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Time-bin / path photonic encoding enc_timebin

Photonic dual-rail qubit in arrival-time bins or waveguide path; loss is the dominant error, and it is heralded — an erasure at a known location — rather than a silent error.

Time-bin / path photonic encoding

enc_timebin · layer 2 Encoding · demonstrated · since 1999
Open on the map

Photonic dual-rail in time or path; loss is the error and it is heralded.

Brief →

Key references

[418] J. Brendel, N. Gisin, W. Tittel, and H. Zbinden, “Pulsed Energy-Time Entangled Twin-Photon… · 1999
[169] K. Alexander et al., “A manufacturable platform for photonic quantum computing,” Nature, v… · 2025
[264] M. C. Löbl, L. A. M. Pettersson, J. Dragašević, S. X. Chen, and O. A. D. Sandberg, “The su… · 2026
[179] S. Bartolucci et al., “Fusion-based quantum computation,” Nat. Commun., vol. 14, Art. no. … · 2023
[420] Toshiba Europe, “News Hub - The Latest Quantum Security News from Toshiba,” Toshiba Quantu…

Design space — attributes

(a) carrier affinity
photon (natural particle, engineered modes)
(b) time · entangling
—
(c) readout
—
(d) mobility
—
(e) control
—
(f) error structure
loss (erasure)
(g) manufacturing
—

Evaluation space — dated attributes

no dated attribute

Origin: Brendel, Gisin, Tittel and Zbinden, Phys. Rev. Lett. 82, 2594 (1999). No mobility of its own — the attribute is 'none', inherited from the host carrier.

Actors & goals — annotations

Fusion-based photonic (FBQC) — PsiQuantum (Omega), Quandela, QuiX · G4 G6

Edges

needs: Single photon (discrete variable)
° one of several that would do · the usual route, not a strict need

Used by 4 machines (3 primary · 1 alternate)

↗ its register card · paper / product / press: links that name the machine (attribution checked on 20 Sep 2026)

Pictures

Every picture carries the story of what it shows and its credit line.

Fig. 1 — Fig. 1: Working principle and fabrication result of LNOI circuit.a CW pump light (λp = 775 nm) is coupled to the chip via grating couplers and split…
Six-panel paper figure of a thin-film lithium-niobate photonic chip that makes and manipulates entangled photon pairs: a render of the circuit from pump splitting through photon-pair generation in periodically poled waveguides to reconfigurable interferometers and projection; a photograph of the packaged chip on a circuit board (1 cm scale bar); micrographs of the poled region and of one Mach–Zehnder interferometer with its heater; the pump settings that yield |00⟩, |11⟩ or a Bell state; and the equivalent two-qubit circuit. From a 2026 Light: Science & Applications paper by Andreas Maeder, Rachel Grange and co-authors. It shows a programmable linear-optical interferometer and its photon source on one chip. “Fig. 1 — Fig. 1: Working principle and fabrication result of LNOI circuit.a CW pump light (λp = 775 nm) is coupled to the chip via grating couplers and split using a…” by Maeder et al., www.nature.com/articles/s41377-025-02150-z, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
Fig. 1 — Figure 1: – Conceptual schematic. a Schematic of the integrated optical circuit. The evolution of the input photon state is illustrated for the thre…
A low-resolution drawing, not a photograph. (a) A thin-film lithium niobate chip: a time-bin photon (two pulses 100 ps apart) enters an electro-optic Mach–Zehnder switch with gold electrodes and a thermal phase shifter, which routes the early or late pulse into the long or short arm of an unbalanced interferometer (100 ps delay, phase electrode θ) feeding two detectors; rows i–iii show three switch drive settings and the resulting measurements. (b) Bloch sphere of the measured time-bin states. It is Fig. 1 of arXiv 2604.16695 (April 2026) by Andrea Bernardi, … Daniele Bajoni, on a gigahertz-rate receiver for time-bin quantum communication. It shows photonic switching at single-switch scale. “Fig. 1 — Figure 1: – Conceptual schematic. a Schematic of the integrated optical circuit. The evolution of the input photon state is illustrated for the three…” by Andrea Bernardi et al., arxiv.org/abs/2604.16695, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
Fig. 6 — Figure SF6: Experimental setup and device. (a) Schematic of the experimental setup. LPF: long-pass filter, BPF: bandpass filter, SNSPD: superconduct…
(a) A drawing of the setup: 775 nm laser pulses pass an unbalanced interferometer with delay τ, making early and late time bins, then a thin-film lithium niobate chip, filters, superconducting nanowire single-photon detectors and a time tagger; (b) a microscope image of a periodically poled waveguide; (c) an optical micrograph of the chip, with couplers, spiral delays and gold electrodes; (d) a photograph of the chip on a copper holder, wire-bonded to a circuit board. It is Figure SF6 of arXiv 2407.04048 by Finco, Miserocchi, Maeder, Kellner, Sabatti, Chapman and Grange. The poled waveguide makes each photon pair in a superposition of early and late: time-bin entanglement. “Fig. 6 — Figure SF6: Experimental setup and device. (a) Schematic of the experimental setup. LPF: long-pass filter, BPF: bandpass filter, SNSPD: superconducting…” by Giovanni Finco et al., arxiv.org/abs/2407.04048, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
enc_timebin · layer 2 Encoding · centrality 1 · demonstrated · since 1999 · updated 2026-09-03

Time-bin / path photonic encoding

Photonic dual-rail qubit in arrival-time bins or waveguide path; loss is the dominant error, and it is heralded — an erasure at a known location — rather than a silent error.

Verdict. The two halves have diverged: path encoding won on chip (PsiQuantum Omega), time-bin survives in fibre, because an on-chip delay costs 0.27 dB per ns. No fusion logical qubit by 2028 leaves it a communication encoding.

Key references

Λ = error-suppression factor per code-distance step; QBI = DARPA Quantum Benchmarking Initiative (Stage A concept → B R&D plan → C government V&V); G1–G7 = the report's goal classes (see Actors & economics).

Identity & lineage

One photon, two orthogonal modes: two arrival-time bins, or two waveguides. Nothing continuous decoheres, so the dominant error is loss — and a lost photon fails to herald, detectable where it happens. Time-bin entanglement came from Brendel, Gisin, Tittel and Zbinden at Geneva in 1999, built for telecom fibre D[418]; path encoding is the on-chip descendant, and Omega says so plainly — "a path-encoded qubit using a heralded photon and two-mode interferometers" D[169]. Attributes: no mobility, no control modality, no fabrication of its own; pure-loss error.

Physics & limits

The floor is a loss budget, not an infidelity: every interferometer, coupler and switch removes amplitude, and the two halves of this node price it differently. A 1 ns time bin needs ~0.15 m of on-chip delay at group index 2, which at PsiQuantum's measured 1.8 ± 0.2 dB/m SiN loss D[169] costs ~0.27 dB — over half a ~0.5 dB per-photon budget, against ~0.04 mdB in fibre. That is why chips are path-encoded and fibres time-bin encoded — one node only in the abstract. The erasure advantage needs a trusted herald: Omega's median on-chip detector efficiency is 93.4% D[169], so about one heralded event in fifteen is an unflagged loss entering the code as ordinary error.

Engineering state of the art

table
Date Figure Who Tag
2025-02 Fusion Bell fidelity 99.22 ± 0.12%, chip-to-chip 99.72 ± 0.04% PsiQuantum D[169]
2025-02 Single-mode SiN loss 1.8 ± 0.2 dB/m; on-chip SNSPD 93.4% PsiQuantum D[169]
2026-06-26 Unencoded 6-ring loss threshold 0.38–0.82% Löbl et al. S[264]

No logical qubit exists in this encoding, and the threshold it must meet is disputed: 2.7% per photon for a boosted 6-ring and 17.4% for a {7,4}-encoded state S[179] against 0.38–0.82% unencoded S[264] — an order of magnitude apart, unreconciled fact.

The main report's "waveguide loss 0.5 dB/m" is not the Omega single-mode silicon-nitride figure, 1.8 ± 0.2 dB/m fact. Loss thresholds for 6-ring fusion differ by an order of magnitude between sources, unreconciled fact. No platform has demonstrated syndrome extraction with heralded loss. The 0.27 dB per nanosecond of on-chip delay is derived from published loss figures, not measured. Not published: whether Toshiba's 2025–26 deployments use time-bin encoding; its public pages do not say so [420].

Manufacturing, materials & supply chain

No fabrication step of its own: the encoding rides its host — PsiQuantum's 300 mm silicon nitride, Quandela's III-V quantum dots, QuiX's SiN meshes. Control is inherited: no per-qubit drive line, but every interferometer needs a phase shifter and every output a detector — thousands of cryogenic channels and their fan-out at 10³–10⁴ qubits. Chokepoints are the detector vendors (Single Quantum, ID Quantique, Photon Spot) and the III-V and SiN foundries Pfact; ECCN 4A906 applies fact.

Role in the stack

The encoding under the fusion-based discrete-variable architecture (PsiQuantum, Quandela, QuiX); it needs only a single-photon source, and its one rival there is the dual-rail encoding, primary on PsiQuantum's Omega and QuiX's Carina. Swapping path for time-bin trades interferometer phase stability for delay-line loss, unaffordable on chip. It also caps fusion: passive linear-optical Bell measurement on a dual-rail qubit succeeds at 50%, at least 75% with unentangled ancillae S[419]. Verification: no group has run a syndrome cycle using heralded loss as an erasure flag, so the advantage is a design argument, not a measurement.

Actors & economics

Who.
Organisation Role Country What they do Evidence
PsiQuantum developer US Path-encoded qubits on Omega D[169]
Quandela developer FR Photonic QPUs on quantum-dot sources Cfact
QuiX Quantum developer NL Carina core delivered to DLR, dual-rail encoded C[340]
Sparrow Quantum supplier DK Only merchant single-photon source vendor P[342]

Money.
2025-04-10 · Sparrow Quantum · Series A · EUR 21.5 M · lead undisclosed · closed P[342]
2025-09-10 · PsiQuantum · Series E · USD 1 B at USD 7 B · BlackRock, Temasek · closed Cfact
2026-07-22 · PsiQuantum · DARPA QBI Stage C expansion · USD 125 M · DARPA · announced Pfact

Market & supply chain. Sparrow is the only merchant source vendor, shipping 20–35% system efficiency against a 71.2% laboratory record P[342], so every fusion architecture pays a multiplexing tax set by one supplier's yield. Pays for G1/G2, is PsiQuantum's G3–G4 substrate, and pays separately for G6 through QKD.

IP & standards. ORCA holds US 12,437,225 on linear-optical encoded GHZ measurements (granted 2025-10-07), the only photonic dual-rail family a dated search surfaced G[407]; no computing-side standard exists.

Roadmaps & track record. PsiQuantum has published no hardware result since Omega, so 2026 progress is legible only through DARPA V&V Pfact. QuiX promised a universal machine for 2026 and delivered the core, not the machine C[340]. Quandela missed its 2025 first-logical-qubit milestone, still targeting 50 by 2028 R.

Strategic reading. Whoever cuts system-level loss wins the fusion race — likely whoever controls detector efficiency and source multiplexing, not whoever designs the encoding, which is free where its host is not. QKD monetises the same physics, so a fusion failure strands PsiQuantum and Quandela but not QKD.

Outlook & open questions

Confirm by 2027: a hardware syndrome cycle consuming heralded loss as an erasure flag; absent that it stays theory. Best case 2029: a small fusion-based logical qubit; worst case, commercially real only in QKD. Open: does anyone reconcile the 0.38% and 2.7% thresholds; can detector efficiency reach herald-trust levels at scale.

References
  1. [169] K. Alexander et al., “A manufacturable platform for photonic quantum computing,” Nature, vol. 641, no. 8064, pp. 876–883, Feb. 2025, doi: 10.1038/s41586-025-08820-7. D
  2. [179] S. Bartolucci et al., “Fusion-based quantum computation,” Nat. Commun., vol. 14, Art. no. 912, Feb. 2023, doi: 10.1038/s41467-023-36493-1. arXiv:2101.09310. S
  3. [264] M. C. Löbl, L. A. M. Pettersson, J. Dragašević, S. X. Chen, and O. A. D. Sandberg, “The subthreshold issue of fusion-based quantum computing,” arXiv:2606.28490, Jun. 2026. S
  4. [340] QuiX Quantum, “QuiX Quantum Secures €15 Million to Deliver First-Generation Universal Photonic Quantum Computer,” Jul. 10, 2025. [Online]. Available: https://www.quixquantum.com/news/quix-quantum-series-a C
  5. [342] M. Abdel-Kareem, “Sparrow Quantum Secures €21.5M ($24M USD) in Series A Funding to Advance Photonic Quantum Chip Production,” Quantum Computing Report, Apr. 10, 2025. [Online]. Available: https://quantumcomputingreport.com/sparrow-quantum-secures-e21-5m-24m-usd-in-series-a-funding-to-advance-photonic-quantum-chip-production/ P
  6. [407] ORCA Computing Limited, “Linear-optical encoded GHZ measurements and fault-tolerant quantum computation and communication,” Justia Patents, Oct. 7, 2025. [Online]. Available: https://patents.justia.com/patent/12437225 G
  7. [418] J. Brendel, N. Gisin, W. Tittel, and H. Zbinden, “Pulsed Energy-Time Entangled Twin-Photon Source for Quantum Communication,” Phys. Rev. Lett., vol. 82, no. 12, pp. 2594–2597, Mar. 1999, doi: 10.1103/PhysRevLett.82.2594. arXiv:quant-ph/9809034. D
  8. [419] F. Ewert and P. van Loock, “3/4-Efficient Bell Measurement with Passive Linear Optics and Unentangled Ancillae,” Phys. Rev. Lett., vol. 113, no. 14, Art. no. 140403, Sep. 2014, doi: 10.1103/PhysRevLett.113.140403. arXiv:1403.4841. S
  9. [420] Toshiba Europe, “News Hub - The Latest Quantum Security News from Toshiba,” Toshiba Quantum Technology. [Online]. Available: https://www.toshiba.eu/quantum/news/ C