We re-examine whether the internal-clock Page–Wootters (PaW) coupling of an optical lattice clock contributes an additional gravitational-redshift bias when the clock's center of mass (COM) is delocalized in a gravitational potential. We first identify and correct a dimensional error in a frequently used branch propagator: the mass-defect interaction must read H_int=(H_c/mc^2)V_k, so the gravitational and motional branch unitaries are U_g/e=exp(± i (omega_0/2mc^2) V_k t), with a dimensionless coefficient beta=omega_0/(2mc^2) ~ 10^23 J^-1. A propagation factor exp(± i V_k t/(hbar m c^2)) used in earlier estimates carries units of 1/J and is numerically ~2/(hbar omega_0)~10^19 too large, producing spurious order-unity phase wrapping. Using the corrected propagator, exact diagonalization on a Fock basis for ^24Mg and ^87Sr lattice clocks shows that (i) the unconditional reduced-COM mean of V_k equals the bare expectation to numerical accuracy (~10^-42 J), a trivial V_k-with-its-own-exponential identity; (ii) the internal–COM entanglement entropy remains below 10^-3 bit at interrogation times up to 1 s; (iii) the only nonzero PaW quantity is a conditional mean shift whose well-conditioned (high-norm) part scales as beta^2 t^2 Cov(V_k,V_k^2)/(mc^2) ~ 10^-22 (Mg, t=50 ms) and oscillates at the optical frequency omega_0 (period ~2 fs); (iv) this conditional shift averages to ~Delta omega/omega_0 ~10^-6 under any phase-insensitive optical readout and is therefore not a clock observable; (v) finite-temperature thermal moments scale the shift as ~T^3 but remain below 10^-25 at 1 mK. We conclude that internal PaW entanglement introduces no resolvable redshift bias at current or near-future optical clocks; Hu-type COM delocalization decoherence remains the leading quantum-relativistic correction. We provide a full parameter-space exclusion map and a sensitivity roadmap. All numerical results in this paper are illustrative/synthetic, computed with the corrected propagator and realistic Mg/Sr parameters.
We re-examine whether the internal-clock Page–Wootters (PaW) coupling of an optical lattice clock contributes an additional gravitational-redshift bias when the clock's center of mass (COM) is delocalized in a gravitational potential. We first identify and correct a dimensional error in a frequently used branch propagator: the mass-defect interaction must read H_int=(H_c/mc^2)V_k, so the gravitational and motional branch unitaries are U_g/e=exp(± i (omega_0/2mc^2) V_k t), with a dimensionless coefficient beta=omega_0/(2mc^2) ~ 10^23 J^-1. A propagation factor exp(± i V_k t/(hbar m c^2)) used in earlier estimates carries units of 1/J and is numerically ~2/(hbar omega_0)~10^19 too large, producing spurious order-unity phase wrapping. Using the corrected propagator, exact diagonalization on a Fock basis for ^24Mg and ^87Sr lattice clocks shows that (i) the unconditional reduced-COM mean of V_k equals the bare expectation to numerical accuracy (~10^-42 J), a trivial V_k-with-its-own-exponential identity; (ii) the internal–COM entanglement entropy remains below 10^-3 bit at interrogation times up to 1 s; (iii) the only nonzero PaW quantity is a conditional mean shift whose well-conditioned (high-norm) part scales as beta^2 t^2 Cov(V_k,V_k^2)/(mc^2) ~ 10^-22 (Mg, t=50 ms) and oscillates at the optical frequency omega_0 (period ~2 fs); (iv) this conditional shift averages to ~Delta omega/omega_0 ~10^-6 under any phase-insensitive optical readout and is therefore not a clock observable; (v) finite-temperature thermal moments scale the shift as ~T^3 but remain below 10^-25 at 1 mK. We conclude that internal PaW entanglement introduces no resolvable redshift bias at current or near-future optical clocks; Hu-type COM delocalization decoherence remains the leading quantum-relativistic correction. We provide a full parameter-space exclusion map and a sensitivity roadmap. All numerical results in this paper are illustrative/synthetic, computed with the corrected propagator and realistic Mg/Sr parameters.
We study the metrological extractability of a relative gravitational-redshift parameter delta encoded in Page–Wootters (PaW) historical states of a clock C and a two-level probe S. Writing the redshifted history state as |Psi(delta)> = exp(-i delta G)|Psi(0)>, with G = dt sum_n n |n><n| (x) H_S, we obtain the exact finite-N quantum Fisher information F_Q = (omega_0 dt)^2 (N-1)(2N-1)/6 tending to (omega_0 T)^2/3 in the continuous limit. Two results follow. First, F_Q vanishes for separable clock-probe states, so that a non-zero joint quantum Fisher information for the relative redshift is equivalent to C–S entanglement entering the correlations; separable-state redshift dynamics, present in earlier PaW treatments, nevertheless survives without providing any joint estimability. Second, a deterministic protocol that reads the clock record n and then performs Ramsey readout on S attains the joint QFI exactly, with no post-selection overhead, yielding a factor ~26 advantage over measuring the reduced probe alone for our toy model. With state-of-the-art optical-lattice clocks (Sr-87, T = 1 s), a 1 m height difference becomes resolvable in tens of shots, placing the PaW resource picture in the same regime as cm-scale clock-array experiments. We contrast this with history-state fidelity, a basis-dependent diagnostic, and discuss decoherence, GHZ scaling and an error budget.
For a finite-mass quantum reference frame, centre-of-mass spreading and gravitational self-interaction define two Newtonian dephasing channels whose engineering crossover is operationally negligible for realistic optical probes; we recast this null result as a rate-based exclusion contour and experimental roadmap. We derive the operational coherence after tracing the frame, for an entangled probe split by internal energy Delta E_S, under (i) the Pikovski-type centre-of-mass wavepacket spread, which is mass-independent and grows as t^2, and (ii) a self-gravity channel in which the internal clock trap-position uncertainty sigma_c produces Gaussian dephasing linear in frame mass m and interrogation time. For a harmonic trap with correlation time tau_c = 1/omega_trap approx 1.6 microseconds (100 kHz trap), the laboratory interrogation T = 1 day satisfies T >> tau_c, so the white-noise Markovian regime applies and channel B grows linearly in t. The 1%-operational-dephasing contour is the analytic line Delta E_S,crit = 0.142 hbar R^2 c^2 /(G m sigma_c T). Optical probes (Delta E_S ~ 10^-19 J) require frame masses of order 10^7 kg, while nuclear/gamma probes (Delta E_S ~ 10^-13 J) require only tens of kg. We compute the engineering crossover m* = sqrt(hbar T R c^2 /(2 G sigma_0 L)) = 0.248 +/- 0.03 kg, the finite-temperature trap width, and Monte-Carlo uncertainty. No new physics is claimed; the result is a decidable null bound and a roadmap for next-generation experiments. PACS numbers: 03.65.Ta, 04.60.m, 03.65.Ud, 04.80.-y
This paper extends the derivation of the Lorentz transformation from count-based observables to the quantum level. First, each ingredient of the Bondi derivation is mapped onto the Page-Wootters (PaW) framework, and the k-factor is extracted as a ratio of PaW clock POVM eigenvalues — completing the derivational chain from Ĥ|Ψ⟩ = 0 to t′ = γ(t − vx/c²). Second, the k-factor is promoted to an operator k̂ = (Ĥ_cm + p̂c)/(mc²), revealing a two-sector decomposition of the Lorentz transformation: a time dilation sector (k̂ + k̂⁻¹ = 2E/(mc²), nonlinear in momentum) and a simultaneity sector (k̂ − k̂⁻¹ = 2pc/(mc²), linear in momentum). Third, the Bondi decomposition is applied to the proper time ω₀τ = ω₀Lm/p of a clock traversing baseline L, showing that the standard result separates into a time dilation contribution ω₀(Lm/p + Lp/(mc²)) and a simultaneity contribution −ω₀Lp/(mc²), which sum to the proper time. The two sectors have different behavior under quantum averaging: the simultaneity sector, being linear in p, is protected — its mean receives no corrections from momentum spread σ_p for any symmetric distribution. The time dilation sector, being nonlinear, produces mean corrections at order σ_p²/(m²c²). This asymmetry clarifies the structure of the Pikovski decoherence mechanism: momentum-dependent dephasing of internal clocks originates entirely from the time dilation sector's nonlinearity, while the simultaneity sector contributes a deterministic offset protected by linearity. All corrections vanish in the classical limit σ_p → 0, recovering the companion paper's Bondi derivation exactly.
I. Introduction The problem of time in quantum gravity is the following. General relativity is a diffeomorphisminvariant theory: there is no preferred time variable, and the Hamiltonian constraint HΨ = 0 impliesthat all physical states are ‘frozen’ in the sense that they carry no manifest time dependence [Dirac1964; DeWitt 1967]. Yet quantum mechanics requires time as a background parameter againstwhich evolution is measured. Reconciling these two facts — the timelessness of diffeomorphisminvariant quantum gravity and the temporal structure required for quantum measurement theory—is the problem of time [Isham 1993; Kuchăř 1992]. The problem is not merely philosophical: ithas concrete consequences for the interpretation of quantum cosmological perturbations, includingthose that seed the CMB anisotropies we observe today.One of the most elegant proposed resolutions is the Page–Wootters (PW) mechanism [Page & Wootters 1983]: time is not a background parameter but a relational quantity, encoded in correlationsbetween a ‘clock’ subsystem and the rest of the universe. Physical time is the reading of the clock,and quantum evolution is the conditional state of the system given a particular clock reading. Thisidea has been placed on rigorous mathematical footing by Höhn, Smith & Lock (HSL) [2021], whoproved the equivalence of three apparently distinct quantization procedures — the Dirac constraintquantization, the Page–Wootters mechanism, and the deparametrized Heisenberg picture — undera precise set of conditions. The HSL result reduces the problem of time, for any system satisfyingthose conditions, to the problem of identifying a clock with the right algebraic properties.In Loop Quantum Cosmology (LQC), relational time has been implemented using material degreesof freedom: a minimally-coupled massless scalar field [Ashtekar, Pawlowski & Singh 2006], a GFTcondensate [Gielen & Sindoni 2016], or dust fields [Brown & Kuchăř 1995; Pawłowski 2014]. Allof these are material clocks: dynamical degrees of freedom that must be added to, or identified1within, the matter content of the model. No worked example exists in which the clock arises fromthe algebraic structure of the kinematic Hilbert space itself — as an invariant of the theory’s gaugegroup rather than an additional field. Such an algebraic clock would be conceptually preferable: itrequires no extension of the Hilbert space, its back-reaction on geometry is controlled, and it is notsubject to the multiple-choice ambiguity [Kuchăř 1992] that afflicts material clock choices when thesystem has multiple degrees of freedom.This paper provides the first such example. The LQC bounce, treated via the Pöschl–Teller (PT)effective potential established in the companion series [Hillard 2026a–d], admits a spectral threshold = √2 that is fixed by the zero of the SU(2) Casimir polynomial W() = ³(²−2)/[C·sinh(/2)],where C = 3.60329… is the Hillard constant [Hillard 2026a, Paper 8]. This zero is a representationtheoretic invariant of the kinematic Hilbert space: it exists prior to any dynamical input, beforeany clock is chosen. We show that instantiates the full HSL relational clock construction. TheClock Decomposition Theorem (Section III) establishes the tensor product factorization H̃_kin H H and constructs the covariant POVM on H. The Temporal Trinity Formula (Section IV)is identified as the HSL clock reading in all three quantization pictures, with explicit clock-changemap T (Appendix A). The result holds exactly: is not a perturbative approximation to a clockbut a genuine algebraic invariant.A relational clock that is an algebraic invariant makes a concrete, falsifiable prediction. The signchange multipole * — the CMB multipole at which the LQC bounce imprint changes sign in thepower spectrum [Hillard 2026a] — is precisely the PW temporal origin in Fourier space: * =/_B. When GFT interactions of strength are included, the PT potential is deformed to U_ =−(2−)sech²(), which remains within the exactly solvable PT family (Section VI). The thresholdshifts to () = √(2−), giving the exact non-perturbative prediction:()/ = √(1 − /2) − 1 −/4 + O(²).* (1)*The sign is negative: GFT interactions push * to lower multipoles. The detection threshold forLiteBIRD is _crit 4/*, placing the prediction within reach of the near-term CMB survey (SectionV). This is the first quantitative prediction from a relational clock interpretation of the LQC bounce.A word on the relationship between this paper and the companion series [Hillard 2026a–d]. ThePWinterpretation of the bounce established here depends on W() only through its functional form:W() ³(²−2). Any function with that polynomial zero structure has a scattering threshold at =√2, and any such function in the Hamiltonian constraint generates the relational clock constructionof Sections III–IV. The validity of the LQG derivation of W in [Hillard 2026a] is therefore not aprerequisite for the results of this paper. The companion series and the present paper are mutuallyreinforcing rather than circularly dependent: [Hillard 2026a] provides the physical motivation andprecise form of W; this paper provides the relational interpretation that explains why has thevalue it has.The paper is organised as follows. Section II reviews the relevant background on LQC, the PTeffective potential, and the HSL formalism. Section III establishes the Clock Decomposition Theorem: spectral factorization (Proposition 1), covariant POVM, and temporal origin. Section IVidentifies the Temporal Trinity Formula in all three HSL pictures (Lemma 1, Proposition 2). Section V derives the physical inner product by group averaging and the full observational prediction(Table 2). Section VI proves that the intertwiner deformation is shape-invariant and promotes theprediction to an exact formula (Proposition 3). Appendix A constructs the clock-change map Texplicitly (Proposition A.1, Corollary A.1).22Main results at a glanceClock Decomposition Theorem (Section III): H̃_kin H H + O(). POVM E(d) =|W()|²/‖W‖² d is covariant. = √2 is the temporal origin.Trinity Formula (Section IV): N_total = N + ln(··k_piv/) is a Dirac observable, clockchange invariant. T acts as identity on (Appendix A).Exact prediction (Sections V–VI): ()/ = √(1−/2) − 1 −/4. LiteBIRD detection threshold:_crit 4/*.First algebraic PW clock in LQC (Section V.4): is an SU(2) Casimir invariant, not amaterial d.o.f. Back-reaction is O(); no Hilbert space extension needed.
Optical ion clocks now approach a regime in which relativistic time dilation is no longer exhausted by a classical proper-time parameter read by a quantum sensor. Recent work by Sorci, Foo, Leibfried, Sanner, and Pikovski shows that harmonically trapped optical clock atoms can exhibit vacuum-induced, squeezing-induced, and intrinsically quantum corrections to the second-order Doppler shift, with time-dilation-induced entanglement between motional and internal clock degrees of freedom becoming observable in strongly squeezed trapped-ion motion. This paper develops the significance of that result within CPT-Coherence Theory. The central principle of CPT-Coherence is Onto-Physical Entanglement: what a system is cannot be separated from how it is allowed to remain coherent under admissible transformation. In the clock setting, this principle appears in branch-visible form. The internal clock transition is not merely an isolated label transported through spacetime; its phase identity is dynamically bound to the motional state through the relativistic proper-time operator. Proper-time interferometry therefore supplies a laboratory instance of a more general structural rule: physical identity is stabilized only through the admissible evolution channels by which it can be read out. The paper distinguishes the relativistic proper time of the ion-clock experiment from the coherence-depth coordinate of CPT-Coherence. The former is a branch-visible observable associated with worldline evolution; the latter is a structural coordinate organizing parent-boundary admissibility, trace access, Schur response, and stable observable descent. Their relation is not identity but projection: quantum proper-time readout provides an experimentally precise interface through which branch observers can test how internal identity, motion, coherence, and record formation remain coupled. We formulate a CPT-Coherence clock-readout sector in which a quantum clock is represented by a triple consisting of an internal identity generator, an external motion algebra, and a proper-time readout operator. The Sorci-Foo-Leibfried-Sanner-Pikovski protocol is then interpreted as a high-precision realization of an OPE readout channel: classical time dilation corresponds to scalar proper-time averaging; vacuum and squeezing shifts correspond to state-dependent branch corrections; and visibility loss corresponds to trace-visible entanglement between identity and admissible motion. This establishes optical ion clocks as a natural experimental grammar for CPT-Coherence, while CPT-Coherence supplies a unifying interpretation of why such experiments probe not simply clock rates, but the inseparability of physical being and physical evolution. Scope note: the quantitative predictions below coincide with those of standard relativistic quantum clock theory; CPT-Coherence contributes an interpretation and vocabulary, together with an explicit falsification target for that interpretation.
In this work, we study the emergence of relativistic effects in a composite quantum clock within the Page-Wootters relational formulation of quantum mechanics. We consider a system with internal and center-of-mass degrees of freedom and analyze the conditioned evolution of the center-of-mass relative to the internal system treated as an internal clock. We show that the internal sector exhibits an effective time dilation. Retaining the full mass-energy structure of the composite particle, a back-reaction of the internal energy on the center-of-mass sector appears through an operator-valued normalization factor. As a direct consequence of the back-reaction, the conditioned center-of-mass dynamics is governed by a Schroodinger equation that is non-local in the clock time, with an effective temporal non-locality of the order of the Compton time of the composite system. Applying the formalism to a center-of-mass momentum superposition, we find that the interferometric visibility acquires a correction that is quadratic in the internal energy, and therefore depends on the absolute distribution of clock energies rather than only on the energy gaps that control the standard time-dilation dephasing. Although this correction is parametrically small, it is a generic feature of relativistic composite clocks and can, in principle, be isolated through a differential visibility measurement comparing clocks with equal transition frequency but different mean internal energies.