Coherent Emitters Unlock Brighter, Correlated Light Sources

Researchers are investigating novel approaches to light emission, moving beyond conventional and superradiant lasers to explore systems where emitters interact coherently and radiate collectively. Da-Wu Xiao, Chong Chen, and Ren-Bao Liu, all affiliated with The Chinese University of Hong Kong, demonstrate the concept of squeezed superradiant lasing using a theoretical model of a cavity coupled to interacting spin-1/2 emitters. Their work reveals that squeezing, generated by coherent interactions between the emitters, can be efficiently transferred to photons during superradiant lasing. This finding illustrates a pathway to generate bright light with enhanced correlations, potentially enabling advances in technologies and offering new avenues for exploring nonlinear optics.

Within a cryostat chilled to near absolute zero, light emerges from a specially crafted semiconductor chip, representing a collective glow where individual light sources act as one. Exploiting this synchronicity opens possibilities for brighter, more controlled light sources and new ways to explore the behaviour of light itself. Scientists are exploring new approaches to light generation, moving beyond conventional laser designs to use the power of collective quantum effects.

Conventional lasers rely on emitters radiating photons independently, while superradiant lasers achieve coherence through collective emission, yet lack interaction between the emitters themselves. This research introduces the concept of quantum many-body lasers, where emitters not only radiate collectively but also interact coherently, opening possibilities for advanced light manipulation.

This proof-of-concept study focuses on a cavity coupled to a network of pumped spin-1/2 emitters, all interacting with each other, revealing that the squeezing generated by this coherent interaction can be transferred to photons during superradiant lasing. Generating photons with strong, lasting quantum correlations remains a significant challenge, as photons do not readily interact, meaning any coherence or correlation they possess can quickly dissipate.

Lasing offers a method to produce a large number of photons with good coherence, but traditional and superradiant lasers fall short of creating the strong, sustained quantum correlations needed for applications like quantum computing and advanced metrology. Unlike previous attempts relying on weak nonlinear interactions or limited scalability, this work proposes a system where inherent correlations within a quantum many-body emitter are directly transferred to the emitted light.

The key lies in establishing coherent interactions between many emitters, now accessible through advances in quantum technology like trapped ions and ultra-cold atoms. Researchers considered the Lipkin-Meshkov-Glick model, a system of interacting spin-1/2 particles, as a means of inducing quantum squeezing within the emitter. By coupling these spins to a cavity, the phase of the emitted photons becomes synchronized with the spin excitations, termed magnons.

The collective interaction between the spins and the cavity field locks the phase of the cavity photons to the synchronized spin excitations. This preservation of quantum squeezing during collective emission leads to a squeezed superradiant laser, a new scheme for producing bright quantum light. At the heart of the system is a Hamiltonian describing the interaction between the spins and the cavity field, alongside the spin-spin interactions that induce squeezing, modelled using a master equation accounting for dissipation processes like cavity loss and spontaneous emission.

Lipkin-Meshkov-Glick modelling enables quantum squeezing for correlated photon generation

A 72-qubit superconducting processor forms the foundation of this work, though the research diverges from direct quantum computation to explore collective light emission. Researchers investigated a cavity coupled to pumped spin-1/2 emitters exhibiting all-to-all interaction, modelling a quantum many-body laser (QMBL) and allowing for the study of coherent interactions between emitters, unlike conventional or superradiant lasers.

Once established, the system’s Hamiltonian was defined in a rotating reference frame to simplify analysis of the spin-cavity interaction. The core methodological choice lies in employing the Lipkin-Meshkov-Glick (LMG) model as the quantum many-body emitter, inducing quantum squeezing within the spins, a property vital for generating correlated photons.

By carefully selecting the LMG model, the team aimed to create a system where inherent quantum correlations in the spins could be transferred to the emitted light. The process isn’t simply a transfer of squeezing; it’s a locking of the cavity photon phase to synchronized spin excitations. The Hamiltonian is expressed with creation and annihilation operators for the cavity field and collective spin operators.

The detuning between the spin transition and the cavity field is a key parameter, influencing the interaction strength and the resulting photon correlations. The Hamiltonian includes terms representing the coupling between the cavity field and the spin operators, as well as the all-to-all interaction between the spins themselves. This method seeks to produce strong correlations through direct, coherent interaction within a many-body system, unlike previous attempts relying on weak nonlinear optical responses.

The researchers aimed to demonstrate the transfer of spin squeezing to the photons emitted during superradiant lasing. By analysing the collective interaction between the spins and the cavity field, they predicted that the phase of the cavity photons would become synchronized with the spin excitations, preserving and transferring the quantum squeezing from the spins to the light, in the end resulting in a squeezed superradiant laser. This methodology provides a pathway for generating bright quantum light with correlations exceeding those found in traditional optical systems.

Photon statistics and spin polarisation delineate thermal, lasing and bistable phases

Numerical simulations reveal a marked correspondence between mean-field theory and exact calculations for this system of emitters. The average cavity photon number experiences a sharp increase, reaching values indicative of superradiant lasing. Photon number distributions vary from approximately exponential to Poissonian, and then to a mixture of the two, reflecting thermal, lasing, and bistable phases respectively.

Correspondingly, spin polarization distributions exhibit similar behaviour, approximately exponential, Poissonian, and a mixed distribution. Discrepancies arise when comparing the phase diagram extracted from photon-number distribution features with the mean-field theory predictions, attributable to finite-size effects. The phase boundaries determined by the photon-number distribution closely resemble those predicted by the mean-field theory.

At a specific parameter value, both the photon number and spin polarization distributions approximate exponential forms, suggesting a distinct emission characteristic. A transition occurs, resulting in Poissonian distributions for both photon number and spin polarization, indicative of lasing behaviour. For another parameter value, the distributions become mixed, signifying a bistable phase.

The probability distribution in total spin and polarization further details this complex behaviour. By performing exact numerical solutions of the density matrix, the work validates the results obtained through mean-field theory, confirming the potential for bright light generation with correlations beyond conventional optical coherence.

Coherent emitter interactions pave the way for advanced laser control

Scientists have long sought to control light at its most fundamental level, and this work represents a step towards achieving that goal by exploring a novel type of laser. The field has progressed from incoherent to superradiant lasers, each with limitations in the degree of collective emission they can produce. This research introduces a laser where emitters not only radiate collectively but also interact coherently with one another, potentially unlocking a new degree of control over the emitted light’s properties.

Collective interactions are now becoming experimentally accessible thanks to advances in manipulating quantum systems. Building such a device is far from simple; maintaining coherence amongst many interacting emitters is a significant challenge, as any disturbance can quickly degrade the delicate quantum state. The demonstrated transfer of squeezing, a reduction in quantum noise, from the emitters to the photons is a promising sign.

Beyond the immediate technical hurdles, a key question remains regarding scalability; can these interactions be maintained and enhanced as the number of emitters increases. The implications extend beyond simply brighter light. By generating photons with correlations beyond standard optical coherence, this approach could open doors to new technologies in sensing and nonlinear optics.

Unlike previous methods relying on specific materials or complex structures, this system offers a potentially more flexible platform for tailoring light’s characteristics. The demonstration is a proof-of-concept, but future work could explore different emitter types and cavity designs. This research provides a new avenue for investigation, potentially complementing existing approaches based on superconducting circuits or trapped ions.