Correlated Emission Lasing in Harmonic Oscillators Coupled via a Single Three-Level Artificial Atom

A single superconducting artificial atom can be used for coupling electromagnetic fields up to the single-photon level due to an easily achieved strong coupling regime. Bringing a pair of harmonic oscillators into resonance with the transitions of a three-level atom converts atomic spontaneous processes into correlated emission dynamics. We present the experimental demonstration of two-mode correlated emission lasing in harmonic oscillators coupled via a fully controllable three-level superconducting quantum system (artificial atom). The correlation of emissions with two different colors reveals itself as equally narrowed linewidths and quenching of their mutual phase diffusion. The mutual linewidth is more than 4 orders of magnitude narrower than the Schawlow-Townes limit. The interference between the different color lasing fields demonstrates that the two-mode fields are strongly correlated.

Figure 1

Device description. (a) Optical micrograph of the device. The meandering structure is a TLR made of a 50-nm-thick Nb film capacitively coupled to external coplanar waveguides. (b) Magnified micrograph of an artificial atom based on the tunable gap superconducting flux qubit geometry. The $\alpha$ loop (with red junctions) allows the flux tunnelling energy to be tuned. (c) Simplified circuit model of the atom capacitively coupled to the TLR through capacitance $C_c$. The atom is effectively coupled to two oscillators with the frequencies ${\omega }_1$ and ${\omega }_2$, schematically shown by the dashed lines. (d) Principles of the device operation. The transitions $|d⟩-|e⟩$ and $|e⟩-|g⟩$ are resonant with the first and second oscillators, respectively. The transition $|g⟩-|d⟩$ is driven by a classical microwave pumping field.

Figure 2

Energy levels of the three-level atom-resonator coupled system. (a), (b) Normalized transmission amplitude through the resonator modes as a function of $\delta \mathrm{\Phi }$. The black dashed lines are theoretical fittings from the full system Hamiltonian. (c) Intensity plot of the phase of a transmission signal as a function of driving frequency and $\delta \mathrm{\Phi }$. The black dashed lines represent theoretical calculations. Our goal is to study the double resonance shown in Fig. 1, which takes place under the conditions marked by the red squares.

Figure 3

Emission spectra of two-mode CEL and quenching of the phase diffusion noise. (a) and (b) Normalized transmission through oscillators 1 and 2 (red closed and open rhombus) and emission spectra from the oscillators (black open triangulars and circles). The emission spectra have the same widths of $\mathrm{\Delta }{\omega }_{e1}=\mathrm{\Delta }{\omega }_{e2}=2\pi \times 0.80\mathrm{MHz}$. (c) Quenching of the mutual phase diffusion. The two frequency emissions are amplified and mixed. The resulting up-converted signal (uncalibrated) is very narrow and limited by the SA bandwidth.

Figure 4

Correlation between the two emission modes. (a) Constructive (red dots) and destructive (black dots) interference peaks of the emissions with two different colors. To observe the interference, the ${\omega }_{e2}$ signal is down-converted to ${\omega }_p-{\omega }_{e2}$ and a variable delay is introduced in the ${\omega }_{e1}$ emission signal before summing it with ${\omega }_p-{\omega }_{e2}$. (b) Interference fringes of the peak amplitudes from (a) as a function of the phase delay between the two signals (red dots). The amplitudes of both signals are adjusted to obtain nearly maximal modulation. The intensity derived from the experimental peaks (red dots) is fitted by a sine function (black curve).