Detection of Quantum Noise from an E|ectrica|[y Driven Two-Leve| System

he electrical noise of mesoscopic devices can be strongly influenced by the quantum motion of electrons. To probe this effect, we have measured the current fluctuations at high frequency (5 to 90 gigahertz) using a superconductorinsulator-superconductor tunnel junction as an on-chip spectrum analyzer. By coupling this frequency-resolved noise detector to a quantum device, we can measure the high-frequency, nonsymmetrized noise as demonstrated for a Josephson junction. The same scheme is used to detect the current fluctuations arising from coherent charge oscil[ations in a two-level system, a superconducting charge qubit. A narrow band peak is observed in the spectral noise density at the frequency of the coherent charge oscillations. Electrical noise, or fluctuations in the current, has proved to be a powerful tool to probe mesoscopic devices (1). At high frequency, it can bear strong signatures of the dynanfics resulting from quantum mechanics. One of the simplest systems to study this effect is a two-level system (TLS) with two coupled quantmn states, [0> and I 1>, that can form a coherent superposition, a 10> + [3 II>, with c~ and [3 complex ntunbers The central idea is illustrated in The idea above is very general and theoretical predictions on narrow band noise exist for Bloch oscillations in a double quantum well (4), charge oscillations in superconducting (5) and semiconducting qubits (6), and electron spin resonance oscillations (7). The experimental detection is difficult as the frequency, f, of the coherent oscillations is typically in the GHz range in order to fulfill the condition hf >> kBT , where k~3T is the thermal energy (8). We report a detection scheme from which we obtain the frequency-resolved spectral density of current noise in the range of 5 to 90 GHz (9). Our detection scheme follows the ideas of (10, 11): a quantum device is coupled on-chip to a detector that converts the high-frequency noise signal into a direct current (DC). The on-chip coupling provides a large frequency bandwidth (-100 GHz), whereas the conversion to DC allows standard amplification of the signal (12). Our detector is a superconductor- To validate our noise detection, we have first measured on a Josephson junction (Jj) for which the high-frequency fluctuations are well known ( For a quantitative description, we consider an SIS junction subject to current fluctuations. The PAT current for a bias eVs~s < 2A is given b