Decoherence-Free Qubit and Chiral Emission from a Giant Molecule in Waveguide QED

Abstract

Combining decoherence protection with directional photon emission in a single waveguide quantum electrodynamics (QED) device remains an open challenge. Here we show that an artificial giant molecule -- strongly interacting artificial atoms coupled to a photonic waveguide at multiple spatially separated points -- achieves both: a fully operational decoherence-free (DF) qubit and state-dependent chiral single-photon emission, arising from the same photon-interference mechanism. Initialization reduces to a local excitation of a single atom, universal single-qubit gates are implemented by modulating a single atomic frequency, and readout exploits state-dependent chiral emission with directionality reaching 100% and low measurement error of 1.2%. The coexistence of decoherence protection and directional emission in a single device positions giant molecules as protected chiral nodes for modular quantum networks in waveguide QED.

Figures (3)

• Figure 1: (a) Giant triatomic molecule coupled to a photonic waveguide. At phase $\phi_{\mathrm{DF}}=\pi$, the molecule supports two decoherence-free logical qubit states $\ket{0_L}$ and $\ket{1_L}$ and a super-radiant state. The tunable detuning $\Delta$ of atom 3 drives transitions and universal gates within the DF subspace. (b) Superconducting-circuit implementation. Tunable couplers mediate intramolecular interactions, microwave XY pulses initialize atom 3, and dedicated flux lines modulate atomic frequencies for single-qubit gates and chiral readout. (c) A local phase-shift gate $\hat{S}(\theta)$ on atom 2, combined with tuning $\phi$ and $\Delta$, enables state-dependent directional emission and projective measurements.
• Figure 2: (a) Maximum chirality $\eta$ over all single-excitation molecular states $\ket{c(0)}$, as a function of $\Delta$ and $\phi$, for $J=0.1\gamma_0$. (b) Quantum circuit used to prepare the maximally chiral state $\ket{c_+}$ with $\eta=+1$.
• Figure 3: (a) Measurement protocol based on chiral emission from the $\ket{0_L}$ and $\ket{1_L}$ states. A photon detected on the right (left) port is assigned to a projective measurement of the $\ket{0_L}$ ($\ket{1_L}$) state. Measurement errors are dominated by imperfect chiral emission from the $\ket{1_L}$ state. (b,c) Photon field amplitudes $\alpha_{L}(t)$ and $\alpha_{R}(t)$ for the states (b) $S_2(\theta)\ket{0_L}$ and (c) $S_2(\theta)\ket{1_L}$, at the point of maximum chirality $(J,\Delta_\text{ch},\phi_\text{ch})\simeq(0.1\gamma_0,-0.132\gamma_0,0.088\pi)$. (d) Maximum measurement error for logical qubit states as a function of the deviations $\delta \theta$ and $\delta \Delta$ from the second measurement protocol values $(\Delta,\phi,\theta) \simeq (-0.1293\gamma_{0},0.0266\pi,0.9974\pi)$. At the optimal point ($\delta \theta=\delta \Delta=0$), the maximum error is $1.27\%$. Dash-dotted curves enclose the regions with maximum measurement error 2%, 3% and 4%. All panels use $\phi=\phi_{\mathrm{ch}}$ and $J=0.1\gamma_0$.