Non-Hermitian Photonics: How Exceptional Points Control Quantum Electrodynamics on a Chip ✦
Reversible control of quantum vacuum fluctuations has been experimentally demonstrated in an integrated microresonator with chiral exceptional points (EPs). A hybrid quantum-photonic platform was developed, based on lithium niobate and gallium arsenide, combining high-quality emitters, low-loss waveguides, an efficient electro-optic effect, and a local piezo actuator. Dynamic tuning of the coupling between ring resonator modes allowed modulation of the spontaneous emission lifetime by a factor of 7 (from 120 to 850 ps). By engineering the local density of states, non-Lorentzian spectral profiles of single photons were generated: a Lorentzian squared, a Fano-like shape, and EP-induced transparency (emission suppression at zero detuning). The results reveal unique effects of cavity quantum electrodynamics near EPs and establish a universal paradigm for non-Hermitian quantum photonics.
Context
Controlling the quantum state of light at the single-photon level is a key challenge in quantum optics, critical for quantum information technologies. In ordinary Hermitian systems, the interaction of an emitter with a resonator is described by the Purcell effect, which enhances spontaneous emission but is limited by a Lorentzian spectral response. Non-Hermitian physics, arising in systems with controlled loss and gain, offers new degrees of freedom through exceptional points—singularities where eigenvalues and eigenvectors coalesce. At these points, the superposition of modes becomes degenerate, and the quantum field inside the resonator acquires non-classical properties, as predicted in the works of Serge Haroche on cavity quantum electrodynamics. This opens the door to unprecedented control over single photons, unattainable in Hermitian electrodynamics.
Methods
To realize non-Hermitian quantum electrodynamics on a chip, the scientists developed a hybrid platform combining lithium niobate (LN) and gallium arsenide (GaAs) with integrated quantum dots. At its heart is a 15-μm-radius LN ring microresonator coupled to a waveguide, one end of which is closed by a Sagnac mirror. An electro-optic phase shifter on LN (using the Pockels effect) dynamically controls the feedback phase, tuning the coupling between counterpropagating modes. GaAs-based quantum dots were transferred onto the resonator via microprinting with sub-100-nm precision. For spectral tuning, the inverse piezoelectric effect in LN was used to shift the emission energy of the quantum dots by 0.6 nm without affecting the resonator. Detailed analysis of optical modes and emission was performed using high-resolution spectroscopy. This architecture, inspired by the pioneering work of Roy Glauber on quantum coherence, enables reconfigurable light–matter interaction at the quantum level.
Results
The experiment demonstrated three distinct regimes of the local density of states (LDOS) depending on the phase φ. At φ = 0, exceptional-point-induced transparency (EPIT) emerges: the spectral density vanishes at zero detuning, leading to emission suppression. At φ = π/2, a Fano-like profile is observed, and at φ = π, a squared-Lorentzian appears. Single photons emitted by the quantum dot inherit this shape: the measured spectra agree well with calculations. Lifetime measurements showed a reversible modulation from 850 picoseconds (in the EPIT regime) to 120 picoseconds (in the squared-Lorentzian regime)—a sevenfold change. The purity of single-photon emission is confirmed by the second-order correlation function g²(0) = 0.020(5), indicating nearly ideal antibunching—essential for quantum computing and quantum measurements. Electric field simulations revealed that standing waves form in the resonator, with nodes at the emitter's location explaining suppression, or with maxima giving enhancement.
Implications
Realizing exceptional points in quantum electrodynamics upends the usual rules of wave–particle duality: now we can not only be near a node or antinode of the field, but purposefully engineer the topology of the standing wave. This provides a qualitatively new tool for controlling spontaneous emission, without requiring precise mode tuning. For quantum technologies, this means the emergence of ultrafast, voltage-controlled single-photon switches and routers, accelerating the development of all-optical quantum networks.
Future development
This technology can be scaled to multi-resonator circuits with integrated quantum-dot arrays, paving the way for non-Hermitian quantum simulators and topological photonic devices. Using traveling-wave electrodes will enable switching speeds in the gigahertz range, essential for quantum gate operation. Moreover, employing other materials, such as diamond with NV centers or semiconductor heterostructures, will expand the platform's functionality.
Impact
This work will impact the field of quantum information, including quantum cryptography and distributed quantum computing, by offering compact integrated single-photon sources with electrical tunability.
Next steps
Next steps include increasing resonator quality factors to enhance effect contrast and developing multi-emitter systems to study collective effects in non-Hermitian environments.
Key open problems
The experiment is directly connected to the fundamental problem of quantum systems interacting with a controlled environment—key both for understanding decoherence and for building quantum processors. Non-Hermitian quantum electrodynamics also touches on issues of spontaneous symmetry breaking in open systems.
🎯 The Purcell effect, describing the enhancement of spontaneous emission in a resonator, was predicted by Edward Purcell in 1946. However, only with the advent of non-Hermitian physics has it become possible not just to enhance, but to completely suppress emission through topology rather than detuning.
Key numbers
- lifetime range: 120 to 850 ps
- Q factor: 14 000
- ring resonator radius: 15 µm
- wavelength tuning range: 0.6 nm
- g²(0): 0.020(5)