Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting Circuits

Fault tolerant quantum computing relies on the ability to detect and correct errors, which in quantum error correction codes is typically achieved by projectively measuring multi-qubit parity operators and by conditioning operations on the observed error syndromes. Here, we experimentally demonstrate the use of an ancillary qubit to repeatedly measure the $ZZ$ and $XX$ parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the real-time stabilization of a Bell state with a fidelity of $F\approx 74\%$ in up to 12 cycles of the feedback loop. We also perform the protocol using Pauli frame updating and, in contrast to the case of real-time stabilization, observe a steady decrease in fidelity from cycle to cycle. The ability to stabilize parity over multiple feedback rounds with no reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits.Comment: 12 pages, 10 figures. Update: Fig. 5 correcte

Digital quantum computation with superconducting qubits

A quantum computer allows solving certain problems which are practically impossible to solve using a computer based on classical physics. The goal of experimental quantum computing is to extend the capabilities of a current quantum processors such that some of the many possible applications of quantum computers can be implemented on real physical devices. Superconducting circuits are a promising platforms for implementing quantum processors. To enable scaling to larger processors, I address challenges of high-fidelity and resource efficient readout and control of superconducting quantum bits. To demonstrate the extended functionality, I present experiments with two-, four- and eight-qubit processors. We used the high-fidelity readout of two qubits and fully automated calibration of single qubit gates and one two qubit operation in a 51 hour long Big Bell test. A Bell test challenges the completeness of quantum mechanics using quantum entanglement and has been carried out on multiple quantum systems in the past. Our experiment was a part of the Big Bell test engaging 100 000 online volunteers. The volunteers were needed to generate the unpredictable input to the experiment instead of the pseudo-, thermal- or quantum-random number generators used in the prior works. The results of the Big Bell test exclude the coexistence of locality and human free will in addition to demonstrating the technical capabilities of the developed experimental setup. As we add more qubits to the processor, new challenges emerge regarding crosstalk, leakage of the quantum gates and control of growing number of qubits. We benchmark the gates and the quantum circuit by executing four-qubit quantum algorithms for preparing maximally entangled state, entanglement swapping and entanglement distillation protocols. For a resource efficient benchmark of a four-qubit quantum processor we use a novel correlation sensitive average readout technique in combination with quantum state tomography to fully characterize the four-qubit quantum state and compare the outcome to a master-equation simulation. In the case of large quantum processors, multiplexing the control and readout can provide a significant reduction in the number of control instruments and cryogenic cabling. On the other hand, multiplexing often comes with a compromise on selectivity and fidelity. In the last experiment discussed in iiithis thesis, we demonstrate a frequency multiplexed readout circuit which allows fast and high-fidelity readout of subset of eight qubits selected by the shape of the modulation pulse. Compared to prior work, our circuit circumvents the mentioned compromise by allowing better readout selectivity due to the use of individual filters for each readout resonator. This approach will likely be used in even larger near term quantum processors, where instrumentation and time efficiency becomes more important

Engineering cryogenic setups for 100-qubit scale superconducting circuit systems

A robust cryogenic infrastructure in form of a wired, thermally optimized dilution refrigerator is essential for solid-state based quantum processors. Here, we engineer a cryogenic setup, which minimizes passive and active heat loads, while guaranteeing rapid qubit control and readout. We review design criteria for qubit drive lines, flux lines, and output lines used in typical experiments with superconducting circuits and describe each type of line in detail. The passive heat load of stainless steel and NbTi coaxial cables and the active load due to signal dissipation are measured, validating our robust and extensible concept for thermal anchoring of attenuators, cables, and other microwave components. Our results are important for managing the heat budget of future large-scale quantum computers based on superconducting circuits.ISSN:2196-076

Improving the Performance of Deep Quantum Optimization Algorithms with Continuous Gate Sets

Variational quantum algorithms are believed to be promising for solving computationally hard problems on noisy intermediate-scale quantum (NISQ) systems. Gaining computational power from these algorithms critically relies on the mitigation of errors during their execution, which for coherence-limited operations is achievable by reducing the gate count. Here, we demonstrate an improvement of up to a factor of 3 in algorithmic performance for the quantum approximate optimization algorithm (QAOA) as measured by the success probability, by implementing a continuous hardware-efficient gate set using superconducting quantum circuits. This gate set allows us to perform the phase separation step in QAOA with a single physical gate for each pair of qubits instead of decomposing it into two CZ gates and single-qubit gates. With this reduced number of physical gates, which scales with the number of layers employed in the algorithm, we experimentally investigate the circuit-depth-dependent performance of QAOA applied to exact-cover problem instances mapped onto three and seven qubits, using up to a total of 399 operations and up to nine layers. Our results demonstrate that the use of continuous gate sets may be a key component in extending the impact of near-term quantum computers.ISSN:2691-339

Unimon qubit

Funding Information: S.K., A.G., O.K., V.V., and M.M. acknowledge funding from the European Research Council under Consolidator Grant No. 681311 (QUESS) and Advanced Grant No. 101053801 (ConceptQ), European Commission through H2020 program projects QMiCS (grant agreement 820505, Quantum Flagship), the Academy of Finland through its Centers of Excellence Program (project Nos. 312300, and 336810), and Business Finland through its Quantum Technologies Industrial grant No. 41419/31/2020. S.K. and M.M. acknowledge Research Impact Foundation for grant No. 173 (CONSTI). E.H. thanks Emil Aaltonen Foundation (grant No. 220056 K) and Nokia Foundation (grant No. 20230659) for funding. We acknowledge the provision of facilities and technical support by Aalto University at OtaNano - Micronova Nanofabrication Center and LTL infrastructure which is part of European Microkelvin Platform (EMP, No. 824109 EU Horizon 2020). We thank the whole staff at IQM and QCD Labs for their support. Especially, we acknowledge the help withthe experimental setup from Roope Kokkoniemi, code and software support from Joni Ikonen, Tuukka Hiltunen, Shan Jolin, Miikka Koistinen, Jari Rosti, Vasilii Sevriuk, and Natalia Vorobeva, and useful discussions with Brian Tarasinski. | openaire: EC/H2020/681311/EU//QUESS | openaire: EC/H2020/101053801/EU//ConceptQSuperconducting qubits seem promising for useful quantum computers, but the currently wide-spread qubit designs and techniques do not yet provide high enough performance. Here, we introduce a superconducting-qubit type, the unimon, which combines the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. In agreement with our quantum models, we measure the qubit frequency, ω01/(2π), and increased anharmonicity α/(2π) at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13 ns single-qubit gates on two qubits with (ω01, α) = (4.49 GHz, 434 MHz) × 2π and (3.55 GHz, 744 MHz) × 2π, respectively. The energy relaxation seems to be dominated by dielectric losses. Thus, improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible useful quantum advantage with noisy systems.Peer reviewe