By extending the concept of interaction-free imaging to the few-atom level, we show that asymptotically on-demand interaction- and measurement-free quantum logic gates can be realized for both single-atom and single-photon qubits. The interaction-free feature suppresses the possibility of qubit decoherence via atomic spontaneous decay, while the elimination of measurements can significantly reduce errors arising from detector inefficiency. We present a general theory of universal quantum Zeno gates, and discuss physical implementations for quantum-information processing with individual atoms and photons. In addition, we propose a loss-tolerant protocol for long-distance quantum communication using quantum Zeno gates incorporated into a Mach-Zehnder interferometer. The efficiency of our Zeno gates is limited primarily by the imprecise control of atom-photon scattering and the finite number of feedback cycles
N due to the limited finesse of the optical ring cavity. We find that the success probability scales as $1\ensuremath{-}O(1/N)$, and for realistic parameters could be as high as 98.4%. Successful generation of atom-atom entanglement can be heralded by detection of the ancillary photon, upon which the fidelity scales as $1\ensuremath{-}O(1/{N}^{2})$, with an achievable fidelity of 99.994%, which comes at the cost of reducing the success probability by the detector efficiency.
An Elitzur–Vaidman bomb tester demonstrates interaction-free sensing: detecting an object without absorbing a photon. The original protocol, however, has a success probability capped at 25%. We present a scalable quantum circuit that harnesses the quantum Zeno effect to overcome this limit. A single probe qubit undergoes repeated cycles of a small rotation followed by a controlled-NOT (CNOT) with an ancilla qubit representing the obstacle. The circuit requires only two qubits, regardless of the number of cycles N and has depth linear in N, compatible with near-term processors that support mid-circuit measurement and conditional reset. We simulate the circuit under ideal conditions and under a depolarizing noise model with gate errors typical of superconducting qubits (CNOT error 2% and single-qubit gate error rate 0.1%). Under noise, the success probability peaks around N=14 (reaching 72.0%), outperforming the original 25% limit by nearly a factor of three, with a well-defined optimal region. The protocol requires approximately 4.4 times fewer photons than the original Elitzur–Vaidman scheme to achieve 99.9% confidence. Its simple repetitive structure and well-defined optimal regime make it a practical benchmark for quantifying gate errors and decoherence on noisy intermediate-scale quantum hardware for quantum sensing applications.
The Zeno effect occurs in quantum systems when a very strong measurement is applied, which can alter the dynamics in non-trivial ways. Despite being dissipative, the dynamics stay coherent within any degenerate subspaces of the measurement. Here we show that such a measurement can turn a single-qubit operation into a two- or multi-qubit entangling gate, even in a non-interacting system. We demonstrate this gate between two effectively non-interacting transmon qubits. Our Zeno gate works by imparting a geometric phase on the system, conditioned on it lying within a particular non-local subspace.