Two-photon Interactions Research Articles

Empowering a qudit-based quantum processor by traversing the dual bosonic ladder

High-dimensional quantum information processing has emerged as a promising avenue to transcend hardware limitations and advance the frontiers of quantum technologies. Harnessing the untapped potential of the so-called qudits necessitates the development of quantum protocols beyond the established qubit methodologies. Here, we present a robust, hardware-efficient, and scalable approach for operating multidimensional solid-state systems using Raman-assisted two-photon interactions. We then utilize them to construct extensible multi-qubit operations, realize highly entangled multidimensional states including atomic squeezed states and Schrödinger cat states, and implement programmable entanglement distribution along a qudit array. Our work illuminates the quantum electrodynamics of strongly driven multi-qudit systems and provides the experimental foundation for the future development of high-dimensional quantum applications such as quantum sensing and fault-tolerant quantum computing.

Violation of Bell inequality by photon scattering on a two-level emitter

Entanglement, the non-local correlations present in multipartite quantum systems, is a key resource for quantum technologies. It is therefore a major priority to develop simple and energy-efficient methods for generating high-fidelity entangled states. In the case of light, entanglement can be realized by interactions with matter but the required nonlinear interaction is often impractically weak. Here we show how a single two-level emitter deterministically coupled to light in a nanophotonic waveguide can be used to realize photonic quantum entanglement by excitation at the single-photon level. Efficient optical coupling enables mediation of two-photon interactions by the emitter, creating a strong nonlinearity that leads to entanglement. We experimentally verify energy–time entanglement by violating a Bell inequality in an interferometric measurement of the two-photon scattering response. The on-chip two-level emitter acts as a passive scatterer, so that no advanced spin control is required. As such, our method may provide a more efficient approach to synthesizing photonic entangled states for quantum simulators or metrology.

Hardware-efficient autonomous error correction with linear couplers in superconducting circuits

Large-scale quantum computers will inevitably need quantum error correction (QEC) to protect information against decoherence. Given that the overhead of such error correction is often formidable, autonomous quantum error correction (AQEC) proposals offer a promising near-term alternative. AQEC schemes work by transforming error states into excitations that can be efficiently removed through engineered dissipation. The recently proposed AQEC scheme by Li , called the Star code, can autonomously correct or suppress all single qubit error channels using two transmons as encoders with a tunable coupler and two lossy resonators as a cooling source. The Star code requires only two-photon interactions and can be realized with linear coupling elements, avoiding experimentally challenging higher-order terms needed in many other AQEC proposals, but needs carefully selected parameters to achieve quadratic improvements in logical states' lifetimes. Here, we theoretically and numerically demonstrate the optimal parameter choices in the Star code. We further discuss adapting the Star code to other planar superconducting circuits, which offers a scalable alternative to single qubits for incorporation in larger quantum computers or error correction codes. Published by the American Physical Society 2024

Robust two-state swap by stimulated Raman adiabatic passage

Efficient initialization and manipulation of quantum states is important for numerous applications and it usually requires the ability to perform high fidelity and robust swapping of the populations of quantum states. Stimulated Raman adiabatic passage (STIRAP) has been known to perform efficient and robust inversion of the ground states populations of a three-level system. However, its performance is sensitive to the initial state of the system. In this contribution we demonstrate that a slight modification of STIRAP, where we introduce a non-zero single-photon detuning, allows for efficient and robust population swapping for any initial state. The results of our work could be useful for efficient and robust state preparation, dynamical decoupling and design of quantum gates in ground state qubits via two-photon interactions.

Competition between Two-Photon Driving, Dissipation, and Interactions in Bosonic Lattice Models: An Exact Solution.

We present an exact solution in arbitrary dimensions for the steady states of a class of quantum driven-dissipative bosonic models, where a set of modes is subject to arbitrary two-photon driving, single-photon loss, and a global Hubbard (or Kerr)-like interaction. Our solutions reveal a wealth of striking phenomena, including the emergence of dissipative phase transitions, nontrivial mode competition physics and symmetry breaking, and the stabilization of many-body SU(1,1) pair-coherent states. Our exact solutions enable the description of spatial correlations, and are fully valid in regimes where traditional mean-field and semiclassical approaches break down.

Measurement of the Second-Order Polarizability of Silver Nanoparticles With Reference-Free Hyper-Rayleigh Scattering for Entangled Photon Pair Interaction

Two-photon interactions of entangled-photon pairs with metallic nanoparticles (NPs) can be enhanced by localized surface-plasmon resonance. Recently, we have described how the properties of this quantum light-matter interaction can be deduced from classical second-harmonic generation measurements performed using a reference-free hyper-Rayleigh scattering method. Herein, we report the results of such classical-light characterization measurements. We obtain a large hyperpolarizability for the NPs, present the dependence of the hyperpolarizability on the NPs' spectral features, and show a dipolar emission pattern for the second-harmonic signal. Our results can be used to optimize entangled-photon pair interactions with metallic NPs to enable first ever detection of this process. Moreover, these results suggest that NPs may be used as source for ultra-broadband entangled-photon pairs through nonphase-matched spontaneous parametric down-conversion.

Two-photon-interaction effects in the bad-cavity limit

Taking advantage of recently developed adiabatic elimination techniques, the authors investigate two-photon light-matter interactions in a system comprised of an oscillator and a collection of qubits operating in the ``bad-cavity'' regime. They derive a general master equation which unveils some unexpected effects of two-photon couplings, like a richer dependence on temperature of the collective quantum phenomena with respect to the one-photon-coupling case.

Nonexclusive particle production by γγ interactions in pp collisions at the LHC

Particle production in two-photon interactions at hadronic collisions is becoming increasingly relevant in the LHC physics programme as a way to improve our understanding of the Standard Model and search for signals of New Physics. A key ingredient for the study of these interactions in $pp$ collisions is the description of the photon content of the proton, which allow us to derive predictions for the cross sections associated to events where occur the proton dissociation (non - exclusive processes) and for those where both incident protons remain intact (exclusive processes). In this paper, a detailed comparison of the different models for the elastic and inelastic photon distributions found in the literature is presented and the current theoretical uncertainty is estimated. The impact on the invariant mass distribution for the dimuon production is analyzed. Moreover, the relative contribution of non - exclusive events is estimated and its dependence on the invariant mass of the pair is presented. We demonstrate that the predictions for production of pairs with large invariant mass is strongly dependent on the model assumed to describe the elastic and inelastic photon distributions and that the ratio between non - exclusive and exclusive cross sections present a mild energy dependence. Finally, our results indicate that a future experimental analysis of the non - exclusive events will be useful to constrain the photon content of proton.

Evidence for X(3872)→J/ψπ^{+}π^{-} Produced in Single-Tag Two-Photon Interactions.

We report the first evidence for X(3872) production in two-photon interactions by tagging either the electron or the positron in the final state, exploring the highly virtual photon region. The search is performed in e^{+}e^{-}→e^{+}e^{-}J/ψπ^{+}π^{-}, using 825 fb^{-1} of data collected by the Belle detector operated at the KEKB e^{+}e^{-} collider. We observe three X(3872) candidates, where the expected background is 0.11±0.10 events, with a significance of 3.2σ. We obtain an estimated value for Γ[over ˜]_{γγ}B(X(3872)→J/ψπ^{+}π^{-}) assuming the Q^{2} dependence predicted by a cc[over ¯] meson model, where -Q^{2} is the invariant mass squared of the virtual photon. No X(3915)→J/ψπ^{+}π^{-} candidates are found.

Photonuclear and Two-Photon Interactions at High-Energy Nuclear Colliders

Ultraperipheral collisions (UPCs) of heavy ions and protons are the energy frontier for electromagnetic interactions. Both photonuclear and two-photon collisions are studied at collision energies that are far higher than those available elsewhere. In this review, we discuss physics topics that can be addressed with UPCs, including nuclear shadowing, nuclear structure, and searches for physics beyond the Standard Model.

Interpreting weak value amplification with a toy realist model

Constructing an ontology for quantum theory is challenging, in part due to measurement back-action. The Aharonov-Albert-Vaidman weak measurement formalism provides a method to predict measurement results (weak values) when back-action is negligible. The weak value appears analogous to a classical conditional mean, yet can be complex and unbounded. We study weak values in the context of a recent quantum optical experiment involving two-photon interactions. The results of the experiment are reinterpreted within a realist ‘stochastic optics’ model of light. We show that the conditional means of the intensities in the model correspond to the experimentally observed weak values and study the breakdown of the model outside the experimentally probed regime in the limit where the weak value predicts ‘anomalous’ results.

Entangled Photon Resonance Energy Transfer in Arbitrary Media.

Inspired by the unique nonclassical character of two-photon interactions induced by entangled photons, we develop a new comprehensive Förster-type formulation for entangled-two-photon resonance energy transfer (E2P-RET) mediated by inhomogeneous, dispersive, and absorptive media with any space-dependent and frequency-dependent dielectric function and with any size of donor/acceptor. In our theoretical framework, two uncoupled particles are jointly excited by the temporally entangled field associated with two virtual photons that are produced by three-level radiative cascade decay in a donor particle. The temporal entanglement leads to frequency anticorrelation in the virtual photon's field, and vanishing of one of the time-ordered excitation pathways. The underlying mechanism leads to more than 3 orders of magnitude enhancement in the E2P-RET rate compared with the uncorrelated photon case. With the power of our new formulation, we propose a way to characterize E2P-RET through an effective rate coefficient KE2P, introduced here. This coefficient shows how energy transfer can be enhanced or suppressed depending on rate parameters in the radiative cascade, and by varying the donor-acceptor frequency differences.

Atomic Bell measurement via two-photon interactions

We introduce a complete Bell measurement on atomic qubits based on two photon interactions with optical cavities and discrimination of coherent states of light. The dynamical system is described by the Dicke model for two three-level atoms interacting in two-photon resonance with a single-mode of the radiation field, which is known to effectively generate a non-linear two-photon interaction between the field and two states of each atom. For initial coherent states with large mean photon number, the field state is well represented by two coherent states at half revival time. For certain product states of the atoms, we prove the coherent generation of GHZ states with two atomic qubits and two orthogonal Schr\"odinger cat states as a third qubit. For arbitrary atomic states, we show that discriminating the two states of the field corresponds to different operations in the Bell basis of the atoms. By repeating this process with a second cavity with a dephased coherent state, we demonstrate the implementation of a complete Bell measurement. Experimental feasibility of our protocols is discussed for cavity-QED, circuit-QED and trapped ions setups.

Constraining the photon coupling of ultra-light dark-matter axion-like particles by polarization variations of parsec-scale jets in active galaxies

Dark matter may consist of axion-like particles with ultra-low masses. Two-photon interactions of these particles affect the polarization of radiation propagating through the dark matter. Coherent oscillations of the Bose condensate of the particles induce periodic changes in the plane of polarisation of emission passing through the condensate. We estimate this effect and analyze MOJAVE VLBA polarization observations of bright downstream features in the parsec-scale jets of active galaxies. Through the non-observation of periodic changes in the polarization angle, we are able to constrain the photon coupling of the ultra-light dark-matter axion-like particles at the level of ≲ 10−12 GeV−1 for masses between ∼ 5× 10−23 eV and ∼ 1.2 × 10−21 eV.

Production of $p\bar {p}$ Pairs in UPC at the LHC

We discuss production of $p\bar{p}$ pairs in two-photon interactions in heavy-ion collisions. We present predictions for the ultraperipheral, ultrarelativistic, heavy-ion collisions (UPC) $^{208}\!Pb \, ^{208}\!Pb \to$ $^{208}\!Pb \, ^{208}\!Pb \,p \bar{p}$. The parameters of vertex form factors are adjusted to the Belle data for the $\gamma \gamma \to p \bar{p}$ reaction. To described the Belle data we include the proton-exchange, the $f_2(1270)$ and $f_2(1950)$ $s$-channel exchanges, as well as the hand-bag mechanism. Then, the total cross section and several differential distributions for experimental cuts corresponding to the LHC experiments are presented. The distribution in $\mathrm{y}_{diff}$, the rapidity distance between the proton and antiproton, is particularly interesting. We find the total cross sections: 100 $\mu$b for the ALICE cuts, 160 $\mu$b for the ATLAS cuts, 500 $\mu$b for the CMS cuts, and 104 $\mu$b taking into account the LHCb cuts. This opens a possibility to study the $\gamma \gamma \to p \bar{p}$ process in UPC at the LHC.

Comment on “ ηc production in photon-induced interactions at the LHC”

In "$\eta_c$ production in photon-induced interactions at the LHC," \cite{Goncalves:2018yxc} Goncalves and Moreira discuss inclusive and exclusive $\eta_c$ production at $pp$ and $pA$ collisions at LHC energies. The exclusive channels are via two-photon and photon-Odderon interactions. This comment points out that there is a large additional source of almost-exclusive $\eta_c$ in ultra-peripheral collisions: from the radiative decay of $J/\psi$ that are produced in photon-nucleon interactions. Although the $J/\psi\rightarrow\gamma\eta_c$ branching ratio is small, the $J/\psi$ production cross-section is large enough that it dominates over the exclusive channels considered in \cite{Goncalves:2018yxc}, and is comparable to the non-exclusive production. In $J/\psi\rightarrow\gamma\eta_c$, the photon is very soft and therefore easy to miss, and the $\eta_c$ will have very similar kinematics to the $J/\psi$.

Ultrastrong-coupling regime of nondipolar light-matter interactions

We present a circuit-QED scheme which allows to reach the ultrastrong coupling regime of a nondipolar interaction between a single qubit and a quantum resonator. We show that the system Hamiltonian is well approximated by a two-photon quantum Rabi model and propose a simple scattering experiment to probe its fundamental properties. In particular, we identify a driving scheme that reveals the change in selection rules characterizing the breakdown of the rotating-wave approximation and the transition from strong to ultrastrong two-photon interactions. Finally, we show that a frequency crowding in a narrow spectral region is observable in the output fluoresce spectrum as the coupling strength approaches the collapse point, paving the way to the direct observation of the onset of the spectral collapse in a solid-state device.

Corrections of two-photon interactions in the fine and hyperfine structure of the P-energy levels of muonic hydrogen

In the framework of the quasipotential method in quantum electrodynamics we calculate corrections to the nuclear structure proportional to $r_N^2$ from two-photon exchange amplitudes in the fine and hyperfine structure of P-states in muonic hydrogen, as well as the photon-photon interaction amplitudes, leading to the exchange of the axial vector meson. In constructing the quasipotential of the muon-nucleus interaction, we use the method of projection operators on states of two particles with a definite spin and total angular momentum. Analytical calculation of the matrix elements is performed and contributions to the fine and hyperfine structure of the $2P_{1/2}$ and $2P_{3/2}$ levels are obtained.

Two-photon production of dilepton pairs in peripheral heavy ion collisions

The STAR collaboration has observed an excess production of $e^+e^-$ pairs in relativistic heavy ion collisions, over the expectations from hadronic production models. The excess pairs have transverse momenta $p_T < 150\ {\rm MeV}/c$ and are most prominent in peripheral gold-gold and uranium-uranium collisions. The pairs exhibit a peak at the $J/\psi$ mass, but include a wide continuum, with pair invariant masses from 400 MeV/c$^2$ up to 2.6 GeV/c$^2$. The ALICE Collaboration observes a similar excess in peripheral lead-lead collisions, but only at the $J/\psi$ mass, without a corresponding continuum. This paper presents a calculation of the cross-section and kinematic for two-photon production of $e^+e^-$ pairs, and find general agreement with the STAR data. The calculation is based on the STARlight simulation code, which is based on the Weizs\"acker-Williams virtual photon approach. The STAR continuum observations are compatible with two-photon production of $e^+e^-$ pairs. The ALICE analysis required individual muon $p_T$ be greater than 1 GeV/c; this eliminated almost all of the pairs from two-photon interactions, while leaving most of the $J/\psi$ decays.

Two-photon quantum Rabi model with superconducting circuits

We propose a superconducting circuit to implement a two-photon quantum Rabi model in a solid-state device, where a qubit and a resonator are coupled by a two-photon interaction. We analyze the input-output relations for this circuit in the strong coupling regime and find that fundamental quantum optical phenomena are qualitatively modified. For instance, two-photon interactions are shown to yield single- or two-photon blockade when a pumping field is either applied to the cavity mode or to the qubit, respectively. In addition, we derive an effective Hamiltonian for perturbative ultrastrong two-photon couplings in the dispersive regime, where two- photon interactions introduce a qubit-state-dependent Kerr term. Finally, we analyze the spectral collapse of the multi-qubit two-photon quantum Rabi model and find a scaling of the critical coupling with the number of qubits. Using realistic parameters with the circuit proposed, three qubits are sufficient to reach the collapse point.