Entanglement-assisted non-local optical interferometry in a quantum network

Nature News · Feb 25, 2026 · Collected from RSS

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MainOptical interferometry is a well-established method for high-resolution imaging with wide-ranging applications from physics and astronomy to biological and medical imaging12,13,14. For instance, astronomical interferometry is routinely used for the observation of stellar objects in which the light signal from multiple physically separated telescopes is combined to increase the imaging resolution1,2,15,16. In such a case, an array of optical receivers forms a synthetic aperture whose resolution scales with their separation (the baseline)13. However, increasing the baseline of receiver arrays in practice is challenging17. In the limit of weak signals typical of the optical domain, the optimal method for observation is direct interference of incident electromagnetic fields3, which is hindered by the exponential loss of signal light associated with optical-fibre-based connections18. Quantum networks19,20,21,22 provide a way to perform non-local interference measurements. The key idea is to use quantum entanglement to effectively teleport the quantum state of the electromagnetic field modes between remote receiver stations (Fig. 1a, right), thereby enabling direct interference4,23. Although a scheme involving entanglement for non-local interference has been recently demonstrated in an all-photonic setting24, the use of quantum memories presents a practical path towards overcoming photon loss through event-ready (heralded) entanglement4 and efficient information processing together with local photon mode erasure, enabling an exponential reduction in the number of required entangled pairs5,6.Fig. 1: A quantum-memory-assisted non-local interferometer based on a quantum network.a, Three remote phase sensing methods and their SNR scalings from left to right: direct interferometric detection, local measurements with an LO, and entanglement-assisted non-local phase sensing. b, The electron-spin-state-dependent cavity reflectance near the SiV optical transition is used for signal photon storage and quantum operations. The dashed line indicates the frequency used for electron spin state readout, photonic entanglement and signal light collection. c, Entangled qubits shared between the stations are used to improve the sensitivity of a non-local interferometer. d–f, Once entanglement has been generated, the steps of the quantum-memory-assisted remote phase sensing protocol are signal light collection through local operations (d), signal photon mode erasure to complete photon state storage (e), and non-local photon heralding through electron state measurement and phase probing through nuclear state measurement (f).Full size imageHere we demonstrate quantum-memory-assisted non-local interferometry with a two-station network separated by a line-of-sight distance of about 6 m (Fig. 1b,c). Our approach uses atom-like defects in solid state21,25,26,27,28,29,30,31, particularly silicon–vacancy centre (SiV) integrated in diamond nanophotonic cavities. These systems recently emerged as a promising platform for quantum networking because of their access to long-lived spin quantum memories, high gate and readout fidelities and strong light–matter interaction, enabling efficient spin–photon operations7,8. These properties have enabled experimental implementations of quantum-memory-enhanced communication32, entanglement generation over a metropolitan-scale deployed fibre9 and blind quantum computation33. In our experiments, each SiV constitutes a two-qubit register with a communication qubit (electron spin) and a memory qubit (29Si nuclear spin). Signal fields are reflected off the fibre-coupled SiV-cavity systems, and an optical fibre network is used for readout, entanglement generation and signal light collection7,8. We use an improved parallel instead of serial9 entanglement scheme to reach higher entanglement rates for both electron–electron and nucleus–nucleus entanglement and demonstrate non-destructive photon heralding, first locally with a time-bin photonic qubit on a single station, then non-locally for a photon in superposition between two remote spatial modes by using remote entanglement. This photon heralding filters out vacuum fluctuations to achieve optimal interferometer sensitivity3. We combine this method with photon mode erasure by interfering the incident fields locally with a coherent state of light to hide the ‘which-path’ information for interferometric measurements. Finally, we integrate these elements to demonstrate the operation of a long-baseline quantum-memory-assisted interferometer with a fibre separation length of up to 1.55 km, five times larger than the current state-of-the-art optical telescope array baseline of 330 m (ref. 2).Non-local phase sensingThe signal in a non-local interferometer, such as the angle of the incident light from a distant object at two detector stations, is typically proportional to the sine or cosine of the differential phase ϕ between the detector stations (Fig. 1c). The goal is, therefore, to determine this differential phase ϕ (from which the spatial information of this distant object can be inferred) with the highest possible efficiency and precision. In conventional systems, two approaches to measure ϕ can be distinguished for thermal light. The first involves direct interference of non-locally collected field modes (Fig. 1a, left), whereas the second involves local measurements (Fig. 1a, middle). In the first approach, direct interference of the signal light collected from each station is enabled by routing the light to a central beam splitter. This method achieves optimal interference visibility with signal-to-noise ratio (SNR) scaling as √μsig (ref. 3), where μsig ≪ 1 is the average photon number of the incident light field. However, for long-baseline measurements, this approach introduces signal attenuation that typically scales exponentially with the distance between the stations18. The second approach consists of interfering the collected field modes with a distributed local oscillator (LO) at each station. The phase difference between the two stations can then be determined by comparing the local measurement results. However, as the signal light is mixed with the LO, the local measurements cannot distinguish the vacuum component of the signal field modes from their single-photon component. The vacuum component of the field (which contains no photons) carries no useful phase information but introduces shot noise (vacuum fluctuation noise), reducing the interference visibility and resulting in an SNR scaling as μsig (ref. 3). Although it is also possible to independently measure the local phase of the incident light at each station using higher-order correlations34,35, for thermal light, this requires the simultaneous arrival of a signal photon at each station, which maintains the unfavourable scaling3.Entangled quantum memories provide a route to achieve optimal non-local measurements without the exponential field attenuation with the baseline size4,5 (Fig. 1a, right). Specifically, pre-generated entanglement between the stations can be used as a resource to perform non-local photon heralding, in which the arrival of a signal photon can be detected without revealing at which station it arrives5. This allows us to distinguish vacuum from signal photons without destroying the phase information ϕ. By keeping only measurement results with successful non-local heralding, vacuum fluctuations can be effectively filtered out to increase the visibility and SNR (see Methods for details).To realize this method experimentally, our approach consists of first ‘arming’ the interferometer by preparing the nuclear qubits in an entangled state between two stations. The entanglement is event-ready as it is heralded independently from the subsequent signal measurement, a key improvement over all-photonic approaches24. We model the distributed signal light with a weak laser pulse with average photon number μsig ≪ 1. The local signal phase at each station is averaged over a uniform distribution (with fixed differential phase ϕ), such that the signal effectively behaves like a two-mode thermal state in the weak-signal regime: \({\rho }_{{\rm{sig}}}\approx |{0}_{{\rm{L}}}{0}_{{\rm{R}}}\rangle \langle {0}_{{\rm{L}}}{0}_{{\rm{R}}}|+{\mu }_{{\rm{sig}}}/2(|{0}_{{\rm{L}}}{1}_{{\rm{R}}}\rangle +{{\rm{e}}}^{{\rm{i}}\phi }|{1}_{{\rm{L}}}{0}_{{\rm{R}}}\rangle )(\langle {0}_{{\rm{L}}}{1}_{{\rm{R}}}|\,+\) \({{\rm{e}}}^{-{\rm{i}}\phi }|{1}_{{\rm{L}}}{0}_{{\rm{R}}}\rangle )\), where |0L, 0R⟩ corresponds to vacuum, and |1L, 0R⟩ corresponds to a single photon arriving at the left station and |0L, 1R⟩ corresponds to that arriving at the right station3 (Methods and Extended Data Fig. 7). The photonic signal is collected through local quantum operations (Fig. 1d) that entangle the photonic state with the qubits at each station. We then erase the photonic mode information6 (Fig. 1e) and subsequently implement non-local, non-destructive photon heralding by measuring the parity of electron qubit spins at the two stations. This correlated parity measurement heralds the arrival of a photon without revealing which station the photon arrived at (Fig. 1f). Finally, the differential phase ϕ of the photonic modes imprinted on the initial Bell state between the nuclear spins is obtained through a two-qubit nuclear parity measurement performed locally.Parallel entanglement generationIn previous work, SiV remote entanglement generation has relied on serial entangling schemes9,33. Here we implement a parallel entangling scheme36,37 with a 7.5 times higher efficiency9. This is realized by connecting the two stations in a Mach–Zehnder interferometer configuration that must be phase-stable with each path reflecting off one SiV-cavity system (Fig. 2a). In this approach, we generate entanglement between electron spin qubits by splitting a weak laser pulse on a beam splitter to send two weak coherent states \({|\alpha /\sqrt{2}\