I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me

01
Okt
2024

# Multipartite entanglement in a Josephson Junction Laser

We analyse the entanglement in a model Josephson photonics system in which a dc voltage-biased Josephson junction couples a collection of cavity modes and populates them with microwave

photons. Using an approximate quadratic Hamiltonian model, we study the Gaussian entanglement that develops between the modes as the Josephson energy of the system is increased. We find that the modes in the system fall into a series of blocks, with bipartite entanglement generated between modes within a given block. Tripartite entanglement between modes within a given block is also widespread, though it is limited to certain ranges of the Josephson energy. The system could provide an alternative route to generating multimode microwave entanglement, an important resource in quantum technologies, without the need for ac excitation.

# Protected Fluxonium Control with Sub-harmonic Parametric Driving

Protecting qubits from environmental noise while maintaining strong coupling for fast high-fidelity control is a central challenge for quantum information processing. Here, we demonstrate

a novel control scheme for superconducting fluxonium qubits that eliminates qubit decay through the control channel by reducing the environmental density of states at the transition frequency. Adding a low-pass filter on the flux line allows for flux-biasing and at the same time coherently controlling the fluxonium qubit by parametrically driving it at integer fractions of its transition frequency. We compare the filtered to the unfiltered configuration and find a five times longer T1, and ten times improved T2-echo time in the protected case. We demonstrate coherent control with up to 11-photon sub-harmonic drives, highlighting the strong non-linearity of the fluxonium potential. We experimentally determine Rabi frequencies and drive-induced frequency shifts in excellent agreement with numerical and analytical calculations. Furthermore, we show the equivalence of a 3-photon sub-harmonic drive to an on-resonance drive by benchmarking sub-harmonic gate fidelities above 99.94 %. These results open up a scalable path for full qubit control via a single protected channel, strongly suppressing qubit decoherence caused by control lines.

26
Sep
2024

# Preserving phase coherence and linearity in cat qubits with exponential bit-flip suppression

Cat qubits, a type of bosonic qubit encoded in a harmonic oscillator, can exhibit an exponential noise bias against bit-flip errors with increasing mean photon number. Here, we focus

on cat qubits stabilized by two-photon dissipation, where pairs of photons are added and removed from a harmonic oscillator by an auxiliary, lossy buffer mode. This process requires a large loss rate and strong nonlinearities of the buffer mode that must not degrade the coherence and linearity of the oscillator. In this work, we show how to overcome this challenge by coloring the loss environment of the buffer mode with a multi-pole filter and optimizing the circuit to take into account additional inductances in the buffer mode. Using these techniques, we achieve near-ideal enhancement of cat-qubit bit-flip times with increasing photon number, reaching over 0.1 seconds with a mean photon number of only 4. Concurrently, our cat qubit remains highly phase coherent, with phase-flip times corresponding to an effective lifetime of T1,eff≃70 μs, comparable with the bare oscillator lifetime. We achieve this performance even in the presence of an ancilla transmon, used for reading out the cat qubit states, by engineering a tunable oscillator-ancilla dispersive coupling. Furthermore, the low nonlinearity of the harmonic oscillator mode allows us to perform pulsed cat-qubit stabilization, an important control primitive, where the stabilization can remain off for a significant fraction (e.g., two thirds) of a 3 μs cycle without degrading bit-flip times. These advances are important for the realization of scalable error-correction with cat qubits, where large noise bias and low phase-flip error rate enable the use of hardware-efficient outer error-correcting codes.

# Towards Error Budgeting for Superconducting Modular Quantum Architecture Designs

This paper addresses frequency crowding constraints in modular quantum architecture design, focusing on the SNAIL-based quantum modules. Two key objectives are explored. First, we present

physics-informed design constraints by describing a physical model for realizable gates within a SNAIL module and building a fidelity model using error budgeting derived from device characteristics. Second, we tackle the allocation problem by analyzing the impact of frequency crowding on gate fidelity as the radix of the module increases. We explore whether the gate fidelity can be preserved with a discrete set of qubit frequencies while adhering to defined separation thresholds. This work offers insights into novel quantum architectures and coupled optimization techniques to mitigate the effects of unstable noise and improve overall gate performance.

24
Sep
2024

# Effect of Etching Methods on Dielectric Losses in Transmons

Superconducting qubits are considered as a promising platform for implementing a fault tolerant quantum computing. However, surface defects of superconductors and the substrate leading

to qubit state decoherence and fluctuations in qubit parameters constitute a significant problem. The amount and type of defects depend both on the chip materials and fabrication procedure. In this work, transmons produced by two different methods of aluminum etching: wet etching in a solution of weak acids and dry etching using a chlorine-based plasma are experimentally studied. The relaxation and coherence times for dry-etched qubits are more than twice as long as those for wet-etched ones. Additionally, the analysis of time fluctuations of qubit frequencies and relaxation times, which is an effective method to identify the dominant dielectric loss mechanisms, indicates a significantly lower impact of two-level systems in the dry-etched qubits compared to the wet-etched ones.

23
Sep
2024

# Quantum Error Correction of Qudits Beyond Break-even

Hilbert space dimension is a key resource for quantum information processing. A large Hilbert space is not only an essential requirement for quantum error correction, but it can also

be advantageous for realizing gates and algorithms more efficiently. There has thus been considerable experimental effort in recent years to develop quantum computing platforms using qudits (d-dimensional quantum systems with d>2) as the fundamental unit of quantum information. Just as with qubits, quantum error correction of these qudits will be necessary in the long run, but to date error correction of logical qudits has not been demonstrated experimentally. Here we report the experimental realization of error-corrected logical qutrits (d=3) and ququarts (d=4) by employing the Gottesman-Kitaev-Preskill (GKP) bosonic code in a circuit QED architecture. Using a reinforcement learning agent, we optimize the GKP qutrit (ququart) as a ternary (quaternary) quantum memory and achieve beyond break-even error correction with a gain of 1.82 +/- 0.03 (1.87 +/- 0.03). This work represents a new way of leveraging the large Hilbert space of a harmonic oscillator for hardware-efficient quantum error correction.

20
Sep
2024

# Thermal spectrometer for superconducting circuits

Superconducting circuits provide a versatile and controllable platform for studies of fundamental quantum phenomena as well as for quantum technology applications. A conventional technique

to read out the state of a quantum circuit or to characterize its properties is based on rf measurement schemes involving costly and complex instrumentation. Here we demonstrate a simple dc measurement of a thermal spectrometer to investigate properties of a superconducting circuit, in this proof-of-concept experiment a coplanar waveguide resonator. A fraction of the microwave photons in the resonator is absorbed by an on-chip bolometer, resulting in a measurable temperature rise. By monitoring the dc signal of the thermometer due to this process, we are able to determine the resonance frequency and the lineshape (quality factor) of the resonator. The demonstrated scheme, which is a simple dc measurement, has a wide band up to 200 GHz, well exceeding that of the typical rf spectrometer. Moreover, the thermal measurement yields a highly frequency independent reference level of the Lorentzian absorption signal, unlike the conventional rf measurement. In the low power regime, the measurement is fully calibration-free. Our technique thus offers an alternative spectrometer for quantum circuits, which is in many ways superior with respect to conventional methods.

16
Sep
2024

# Disentangling the Impact of Quasiparticles and Two-Level Systems on the Statistics of Superconducting Qubit Lifetime

Temporal fluctuations in the superconducting qubit lifetime, T1, bring up additional challenges in building a fault-tolerant quantum computer. While the exact mechanisms remain unclear,

T1 fluctuations are generally attributed to the strong coupling between the qubit and a few near-resonant two-level systems (TLSs) that can exchange energy with an assemble of thermally fluctuating two-level fluctuators (TLFs) at low frequencies. Here, we report T1 measurements on the qubits with different geometrical footprints and surface dielectrics as a function of the temperature. By analyzing the noise spectrum of the qubit depolarization rate, Γ1=1/T1, we can disentangle the impact of TLSs, non-equilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs on the variance in Γ1. We find that Γ1 variances in the qubit with a small footprint are more susceptible to the QP and TLS fluctuations than those in the large-footprint qubits. Furthermore, the QP-induced variances in all qubits are consistent with the theoretical framework of QP diffusion and fluctuation. We suggest these findings can offer valuable insights for future qubit design and engineering optimization.

# Observation of Interface Piezoelectricity in Superconducting Devices on Silicon

The evolution of superconducting quantum processors is driven by the need to reduce errors and scale for fault-tolerant computation. Reducing physical qubit error rates requires further

advances in the microscopic modeling and control of decoherence mechanisms in superconducting qubits. Piezoelectric interactions contribute to decoherence by mediating energy exchange between microwave photons and acoustic phonons. Centrosymmetric materials like silicon and sapphire do not display piezoelectricity and are the preferred substrates for superconducting qubits. However, the broken centrosymmetry at material interfaces may lead to piezoelectric losses in qubits. While this loss mechanism was predicted two decades ago, interface piezoelectricity has not been experimentally observed in superconducting devices. Here, we report the observation of interface piezoelectricity at an aluminum-silicon junction and show that it constitutes an important loss channel for superconducting devices. We fabricate aluminum interdigital surface acoustic wave transducers on silicon and demonstrate piezoelectric transduction from room temperature to millikelvin temperatures. We find an effective electromechanical coupling factor of K2≈2×10−5% comparable to weakly piezoelectric substrates. We model the impact of the measured interface piezoelectric response on superconducting qubits and find that the piezoelectric surface loss channel limits qubit quality factors to Q∼104−108 for designs with different surface participation ratios and electromechanical mode matching. These results identify electromechanical surface losses as a significant dissipation channel for superconducting qubits, and show the need for heterostructure and phononic engineering to minimize errors in next-generation superconducting qubits.

# Revealing the Origin and Nature of the Buried Metal-Substrate Interface Layer in Ta/Sapphire Superconducting Films

Despite constituting a smaller fraction of the qubits electromagnetic mode, surfaces and interfaces can exert significant influence as sources of high-loss tangents, which brings forward

the need to reveal properties of these extended defects and identify routes to their control. Here, we examine the structure and composition of the metal-substrate interfacial layer that exists in Ta/sapphire-based superconducting films. Synchrotron-based X-ray reflectivity measurements of Ta films, commonly used in these qubits, reveal an unexplored interface layer at the metal-substrate interface. Scanning transmission electron microscopy and core-level electron energy loss spectroscopy identified an approximately 0.65 \ \text{nm} \pm 0.05 \ \text{nm} thick intermixing layer at the metal-substrate interface containing Al, O, and Ta atoms. Density functional theory (DFT) modeling reveals that the structure and properties of the Ta/sapphire heterojunctions are determined by the oxygen content on the sapphire surface prior to Ta deposition, as discussed for the limiting cases of Ta films on the O-rich versus Al-rich Al2O3 (0001) surface. By using a multimodal approach, integrating various material characterization techniques and DFT modeling, we have gained deeper insights into the interface layer between the metal and substrate. This intermixing at the metal-substrate interface influences their thermodynamic stability and electronic behavior, which may affect qubit performance.