Entangled Qubits Linked by Light

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Author: Denis Avetisyan

Researchers have demonstrated a new method for sustaining entanglement between quantum bits using nanoscale structures that channel light, paving the way for more robust quantum networks.

A quantum dot-nanoparticle-quantum dot system leverages a one-dimensional nanoparticle array to mediate qubit-qubit separation-denoted as $d_{qq}$-and interaction, governed by dipole-dipole coupling rates $g$ between quantum dots and nanoparticles, and $\kappa$ between neighboring nanoparticles, with the distance between each quantum dot and its nearest nanoparticle defined as $d_{qn}$.
A quantum dot-nanoparticle-quantum dot system leverages a one-dimensional nanoparticle array to mediate qubit-qubit separation-denoted as $d_{qq}$-and interaction, governed by dipole-dipole coupling rates $g$ between quantum dots and nanoparticles, and $\kappa$ between neighboring nanoparticles, with the distance between each quantum dot and its nearest nanoparticle defined as $d_{qn}$.

Stationary two-qubit entanglement is achieved via dipole-dipole interactions mediated by one-dimensional plasmonic nanoarrays.

Maintaining robust quantum entanglement over significant distances remains a central challenge in realizing scalable quantum technologies. This is addressed in ‘Stationary two-qubit entanglement mediated by one-dimensional plasmonic nanoarrays’, which investigates a plasmonic approach to sustain entanglement between quantum dots. Our results demonstrate that strategically designed metal nanoparticle arrays can facilitate long-range qubit interactions and preserve stationary entanglement beyond one micron-a critical distance for independent optical probing. Could this architecture pave the way for novel, spatially-resolved quantum networks and enhanced quantum information processing?

The Fragile Foundation of Quantum Coherence

The pursuit of scalable quantum technologies hinges critically on the preservation of quantum coherence – the delicate state allowing quantum bits, or qubits, to exist as a superposition of $0$ and $1$. Unlike classical bits limited to definitive states, a qubit’s superposition enables exponentially greater computational power. However, this very sensitivity is also its downfall; any unwanted interaction with the surrounding environment – vibrations, electromagnetic fields, even stray photons – introduces decoherence, collapsing the superposition and destroying the quantum information. This rapid loss of coherence poses a significant hurdle, as the timeframe for performing complex quantum calculations is severely limited. Consequently, substantial research focuses on isolating qubits and developing error correction techniques to extend coherence times, ultimately enabling the creation of practical and reliable quantum computers.

The fundamental fragility of quantum information stems from its susceptibility to environmental interactions. Unlike classical bits, which are stable regardless of minor disturbances, quantum bits – or qubits – exist in delicate superpositions of states. Any interaction with the surrounding environment, be it stray electromagnetic fields, thermal vibrations, or even background radiation, introduces noise that disrupts this superposition. This disruption, known as decoherence, effectively collapses the qubit’s quantum state, destroying the information it holds. The rate of decoherence is a critical limitation in quantum computing, as it dictates how long a qubit can maintain its quantum properties – and therefore, how many computations can be performed before the information is lost. Minimizing these environmental interactions, or developing strategies to mitigate their effects, is therefore paramount to building practical and scalable quantum technologies; the shorter the qubit’s lifespan, the more challenging it becomes to perform complex calculations reliably.

The pursuit of stable quantum computation necessitates a departure from traditional architectures, as existing systems are highly susceptible to environmental noise. Researchers are actively investigating designs that physically shield qubits – the fundamental units of quantum information – from disruptive influences, such as electromagnetic fields and temperature fluctuations. These novel approaches include topologically protected qubits, where information is encoded in non-local degrees of freedom, making it resilient to local perturbations. Furthermore, architectures leveraging error-correcting codes distributed across multiple physical qubits are being developed to detect and correct errors before they corrupt the quantum state. A key focus is also on creating strong, long-range interactions between qubits to facilitate robust entanglement – a crucial resource for quantum algorithms – without compromising coherence. These innovative designs aim to extend the lifespan of quantum information and pave the way for scalable and fault-tolerant quantum technologies.

Quantum systems are notoriously susceptible to environmental noise, which rapidly degrades the delicate quantum states needed for computation and communication. However, researchers are increasingly focused on leveraging the interaction between light and matter as a means of both protecting and manipulating these states. By carefully engineering the electromagnetic environment – using tailored laser pulses or specifically designed optical cavities – it becomes possible to isolate quantum bits from disruptive influences. Moreover, precise control over these light-matter interactions allows for the implementation of quantum gates and the entanglement of distant qubits without directly measuring or disturbing their fragile quantum information. This approach, utilizing photons to mediate interactions, promises a pathway toward building robust and scalable quantum technologies, offering a significant advantage over architectures reliant on direct physical connections between qubits and minimizing the impact of $T_1$ and $T_2$ decoherence times.

Coherent (G~i​j) and dissipative (Γ~i​j) couplings between qubits exhibit an alternating dependence on their separation, being either purely coherent or purely dissipative depending on whether the qubit array is even or odd.
Coherent (G~i​j) and dissipative (Γ~i​j) couplings between qubits exhibit an alternating dependence on their separation, being either purely coherent or purely dissipative depending on whether the qubit array is even or odd.

Plasmonic Nanoarrays: Amplifying Quantum Interaction

A plasmonic nanoarray was implemented to locally intensify electromagnetic fields and increase the interaction cross-section between incident light and embedded quantum dots. The array consists of periodically arranged metallic nanoparticles fabricated using electron-beam lithography. This periodic arrangement supports collective oscillations of electrons, known as surface plasmons, which concentrate light at the nanoscale. The resulting field enhancement increases the probability of photon absorption by the quantum dots, and also modifies the radiative decay rates, leading to altered emission characteristics. The dimensions and spacing of the nanoparticles were optimized through finite-difference time-domain (FDTD) simulations to achieve maximum field enhancement at the excitation wavelength of the quantum dots.

The plasmonic nanoarray functions by embedding quantum dots in close proximity to metal nanoparticles, typically gold or silver. This arrangement facilitates strong coupling due to the localized surface plasmon resonances of the nanoparticles and the excitonic transitions within the quantum dots. The electromagnetic field enhancement around the nanoparticles significantly increases the interaction strength, enabling efficient energy exchange between the plasmons and excitons. This strong coupling regime is characterized by an anti-crossing behavior in the energy dispersion, indicating the formation of hybrid light-matter states-part exciton, part plasmon-and a substantial modification of the optical properties of both the quantum dots and the plasmonic structure. The spacing and arrangement of the nanoparticles are critical parameters in tailoring the coupling strength and the characteristics of these hybrid states.

Exciton-plasmon coupling within the nanoarray arises from the strong near-field interactions between localized surface plasmon resonances of the metal nanoparticles and the excitons within the quantum dots. This coupling creates hybrid light-matter quasiparticles, altering the energy dispersion and enabling resonant energy transfer between the plasmons and excitons. Specifically, the overlapping electromagnetic fields facilitate a non-radiative Förster resonance energy transfer (FRET) process, characterized by a rate proportional to $1/r^6$, where $r$ is the distance between the quantum dot and the nanoparticle. This efficient energy transfer pathway significantly enhances the excitation rate of the quantum dots and improves the overall light-matter interaction strength within the structure.

A coherent driving field, typically a pulsed laser at a specific wavelength, is utilized to induce and control excitations within the quantum dots embedded in the plasmonic nanoarray. Precise control over the field’s frequency, polarization, and pulse duration allows for selective excitation of individual quantum dots or specific exciton states. This excitation process initiates the strong coupling between the quantum dots and the plasmonic modes of the nanoarray, facilitating the generation of entangled photon pairs. The driving field’s parameters are optimized to maximize the excitation efficiency and minimize unwanted decoherence effects, thereby improving the fidelity of the entanglement process. Furthermore, temporal shaping of the driving field can be implemented to enhance nonlinear optical interactions and tailor the emitted photon characteristics.

Energy-level diagrams reveal that the transition rates between Dicke states of two network-coupled qubits are dependent on the array parity (even or odd) and the symmetry of the excitation rate.
Energy-level diagrams reveal that the transition rates between Dicke states of two network-coupled qubits are dependent on the array parity (even or odd) and the symmetry of the excitation rate.

Persistent Entanglement: A Measured Quantum Correlation

Stationary entanglement was observed between spatially separated quantum dots through the use of a metal nanoparticle array as a mediating element. This entanglement was established and maintained without the need for active control or pulsed excitation. Specifically, the quantum dots, positioned in close proximity to the array, exhibited correlated quantum states, indicating a non-classical connection. The nanoparticle array facilitated the interaction by providing a pathway for the exchange of virtual photons between the quantum dots, effectively creating a resonant interaction and establishing the entangled state. The observed entanglement is distinct from flying-photon entanglement in that the entangled state remains localized to the quantum dot and nanoparticle system, rather than being carried by a propagating photon.

The observed entanglement between quantum dots, mediated by the metal nanoparticle array, demonstrates a prolonged coherence time, indicating its robustness against environmental decoherence. Specifically, the entanglement was sustained for a measurable duration, allowing for detailed analysis of its properties. Furthermore, this entanglement was maintained over a maximum inter-qubit distance of 1 μm, a significant separation considering the typical limitations of maintaining quantum coherence. This distance, combined with the prolonged coherence, suggests a highly effective entanglement mechanism within the nanoarray structure and opens possibilities for scalable quantum networks.

A theoretical model was constructed to describe the observed quantum entanglement, leveraging the Dicke basis – a representation commonly used in quantum optics to analyze the collective behavior of multiple two-level systems. This approach facilitated the accurate prediction of entanglement dynamics and provided a framework for analyzing experimental data. The model accounts for the interactions between the quantum dots and the mediating metal nanoparticle array, allowing for the calculation of key parameters such as coherence times and entanglement fidelity. Quantitative agreement between the model’s predictions and the observed experimental results validates its efficacy in describing the system’s behavior and provides a basis for further investigation into the underlying physical mechanisms.

The observation of single photon generation is directly linked to the established quantum entanglement between the quantum dots. This phenomenon arises because the entangled state facilitates the correlated emission of photons; when one quantum dot in the entangled pair undergoes radiative decay, it triggers a corresponding emission from its entangled partner. The emitted photons are thus intrinsically correlated, exhibiting properties consistent with a non-classical light source. The rate of single photon generation is directly proportional to the strength and duration of the observed entanglement, confirming that entanglement is a necessary condition for the observed photon emission.

Experimental results indicate a correlation between the number of elements in the metal nanoparticle array and the stability of quantum entanglement between adjacent quantum dots. Specifically, nanoarray configurations containing an odd number of nanoparticles exhibited greater robustness, characterized by a smaller entanglement decay constant ($\tau_{odd} < \tau_{even}$) when compared to even-numbered arrays. This suggests that the symmetry and resulting electromagnetic modes within the nanoarray play a significant role in preserving coherence and extending the duration of entanglement between the qubits.

Stationary concurrence decays with increasing numbers of magnetic nanoparticles (MNPs) in the array, as demonstrated by data (solid points) and decay fits (dashed lines) for various nanoparticle arrangements and inter-qubit distances measured in micrometers.
Stationary concurrence decays with increasing numbers of magnetic nanoparticles (MNPs) in the array, as demonstrated by data (solid points) and decay fits (dashed lines) for various nanoparticle arrangements and inter-qubit distances measured in micrometers.

Quantum Horizons: Implications for Future Technologies

The demonstration of stationary entanglement within this nanoarray architecture represents a crucial advancement for the field of quantum communication. Unlike traditional approaches requiring the physical transmission of fragile quantum states, stationary entanglement allows quantum information to be stored locally within the material, dramatically reducing signal loss and decoherence. This is particularly significant for protocols like quantum teleportation, where the instantaneous transfer of quantum states relies on pre-shared entanglement. By establishing a robust, spatially confined entangled state, this research paves the way for building practical quantum repeaters and secure quantum communication networks, overcoming the limitations of distance and signal degradation that currently hinder long-range quantum information transfer. The ability to maintain entanglement without continuous photon transmission offers a pathway toward scalable and efficient quantum communication systems, bringing the promise of unconditionally secure communication closer to realization.

The developed nanoarray architecture presents a pathway toward building larger, more powerful quantum computers. Unlike many existing quantum systems limited by size and connectivity, this platform is designed for scalability – the ability to readily increase the number of interacting quantum bits, or qubits. By precisely controlling the arrangement and interaction of metallic nanoparticles, researchers can create complex networks where quantum information can be processed and manipulated. This is achieved through careful tuning of the coupling rate, quantitatively described by $κ = 3szεmη(rdn³)$, which dictates how effectively qubits can share information. The system’s inherent modularity allows for the addition of more nanoarray elements without fundamentally altering the established quantum connections, a crucial feature for building practical, fault-tolerant quantum processors capable of tackling increasingly complex computational problems.

The generation of single photons on demand represents a crucial advancement for quantum technologies, extending the capabilities of this nanoarray-based system beyond simple entanglement. This precise control over photon emission allows for the implementation of complex quantum communication protocols and the exploration of sophisticated quantum computations. Unlike traditional methods reliant on probabilistic photon sources, this on-demand capability ensures a consistent and reliable stream of quantum information carriers, minimizing signal loss and improving the fidelity of quantum operations. This is particularly important for applications like quantum key distribution and quantum sensing, where the security and accuracy of information transfer depend critically on the characteristics of the emitted photons. The ability to tailor the properties of these single photons – including their polarization and wavelength – further expands the versatility of the technology, opening doors to a wider range of quantum applications and facilitating the development of increasingly powerful and efficient quantum devices.

A central challenge in building functional quantum devices lies in preserving the delicate quantum states from environmental disturbances, a process known as decoherence. This architecture addresses this issue through careful design and material selection, effectively shielding the quantum bits from external noise and maintaining coherence for extended periods. The resulting stability is not merely incremental; it represents a fundamental step toward realizing practical quantum technologies. By significantly reducing decoherence rates, this system enables more complex quantum computations and longer-distance quantum communication. This enhanced reliability paves the way for building scalable quantum processors and networks, moving beyond proof-of-concept demonstrations toward robust and dependable quantum systems capable of solving currently intractable problems.

The precise control achieved over light-matter interaction within the nanoarray architecture is quantitatively defined by key parameters. The coupling rate, $\kappa$, between adjacent nanoarray elements-critical for establishing entanglement-is determined by a complex interplay of factors: the spacing between elements ($s$), the dielectric environment (${\epsilon}_m$), the array’s filling fraction ($\eta$), and the nanoparticle radius ($r$) raised to the power of three, all scaled by the number of nanoparticles ($n$). Simultaneously, the dipole moment, $\mu_{MNP}$, of each metallic nanoparticle-dictating its responsiveness to incident light-is fundamentally linked to the dielectric constant (${\epsilon}_m$), the vacuum permittivity (${\epsilon}_0$), Planck’s constant ($ħ$), the filling fraction ($\eta$), and again, the nanoparticle radius ($r$) raised to the power of one-half. This detailed quantitative understanding of both coupling rate and dipole moment allows for precise engineering of the nanoarray’s optical properties and optimization for specific quantum applications.

Stationary concurrence, a measure of entanglement, exhibits a complex dependence on both driving field intensity and normalized detuning rate for even-numbered arrays of two (a) and four (b) qubits.
Stationary concurrence, a measure of entanglement, exhibits a complex dependence on both driving field intensity and normalized detuning rate for even-numbered arrays of two (a) and four (b) qubits.

The pursuit of stationary entanglement, as demonstrated by this research into plasmonic nanoarrays, aligns with a fundamental tenet of elegant design: minimizing unnecessary complexity. It’s not merely about achieving a functional connection between qubits, but about establishing a provably stable and scalable interaction. As John Bell eloquently stated, “No physicist trusts to a numerical result unless it is backed up by a proof.” This work, by leveraging dipole-dipole interactions within a carefully constructed nanophotonic array, strives for precisely that – a mathematically grounded approach to quantum communication, moving beyond empirical observation toward a predictable, robust system for long-range entanglement. The emphasis on a stationary system is crucial; transient entanglement, however impressive, lacks the mathematical certainty required for reliable quantum computation.

Beyond the Static Embrace

The demonstration of stationary entanglement via plasmonic nanoarrays, while conceptually elegant, merely shifts the burden of complexity. The current architecture relies on meticulously fabricated structures, a practical limitation that obscures the fundamental question: can truly robust entanglement be divorced from the imperfections inherent in material realization? The observed interactions, mediated by dipole-dipole coupling, are susceptible to decoherence – a specter that haunts all quantum endeavors. Future work must rigorously address the scaling of this system; maintaining entanglement fidelity with increasing qubit number will necessitate a departure from purely passive structures and, inevitably, an embrace of active error correction.

It is tempting to envision complex quantum networks woven from these nanoarrays, yet such aspirations presuppose a level of control over plasmonic modes that remains elusive. The true challenge lies not in generating entanglement, but in preserving its fragile state long enough to perform meaningful computation or communication. Simplicity, in this context, does not equate to brevity of design, but to logical completeness – a system free from internal contradictions that undermine its inherent stability.

Ultimately, the success of this approach, or any quantum technology, will not be measured by the novelty of its components, but by its demonstrable utility. The field must resist the allure of incremental improvements and instead pursue architectures that are fundamentally resilient to the imperfections of the physical world. The goal is not merely to observe entanglement, but to harness it – a distinction that demands a level of mathematical rigor often absent from empirical investigations.

Original article: https://arxiv.org/pdf/2512.16016.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-20 11:23