Quantum computers could one day achieve significantly higher computing speeds than conventional digital computers in performing certain types of tasks. Superconducting circuits belong to the most promising candidates for implementing quantum bits, known as qubits, with which quantum computers can store and process information. The high error rates associated with previously available qubits have up to now limited the size and efficiency of quantum computers. Dr. Gianluigi Catelani of the Peter Grünberg Institute (PGI-2) in Jülich, together with his colleagues has now found a way to prolong the time in which the superconducting circuits are able to store a “0” or a “1” without errors. Beside Catelani, the team comprises researchers working in the USA (Massachusetts Institute of Technology, Lincoln Laboratory, and the University of California, Berkeley), Japan (RIKEN), and Sweden (Chalmers University of Technology).
Illustration of the filtering of unwanted quasiparticles (red spheres) from a stream of superconducting electron pairs (blue spheres) using a microwave-driven pump. Copyright: Philip Krantz, Krantz NanoArt
When superconducting materials are cooled below a material-specific critical temperature, electrons come together to form pairs; then current can flow without resistance. However, so far it has not been possible to build superconducting circuits in which all electrons bundle together. Single electrons remain unpaired and are unable to flow without resistance. Due to these so-called quasiparticles, energy is lost and this limits the length of time that the circuits can store data.
Researchers have now developed and tested a technique that can temporarily remove unpaired electrons away from the circuit; with the help of microwave pulses, they are in effect “pumped out”. This results in a three-fold improvement in the lifespan of the qubits.
“The technique can in principle be put to immediate use for all superconducting qubits”, explained Catelani, who, as a theoretical physicist has contributed to the analysis and interpretation of the experimental data. However, he emphasised that the lifespan of qubits is only one of many hurdles in the development of complex quantum computers. Moreover, the new technique means that the quasiparticles are not permanently removed, but flow back again and again. The scientists have another solution ready to solve this problem: the pumping technique can be combined with another method that permanently traps the quasiparticles. Catelani, together with his colleagues from Jülich and Yale, has already analysed and tested such a quasiparticle “trap”.
Physical Review B - Normal-metal quasiparticle traps for superconducting qubits
The presence of quasiparticles in superconducting qubits emerges as an intrinsic constraint on their coherence. While it is difficult to prevent the generation of quasiparticles, keeping them away from active elements of the qubit provides a viable way of improving the device performance. Here we develop theoretically and validate experimentally a model for the effect of a single small trap on the dynamics of the excess quasiparticles injected in a transmon-type qubit. The model allows one to evaluate the time it takes to evacuate the injected quasiparticles from the transmon as a function of trap parameters. With the increase of the trap size, this time decreases monotonically, saturating at the level determined by the quasiparticles diffusion constant and the qubit geometry. We determine the characteristic trap size needed for the relaxation time to approach that saturation value.
Science - Suppressing relaxation in superconducting qubits by quasiparticle pumping
Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. We investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. A 70% reduction in the quasiparticle density results in a threefold enhancement in qubit relaxation times and a comparable reduction in coherence variability.
SOURCES- Alphagalileo, Physical Review B