QMPD-2 – Quantum Multi Photon Detector

QMPD-2 (Quantum Multi Photon Detector) is the SiPM detector head of the QuantumBox photon-number-resolving chain, paired with the optional QMPD-1 bias & control unit (both described below).

Built around a room-temperature Hamamatsu S13360 1.3 × 1.3 mm² SiPM, the QMPD-2 resolves the exact number of photons in each light pulse — true photon-number resolution (PNR) — over a 10–200 p.e. dynamic range, with no cooling required.

A key strength of the QMPD-2 is its mechanical compatibility with the Thorlabs® 30 mm Cage System: it mounts into standard 30 mm cage assemblies and works with common user-replaceable terminated-fiber adapters (FC / SMA / PC / LC / ST), with an adjustable fiber-to-SiPM distance — so it integrates into an existing optical bench with no custom optomechanics.

The detector head can be powered and read directly by a DAQ141-6 digitizer — which supplies the SiPM bias, the ±6 V preamplifier rails, temperature feedback and digital gain control over a single M8 cable — or made fully stand-alone with the QMPD-1 unit, for example when used with the 32-channel DAQ141.

Why QMPD-2

• Hamamatsu S13360 SiPM, 1.3 × 1.3 mm² — true photon-number resolution
• Resolves the exact photon number in every pulse, over a 10–200 p.e. range
• Room-temperature operation — no cooling, no cryogenics
• Variable-gain front-end (TIA + ×6) matched to the optical dynamic range
• Single-photon resolution with < 0.3 p.e. pk-pk baseline noise
• Compatible with the Thorlabs® 30 mm cage system, user-replaceable FC / SMA / PC / LC / ST fiber adapters
• Powered and read directly by a DAQ141-6, or stand-alone with the QMPD-1
• Part of the QuantumBox photon-counting and time-tagging chain

QMPD-2 — Detector Head

The QMPD-2 is the optical front-end of the QuantumBox chain. Inside the head, light delivered through the fiber adapter illuminates the Hamamatsu S13360 SiPM; the resulting current pulse is converted to a voltage by the variable-gain transimpedance amplifier (TIA), amplified by a fixed ×6 output stage, and presented on the MCX output as a fast analog pulse whose area is proportional to the number of detected photons.

All bias and control signals reach the head through a single M8 cable: the programmable SiPM bias, the ±6 V ultra-low-noise preamplifier rails, the digital gain selection, an analog temperature-feedback line and a one-wire bus carrying the calibration EEPROM and board-temperature data. The same cable is driven either by a DAQ141-6 — which integrates the full front-end supply for up to six heads — or by a QMPD-1 unit when the head is used stand-alone or with the 32-channel DAQ141.

The QMPD-2 detector head: a fiber-coupled S13360 SiPM in a 30 mm cage-compatible cylinder housing, with MCX signal output and a single M8 power/control connector.

The QMPD-2 detector head: a fiber-coupled S13360 SiPM in a 30 mm cage-compatible cylinder housing, with MCX signal output and a single M8 power/control connector.

Key Features

• Hamamatsu S13360 SiPM, 1.3 × 1.3 mm² active area
• Selectable 25 µm or 50 µm cell pitch
• True PNR via multi-cell Geiger-mode matrix
• Room-temperature operation — no cooling needed
• On-board temperature monitor
• Variable-gain TIA + fixed ×6 output stage
• Selectable ranges: 20 / 50 / 150 / 200 photons (10–200 p.e.)
• Baseline noise < 0.3 p.e. pk-pk, temperature-compensated gain
• MCX signal output · M8 power/control

Variable-Gain Front-End

A variable-gain transimpedance amplifier with selectable feedback (20 / 50 / 150 / 200 photon ranges) feeds a fixed ×6 output stage. Selecting the TIA gain matches the optical dynamic range to the measurement — from single photons up to large multi-photon pulses — keeping the photon-number peaks well separated.

Figure 1 — QMPD-2 front-end: a variable-gain transimpedance amplifier with selectable feedback (20 / 50 / 150 / 200 photon ranges) feeding a fixed ×6 output stage.

Figure 1 — QMPD-2 front-end: a variable-gain transimpedance amplifier with selectable feedback (20 / 50 / 150 / 200 photon ranges) feeding a fixed ×6 output stage.

Optomechanics & Fiber Coupling

The QMPD-2 is mechanically compatible with the Thorlabs® 30 mm cage system: its cage-cylinder housing mounts into standard 30 mm cage assemblies with an adjustable fiber-to-SiPM distance. Light is delivered through user-replaceable terminated-fiber adapters in all common terminations — FC, SMA, PC, LC, ST — so the same head can be matched to virtually any fiber in the lab.

Connectors & Interfaces

Characterization & Performance

The plots below were acquired with a QMPD-2 head illuminated by a 6 ns pulsed laser through a fiber, with the front-end gain set for a 50 p.e. full-scale range. They show the single-pulse response, the photon-number resolution and the baseline noise that define the detector’s single-photon performance.

Single-Pulse Response

A single laser pulse produces a fast SiPM signal whose area is proportional to the number of detected photons. The leading edge rises in about 10 ns, the pulse reaches a full width at half maximum of roughly 35 ns, and an exponential tail returns to baseline in approximately 100 ns.

Figure 2 — Laser-triggered single pulse. Top trace: 6 ns laser trigger reference; bottom trace: detector output (~10 ns rise, ~35 ns FWHM). Front-end gain set for 50 p.e. full-scale.

Figure 2 — Laser-triggered single pulse. Top trace: 6 ns laser trigger reference; bottom trace: detector output (~10 ns rise, ~35 ns FWHM). Front-end gain set for 50 p.e. full-scale.

Photon-Number Resolution

Acquired in persistence mode over many laser shots, the pulse amplitude does not form a continuous blur but clusters into discrete, well-separated bands. Each band corresponds to an exact number of detected photons — direct evidence of true photon-number resolution. The QMPD-2 keeps these peaks distinct across its full 10–200 p.e. range.

Figure 3 — Multiphoton persistence (50 p.e. full-scale gain): each discrete amplitude band is one additional detected photon.

Figure 3 — Multiphoton persistence (50 p.e. full-scale gain): each discrete amplitude band is one additional detected photon.

Figure 4 — Persistence of the first two photon levels; the cursors measure the single-photon-equivalent (SPE) step — the amplitude added by each individual photon at this gain (≈ 42.8 mV).

Figure 4 — Persistence of the first two photon levels; the cursors measure the single-photon-equivalent (SPE) step — the amplitude added by each individual photon at this gain (≈ 42.8 mV).

Baseline Noise

With the detector in the dark, the baseline noise measures 13 mV peak-to-peak (1.5 mV rms). Relative to the 42.8 mV single-photon step this is below 0.3 p.e. peak-to-peak, so adjacent photon-number peaks stay cleanly separated and the single-photon level is unambiguously resolved.

Multi-Photon Spectrum Analysis — Poisson-Constrained Fit

The pulse-height (charge) spectrum is fitted with a single, joint model: a sum of Gaussian peaks whose areas are constrained by a Poisson distribution. The macroscopic shape of the spectrum (the “belly”) emerges naturally as the Poisson envelope of the peaks — it is not modelled with an ad-hoc polynomial baseline. This collapses the free parameters from 3 + 3N (66, for 21 independent peaks plus a polynomial) to just 8.

On a representative dataset the fit converged on 21 resolved peaks (n = 0…20) with a mean photon number ⟨n⟩ = 8.244 ± 0.008:

• Gain (linearity): 202.823 bin/photon; an independent linear fit on the first six photon centroids agrees to within 0.77 % across the full dynamic range
• Width law σₙ² = σₑ² + n·σ_g² describes the broadening of all 20 non-pedestal peaks (electronic noise σₑ = 3.29 bin, per-photon gain spread σ_g = 9.54 bin)
• Fano factor F = 0.997, confirming the Poisson nature of the source

Measured spectrum (black), Poisson-constrained joint fit (red), individual Gaussians (blue) and the Poisson envelope connecting the peak amplitudes (purple). Linear scale.

Measured spectrum (black), Poisson-constrained joint fit (red), individual Gaussians (blue) and the Poisson envelope connecting the peak amplitudes (purple). Linear scale.

The same fit on a logarithmic scale: the Poisson envelope is visible as the smooth modulation of the peak heights, peaking at n ≈ 8.2.

The same fit on a logarithmic scale: the Poisson envelope is visible as the smooth modulation of the peak heights, peaking at n ≈ 8.2.

Centroid vs. photon number: the gain is linear at 202.823 bin/photon, with residuals staying below 0.77 % across the full range.

Centroid vs. photon number: the gain is linear at 202.823 bin/photon, with residuals staying below 0.77 % across the full range.

Observed multiplicity (peak area / N_tot, grey) vs. the Poisson distribution at the fitted ⟨n⟩ = 8.244 (red), Fano factor 0.997.

Observed multiplicity (peak area / N_tot, grey) vs. the Poisson distribution at the fitted ⟨n⟩ = 8.244 (red), Fano factor 0.997.

QMPD-1 — Bias & Control Unit

The QMPD-1 is the power supply and controller for the QMPD-2 detector head, designed to run the detector fully stand-alone, with no DAQ141-6 in the system. While a DAQ141-6 can power and control the head directly, the QMPD-1 supplies everything the detector needs from a single compact box — making it the natural companion when the QMPD-2 is used with the 32-channel DAQ141 or in any setup without an integrated front-end supply.

Over the single M8 cable it delivers the programmable SiPM high-voltage bias, the ±6 V ultra-low-noise preamplifier rails and the digital front-end gain control, while continuously monitoring output voltage and current:

• The high-voltage section provides a programmable SiPM bias from 20 to 85 V at up to 10 mA, with very low ripple (< 0.1 mVpp typical) and high-resolution voltage and current read-back. Programmable temperature compensation corrects the bias against the SiPM temperature coefficient, keeping the gain — and therefore the spacing of the photon-number peaks — stable over temperature.
• Front-end gain is set digitally: the QMPD-1 selects the variable-gain TIA feedback of the QMPD-2 (the 20 / 50 / 150 / 200-photon ranges), matching the optical dynamic range to the measurement with no manual intervention on the detector head.

The QMPD-1 bias & control unit: stand-alone power supply and controller for the QMPD-2 — HV bias, ±6 V preamplifier rails, digital gain control and monitoring over a single M8 cable.

The QMPD-1 bias & control unit: stand-alone power supply and controller for the QMPD-2 — HV bias, ±6 V preamplifier rails, digital gain control and monitoring over a single M8 cable.

QMPD-1 Features

• 20 ÷ 85 V (10 mA) programmable bias output, suitable for large SiPM matrices
• Very low ripple (< 0.1 mVpp typical)
• ±6 V ultra-low-noise preamplifier supply
• Digital front-end gain control of the QMPD-2
• Analog and digital control (UART, I²C, analog & USB)
• High-resolution voltage and current monitoring
• Programmable temperature compensation
• Multi-unit addressing over the I²C bus
• ZEUS 2 control software · open-source C & Python libraries (GitHub)

ZEUS 2 Control Software

Both the HV bias and the front-end gain are managed from ZEUS 2, the control application for the QMPD-1. Connected over USB (or Ethernet / I²C), ZEUS 2 lets the user set and read back the SiPM bias voltage and the detector gain range, monitor live voltage, current and temperature, enable temperature compensation and store per-device calibration. The same functions are exposed programmatically through open-source C and Python libraries, and the I²C bus allows several QMPD-1 units to be addressed together — so a multi-detector array can be configured and monitored from one ZEUS 2 session or script.

ZEUS 2 live monitoring: per-channel SiPM bias voltage, current and temperature, each with an editable set-point and a configurable danger threshold.

ZEUS 2 live monitoring: per-channel SiPM bias voltage, current and temperature, each with an editable set-point and a configurable danger threshold.

ZEUS 2 I–V curve tool: an automated bias sweep that traces the SiPM current-vs-voltage characteristic for breakdown-voltage extraction and diagnostics.

ZEUS 2 I–V curve tool: an automated bias sweep that traces the SiPM current-vs-voltage characteristic for breakdown-voltage extraction and diagnostics.

Full Specifications

QMPD-2 — Detector Head

QMPD-1 — Bias & Control Unit

Part of the QuantumBox System

The QMPD-2 and QMPD-1 are components of the QuantumBox photon-number-resolving detection chain:

• With DAQ141-6 — the digitizer powers and controls the QMPD-2 directly; no QMPD-1 needed.
• With DAQ141 — each QMPD-2 head is powered and controlled by a QMPD-1 unit, scaling to large multi-detector and twin-beam arrays.