Cavity QED System Research Articles

Parametrized protocol achieving the Heisenberg limit in the optical domain via dispersive atom–light interactions

The strong and collective atom–light interactions in cavity-QED systems perform manifold benefits in quantum-enhanced measurements. Here, we study the time-reversal protocol that has been proposed to sense small displacements of the light field, and report the sensitivity of the scheme that could be speeded up to attain the Heisenberg limit (HL). We show the holonomic unitary parametrization process of the scheme and one only need to choose appropriate initial states to pursue the ultimate sensitivity. The scheme may pave an experimentally feasible way to achieve Heisenberg-limited metrology with nonclassical states.

Atom-light entanglement for precise field sensing in the optical domain

Macroscopic arrays of cold atoms trapped in optical cavities can reach the strong atom-light collective coupling regime thanks to the simultaneous interactions of the cavity mode with the atomic ensemble. In a recent work we reported a protocol that takes advantage of the strong and collective atom-light interactions in cavity QED systems for precise electric field sensing in the optical domain. We showed that it can provide between $10$-$20$~dB of metrological gain over the standard quantum limit in current cavity QED experiments operating with long-lived alkaline-earth atoms. Here, we give a more in depth discussion of the protocol using both exact analytical calculations and numerical simulations, and describe the precise conditions under which the predicted enhancement holds after thoroughly accounting for both photon loss and spontaneous emission, natural decoherence mechanisms in current experiments. The analysis presented here not only serves to benchmark the protocol and its utility in cavity QED arrays but also sets the conditions required for its applicability in other experimental platforms such as arrays of trapped ions.

Resilient Mølmer-Sørensen gate with cavity QED

We propose a simple scheme for implementing resilient two-qubit entangling gates in universal cavity QED systems. The technique employs suitably designed laser pulse with modulated amplitude and phase to decrease residual qubit-cavity entanglement that reduces the fidelity of the desired entangling operation. Numerical simulations suggest that the optimized gates are robust against errors in gate time, frequency detuning, atom-field coupling constant and pulse amplitude. Moreover, the optimized gates provide increased robustness to cavity decay as well as strong suppression of atomic spontaneous emissions. With modest experimental parameters the scheme could enable a high gate fidelity exceeding 99% in realistic scenarios.

Free-standing silicon nitride nanobeams with an efficient fiber-chip interface for cavity QED

We present the design, fabrication and characterization of high quality factor silicon nitride nanobeam PhC cavities at visible wavelengths for coupling to diamond color centers in a cavity QED system. We demonstrate devices with a quality factor of ∼24, 000 (±250) around the zero-phonon line of the germanium-vacancy center in diamond. We also present an efficient fiber-to-waveguide coupling platform for suspended nanophotonics. By gently changing the corresponding effective indices at the fiber-waveguide interface, we achieve a coupling efficiency of ∼96% (±2%) at the cavity resonance.

Uniqueness of the Phase Transition in Many-Dipole Cavity Quantum Electrodynamical Systems.

The possibility of a superradiant phase transition in light-matter systems is the subject of much debate, due to numerous apparently conflicting no-go and counter no-go theorems. Using an arbitrary-gauge approach we show that a unique phase transition does occur in archetypal many-dipole cavity QED systems, and that it manifests unambiguously via a macroscopic gauge-invariant polarization. We find that the gauge choice controls the extent to which this polarization is included as part of the radiative quantum subsystem and thereby determines the degree to which the abnormal phase is classed as superradiant. This resolves the long-standing paradox of no-go and counter no-go theorems for superradiance, which are shown to refer to different definitions of radiation.

Quantized quasinormal-mode description of nonlinear cavity-QED effects from coupled resonators with a Fano-like resonance

We employ a recently developed quantization scheme for quasinormal modes (QNMs) to study a nonperturbative open cavity-QED system consisting of a hybrid metal-dielectric resonator coupled to a quantum emitter. This hybrid cavity system allows one to explore the complex coupling between a low $Q$ (quality factor) resonance and a high $Q$ resonance, manifesting in a striking Fano resonance, an effect that is not captured by traditional quantization schemes using normal modes or a Jaynes-Cummings (JC) type model. The QNM quantization approach rigorously includes dissipative coupling between the QNMs, and is supplemented with generalized input-output relations for the output electric field operator for multiple modes in the system, and correlation functions outside the system. The role of the dissipation-induced mode coupling is explored in the strong coupling regime between the photons and emitter beyond the first rung of the JC dressed-state ladder. Important differences in the quantum master equation and input-output relations between the QNM quantum model and phenomenological dissipative JC models are found. In a second step, numerical results for the Fock distributions and system as well as output correlation functions obtained from the quantized QNM model for the hybrid structure are compared with results from a phenomenological approach. We demonstrate explicitly how the quantized QNM model manifests in multiphoton quantum correlations beyond what is predicted by the usual JC models.

Coupling silicon vacancy centers in a thin diamond membrane to a silica optical microresonator.

We report the development of a composite cavity QED system, in which silicon vacancy centers in a diamond membrane as thin as 100 nm couple to optical whispering gallery modes (WGMs) of a silica microsphere with a diameter of order 50 µm. The membrane induces a linewidth broadening of 3 MHz for equatorial and off-resonant WGMs, while the overall linewidth of the composite system remains below 40 MHz. Photoluminescence experiments in the cavity QED setting demonstrate the efficient coupling of optical emissions from silicon vacancy centers into the WGMs. Additional analysis indicates that the composite system can be used to achieve the good cavity limit in cavity QED, enabling an experimental platform for applications such as state transfer between spins and photons.

Autonomous Maxwell's demon in a cavity QED system

We present an autonomous Maxwell's demon scheme. It is first analysed theoretically in term of information exchange in a closed system and then implemented experimentally with a single Rydberg atom and a high-quality microwave resonator. The atom simulates both a qubit interacting with the cavity, and a demon carrying information on the qubit state. While the cold qubit crosses the hot cavity, the demon prevents energy absorption from the cavity mode, apparently violating the second law of thermodynamics. Taking into account the change of the mutual information between the demon and the qubit-cavity system gives rise to a generalized expression of the second law that we establish and measure. Finally, considering the closed qubit-cavity-demon system, we establish and measure that the generalized second law can be recast into an entropy conservation law, as expected for a unitary evolution.

Extremal photon statistics signal the extremal entanglement

Entanglement and photon statistics are investigated in a cavity QED system with three two-level atoms trapped in a modestly driven cavity with weak dissipation. It is shown that the local maximal steady-state entanglement of any two atoms perfectly corresponds to the strongest photon antibunching point. In particular, the local maximal entanglement of two atoms is also consistent with the strongest photon bunching point. It provides a new angle to understand both photon statistics and entanglement. Our work gives strong support for employing extremal photon statistics to signal the extremal atomic entanglement in this system.

Transfer-Matrix Approach to Determining the Linear Response of All-Fiber Networks of Cavity-QED Systems

A semiclassical model is presented for characterizing the linear response of elementary quantum optical systems involving cavities, optical fibers, and atoms. Formulating the transmission and reflection spectra using a scattering-wave (transfer matrix) approach, the calculations become easily scalable. To demonstrate how useful this method is, we consider the example of a simple quantum network, i.e., two cavity-QED systems connected via an optical fiber. Differences between our quasi-exact transfer matrix approach and a single-mode, linearized quantum-optical model are demonstrated for parameters relevant to recent experiments with coupled nanofiber-cavity-QED systems.

Enhancing atom-field interaction in the reduced multiphoton Tavis-Cummings model

We propose a scheme to enhance the light-matter coupling in a cavity QED system where two $\mathrm{\ensuremath{\Lambda}}$ atoms are weakly coupled to a single-mode cavity via multiphoton interaction. By introducing a second-order nonlinear (parametric) driving of the cavity, the original $N$-photon exchanges between the atoms, and the cavity can be reduced to either an effective single- or two-photon transition depending on the odevity of $N$, with the effective atom-field interaction strength being significantly enhanced. This results in an $N$-dependent effective coupling strength on top of the exponential enhancement induced only by the parametric driving. As an example, we demonstrate that our proposal can be exploited for the preparation of high-fidelity steady-state entanglement. The proposed scheme could provide a feasible alternative for diverse applications in quantum technologies.

Protocol for Precise Field Sensing in the Optical Domain with Cold Atoms in a Cavity.

In the context of quantum metrology, optical cavity-QED platforms have primarily been focused on the generation of entangled atomic spin states useful for next-generation frequency and time standards. Here, we report a complementary application: the use of optical cavities to generate nonclassical states of light for electric field sensing below the standard quantum limit. We show that cooperative atom-light interactions in the strong collective coupling regime can be used to engineer generalized atom-light cat states which enable quantum enhanced sensing of small displacements of the cavity field even in the presence of photon loss. We demonstrate that metrological gains of 10-20dB below the standard quantum limit are within reach for current cavity-QED systems operating with long-lived alkaline-earth atoms.

Generation of macroscopic entangled cat states in a longitudinally coupled cavity-QED model

Macroscopic entangled cat states not only are significant in the demonstration of the fundamentals of quantum physics, but also have wide applications in modern quantum technologies such as continuous-variable quantum information processing and quantum metrology. Here we propose a scheme for generation of macroscopic entangled cat states in a molecular cavity-QED system, which is composed of an organic molecule (including electronic and vibrational states) coupled to a single-mode cavity field. By simultaneously modulating the resonance frequencies of the molecular vibration and the cavity field, the molecular vibrational displacement can be enhanced significantly and hence macroscopic entangled cat states between the molecular vibrational mode and the cavity mode can be created. We also study quantum coherence effects in the generated states by calculating the joint Wigner function and the degree of entanglement. The dissipation effects are included by considering the state generation in the open-system case. Our results will pave the way to the study of quantum physics and quantum chemistry in molecular cavity-QED systems.

Efficient ion-photon qubit SWAP gate in realistic ion cavity-QED systems without strong coupling.

We present a scheme for deterministic ion-photon qubit exchange, namely a SWAP gate, based on realistic cavity-QED systems with 171Yb+, 40Ca+ and 138Ba+ ions. The gate can also serve as a single-photon quantum memory, in which an outgoing photon heralds the successful arrival of the incoming photonic qubit. Although strong coupling, namely having the single-photon Rabi frequency be the fastest rate in the system, is often assumed essential, this gate (similarly to the Duan-Kimble C-phase gate) requires only Purcell enhancement, i.e. high single-atom cooperativity. Accordingly, it does not require small mode volume cavities, which are challenging to incorporate with ions due to the difficulty of trapping them close to dielectric surfaces. Instead, larger cavities, potentially more compatible with the trap apparatus, are sufficient, as long as their numerical aperture is high enough to maintain small mode area at the ion's position. We define the optimal parameters for the gate's operation and simulate the expected fidelities and efficiencies, demonstrating that efficient photon-ion qubit exchange, a valuable building block for scalable quantum computation, is practically attainable with current experimental capabilities.

Engineering indefinitely long-lived localization in cavity-QED arrays

By exploiting the nonlinear nature of the Jaynes Cumming's interaction, one can get photon population trapping in cavity-QED arrays. However, the unavoidable dissipative effects in a realistic system would destroy the self-trapped state by continuous photon leakage. To circumvent this issue, we show that a careful engineering of drive, dissipation and Hamiltonian results in achieving indefinitely sustained self-trapping. We show that the intricate interplay between drive, dissipation, and light-matter interaction results in requiring an optimal window of drive strengths in order to achieve such non-trivial steady states. We treat the two-cavity and four-cavity cases using exact open quantum many-body calculations. Additionally, in the semiclassical limit we scale up the system to a long 1-D chain and demonstrate localization de-localization transition in a driven-dissipative system. Although, our analysis is performed keeping cavity-QED systems in mind, our work is applicable to other driven-dissipative systems where nonlinearity plays a defining role.

Improvement of Three-Photon Blockade Using Cavity Electromagnetically Induced Transparency

We present a scheme to improve the three-photon blockade in a two-atom cavity QED system in a strong coupling regime. By using electromagnetically induced transparency technique, we show that the frequency range to realize the three-photon blockade can be broadened in the presence of the control field in the case of both in-phase and out-of-phase radiations. We also show that the three-order field correlation function g(3)(0) can be decreased considerably in the presence of the control field especially in the case of out-of-phase radiations. The scheme presented in this paper provides a good candidate to realize and improve the three-photon blockade, as well as generating anti-bunched photon pairs.

Origin of the slow growth of entanglement entropy in long-range interacting spin systems

Long-range interactions allow far-distance quantum correlations to build up very fast. Nevertheless, numerical simulations demonstrated a dramatic slowdown of entanglement entropy growth after a sudden quench. In this work, we unveil the general mechanism underlying this counterintuitive phenomenon for $d$-dimensional quantum spin systems with slowly-decaying interactions. We demonstrate that the semiclassical rate of collective spin squeezing governs the dynamics of entanglement, leading to a universal logarithmic growth in the absence of semiclassical chaos. In fact, the standard quasiparticle contribution is shown to get suppressed as the interaction range is sufficiently increased. All our analytical results agree with numerical computations for quantum Ising chains with long-range couplings. Our findings thus identify a qualitative change in the entanglement production induced by long-range interactions, and are experimentally relevant for accessing entanglement in highly-controllable platforms, including trapped ions, atomic condensates and cavity-QED systems.

Quantum trajectory theory of few-photon cavity-QED systems with a time-delayed coherent feedback

We describe an efficient approach to modelling cavity quantum electrodynamics (QED) with a time-delayed coherent feedback using quantum trajectory simulations. An analytical set of equations is derived to exploit the advantages of trajectories in the presence of the non-Markovian dynamics, where adjustments to the standard stochastic dynamics are discussed. In the weak excitation regime, we first verify that our approach recovers known results obtained with other simulation methods and demonstrate how a coherent feedback loop can increase the photon lifetime in typical cavity-QED systems. We then explore the nonlinear few-photon regime of cavity-QED, under the restriction of at most one photon at a time in the feedback loop. In particular, we show how feedback affects the cavity photoluminescence (populations versus laser detuning), and describe how one must account for conditioning in the presence of feedback, specifically the system observables must be conditioned on no photon detections at the feedback output channel occurring.

N-Phonon Bundle Emission via the Stokes Process.

We demonstrate theoretically the bundle emission of n strongly correlated phonons in an acoustic cavity QED system. The mechanism relies on Stokes resonances that generate super-Rabi oscillations between states with a large difference in their number of excitations, which, combined with dissipation, transfer coherently pure n-phonon states outside of the cavity. This process works with close to perfect purity over a wide range of parameters and is tunable optically with well-resolved operation conditions. This broadens the realm of quantum phononics, with potential applications for on-chip quantum information processing, quantum metrology, and engineering of new types of quantum devices, such as optically heralded n-phonon guns.

Time-asymmetric quantum fluctuations in intensity-amplitude correlation for a driven cavity QED system.

The intensity-amplitude correlation functions for a driven cavity QED system with two non-identical atoms are investigated in this paper. With the support of conditional homodyne detection, one can detect the time-dependent intensity-amplitude correlation functions experimentally. We find time-asymmetry in this correlation when the driving field is tuned to be resonant with the two-photon excitation state, which brings non-Gaussian fluctuations. The physical origin of these phenomena is the distinction of the third-order moment based on complete-collapse and partial-collapse, which corresponds to the measuring sequence of the intensity and amplitude. Finally, we also examined the nonclassical features of the system, which always exhibits photon bunching. The squeezing occurs in the region of weak driving and disappears with the increase of driving strength. Hence, a new classical inequality based on the technique of homodyne cross-correlation measurement is introduced to determine the nonclassicality of the non-Gaussian system in the region of unsqueezing.