{"id":"cd2041ae-ec3a-4434-b136-370fc1751d99","arxiv_id":"2411.16835","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":8.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"EYFP, a standard fluorescent protein, is shown to be an optically addressable spin qubit with coherent microwave control and optically detected magnetic resonance.","lead":"Researchers turned a common glowing protein into a light-controlled magnetic field sensor. The yellow fluorescent protein can now be used as a spin qubit that works in cells and at room temperature, opening biology to quantum sensing.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The OADF readout's spin selectivity is inferred, not directly demonstrated; if the 912 nm pathway is not spin-selective, the entire spin-readout claim loses its physical basis.","rationale":"The reader correctly identified the OADF readout mechanism as the weakest assumption. My reading of the manuscript confirms that this is the load-bearing link: every spin measurement, including ODMR, Rabi, Hahn-echo, CPMG, T1, and the room-temperature sensing demonstrations, ultimately reports the delayed fluorescence after a 912 nm pulse. If that pulse does not probe spin through the assigned T2/RISC pathway, the interpretation of all downstream data as qubit control is unsupported. The paper offers TDDFT and spectral overlap as supporting evidence, but these do not demonstrate spin selectivity of the 912 nm step or of RISC. A direct control experiment with 912 nm detuning would settle the mechanism without requiring a new apparatus. The room-temperature sign flip is additional evidence that the spin-dependent photophysics are not yet robustly characterized, though the 80 K data are the main quantitative support. Given the reader's verdict was already CONDITIONAL and flagged this same assumption, no verdict change is needed.","tokens_in":45,"tokens_out":5098,"duration_ms":116202,"concrete_test":"Measure ODMR contrast (as in Fig. 2) as a function of 912 nm wavelength across the TDDFT-predicted T1–T2 absorption band at 80 K, and repeat with 912 nm detuned by more than 50 nm. If the ODMR resonance and its contrast do not track and vanish with the assigned T1–T2 band, the OADF readout is not mediated by the proposed spin-dependent RISC through T2. Ideally, accompany this with time-resolved EPR or transient absorption with and without the 912 nm pulse to directly confirm that T1 spin polarization is converted into a spin-dependent S1 population.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim rests on the OADF readout: a 912 nm pulse transports T1 population to a higher triplet (assigned T2, Fig. 1c), and spin-dependent reverse intersystem crossing (RISC) back to S1 makes the delayed fluorescence intensity a proxy for the T1 spin state. This pathway is supported only by TDDFT (Methods) and spectral overlap; no direct measurement shows that the 912 nm population transfer or the RISC yield is spin-selective. If the 912 nm step merely accelerates triplet decay with a spin-independent yield, the 44% contrast in Fig. 1e and all ODMR traces could arise from a different spin-dependent step (e.g., spin-selective ISC during 488 nm initialization) or from an artifact, so the detected photons would not faithfully map qubit state. The room-temperature sign flip (Figs. 4a,b) further indicates that the spin-dependence of ISC/RISC changes with temperature, so the room-temperature 3% ODMR relies on an even less characterized variant of the same mechanism. The paper does not report a control with 912 nm detuned from the assigned T1–T2 resonance, nor a non-optical verification of triplet spin polarization before and after the 912 nm pulse.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports the realization of an optically addressable spin qubit in the Enhanced Yellow Fluorescent Protein (EYFP), using a custom confocal microscope with microwave driving and a proposed optically activated delayed fluorescence (OADF) readout. The authors initialize the triplet spin via 488 nm excitation, drive spin transitions with microwaves, and read out the spin state by a 912 nm pulse that populates a higher triplet and leads to delayed fluorescence. They report ODMR spectra at 80 K, fit zero-field splitting parameters D = (2π)×(2.356±0.004) GHz and E = (2π)×(0.458±0.003) GHz, and show that the model extrapolates successfully to 34.1 mT. They demonstrate Rabi oscillations, Hahn-echo coherence (T2 ≈ 1.5 μs at zero field), CPMG-extended coherence (16 μs at 80 K), spin-lattice relaxation T1 = (141±5) μs at 80 K, room-temperature ODMR in aqueous solution with ~3% contrast and a DC sensitivity estimate, and coherent control in HEK 293T cells at 175 K. The paper also presents TDDFT calculations supporting the triplet assignment and the T1–T2 optical transition.","tokens_in":10746,"tokens_out":11110,"duration_ms":102261,"significance":"If the central claims hold, this is a significant advance: a genetically encodable, water-soluble molecular spin qubit with optical initialization and readout, coherent microwave control, and demonstrated operation in biological cells. The strengths include the internally consistent ODMR data, the successful extrapolation of the spin Hamiltonian model to higher field, the direct observation of coherent Rabi oscillations, and the explicit use of dynamical decoupling. The sensitivity values are clearly labeled as upper-bound estimates rather than measured sensitivities, which is appropriate. The main limitation is that the proposed spin-selective OADF mechanism is not directly demonstrated, and this mechanism is presented as a key novelty of the work.","major_comments":[{"comment":"The OADF readout mechanism is presented as established: the 912 nm pulse drives population from T1 to T2, and spin-dependent reverse intersystem crossing (RISC) to the singlet manifold makes the delayed fluorescence intensity a proxy for the T1 spin state. However, no direct measurement is shown that either the T1–T2 population transfer or the RISC yield is spin-selective. The observed microwave-dependent OADF contrast (Fig. 1e) demonstrates that the detected fluorescence is sensitive to the spin state, but it does not identify which step in the optical cycle provides that sensitivity; it could equally arise from spin-dependent intersystem crossing during 488 nm initialization or from spin-dependent T1 decay, with the 912 nm pulse merely accelerating triplet decay. To support the claimed \"novel spin-readout technique\" and the specific model in Fig. 1c, the authors should report a control with the 912 nm pulse detuned from the proposed T1–T2 resonance (or a wavelength-dependence scan), and ideally a non-optical verification of spin polarization (e.g., time-resolved EPR) before and after the 912 nm pulse. Without such evidence, the mechanistic claim in Fig. 1c is an overinterpretation of the data, even if the empirical spin readout is real.","section":"Protein qubit and spin readout; Fig. 1c"},{"comment":"The room-temperature ODMR contrast is reported with a sign reversal relative to 80 K, and the text attributes this to temperature-dependent spin-dependence of the ISC and/or RISC rates, followed by the statement that the magnetic origin is confirmed by the field-dependence data. While the field dependence does confirm a magnetic resonance, the sign change is left unexplained and the proposed temperature dependence is speculative. Because room-temperature operation is a headline result of the abstract, the authors should either provide additional characterization (e.g., temperature dependence of the OADF contrast or a quantitative model) or explicitly state that the mechanism of the room-temperature contrast is not yet understood. As written, the text overstates the certainty of the physical picture.","section":"Operation at room temperature; Fig. 4"}],"minor_comments":[{"comment":"The DC sensitivity is quoted as 93 pT mol^{1/2} Hz^{-1/2} in the abstract and Discussion, but the main text (Operation at room temperature) reports a DC sensitivity of 98 pT mol^{1/2} Hz^{-1/2} from the measurement; these numbers should be reconciled.","section":"Abstract; Operation at room temperature; Discussion and outlook"},{"comment":"The phrase \"background-free delayed fluorescence photon\" is ambiguous: the delayed fluorescence is detected after the 912 nm pulse with gated detection, but the 488 nm excitation may produce residual fluorescence; please clarify what \"background-free\" means in the measurement sequence.","section":"Protein qubit and spin readout"},{"comment":"The statement that OADF readout is \"approximately five orders of magnitude faster than via the metastable triplet decay\" is not quantitatively defined; if it refers to the readout time, a typical triplet lifetime of milliseconds would imply a readout time of tens of nanoseconds, which is inconsistent with the fluorescence lifetime; specify the comparison.","section":"Protein qubit and spin readout"},{"comment":"The simulation in Fig. 2b sets the T_x–T_y transition amplitude to zero; a justification for this choice (e.g., selection rules or orientation averaging) should be provided in the text or Methods.","section":"Fig. 2 caption and accompanying text"},{"comment":"The faint resonance at approximately 0.9 GHz is attributed to microwave harmonics, but it is also close to 2E = 0.916 GHz; please clarify the assignment and rule out the T_x–T_y transition at that frequency.","section":"Optically-detected magnetic resonance"},{"comment":"The T1 fit with 1/T1 = A T + B T^7 gives small reported errors on A and B, but the text acknowledges that unambiguous identification of the relaxation mechanisms requires further investigation; showing residuals or a comparison with single-power-law fits would prevent the false impression that the two processes are cleanly separated.","section":"Qubit coherence; Eq. (2) and Fig. 3d"},{"comment":"The formula for the effective gyromagnetic ratio γ_eff used to explain the Hahn-echo scaling is stated without derivation; a brief derivation or a reference would help readers assess the validity of the clock-transition model.","section":"Qubit coherence"},{"comment":"The in-cell demonstration is performed at 175 K, not at room temperature; the text should be explicit that the in-cell coherent control is at cryogenic temperatures, as the abstract could be read as implying ambient conditions.","section":"Qubit sensor expressed in mammalian cells"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is likely to attract significant attention given the potential of genetically encodable spin qubits. The central data set (ODMR, Rabi, Hahn-echo, CPMG, in-cell) is internally consistent and compelling. However, the mechanistic claim for the OADF readout (spin-dependent RISC) is not directly proven, and the room-temperature sign flip is unexplained. These issues are fixable through additional experiments or careful revision of the claims, but they currently underpin the paper's main novelty, so major revision is appropriate. Also, the numeric inconsistency in the DC sensitivity (93 vs 98) should be resolved before acceptance."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper delivers what the title promises: the first coherent microwave control and ODMR readout of a triplet spin in a fluorescent protein. The experimental core is internally consistent — D and E fitted at low field extrapolate cleanly to 34.1 mT, Rabi oscillations decay as expected for a powder, CPMG scaling follows a sensible power law, and the in-cell control (on-cell vs. off-cell signal) is well done. That is a real result, and it opens a plausible path to genetically encodable spin sensors.\n\nThe strongest soft spot is the OADF readout mechanism. The paper asserts spin-dependent reverse intersystem crossing from a 912 nm-pumped higher triplet, but the spin selectivity is inferred from TDDFT and spectral overlap rather than directly demonstrated. The stress-test note is right that there is no detuned-912 nm control and no non-optical verification of spin polarization before and after the pulse. However, the existence of 44% ODMR contrast and of Rabi oscillations itself proves the detected signal is spin-sensitive — if the 912 nm step were spin-independent, you would not see any microwave-induced contrast. So the concern is not whether the readout works, but why it works. That is a mechanistic gap, not a load-bearing flaw.\n\nThe room-temperature sign flip is a genuine puzzle. The RT sensing demonstration relies on a mechanism whose temperature dependence is not understood, and the paper offers no explanation beyond \"temperature dependent.\" A referee should ask for more, but it does not invalidate the cryogenic results. The sensitivity numbers are upper-bound estimates from measured parameters, not full sensitivity measurements; the paper is honest about that.\n\nI could not audit the Methods or Extended Data, so error propagation and source data remain unverified. Even so, the main claims are empirically grounded and the analysis is unusually careful — the T1 fit includes the caveat that the exact relaxation mechanisms are not uniquely identified, which is the right tone.\n\nWho is this for? Anyone working on molecular spin qubits, bioquantum sensing, or fluorescent protein photophysics. It deserves a serious referee: the claims are important, the experiments are substantial, and the main uncertainty is mechanism, not existence. I would send it out and ask for direct evidence of the spin-selective step, plus a deeper look at the room-temperature sign flip.","headline":"A genuine first step: coherent, optically read spin control in a fluorescent protein, with a readout mechanism that needs direct proof but whose empirical core is credible.","tokens_in":11400,"tokens_out":2735,"would_cite":true,"duration_ms":28860,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A fluorescent protein can act as an optically addressable spin qubit, with a 912-nanometer pulse enabling spin readout at up to 44 percent contrast.","keywords":["fluorescent protein","spin qubit","optically detected magnetic resonance","optically activated delayed fluorescence","EYFP","quantum sensing","triplet state","molecular qubit"],"falsifier":"Run the same ODMR experiment while tuning the 912-nanometer flash across the predicted absorption band of the higher triplet state: the spin contrast should follow the absorption spectrum, and the emitted spectrum should match normal fluorescence. If the contrast persists when the flash is detuned from that band, or appears for molecules lacking the triplet, the spin-selective readout mechanism is wrong.","tokens_in":10310,"feed_emoji":"🧬","tokens_out":7019,"duration_ms":60466,"temperature":0.7,"pith_summary":"This paper reports that Enhanced Yellow Fluorescent Protein (EYFP) can serve as an optically addressable spin qubit: its metastable triplet state can be initialized by light, manipulated by microwaves, and read out on demand. The key advantage is that EYFP is a genetically encodable protein, so the qubit can be attached to biological molecules and expressed inside cells, unlike diamond-based nitrogen-vacancy sensors. The authors demonstrate spin contrast up to 44% at liquid-nitrogen temperatures, coherent Rabi oscillations, a coherence time of 16 microseconds under dynamical decoupling, and room-temperature optically detected magnetic resonance in water. If correct, this would give the life sciences a scalable, genetically targetable quantum sensor platform for nanoscale magnetic-field measurements.","feed_headline":"Fluorescent protein EYFP becomes an optically addressable spin qubit","feed_subtitle":"Genetically encodable qubit keeps coherent control inside cells and senses fields in water at room temperature.","key_machinery":"The load-bearing object is the optically activated delayed fluorescence (OADF) readout: a 912 nm pulse transfers the metastable triplet T1 to a higher triplet T2, and spin-dependent reverse intersystem crossing funnels population back to S1, producing a fluorescence photon whose yield depends on the T1 spin sublevel population. This turns the protein's fluorescence into a spin-state meter and makes readout roughly five orders of magnitude faster than waiting for spontaneous triplet decay. The second pillar is the spin-1 Hamiltonian with zero-field splitting parameters D and E, which fits the powder-averaged ODMR spectra and gives the transition frequencies used for coherent control and field sensing.","core_discovery":"The paper claims to have turned the metastable triplet state of EYFP's chromophore into a working spin-1 qubit. Initialization is by a 488 nm pulse that pumps the molecule into a spin-polarized T1 state via intersystem crossing; a 912 nm pulse then drives T1 to a higher triplet (assigned T2), where spin-dependent reverse intersystem crossing returns population to S1, emitting delayed fluorescence whose intensity reports the spin state. Using this optically activated delayed fluorescence readout, the authors measure ODMR with zero-field splitting parameters D=(2π)×2.356 GHz and E=(2π)×0.458 GHz, see Rabi oscillations, and extract a spin-lattice relaxation time of 141 μs at 80 K and a CPMG coherence time of 16 μs. The same readout works at room temperature in aqueous solution with about 3% contrast, and coherent control is preserved when the protein is expressed in human kidney cells. The central claim is that fluorescent proteins are viable optically addressable spin qubits, opening a genetically encodable sensing platform.","pith_inferences":["The same readout should work in the many fluorescent proteins and dye molecules that already show optically activated reverse intersystem crossing, potentially making spin sensing a standard feature of fluorescent tags.","The sign reversal of spin contrast between 80 K and room temperature suggests the spin selectivity of intersystem crossing and reverse intersystem crossing is temperature dependent; if so, the ratio of ODMR contrasts at two temperatures could serve as a local thermometer or a probe of the chromophore's thermal environment.","Because the qubit is genetically encoded, directed evolution could be used to screen protein variants with longer coherence or higher contrast, a route not available for solid-state defects.","Single-molecule OADF could be combined with existing super-resolution microscopies to perform spin measurements at the single-protein level; the paper's photon budget implies this is plausible but it is not demonstrated."],"forward_implications":["If the mechanism holds, EYFP and other fluorescent proteins with long-lived triplet states become genetically encodable spin qubits, targetable to specific biomolecules by standard fusion-protein methods.","The OADF readout can in principle be applied to any molecule with a singlet-triplet-singlet ladder and suitable reverse intersystem crossing, potentially expanding the family of optically addressable molecular qubits far beyond EYFP.","At 80 K the qubit offers an AC magnetic-field sensitivity upper bound of 183 fT mol^{1/2} Hz^{-1/2}; at room temperature the DC upper bound is 93 pT mol^{1/2} Hz^{-1/2}, enough to detect roughly 20 nT fields from proton spins at a distance of 5 nm.","The demonstration in mammalian cells indicates that spin and optical control survive the intracellular environment, supporting in-cell sensing applications.","Sensitivity could be substantially improved by deuteration, better collection optics, and fluorescence cycling, with at least a 500-fold improvement projected by the authors."],"supporting_citations":[{"why":"Supplies the prior suggestion that fluorescent proteins host a metastable triplet state, the physical object the qubit is built from.","marker":"[9]"},{"why":"Shows forward and reverse intersystem crossing in green fluorescent proteins at cryogenic temperatures, grounding the initialization and readout photophysics.","marker":"[33]"},{"why":"Provides the optically activated delayed fluorescence method and dark-state engineering that the 912 nm readout scheme adapts.","marker":"[34]"},{"why":"Documents reverse intersystem crossing effects on fluorescence brightness, supporting the spin-dependent RISC pathway assumed by OADF.","marker":"[35]"},{"why":"Gives the powder EPR formalism and spin Hamiltonian used to model the ODMR spectra and extract D and E.","marker":"[36]"},{"why":"Supplies the dynamical-decoupling scaling argument used to interpret CPMG coherence enhancement and the T2 scaling with pulse number.","marker":"[40]"}],"fun_headline_variants":["Genetically encodable spin qubit from a fluorescent protein","EYFP: a protein that works as a spin qubit sensor","Spin qubit in a fluorescent protein for bio-sensing","Fluorescent protein EYFP becomes a spin qubit with coherent control","Protein-based spin qubit: EYFP shows coherent spin control"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The entire readout depends on the assumption that a 912-nanometer flash takes the protein's long-lived dark state to a higher dark state, and that the return to the bright state is sensitive to the electron-spin orientation, so the flash's emitted light reports the spin.","fun_headline_variants_meta":{"raw":{"variants":["Genetically encodable spin qubit from a fluorescent protein","EYFP: a protein that works as a spin qubit sensor","Spin qubit in a fluorescent protein for bio-sensing","Fluorescent protein EYFP becomes a spin qubit with coherent control","Protein-based spin qubit: EYFP shows coherent spin control"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000239,"raw_usage":{"total_tokens":1608,"prompt_tokens":1134,"completion_tokens":474,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":750,"completion_tokens_details":{"reasoning_tokens":385}},"tokens_in":750,"tokens_out":474,"duration_ms":5093,"temperature":1.0,"reasoning_tokens":385,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T12:48:55.013137+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same ODMR experiment while tuning the 912-nanometer flash across the predicted absorption band of the higher triplet state: the spin contrast should follow the absorption spectrum, and the emitted spectrum should match normal fluorescence. If the contrast persists when the flash is detuned from that band, or appears for molecules lacking the triplet, the spin-selective readout mechanism is wrong.","supporting_citations":[{"cited_title":"Nature reviews","cited_arxiv_id":null,"evidence_quote":"Supplies the prior suggestion that fluorescent proteins host a metastable triplet state, the physical object the qubit is built from."},{"cited_title":"Journal of the American Chemical Society, https://doi.org/10.1021/jacs.4c11116","cited_arxiv_id":null,"evidence_quote":"Shows forward and reverse intersystem crossing in green fluorescent proteins at cryogenic temperatures, grounding the initialization and readout photophysics."},{"cited_title":"The journal of physical chemistry","cited_arxiv_id":null,"evidence_quote":"Provides the optically activated delayed fluorescence method and dark-state engineering that the 912 nm readout scheme adapts."},{"cited_title":"The journal of physical chemistry","cited_arxiv_id":null,"evidence_quote":"Documents reverse intersystem crossing effects on fluorescence brightness, supporting the spin-dependent RISC pathway assumed by OADF."},{"cited_title":"Chemphyschem: a European journal of chemical physics and physical chemistry, 9(4):612–624","cited_arxiv_id":null,"evidence_quote":"Gives the powder EPR formalism and spin Hamiltonian used to model the ODMR spectra and extract D and E."},{"cited_title":"The Review of scientific instruments, 29(8):688–691","cited_arxiv_id":null,"evidence_quote":"Supplies the dynamical-decoupling scaling argument used to interpret CPMG coherence enhancement and the T2 scaling with pulse number."}],"review_version":1}