National Physical Laboratory


Solid State Quantum Information ProcessingSolid State Quantum Information Processing
 

Applying precision frequency metrology techniques and analysis to our solid-state system to measure noise due to TLFs within a variety of systems.

NPL is developing new measurement techniques for investigating noise within the dielectric environment of quantum devices.

This noise is parameterised as a bath of two level fluctuators (TLFs), located at the interface between the superconductor and a dielectric (such as the substrate or an oxidised surface). Such interfaces are unavoidable for many devices but are also prominent in Josephson Junctions - where a dielectric layer separates two superconductors to form a weak link. Josephson junctions are the building blocks of Superconducting QUantum Interference Devices (SQUIDs) and qubits.

Amorphous interfaces are found at the boundary between superconductors and substrates
Amorphous interfaces are found at the boundary between superconductors and substrates, or when an oxide layer coats the superconductor.
In a simple picture dangling bonds exist within the amorphous structure, which manifest as two level fluctuators.

TLFs are a contender for producing the 1/f type noise seen in SQUIDs and other electronics. 1/f noise is a signal which scales as 1/f in its power spectral density (PSD). This noise is problematic for any device operating at low frequencies or DC. TLFs are also problematic at higher frequencies since they can be resonantly excited by microwave frequencies. This leads to resonant coupling to qubits, making them a source for decoherence.

Current distribution within lumped element resonator as excitation frequency varies
Current distribution within lumped element resonator as excitation frequency varies. Red indicates high current density and blue indicates low current density. When at resonance most of current is within the inductor, this shows the micro-resonator to behave as a lumped element tank circuit.

Superconducting resonators couple to the dielectric environment and are ideal probes of TLFs. The Resonator centre frequency and Q are found to depend on the TLF population. TLFs can be excited thermally or by resonant photon absorption. Recent work has studied losses due to TLFs within a variety of superconductor and substrate combinations. The TLFs become saturated (probability of being excited =1/2) at high temperatures and high microwave powers. When saturated, TLF resonant absorption is suppressed, resulting in a lower contribution to noise.

We are implementing precision frequency metrology techniques for high accuracy, real time (high bandwidth) readout of resonators. The Readout also provides information on how the resonant frequency varies; these variations are due to coupled noise sources. Analysis of such variations provides a direct probe of noise sources coupled to the resonator.

Allan deviation data
Allan deviation data is used to determine read out resolution in Hz obtainable within the measurement time tau. Gradients are useful for determining types of noise processes dominant over various time scales.

A greater understanding of noise sources should be obtained by studying coupled noise over a range of temperatures and microwave powers, for a variety of superconductor and dielectric combinations. The motivation of this is to develop improved quantum devices and exploit effects that can be gained by coupling various devices to microwave resonators.

People working on this project

External collaborators

Recent publications

Recent posters and other media

  • Superconducting lumped element resonators as probes of dielectrics
    Presented at Applied Superconductivity Conference 2010

  • Pound-Drever-Hall readout of Superconducting lumped element resonators
    Presented at Mesoscopic Superconductivity & Vortex Imaging 2011

  • High Precision readout of superconducting resonators
    Presented at Condensed Matter and Materials Physics 2011

Useful links

Superconducting Devices