[Paper Review] On-Demand Directional Microwave Photon Emission Using Waveguide Quantum Electrodynamics

Bharath Kannan, Aziza Almanakly+17 more|arXiv (Cornell University)|Mar 2, 2022

Quantum Information and Cryptography4 citations

TL;DR

This paper demonstrates on-demand, high-fidelity, directional microwave photon emission using a superconducting artificial molecule coupled to a bidirectional waveguide. By engineering quantum interference between two emitter qubits, the system generates single photons that propagate exclusively in a chosen direction, enabling scalable, reciprocal quantum interconnects for modular quantum networks with reduced crosstalk and loss.

ABSTRACT

Routing quantum information between non-local computational nodes is a foundation for extensible networks of quantum processors. Quantum information transfer between arbitrary nodes is generally mediated either by photons that propagate between them, or by resonantly coupling nearby nodes. The utility is determined by the type of emitter, propagation channel, and receiver. Conventional approaches involving propagating microwave photons have limited fidelity due to photon loss and are often unidirectional, whereas architectures that use direct resonant coupling are bidirectional in principle, but can generally accommodate only a few local nodes. Here we demonstrate high-fidelity, on-demand, directional, microwave photon emission. We do this using an artificial molecule comprising two superconducting qubits strongly coupled to a bidirectional waveguide, effectively creating a chiral microwave waveguide. Quantum interference between the photon emission pathways from the molecule generates single photons that selectively propagate in a chosen direction. This circuit will also be capable of photon absorption, making it suitable for building interconnects within extensible quantum networks.

Motivation & Objective

• Address the challenge of efficient, directional quantum information transfer between non-local quantum nodes in superconducting quantum processors.
• Overcome limitations of conventional unidirectional waveguides that require lossy circulators and suffer from low fidelity due to photon loss.
• Develop a bidirectional waveguide-based architecture that enables on-demand, directional photon emission without non-reciprocal components.
• Enable high-connectivity, scalable quantum networks by combining high-fidelity photon emission with directional control via quantum interference.
• Demonstrate both photon emission and absorption capabilities, making the system suitable for bidirectional interconnects in quantum networks.

Proposed method

• Engineer a superconducting artificial molecule composed of two strongly coupled qubits (emitters) coupled to a common bidirectional waveguide.
• Utilize quantum interference between two distinct emission pathways from the molecule to create chiral photon emission: one path favors leftward, the other rightward propagation.
• Implement a four-level system where the initial superposition state |ψqb±⟩ = (|gg⟩ + |ψ±⟩)/√2 determines the direction of emitted microwave photons.
• Model the system using a master equation in the superoperator formalism, with Hamiltonian H = g_eff(σ⁻₃σ⁺₁ + σ⁺₃σ⁻₁) + g_eff(σ⁻₄σ⁺₂ + σ⁺₄σ⁻₂), describing effective coupling between emitter and data qubits.
• Solve the master equation numerically using the Liouvillian superoperator formalism to simulate time evolution of the density matrix and compute observables.
• Use input-output relations to compute the field amplitude in left- and right-going waveguide modes: ⟨a_L⟩ = √(γ/2)(⟨σ⁻₁⟩ + i⟨σ⁻₂⟩), ⟨a_R⟩ = √(γ/2)(⟨σ⁻₁⟩ - i⟨σ⁻₂⟩), enabling prediction of directional emission.

Experimental results

Research questions

• RQ1 Can on-demand, directional microwave photon emission be achieved in a bidirectional waveguide without non-reciprocal components?
• RQ2 How does quantum interference between two emitter qubit pathways enable chiral photon emission in a superconducting circuit?
• RQ3 What is the fidelity and directionality of photon emission when the initial state of the emitter qubits is engineered as a superposition state?
• RQ4 Can the system also support directional photon absorption, enabling bidirectional interconnect functionality?
• RQ5 To what extent does the effective coupling strength g_eff/2π ≈ 1.28 MHz determine the temporal shape and directionality of the emitted photon wavepacket?

Key findings

• The system achieves on-demand, directional microwave photon emission with high fidelity by exploiting quantum interference in a superconducting artificial molecule.
• When initialized in the state |ψqb⁻⟩, the emitted photon field amplitude is predominantly left-going (⟨a_L⟩ ≠ 0, ⟨a_R⟩ = 0), while |ψqb⁺⟩ produces a right-going wavepacket.
• The temporal shape of the emitted photon wavepacket is predicted by the analytical solution ⟨a_L⟩ ∝ -g_eff√γ / Γ × e^(-γt/4) sinh(Γt/2), with Γ = 2√((γ/4)² - g_eff²).
• The effective coupling strength between emitter and data qubits is extracted as g_eff/2π ≈ 1.28 MHz, consistent with experimental fits to the photon emission envelope.
• The system exhibits high directionality: for the |ψqb⁻⟩ state, the right-going amplitude ⟨a_R⟩ is zero in the ideal model, confirming unidirectional emission.
• The model predicts that the system can also absorb incoming photons directionally, making it suitable for bidirectional quantum interconnects in scalable networks.