{"id":446770,"date":"2026-02-26T14:08:12","date_gmt":"2026-02-26T14:08:12","guid":{"rendered":"https:\/\/www.newsbeep.com\/uk\/446770\/"},"modified":"2026-02-26T14:08:12","modified_gmt":"2026-02-26T14:08:12","slug":"entanglement-assisted-non-local-optical-interferometry-in-a-quantum-network","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/uk\/446770\/","title":{"rendered":"Entanglement-assisted non-local optical interferometry in a quantum network"},"content":{"rendered":"<p>Experimental setup<\/p>\n<p>The experiment encompasses two labs, each containing one station with an SiV inside a dilution refrigerator (BlueFors BF-LD250) at about 100\u2009mK and connected as shown in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig6\" rel=\"nofollow noopener\" target=\"_blank\">1<\/a>. Light signals are prepared in the laser setup, including for entanglement and erasure interferometer locking (NewFocus TLB-6700 Velocity), right station SiV readout, entanglement qubit generation, signal light generation, and erasure LO generation (MSquared SolsTiS Ti:Sapphire), left station SiV readout and filter cavity locking (Toptica DLPro) and SiV de-ionization (Thorlabs Green diode LP520-SF15). All free space and in-fibre acousto-optic modulators (AOM) are driven with 215\u2009MHz. The entanglement qubit, signal light and erasure LO pulses are shaped by modulating the radiofrequency signal sent to in-fibre AOMs. We bridge the frequency difference between the SiVs at each station of \u0394fL\u2212R\u00a0\u2248\u00a010\u2009GHz (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig13\" rel=\"nofollow noopener\" target=\"_blank\">8<\/a>) by generating sidebands with electro-optic modulators (EOM) driven with a radiofrequency signal at \u0394fL\u2212R inside the entanglement interferometer and filtering the light with a Fabry\u2013Perot cavity<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 9\" title=\"Knaut, C. M. et al. Entanglement of nanophotonic quantum memory nodes in a telecom network. Nature 629, 573&#x2013;578 (2024).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR9\" id=\"ref-link-section-d162203309e4941\" rel=\"nofollow noopener\" target=\"_blank\">9<\/a>. Free space AOMs at the left and right stations act both as a switch between the entanglement path and signal-erasing path, as well as frequency shifters for entanglement interferometer phase locking. Photon counts for erasure are detected with pairs of superconducting nanowire single-photon detectors (SNSPD) (Photon Spot) at each station, and entanglement photon heralding clicks are detected with a single-photon avalanche photodiode (APD). We note that the erasure SNSPDs are not instantaneously photon-number-resolving but can effectively resolve photon number when the detector deadtime is much lower than the photon length. All counts are logged with a time tagger (Swabian Instruments Time Tagger Ultra), and two Zurich Instrument HDAWG 2.4 GSa\/s arbitrary waveform generators are used for sequence logic, control of the AOMs and EOMs, as well as MW and radiofrequency pulse generation for SiV control.<\/p>\n<p>SNR and Fisher information<\/p>\n<p>The SNR can be evaluated through the Fisher information \\({{\\mathcal{F}}}_{I}\\) of the measurement, which is equivalent to (SNR)2 and bounds the \u03d5 estimation variance as \\(\\mathrm{var}({\\phi }_{\\mathrm{est}})\\ge 1\/{{\\mathcal{F}}}_{I}\\) (refs.\u2009<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 3\" title=\"Tsang, M. Quantum nonlocality in weak-thermal-light interferometry. Phys. Rev. Lett. 107, 270402 (2011).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR3\" id=\"ref-link-section-d162203309e5020\" rel=\"nofollow noopener\" target=\"_blank\">3<\/a>,<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 55\" title=\"Giovannetti, V., Lloyd, S. &amp; Maccone, L. Advances in quantum metrology. Nat. Photon. 5, 222&#x2013;229 (2011).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR55\" id=\"ref-link-section-d162203309e5023\" rel=\"nofollow noopener\" target=\"_blank\">55<\/a>): <\/p>\n<p>$${{\\mathcal{F}}}_{I}=\\sum _{y}\\frac{1}{P(y| \\phi )}{\\left(\\frac{{\\rm{\\delta }}P(y| \\phi )}{{\\rm{\\delta }}\\phi }\\right)}^{2},$$<\/p>\n<p>\n                    (7)\n                <\/p>\n<p>where for our experiment P(y|\u03d5) is the probability of obtaining a nuclear two-qubit parity measurement outcome y for a given \u03d5. The probabilities P(y|\u03d5) are<\/p>\n<p>$$\\begin{array}{c}P(\\mathrm{discarded}|\\phi )=1-{P}_{\\mathrm{succ}}\\\\ P(\\pm \\,\\mathrm{parity}|\\phi )={P}_{\\mathrm{succ}}(1\\pm V\\cos (\\phi ))\/2,\\end{array}$$<\/p>\n<p>\n                    (8)\n                <\/p>\n<p>where V is the visibility of the measurement and Psucc is the probability to herald a photon (including the photon presence probability itself). Using equations (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Equ7\" rel=\"nofollow noopener\" target=\"_blank\">7<\/a>) and (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Equ8\" rel=\"nofollow noopener\" target=\"_blank\">8<\/a>) for small visibility V2\u2009\u226a 1, we get \\({{\\mathcal{F}}}_{I}\\propto {P}_{\\mathrm{succ}}{V}^{2}\\).<\/p>\n<p>For our implemented protocol, \\({P}_{\\mathrm{succ}}={\\eta }_{\\mathrm{erasure}}{\\eta }_{\\mathrm{herald}}P({n}_{\\mathrm{photon}}\\ge 1)\\), where \u03b7erasure(herald) are constant factors given by the erasure (signal photon heralding) efficiency. The signal photon heralding efficiency is limited to 50% by the use of amplitude-based SMSPG, but can be increased to 100% by using phase-based gates instead (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"supplementary material anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#MOESM1\" rel=\"nofollow noopener\" target=\"_blank\">Supplementary Information<\/a>). The sequence heralds whether there was at least one signal photon but does not distinguish between single and multi-photon events. As the protocol fails when more than one photon is collected, the visibility is given by \\(V=\\bar{V}P({n}_{\\mathrm{photon}}=1|{n}_{\\mathrm{photon}}\\ge 1)\\) (Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4e<\/a>, dashed red curve), where \\(\\overline{V}\\) is the constant overhead factor due to fidelity reduction from imperfect photon erasure, gate errors and initial qubit state fidelities (Extended Data Table <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"table anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Tab1\" rel=\"nofollow noopener\" target=\"_blank\">1<\/a>). For a light signal obeying Poissonian photon-number statistics (as our signal results from an attenuated laser with scrambled local phase but constant intensity) with average photon number \u03bcsig arriving at the stations, the Fisher information is <\/p>\n<p>$${{\\mathcal{F}}}_{I}={\\eta }_{\\mathrm{erasure}}{\\eta }_{\\mathrm{herald}}{\\bar{V}}^{2}\\frac{{\\mu }_{\\mathrm{sig}}^{2}{{\\rm{e}}}^{-2{\\mu }_{\\mathrm{sig}}}}{1-{{\\rm{e}}}^{-{\\mu }_{\\mathrm{sig}}}},$$<\/p>\n<p>\n                    (9)\n                <\/p>\n<p>which reduces to \\({{\\mathcal{F}}}_{I}\\propto {\\mu }_{\\mathrm{sig}}\\) for small signal \u03bcsig \u226a 1.<\/p>\n<p>Without signal photon heralding, \\({P}_{\\mathrm{succ}}={\\eta }_{\\mathrm{erasure}}\\) and \\(V={\\eta }_{{\\rm{herald}}}\\) \\(\\overline{V}P({n}_{{\\rm{photon}}}=1)\\) (Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4e<\/a>, solid blue curve), so that the resulting Fisher information is<\/p>\n<p>$${{\\mathcal{F}}}_{I}={\\eta }_{\\mathrm{erasure}}{({\\eta }_{\\mathrm{herald}}\\bar{V})}^{2}{\\mu }_{\\mathrm{sig}}^{2}{{\\rm{e}}}^{-2{\\mu }_{\\mathrm{sig}}},$$<\/p>\n<p>\n                    (10)\n                <\/p>\n<p>which reduces to \\({{\\mathcal{F}}}_{I}\\propto {\\mu }_{\\mathrm{sig}}^{2}\\) for \u03bcsig \u226a 1. This precisely shows that the key feature that enables SNR scaling enhancement is the non-destructive non-local signal photon heralding. This step, enabled by pre-generated entanglement, is what gives the remote phase sensing protocol its non-local character.<\/p>\n<p>By contrast, when using non-local signal photon heralding, mis-heralding events (with probability \u03b5mh) corrupt the signal, modifying Psucc to \\({\\eta }_{{\\rm{erasure}}}P({\\rm{herald}}\\cup \\text{mis-herald})\\) and V to \\(\\bar{V}P({n}_{{\\rm{photon}}}\\,=\\) \\(1|({\\rm{herald}}\\cup \\text{mis-herald}))\\) (Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4e<\/a>, solid red curve). This results in<\/p>\n<p>$${{\\mathcal{F}}}_{I}={\\eta }_{\\mathrm{erasure}}{\\eta }_{\\mathrm{herald}}{\\bar{V}}^{2}\\frac{{\\mu }_{\\mathrm{sig}}^{2}{{\\rm{e}}}^{-2{\\mu }_{\\mathrm{sig}}}}{1-{{\\rm{e}}}^{-{\\mu }_{\\mathrm{sig}}}\\,(1-{\\varepsilon }_{\\mathrm{mh}}\/{\\eta }_{\\mathrm{herald}})},$$<\/p>\n<p>\n                    (11)\n                <\/p>\n<p>which scales as \\({{\\mathcal{F}}}_{I}\\propto {\\mu }_{\\mathrm{sig}}\\) for \\({\\mu }_{\\mathrm{sig}}\\gtrsim {\\widetilde{\\varepsilon }}_{\\mathrm{mh}}\\) (where \\({\\widetilde{\\varepsilon }}_{\\mathrm{mh}}={\\varepsilon }_{\\mathrm{mh}}\/{\\eta }_{\\mathrm{herald}}\\) is the effective mis-heralding probability) but curves down to \\({{\\mathcal{F}}}_{I}\\propto {\\mu }_{\\mathrm{sig}}^{2}\\) for \\({\\mu }_{\\mathrm{sig}}\\lesssim {\\widetilde{\\varepsilon }}_{\\mathrm{mh}}\\) (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig10\" rel=\"nofollow noopener\" target=\"_blank\">5<\/a> and <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"supplementary material anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#MOESM1\" rel=\"nofollow noopener\" target=\"_blank\">Supplementary Information<\/a>). The visibility improvement from signal photon heralding can be seen both in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4d<\/a> and Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig7\" rel=\"nofollow noopener\" target=\"_blank\">2<\/a>.<\/p>\n<p>SMPHONE gate error detection<\/p>\n<p>Similarly to the PHONE gate<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 8\" title=\"Stas, P.-J. et al. Robust multi-qubit quantum network node with integrated error detection. Science 378, 557&#x2013;560 (2022).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR8\" id=\"ref-link-section-d162203309e6348\" rel=\"nofollow noopener\" target=\"_blank\">8<\/a>,<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 9\" title=\"Knaut, C. M. et al. Entanglement of nanophotonic quantum memory nodes in a telecom network. Nature 629, 573&#x2013;578 (2024).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR9\" id=\"ref-link-section-d162203309e6351\" rel=\"nofollow noopener\" target=\"_blank\">9<\/a>, the SMPHONE gate entangles a photonic qubit with the nuclear spin\u2014but in the Fock basis instead of the time-bin basis\u2014mediated by the electron spin. Starting the nucleus and the photon in superposition states (|\u2193\u27e9\u2009+\u2009|\u2191\u27e9)n\/\u221a2 and (|0\u27e9\u2009+\u2009|1\u27e9)phot\/\u221a2 and the electron in the |\u2191\u27e9e state, we implement the SMPHONE gate (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig8\" rel=\"nofollow noopener\" target=\"_blank\">3a<\/a>): <\/p>\n<p>$${(|0\\rangle +|1\\rangle )}_{\\mathrm{phot}}{|{\\rm{\\uparrow }}\\rangle }_{{\\rm{e}}}{(|{\\rm{\\downarrow }}\\rangle +|{\\rm{\\uparrow }}\\rangle )}_{{\\rm{n}}}\\to {|{\\rm{\\uparrow }}\\rangle }_{{\\rm{e}}}({|0\\rangle }_{\\mathrm{phot}}{|+\\rangle }_{{\\rm{n}}}+{|1\\rangle }_{\\mathrm{phot}}{|{\\rm{\\downarrow }}\\rangle }_{{\\rm{n}}}\\,\/\\sqrt{2}).$$<\/p>\n<p>\n                    (12)\n                <\/p>\n<p>Here the nucleus is entangled with the photon and the electron is always in the |\u2191\u27e9 state, unless a MW error occurred during the SMPHONE gate operation. We note, however, that the nucleus does not directly interface with light, and the nucleus\u2013photon entanglement generation is mediated by the electron (which does interface with light), so that MW errors on the electron translate to errors on the nucleus\u2013photon entangled state. Therefore, by measuring the electron state we can detect these MW errors and post-select on |\u2191\u27e9 results to boost the nucleus entanglement fidelity (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig8\" rel=\"nofollow noopener\" target=\"_blank\">3b<\/a>). As measuring the electron in the |\u2191\u27e9 state (as opposed to the |\u2191\u27e9 state) does not cause decoherence of the 29Si nucleus state<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 8\" title=\"Stas, P.-J. et al. Robust multi-qubit quantum network node with integrated error detection. Science 378, 557&#x2013;560 (2022).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR8\" id=\"ref-link-section-d162203309e6556\" rel=\"nofollow noopener\" target=\"_blank\">8<\/a>, we can perform error detection mid-circuit, as in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4b<\/a>.<\/p>\n<p>Entanglement interferometer phase<\/p>\n<p>The entanglement interferometer phase \u03b4\u03c6e stability (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig9\" rel=\"nofollow noopener\" target=\"_blank\">4c<\/a>) is limited by noise from the fibre link between the two labs in which the stations are located and vibrations from the pulse tube motor-head of the dilution refrigerators. We reduce phase noise introduced in the fibre link by packaging the fibre in a rubber tube filled with sand for vibration damping (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig9\" rel=\"nofollow noopener\" target=\"_blank\">4a<\/a>). We limit the phase noise introduced by the pulse tube motor-head by clamping the motor-head between aluminium plates padded with foam. To reduce vibrations guided to the dilution refrigerator through the flexline connecting to the pulse tube head, we clamp the flexline in bags of sand that further damp vibrations (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig9\" rel=\"nofollow noopener\" target=\"_blank\">4b<\/a>). With this passive stabilization, the interferometer phase auto-correlation time increases from about 4\u2009ms to 500\u2009ms (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig9\" rel=\"nofollow noopener\" target=\"_blank\">4c<\/a>). When we add the two spools of 1.5\u2009km for the long-baseline operation (Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig5\" rel=\"nofollow noopener\" target=\"_blank\">5<\/a>), the auto-correlation time of the entanglement interferometer decreases again (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig9\" rel=\"nofollow noopener\" target=\"_blank\">4c<\/a>, inset).<\/p>\n<p>We then lock the interferometer phase by alternating phase probing with SiV readout every 50\u2009\u03bcs. A field-programmable gate array integrates the phase probing light for 1\u2009ms and locks the interferometer phase by adjusting the drive frequency of acousto-optic modulators in each arm, resulting in a locked optical interference visibility of around 0.93 (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig9\" rel=\"nofollow noopener\" target=\"_blank\">4d<\/a>).<\/p>\n<p>Quantum-memory-assisted interferometry details<\/p>\n<p>After generating entanglement and collecting the signal in the non-local phase sensing protocol, with the resulting state in equation (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"equation anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Equ6\" rel=\"nofollow noopener\" target=\"_blank\">6<\/a>), we erase the photonic spatial mode<\/p>\n<p>$$[|{+}_{{{\\rm{e}}}_{{\\rm{L}}}}{+}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle +\\sqrt{{\\mu }_{\\mathrm{sig}}\/2}(|{+}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle +{{\\rm{e}}}^{{\\rm{i}}\\phi }|{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{+}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle )]{| {\\varPsi }^{-}\\rangle }_{{{\\rm{n}}}_{{\\rm{L}}},{{\\rm{n}}}_{{\\rm{R}}}}.$$<\/p>\n<p>\n                    (13)\n                <\/p>\n<p>Then, with local CnNOTe and \u03c0\/2 pulses at each station, we transform the state to<\/p>\n<p>$$\\begin{array}{c}(|{{\\rm{\\downarrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle |{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle -|{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle |{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle )\/\\sqrt{2}\\\\ \\,+\\,\\sqrt{\\mu }\/2[|{{\\rm{\\downarrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle (|{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle -{{\\rm{e}}}^{{\\rm{i}}\\phi }|{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle )\\\\ \\,+\\,|{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle ({{\\rm{e}}}^{{\\rm{i}}\\phi }|{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle -|{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle )\\\\ \\,+\\,|{{\\rm{\\downarrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle (1+{{\\rm{e}}}^{{\\rm{i}}\\phi })|{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle +|{{\\rm{\\uparrow }}}_{{{\\rm{e}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{e}}}_{{\\rm{R}}}}\\rangle (1-{{\\rm{e}}}^{{\\rm{i}}\\phi })|{{\\rm{\\uparrow }}}_{{{\\rm{n}}}_{{\\rm{L}}}}{{\\rm{\\downarrow }}}_{{{\\rm{n}}}_{{\\rm{R}}}}\\rangle ],\\end{array}$$<\/p>\n<p>\n                    (14)\n                <\/p>\n<p>so the electron two-qubit parity is even (|\u2191\u2191\u27e9 or |\u2193\u2193\u27e9) only if a signal photon was present, and the probability of measuring these states scales with the probability of at least one signal photon arriving (Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4c<\/a>). We note that the mis-heralding probability is higher for heralding on the |\u2193e\u2193e\u27e9 than the |\u2191e\u2191e\u27e9 electron state due to experimental errors accumulating coherently in the |\u2193e\u2193e\u27e9 state. The nuclear two-qubit parity oscillation curves with and without non-local signal photon heralding in function of signal phase separated by signal strength are shown in Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig11\" rel=\"nofollow noopener\" target=\"_blank\">6<\/a>. These curves are combined to plot the curve in Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4e<\/a>. Figure <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4c<\/a> has 9,898 successful experimental trials for a 16\u2009h 24\u2009min run time. Figure <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4d<\/a> has 6,645 successful experimental trials with and 16,270 without non-local heralding for a 155\u2009h 43\u2009min run time. Figure <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig4\" rel=\"nofollow noopener\" target=\"_blank\">4e<\/a> has 3,167 successful experimental trials with and 7,400 without non-local heralding for a 74\u2009h 58\u2009min run time.<\/p>\n<p>The phases of the LO pulses used in the photon erasure step at the left and right stations are also imprinted onto the nuclear state, so that the relevant phase is \u0394\u03a6L\u2212R\u00a0=\u00a0\u03b4\u03d5L\u00a0\u2212\u00a0\u03b4\u03d5R with \u03b4\u03d5L the differential phase between the signal and the LO pulses at the left station and \u03b4\u03d5R at the right station. \u0394\u03a6L\u2212R reduces to simply \u03d5 when the phases of the LO pulses are locked to one another. However, it is enough to simply know the phases through calibration measurements, which we perform every four experimental trials (<a data-track=\"click\" data-track-label=\"link\" data-track-action=\"supplementary material anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#MOESM1\" rel=\"nofollow noopener\" target=\"_blank\">Supplementary Information<\/a>).<\/p>\n<p>Signal state preparation<\/p>\n<p>A weak coherent state, used to emulate the signal light, is split on a beam splitter and sent to both stations. The optical phases \u03b4\u03d5L and \u03b4\u03d5R at the left and right stations, respectively, are not phase-locked and therefore fluctuate freely. After interacting with the SiVs, the signal undergoes a photon-erasure step, during which it is combined with an LO. The LOs at the two stations are also not phase-locked.<\/p>\n<p>Rather than stabilizing the phases, we probe them using bright reference lasers for each experimental shot. This allows us to determine the phase at each station for every shot. The phase of the LO at each station can be taken as the zero reference, as we measure all quantities relative to it.<\/p>\n<p>For local phases \u03b4\u03d5L and \u03b4\u03d5R at the left and right stations, respectively, and differential phase \u0394\u03a6L\u2212R\u00a0=\u00a0\u03b4\u03d5L\u00a0\u2212\u00a0\u03b4\u03d5R, the photonic state density matrix in the {|00\u27e9,\u2009|01\u27e9,\u2009|10\u27e9} basis for weak signals \u03bc \u226a 1 is <\/p>\n<p>$$\\begin{array}{c}{\\rho }_{{\\rm{sig}}}=|{\\alpha }_{L}\\rangle \\otimes |{\\alpha }_{{\\rm{R}}}\\rangle =|\\sqrt{\\mu \/2}\\,{{\\rm{e}}}^{{\\rm{\\delta }}{\\phi }_{{\\rm{L}}}}\\rangle \\otimes |\\sqrt{\\mu \/2}\\,{{\\rm{e}}}^{{\\rm{\\delta }}{\\phi }_{{\\rm{R}}}}\\rangle \\\\ \\,\\approx \\,\\left(\\begin{array}{ccc}1 &amp; {{\\rm{e}}}^{-{\\rm{i}}{\\rm{\\delta }}{\\phi }_{{\\rm{R}}}}\\sqrt{\\mu \/2} &amp; {{\\rm{e}}}^{-{\\rm{i}}{\\rm{\\delta }}{\\phi }_{{\\rm{L}}}}\\sqrt{\\mu \/2}\\\\ {{\\rm{e}}}^{{\\rm{i}}{\\rm{\\delta }}{\\phi }_{{\\rm{R}}}}\\sqrt{\\mu \/2} &amp; \\mu \/2 &amp; {{\\rm{e}}}^{-{\\rm{i}}({\\rm{\\delta }}{\\phi }_{{\\rm{L}}}-{\\rm{\\delta }}{\\phi }_{{\\rm{R}}})}\\mu \/2\\\\ {{\\rm{e}}}^{{\\rm{i}}{\\rm{\\delta }}{\\phi }_{{\\rm{L}}}}\\sqrt{\\mu \/2} &amp; {{\\rm{e}}}^{{\\rm{i}}({\\rm{\\delta }}{\\phi }_{{\\rm{L}}}-{\\rm{\\delta }}{\\phi }_{{\\rm{R}}})}\\mu \/2 &amp; \\mu \/2\\end{array}\\right).\\end{array}$$<\/p>\n<p>During data analysis, we group the measurements according to the same differential phase \u0394\u03a6L\u2212R\u00a0=\u00a0\u03b4\u03d5L\u00a0\u2212\u00a0\u03b4\u03d5R between the stations. On averaging, terms depending on the individual local phases \u03b4\u03d5L and \u03b4\u03d5R cancel out, whereas terms depending on the fixed differential phase remain (Extended Data Fig. <a data-track=\"click\" data-track-label=\"link\" data-track-action=\"figure anchor\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#Fig12\" rel=\"nofollow noopener\" target=\"_blank\">7<\/a>), so that the density matrix becomes <\/p>\n<p>$$\\begin{array}{c}{\\mathop{\\rho }\\limits^{ \\sim }}_{\\mathrm{sig}}\\approx \\left(\\begin{array}{ccc}1 &amp; 0 &amp; 0\\\\ 0 &amp; \\mu \/2 &amp; {{\\rm{e}}}^{-{\\rm{i}}\\Delta {\\varPhi }_{{\\rm{L}}-{\\rm{R}}}}\\mu \/2\\\\ 0 &amp; {{\\rm{e}}}^{{\\rm{i}}\\Delta {\\varPhi }_{{\\rm{L}}-{\\rm{R}}}}\\mu \/2 &amp; \\mu \/2\\end{array}\\right)\\approx {\\rho }_{\\mathrm{th}},\\end{array}$$<\/p>\n<p>which is approximately the density matrix of a weak thermal state (with mean photon number \u03bc and complex visibility \\(g={{\\rm{e}}}^{{\\rm{i\\Delta }}{\\varPhi }_{{\\rm{L}}-{\\rm{R}}}}\\)) (ref.\u2009<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 3\" title=\"Tsang, M. Quantum nonlocality in weak-thermal-light interferometry. Phys. Rev. Lett. 107, 270402 (2011).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR3\" id=\"ref-link-section-d162203309e8615\" rel=\"nofollow noopener\" target=\"_blank\">3<\/a>). Although the photon number statistics of the light remain Poissonian rather than thermal, in the low-mean-photon-number regime and to first photon order, the corresponding density matrices are effectively the same.<\/p>\n<p>We note that in our work, the generated signal visibility is unity, but this is often not the case in practical astronomical imaging<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 3\" title=\"Tsang, M. Quantum nonlocality in weak-thermal-light interferometry. Phys. Rev. Lett. 107, 270402 (2011).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR3\" id=\"ref-link-section-d162203309e8622\" rel=\"nofollow noopener\" target=\"_blank\">3<\/a>. The value of the visibility can, in principle, also be estimated with our protocol, although care must be taken to distinguish the intrinsic signal visibility from the measurement protocol infidelity, as well as from environmental noise sources. With additional quantum resources, more advanced quantum-processing techniques<a data-track=\"click\" data-track-action=\"reference anchor\" data-track-label=\"link\" data-test=\"citation-ref\" aria-label=\"Reference 10\" title=\"Mokeev, A., Saif, B., Lukin, M. D. &amp; Borregaard, J. Enhancing optical imaging via quantum computation. Preprint at &#010;                https:\/\/arxiv.org\/abs\/2509.09465v3&#010;                &#010;               (2025).\" href=\"http:\/\/www.nature.com\/articles\/s41586-026-10171-w#ref-CR10\" id=\"ref-link-section-d162203309e8626\" rel=\"nofollow noopener\" target=\"_blank\">10<\/a> can be used to extract the intrinsic visibility from unknown and fluctuating noise contributions without needing to reconstruct the noise itself.<\/p>\n","protected":false},"excerpt":{"rendered":"Experimental setup The experiment encompasses two labs, each containing one station with an SiV inside a dilution refrigerator&hellip;\n","protected":false},"author":2,"featured_media":446771,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[24],"tags":[4230,4231,2302,4835,26783,30253,90,56,54,55],"class_list":{"0":"post-446770","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-physics","8":"tag-humanities-and-social-sciences","9":"tag-multidisciplinary","10":"tag-physics","11":"tag-quantum-information","12":"tag-quantum-metrology","13":"tag-quantum-optics","14":"tag-science","15":"tag-uk","16":"tag-united-kingdom","17":"tag-unitedkingdom"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/446770","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/comments?post=446770"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/446770\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media\/446771"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media?parent=446770"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/categories?post=446770"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/tags?post=446770"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}