Abstract Superconducting fluxonium qubits provide a promising alternative to transmons on the path toward large-scale superconductor-based quantum computing due to their better coherence and larger anharmonicity. A major challenge for multi-qubit fluxonium devices is the experimental demonstration of a scalable crosstalk-free multi-qubit architecture with high-fidelity single-qubit and two-qubit gates, single-shot readout, and state initialization. Here, we present a two-qubit fluxonium-based quantum processor with a tunable coupler element. We experimentally demonstrate fSim-type and controlled-Z-gates with 99.55 and 99.23% fidelities, respectively. The residual ZZ interaction is suppressed down to the few kHz levels. Using a galvanically coupled flux control line, we implement high-fidelity single-qubit gates and ground state initialization with a single arbitrary waveform generator channel per qubit. 

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High fidelity two-qubit gates on fluxoniums using a tunable coupler

Identifying, quantifying, and suppressing decoherence mechanisms in qubits are important steps towards the goal of engineering a quantum computer or simulator. Superconducting circuits offer flexibility in qubit design; however, their performance is adversely affected by quasiparticles (broken Cooper pairs). Developing a quasiparticle mitigation strategy compatible with scalable, high-coherence devices is therefore highly desirable. Here we experimentally demonstrate how to control quasiparticle generation by downsizing the qubit, capping it with a metallic cover, and equipping it with suitable quasiparticle traps. Using a flip-chip design, we shape the electromagnetic environment of the qubit above the superconducting gap, inhibiting quasiparticle poisoning. Our findings support the hypothesis that quasiparticle generation is dominated by the breaking of Cooper pairs at the junction, as a result of photon absorption by the antenna-like qubit structure. We achieve record low charge-parity switching rate (<1Hz). Our aluminium devices also display improved stability with respect to discrete charging events. 

/pdf/engineering-superconducting-qubits-to-reduce-quasiparticles-9o7z74b6.pdf

Engineering superconducting qubits to reduce quasiparticles and charge noise

Abstract Quantum error correction (QEC) aims to protect logical qubits from noises by using the redundancy of a large Hilbert space, which allows errors to be detected and corrected in real time 1 . In most QEC codes 2–8 , a logical qubit is encoded in some discrete variables, for example photon numbers, so that the encoded quantum information can be unambiguously extracted after processing. Over the past decade, repetitive QEC has been demonstrated with various discrete-variable-encoded scenarios 9–17 . However, extending the lifetimes of thus-encoded logical qubits beyond the best available physical qubit still remains elusive, which represents a break-even point for judging the practical usefulness of QEC. Here we demonstrate a QEC procedure in a circuit quantum electrodynamics architecture 18 , where the logical qubit is binomially encoded in photon-number states of a microwave cavity 8 , dispersively coupled to an auxiliary superconducting qubit. By applying a pulse featuring a tailored frequency comb to the auxiliary qubit, we can repetitively extract the error syndrome with high fidelity and perform error correction with feedback control accordingly, thereby exceeding the break-even point by about 16% lifetime enhancement. Our work illustrates the potential of hardware-efficient discrete-variable encodings for fault-tolerant quantum computation 19 . 

/pdf/beating-the-break-even-point-with-a-discrete-variable-1hgmw71x.pdf

Beating the break-even point with a discrete-variable-encoded logical qubit

In superconducting quantum circuits, chiral routing quantum information is often realized with the ferrite circulators, which are usually buck, lossy and require strong magnetic fields. To overcome these problems, we propose to realize chiral quantum networks by considering giant atoms interacting with photonic crystal waveguide (PCW). By assuming each coupling point modulated with time, the interaction become momentum-dependent, and giant atoms will chirally emit photons due to interference effects. Our analyze indicates that the chiral factor can approach 1, and both the emission direction and rate can be freely tuned by the modulating signals. By tailoring the chiral decay parameters, we demonstrate that high-fidelity state transfer between two remote giant atoms can be realized. Our proposal can be exploited as tunable toolbox with giant atoms for quantum information processing in future chiral quantum networks.

Chiral Quantum Network with Giant Atoms

In superconducting quantum circuits (SQCs), chiral routing quantum information is often realized with the ferrite circulators, which are usually bulky, lossy and require strong magnetic fields. To overcome those problems, we propose a novel method to realize chiral quantum networks by exploiting giant atom effects in SQC platforms. By assuming each coupling point being modulated with time, the interaction becomes momentum-dependent, and giant atoms will chirally emit photons due to interference effects. The chiral factor can approach 1, and both the emission direction and rate can be freely tuned by the modulating signals. We demonstrate that a high-fidelity state transfer between remote giant atoms can be realized. Our proposal can be integrated on the superconducting chip easily, and has the potential to work as a tunable toolbox for quantum information processing in future chiral quantum networks. 

Chiral quantum network with giant atoms

By using the dry etching process of tantalum (Ta) film, we had obtained transmon qubit with the best lifetime (T1) 503 us, suggesting that the dry etching process can be adopted in the following multi-qubit fabrication with Ta film. We also compared the relaxation and coherence times of transmons made with different materials (Ta, Nb and Al) with the same design and fabrication processes of Josephson junction, we found that samples prepared with Ta film had the best performance, followed by those with Al film and Nb film. We inferred that the reason for this difference was due to the different loss of oxide materials located at the metal-air interface. 

/pdf/towards-practical-quantum-computers-transmon-qubit-with-a-2obyzxca.pdf

Towards practical quantum computers: transmon qubit with a lifetime approaching 0.5 milliseconds

/pdf/towards-practical-quantum-computers-transmon-qubit-with-a-3qmgcjx0.pdf

Guan-Shiung Wang

Papers

Towards practical quantum computers: transmon qubit with a lifetime approaching 0.5 milliseconds

Towards practical quantum computers: transmon qubit with a lifetime approaching 0.5 milliseconds