phase- and amplitude-controlled pulses are sufficient for arbitrary qubit control from the ground state.
• Three pulses enable arbitrary qubit control from any initial state.
• Leakage is suppressed up to the first order of perturbation without increasing pulse-area cost.
• Experimental results show good agreement with theoretical predictions.

Conclusions:

• The proposed pulse control strategy effectively suppresses leakage in three-level quantum systems.
• This method offers an efficient approach to achieving high-fidelity qubit operations.
• Experimental validation confirms the practical applicability of the technique for quantum information processing.