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Cavity-based architecture to preserve quantum coherence and entanglement
Zhong-Xiao Man, Yun-Jie Xia, Rosario Lo Franco
TL;DR
Environmental coupling threatens quantum coherence and entanglement, motivating protection strategies for quantum information processing. The paper analyzes two coupled lossy cavities as an engineered environment that switches qubit dynamics between Markovian and non-Markovian regimes. The architecture preserves coherence and substantially extends entanglement lifetimes, with steady-state entanglement in the perfect-cavity limit and straightforward extension to many qubits.
Problem
Environmental coupling causes quantum coherence and entanglement to decay, so protecting these resources is essential for quantum information processing.
Method
The paper engineers each qubit’s environment with two coupled cavities and models the resulting dissipative dynamics, including experimentally relevant circuit-QED parameters.
Results
Entanglement lasts orders of magnitude longer than without local cavity couplings and can reach steady-state preservation when the cavities are perfect.
Takeaways & Limitations
The independent-qubit cavity architecture supports individual operations, scalability to many qubits, and control of non-Markovian dynamics for preserving quantum resources.
Takeaways & Limitations
The analysis includes a single-excitation initial-state assumption and notes that ultra-strong coupling requires relaxing the rotating-wave approximation.
Abstract
from arXiv · showhide
Quantum technology relies on the utilization of resources, like quantum coherence and entanglement, which allow quantum information and computation processing. This achievement is however jeopardized by the detrimental effects of the environment surrounding any quantum system, so that finding strategies to protect quantum resources is essential. Non-Markovian and structured environments are useful tools to this aim. Here we show how a simple environmental architecture made of two coupled lossy cavities enables a switch between Markovian and non-Markovian regimes for the dynamics of a qubit embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely preserved if the cavity without qubit is perfect. We then focus on entanglement control of two independent qubits locally subject to such an engineered environment and discuss its feasibility in the framework of circuit quantum electrodynamics. With up-to-date experimental parameters, we show that our architecture allows entanglement lifetimes orders of magnitude longer than the spontaneous lifetime without local cavity couplings. This cavity-based architecture is straightforwardly extendable to many qubits for scalability.
Introduction
Quantum entanglement is essential for quantum information technology but is degraded by environmental coupling, motivating strategies that protect it. The paper proposes coupled cavities as a simple, experimentally realizable engineered environment for controlling qubit dynamics and preserving quantum resources.
- Motivation: Environmental coupling can cause entanglement decay or abrupt disappearance, threatening reliable quantum information processing.The abrupt loss is termed entanglement sudden death.
- Prior approaches: Existing approaches include entanglement concentration, decoherence-free subspaces, error correction, dynamical decoupling, and measurement-based protection.These methods can require many identically decohered states, symmetry, measurements, or probabilistic success, and may not uniformly prevent disentanglement.
- Structured environments: Non-Markovian environments can produce spontaneous or locally induced entanglement revivals, although these revivals eventually decay.Structured environments are therefore useful for protecting quantum superpositions and composite-system entanglement.
- Proposed architecture: The paper studies independent qubits locally interacting with an engineered environment because this configuration supports the individual control required in quantum hardware.The proposed architecture uses a qubit embedded in one cavity coupled to a second cavity.
- Proposed architecture: Adjusting the coupling between the two cavities enables transitions between Markovian and non-Markovian qubit dynamics while supporting coherence and entanglement preservation.The architecture is presented as simple and realizable with current experimental technologies, including circuit quantum electrodynamics.
Results
The coupled-cavity architecture controls single-qubit coherence and two-qubit entanglement by switching between Markovian and non-Markovian dynamics. Perfect secondary cavities can trap coherence and entanglement, while experimentally realistic parameters substantially extend entanglement lifetimes.
- Architecture and approach: The study analyzes single-qubit coherence and two-qubit entanglement using coupled-cavity architectures, with the two-qubit setup consisting of independent local environments.The dynamics are modeled through cavity couplings, photon decay rates, detuning, and concurrence.
- Single-qubit coherence: Increasing the inter-cavity coupling J switches weakly coupled qubit dynamics from Markovian to non-Markovian, while strongly coupled dynamics can transition from non-Markovian to Markovian and back.The transitions are identified through coherence oscillations and the non-Markovianity quantifier N.
- Single-qubit coherence: Γ2 = 0 enables long-time coherence trapping through a decay-free bound state shared by the qubit and secondary cavity.The bound state is |ψ−⟩ = J|10⟩ − κ|01⟩, and trapping requires a nonzero initial component of this state.
- Single-qubit coherence: Detuning accelerates coherence decay, with the fastest decay occurring around δ = J under non-resonant interaction.The detuning dependence is examined for both weak and strong qubit-cavity coupling regimes.
- Two-qubit entanglement: Increasing local cavity couplings converts weak-regime entanglement decay from Markovian to non-Markovian and shifts revivals to earlier times while concurrence remains relatively large.Without coupled cavities, the α = 9/10 initial state exhibits entanglement sudden death; additional cavities prolong disappearance and enhance concurrence.
- Experimental feasibility: The proposed analysis is framed for circuit quantum electrodynamics, but ultra-strong qubit-cavity coupling requires relaxing the rotating-wave approximation.Counter-rotating terms must be included when the coupling becomes comparable to the cavity frequency.
Discussion
The proposed coupled-cavity environment controls qubit memory effects and protects coherence and entanglement. Its steady-state entanglement, experimental feasibility, and scalability support use in quantum information architectures.
- Discussion: Two coupled lossy cavities switch a qubit between Markovian and non-Markovian dynamics, while low leakage from the cavity without the qubit maintains coherence.The architecture is also interpreted as a photonic band gap inhibiting spontaneous emission.
- Discussion: A perfect cavity without the qubit enables steady-state entanglement by combining a qubit-environment bound state with non-Markovian dynamics.This mechanism can preserve a substantial fraction of the initially shared entanglement under current experimental parameters.
- Discussion: The architecture is straightforwardly extendable to many independent, noninteracting qubits, supporting individual operations and scalable quantum protocols.Its scalability follows from preserving long-lived entanglement without requiring interactions between the qubits.
- Discussion: The estimated entanglement lifetimes should be long enough for various quantum tasks and exceed the natural lifetimes listed for systems without cavity coupling.Table 1 gives natural entanglement lifetimes of 669 ns to 6.69 µs as the comparison range.
Methods
The methods quantify qubit evolution through Laplace-transform-derived density-matrix functions and quantify two-qubit entanglement using concurrence.
- Methods: The single-qubit density-matrix functions u_t and z_t are obtained using the inverse Laplace transform of L(s).These functions appear in the single-qubit density-matrix evolution.
- Methods: Concurrence quantifies entanglement for an arbitrary two-qubit state ρAB.The calculation uses eigenvalues of ρAB(σ_y ⊗ σ_y)ρAB* in the computational basis.
- Methods: The concurrence construction orders four eigenvalues associated with the transformed two-qubit density matrix in decreasing order.The computational basis is {|11⟩,|10⟩,|01⟩,|00⟩}.