Enhanced coherence of all-nitride superconducting qubits epitaxially grown on silicon substrate

Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminum-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of ~5.2 meV for NbN, compared to ~0.3 meV for aluminum, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time $${T}_{1}=16.3\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 = 16.3 μ s and a spin-echo dephasing time $${T}_{2}=21.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 2 = 21.5 μ s . These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to the previously reported $${T}_{1}\approx {T}_{2}\approx 0.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 ≈ T 2 ≈ 0.5 μ s of NbN-based qubits on MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.

Saving superconducting quantum processors from decay and correlated errors generated by gamma and cosmic rays

Error-corrected quantum computers can only work if errors are small and uncorrelated. Here, I show how cosmic rays or stray background radiation affects superconducting qubits by modeling the phonon to electron/quasiparticle down-conversion physics. For present designs, the model predicts about 57% of the radiation energy breaks Cooper pairs into quasiparticles, which then vigorously suppress the qubit energy relaxation time (T1 ~ 600 ns) over a large area (cm) and for a long time (ms). Such large and correlated decay kills error correction. Using this quantitative model, I show how this energy can be channeled away from the qubit so that this error mechanism can be reduced by many orders of magnitude. I also comment on how this affects other solid-state qubits.

Atomistic Band-Structure Computation for Investigating Coulomb Dephasing and Impurity Scattering Rates of Electrons in Graphene

In this paper, by introducing a generalized quantum-kinetic model which is coupled self-consistently with Maxwell and Boltzmann transport equations, we elucidate the significance of using input from first-principles band-structure computations for an accurate description of ultra-fast dephasing and scattering dynamics of electrons in graphene. In particular, we start with the tight-binding model (TBM) for calculating band structures of solid covalent crystals based on localized Wannier orbital functions, where the employed hopping integrals in TBM have been parameterized for various covalent bonds. After that, the general TBM formalism has been applied to graphene to obtain both band structures and wave functions of electrons beyond the regime of effective low-energy theory. As a specific example, these calculated eigenvalues and eigen vectors have been further utilized to compute the Bloch-function form factors and intrinsic Coulomb diagonal-dephasing rates for induced optical coherence of electron-hole pairs in spectral and polarization functions, as well as the energy-relaxation time from extrinsic impurity scattering of electrons for non-equilibrium occupation in band transport.

Enhanced-coherence all-nitride superconducting qubit epitaxially grown on Si substrate

We have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on Si substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation (resulting in fewer two-level fluctuators), feasibility for epitaxial tunnel barriers (further reducing energy relaxation and dephasing), and a larger superconducting gap of ~ 5.2 meV for NbN compared to ~ 0.3 meV for Al (suppressing the excitation of quasiparticles). Replacing conventional MgO by a Si substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time \({T}_{1}=16.3 {\mu }\text{s}\) and a spin-echo dephasing time \({T}_{2}=21.5 {\mu }\text{s}\). These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to previously reported NbN-based qubits with MgO substrates (\({T}_{1}\approx {T}_{2}\approx 0.5 {\mu }\text{s}\)). These results are an important step towards constructing a new platform for superconducting quantum hardware.

Influence of Hot Carrier and Thermal Components on Photovoltage Formation across the p–n Junction

In the present work we reveal the existence of the hot carrier photovoltage induced across a p–n junction in addition to the classical carrier generation-induced and thermalization-caused photovoltages. On the basis of the solution of the differential equation of the first-order linear time-invariant system, we propose a model enabling to disclose the pure value of each photovoltage component. The hot carrier photovoltage is fast since it is determined by the free carrier energy relaxation time (which is of the order of 10−12 s), while the thermal one, being conditioned by the junction temperature change, is relatively slow; and both of them have a sign opposite to that of the electron-hole pair generation-induced component. Simultaneous coexistence of the components is evidenced experimentally in GaAs p–n junction exposed to pulsed 1.06 μm laser light. The work is remarkable in two ways: first, it shows that creation of conditions unfavorable for the rise of hot carrier photovoltage might improve the efficiency of a single junction solar cell, and second, it should inspire the photovoltaic society to revise the Shockley–Queisser limit by taking into account the damaging impact of the hot carrier photovoltage.

Hot electron energy relaxation time in vanadium nitride superconducting film structures under THz and IR radiation

The paper presents the experimental results of studying the dynamics of electron energy relaxation in structures made of thin (d ≈ 6 nm) disordered superconducting vanadium nitride (VN) films converted to a resistive state by high-frequency radiation and transport current. Under conditions of quasi-equilibrium superconductivity and temperature range close to critical (~ Tc), a direct measurement of the energy relaxation time of electrons by the beats method arising from two monochromatic sources with close frequencies radiation in sub-THz region (ω ≈ 0.140 THz) and sources in the IR region (ω ≈ 193 THz) was conducted. The measured time of energy relaxation of electrons in the studied VN structures upon heating of THz and IR radiation completely coincided and amounted to (2.6–2.7) ns. The studied response of VN structures to IR (ω ≈ 193 THz) picosecond laser pulses also allowed us to estimate the energy relaxation time in VN structures, which was ~ 2.8 ns and is in good agreement with the result obtained by the mixing method. Also, we present the experimentally measured volt-watt responsivity (S~) within the frequency range ω ≈ (0.3–6) THz VN HEB detector. The estimated values of noise equivalent power (NEP) for VN HEB and its minimum energy level (δE) reached NEP@1MHz ≈ 6.3 × 10–14 W/√Hz and δE ≈ 8.1 × 10–18 J, respectively.

Photoluminescent Spectral Broadening of Lead Halide Perovskite Nanocrystals Investigated by Emission Wavelength Dependent Lifetime

Despite intensive efforts, the fluorescence of perovskite nanocrystals (NCs) still suffers from a poor color purity, which limits the applications in light emitting and multicolor display. A deep understanding on the fundamental of the photoluminescent (PL) spectral broadening is thus of great significance. Herein, the PL decay curves of the CsPbClxBr3-x NCs are monitored at different wavelengths covering the entire PL band. Moreover, energy relaxation time τ and radiative recombination time β are obtained by numerical fittings. The dependences of τ and 1/β on the detection wavelength agree well with the steady-state PL spectrum, indicating the observed PL broadening is an intrinsic effect due to the resonance and off-resonance exciton radiative recombination processes. This work not only provides a new analysis method for time-resolved PL spectra of perovskites, but also gains a deep insight into the spectral broadening of the lead halide perovskite NCs.