Large scale quantum computing motivates the invention of two-qubit gate schemes that not only maximize the gate fidelity but also draw minimal resources. In the case of superconductingqubits, the weak anharmonicity of transmons imposes profound constraints on the gate design, leading to increased complexity of devices and control protocols. Here we demonstrate a resource-efficient control over the interaction of strongly-anharmonic fluxonium qubits. Namely, applying an off-resonant drive to non-computational transitions in a pair of capacitively-coupled fluxoniums induces a ZZ-interaction due to unequal ac-Stark shifts of the computational levels. With a continuous choice of frequency and amplitude, the drive can either cancel the static ZZ-term or increase it by an order of magnitude to enable a controlled-phase (CP) gate with an arbitrary programmed phase shift. The cross-entropy benchmarking of these non-Clifford operations yields a sub 1% error, limited solely by incoherent processes. Our result demonstrates the advantages of strongly-anharmonic circuits over transmons in designing the next generation of quantum processors.
We demonstrate a controlled-Z gate between capacitively coupled fluxonium qubits with transition frequencies 72.3 MHz and 136.3 MHz. The gate is activated by a 61.6 ns long pulse atthe frequency between non-computational transitions |10⟩−|20⟩ and |11⟩−|21⟩, during which the qubits complete only 4 and 8 Larmor periods, respectively. The measured gate error of (8±1)×10−3 is limited by decoherence in the non-computational subspace, which will likely improve in the next generation devices. Although our qubits are about fifty times slower than transmons, the two-qubit gate is faster than microwave-activated gates on transmons, and the gate error is on par with the lowest reported. Architectural advantages of low-frequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual ZZ-coupling rate (here 46 kHz), and absence of either excessive parameter matching or complex pulse shaping requirements.
Interfacing stationary qubits with propagating photons is a fundamental problem in quantum technology. Cavity quantum electrodynamics (CQED) invokes a mediator degree of freedom inthe form of a far-detuned cavity mode, the adaptation of which to superconducting circuits (cQED) proved remarkably fruitful. The cavity both blocks the qubit emission and it enables a dispersive readout of the qubit state. Yet, a more direct (cavityless) interface is possible with atomic clocks, in which an orbital cycling transition can scatter photons depending on the state of a hyperfine or quadrupole qubit transition. Originally termed „electron shelving“, such a conditional fluorescence phenomenon is the cornerstone of many quantum information platforms, including trapped ions, solid state defects, and semiconductor quantum dots. Here we apply the shelving idea to circuit atoms and demonstrate a conditional fluorescence readout of fluxonium qubit placed inside a matched one-dimensional waveguide. Cycling the non-computational transition between ground and third excited states produces a microwave photon every 91 ns conditioned on the qubit ground state, while the qubit coherence time exceeds 50 us. The readout has a built-in quantum non-demolition property, allowing over 100 fluorescence cycles in agreement with a four-level optical pumping model. Our result introduces a resource-efficient alternative to cQED. It also adds a state-of-the-art quantum memory to the growing toolbox of waveguide QED.
The non-dissipative non-linearity of a Josephson junction converts macroscopic superconducting circuits into artificial atoms, enabling some of the best controlled quantum bits (qubits)today. Three fundamental types of superconducting qubits are known, each reflecting a distinct behavior of quantum fluctuations in a Cooper pair condensate: single charge tunneling (charge qubit), single flux tunneling (flux qubit), and phase oscillations (phase qubit). Yet, the dual nature of charge and flux suggests that circuit atoms must come in pairs. Here we introduce the missing one, named „blochnium“. It exploits a coherent insulating response of a single Josephson junction that emerges from the extension of phase fluctuations beyond the 2π-interval. Evidence for such effect was found in an out-of-equilibrium dc-transport through junctions connected to high-impedance leads, although a full consensus is absent to date. We shunt a weak junction with an exceptionally high-value inductance — the key technological innovation behind our experiment — and measure the rf-excitation spectrum as a function of external magnetic flux through the resulting loop. The junction’s insulating character manifests by the vanishing flux-sensitivity of the qubit transition between the ground and the first excited states, which nevertheless rapidly recovers for transitions to higher energy states. The spectrum agrees with a duality mapping of blochnium onto transmon, which replaces the external flux by the offset charge and introduces a new collective quasicharge variable in place of the superconducting phase. Our result unlocks the door to an unexplored regime of macroscopic quantum dynamics in ultrahigh-impedance circuits, which may have applications to quantum computing and quantum metrology of direct current.
Josephson effect is usually taken for granted because quantum fluctuations of the superconducting phase-difference are stabilized by the low-impedance embedding circuit. To realizethe opposite regime, we shunt a weak Josephson junction with a nearly ideal kinetic inductance, whose microwave impedance largely exceeds the resistance quantum, reaching above 160 kOhm. Such an extraordinary value is achieved with an optimally designed Josephson junction chain released off the substrate to minimize the stray capacitance. The low-energy spectrum of the resulting free-standing superconducting loop spectacularly loses magnetic flux sensitivity, explained by replacing the junction with a 2e-periodic in charge capacitance. This long-predicted quantum non-linearity dramatically expands the superconducting electronics toolbox with applications to metrology and quantum information
We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T2 > 100 microseconds (T2 > 300 microseconds in onedevice). Moreover, given the state of the art values of the surface loss tangent and the 1/f flux noise amplitude, coherence can be further improved beyond 1 millisecond. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit — over 100 here — must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.
Quantum control of atomic systems is largely enabled by the rich structure of selection rules in the spectra of most real atoms. Their macroscopic superconducting counterparts havebeen lacking this feature, being limited to a single transition type with a large dipole. Here we report a superconducting artificial atom with tunable transition dipoles, designed such that its forbidden (qubit) transition can dispersively interact with microwave photons due to the virtual excitations of allowed transitions. Owing to this effect, we have demonstrated an in-situ tuning of qubit’s energy decay lifetime by over two orders of magnitude, exceeding a value of 2 ms, while keeping the transition frequency fixed around 3,5 GHz