Single-Photon Detectors (SPADs)

Role in QLT Fabrication

Single-photon avalanche diodes (SPADs) are the eyes of QLT's quantum photonic processor. Every quantum metric — g²(0) autocorrelation proving single-photon emission, Hong-Ou-Mandel (HOM) dip visibility proving photon indistinguishability, coincidence rates proving entanglement, and heralding efficiency proving source quality — depends on detecting individual photons with picosecond timing resolution at room temperature.

QLT's architecture requires two complementary detector technologies: silicon SPADs for visible-wavelength herald detection (from SPDC pump at 775 nm) and InGaAs/InP SPADs for telecom C-band (1550 nm) signal detection. Together with a time-to-digital converter (TDC), these detectors form the quantum characterization backbone that validates every fabrication step from waveguide loss measurement through final system-level boson sampling demonstrations.

• g²(0) autocorrelation ● proves non-classical light: g²(0) < 0.5 confirms single-photon emission
• HOM dip visibility ● measures photon indistinguishability: target V > 90%
• Coincidence counting ● verifies entanglement and multi-photon interference
• Heralding efficiency ● validates source brightness and coupling quality
• Photon count rate ● measures quantum channel throughput (~10⁵–10⁶ counts/s)
• SPAD timing jitter ● characterizes detector timing resolution (< 100 ps FWHM)

Key Quantum Measurements Enabled

Technical Specifications

Silicon SPAD (Visible / Herald Channel)

InGaAs/InP SPAD (Telecom C-Band / 1550 nm)

System Timing Resolution

TIMING BUDGET — SPAD + TDC System:

Source of uncertainty         │ Value              │ Bottleneck?
──────────────────────────────┼────────────────────┼───────────
TDC digital resolution        │ 1 ps               │ No
TDC RMS jitter                │ 34 ps              │ No
TDC FWHM jitter               │ 80 ps              │ No
Si SPAD jitter (Excelitas)    │ 350 ps FWHM        │ YES — herald
Si SPAD jitter (MPD 20μm)    │ 35 ps FWHM         │ No
InGaAs SPAD jitter (id230)   │ 200 ps FWHM        │ YES — signal
──────────────────────────────┼────────────────────┼───────────
System timing (herald path)   │ √(80² + 350²)      │ ≈ 359 ps FWHM
System timing (signal path)   │ √(80² + 200²)      │ ≈ 215 ps FWHM
Coincidence window (HOM)      │ 1–10 ns            │ Adequate ✓

NOISE EQUIVALENT POWER (Si SPAD):
NEP = (hν / PDE) × √(2 × DCR)
    = (2.84e-19 / 0.70) × √(2 × 25)
    ≈ 2.9 × 10⁻¹⁸ W/√Hz   → excellent

Process Integration

QLT QUANTUM TEST LAB ● SPAD Detector Setup (Step 07):

CONFIGURATION A: Heralded Single-Photon Characterization
├── SPDC source (ppKTP, 775 nm pump)
│   ├── Signal (1550 nm) → fiber → chip input
│   └── Idler (775 nm herald) → fiber → Si SPAD (Excelitas)
├── Chip output → fiber → InGaAs SPAD (id230)
├── Both SPADs → SMA cables → Time Tagger 20 (TDC)
├── TDC → USB → Python (TimeTagger API)
│
├── Measurement 1: Count rate (singles on each channel)
├── Measurement 2: Coincidence rate (herald-signal pairs)
├── Measurement 3: g²(0) (Hanbury-Brown-Twiss correlation)
└── Measurement 4: Heralding efficiency = coincidence/herald

CONFIGURATION B: Hong-Ou-Mandel Dip
├── Two indistinguishable photons → two chip inputs
├── On-chip beam splitter (MZI)
├── Two outputs → 2 × InGaAs SPAD (id230)
├── Both SPADs → TDC channels 1 & 2
├── Scan: optical delay between input photons
├── Record: coincidence rate vs. delay
└── Result: HOM dip → visibility V = (R_max - R_min)/R_max
    Target: V > 90% (proves photon indistinguishability)

CONFIGURATION C: Boson Sampling Verification
├── 4–8 photons into mesh (from multiplexed source)
├── 8 mesh outputs → 8 × InGaAs SPAD
├── All 8 SPADs → TDC (8 channels)
├── Record: all multi-fold coincidence combinations
├── Compare: measured vs. permanent matrix theory
└── Result: validates quantum computational advantage

Vendor Options & Pricing

Silicon SPAD Modules (Visible / Herald)

InGaAs SPAD Modules (1550 nm / Signal)

SNSPD Upgrade Path (Future)

Total System Budget

QLT SINGLE-PHOTON DETECTOR LAB:

Herald channel (visible):
├── 2 × Excelitas SPCM-AQRH          $7,000–$10,000
├── 2 × MPD PDM (low-jitter option)   $16,000–$24,000

Signal channel (1550 nm):
├── 4 × ID Quantique id Qube NIR      $32,000–$48,000
├── 4 × ID Quantique id230            $60,000–$100,000

Time-to-digital converter:
├── Swabian Time Tagger 20 (8 ch)     $8,000–$10,000

Accessories:
├── FC/PC fiber patch cords (×16)      $800
├── SMA cables 50Ω (×8)               $200
├── Optical attenuators (×4)           $400
├── BNC/SMA adapters                   $100

═══════════════════════════════════════════
MINIMUM CONFIG (4-ch InGaAs + 2-ch Si):
  $55,000–$80,000

FULL 8-CHANNEL CONFIG:
  $120,000–$200,000

WITH SNSPD UPGRADE (Phase 3):
  $1,000,000–$2,000,000
═══════════════════════════════════════════

Facility Requirements

Safety & Handling