Frequently Asked Questions: Balanced Detection and Squeezed Light

A balanced photodetector uses two photodiodes whose photocurrents are subtracted electronically. Noise that is common to both optical inputs - laser intensity noise and other classical fluctuations - cancels in the subtraction, leaving only the differential signal. The key performance figures are quantum efficiency, common-mode rejection ratio (CMRR), and dark noise clearance.

Balanced photodetectors are the standard detection tool for balanced homodyne detection, squeezed light measurement, and other shot-noise-limited measurements in quantum optics. Noisy Labs’ High Quantum Efficiency (HQE) Balanced Detectors reach a quantum efficiency of >95% , a common mode rejection ratio of 50 dB at 1 MHz, and a dark noise clearance of more than 20 dB in the MHz frequency range - enough to directly detect at least 10 dB of squeezed light.

A single photodetector measures the total intensity of one beam, so its output contains the full classical noise of the laser. A balanced photodetector splits the measurement across two photodiodes and subtracts their photocurrents, cancelling the noise common to both paths.

In practice this means a single detector can only analyze the amplitude quadrature of a laser beam and perform, for example, power monitoring and intensity measurements. Measurements below the classical noise floor can only be performed if the laser impinging on the diode is already quantum noise limited. A balanced detector is required when the quantity of interest is in an arbitrary quadrature, e.g. in the phase quadrature or at a random phase angle.

The noise reduction comes from common-mode rejection. Classical laser noise appears identically in both photodiodes of a balanced detector and is suppressed by the subtraction; a CMRR of 50 dB at 1 MHz corresponds to a 100,000-fold suppression of common-mode noise power. What remains is the shot noise of the detected light plus the detector’s electronic noise.

A single detector offers no such cancellation: its noise floor is set by the full classical intensity noise of the source, which in most laboratory lasers sits well above shot noise in the relevant frequency bands. This is why shot-noise-limited measurements are most often performed with balanced detection.

An avalanche photodiode (APD) multiplies photocurrent internally through avalanche gain, which makes it suitable for detecting very weak light and, in Geiger mode, for counting single photons. The avalanche process adds excess noise and the timing/gain behaviour is statistical.

A balanced detector uses two PIN photodiodes with high quantum efficiency and relies on a strong local oscillator to lift the signal above electronic noise. It is the right choice for continuous-variable measurements: quadrature detection, squeezed light characterization, and shot-noise-limited measurements. As a rule of thumb: APDs count discrete photons; balanced detectors measure field quadratures and noise.

Quantum efficiency (QE) is the fraction of incident photons that a photodetector converts into photoelectrons. A QE of 95% means 5% of the light is effectively lost in detection.

In quantum optics, detection QE acts exactly like optical loss. For squeezed light measurement this is critical: every percent of loss mixes vacuum noise back into the measurement and reduces the observable squeezing. This is why detectors built for quantum optics, such as the Noisy Labs High Quantum Efficiency (HQE) Balanced Detector, are specified at >95% QE - enough to directly detect at least 10 dB of squeezed light.

The common-mode rejection ratio (CMRR) of a balanced photodetector quantifies how strongly it suppresses signals that appear identically on both photodiodes. Noisy Labs balanced detectors surpass 50 dB CMRR at 1 MHz; 50 dB of common mode suppression means common-mode noise power is reduced by a factor of 100,000.

CMRR is determined by how well the two photodiodes are matched in responsivity and how precisely the optical and electronic balancing is maintained. It is one of the four specifications that define a balanced detector’s real-world performance, alongside quantum efficiency, bandwidth and dark noise clearance.

In homodyne detection, the signal field is interfered with a strong reference beam - the local oscillator (LO) - on a 50/50 beam splitter, and the two outputs are sent to a balanced photodetector. The difference photocurrent is proportional to one quadrature of the signal field, and the measured quadrature is selected by the LO phase.

The strong LO amplifies the weak signal above the detector’s electronic noise. Whether the measurement is actually shot-noise-limited depends on the detector’s dark noise clearance: Noisy Labs balanced detectors provide more than 20 dB of clearance in the MHz frequency range, leaving the shot noise well above the electronic noise floor.

The standard method is to illuminate a balanced photodetector with a local oscillator and record the noise spectrum of the difference signal. The signature of shot noise is linear scaling with optical power: doubling the LO power raises the noise level by 3 dB. Classical intensity noise scales by 6 dB per doubling, and electronic noise does not change at all, so the scaling test distinguishes all three contributions.

A reliable shot noise measurement also requires sufficient dark noise clearance - the margin between the shot noise level and the detector’s electronic noise. With 20 dB clearance, the electronic contribution to the measured noise power is at the 1% level. Smaller clearances require the subtraction of electronic noise from the shot noise spectrum to verify linearity.

Squeezed light reduces the quantum noise in one field quadrature below the shot-noise limit. It is used in quantum-enhanced sensing and spectroscopy, quantum metrology, continuous-variable quantum key distribution (CV-QKD), optical quantum computing research, and gravitational-wave detection. One established metrology application is calibrating the detection efficiency of photosensors without the need for a calibrated lamp or standard candle.

Generating and detecting squeezed light requires a low-loss measurement chain, because every percent of optical loss reduces the observable squeezing. Noisy Labs supplies both sides of this chain: the Squeeze Laser - the world’s first commercial squeeze laser, a turn-key CW source of squeezed vacuum states with squeeze factors of up to 10 dB at 1064 nm and 1550 nm - and HQE balanced detectors with >95% quantum efficiency for measuring it.

In an optical measurement, the vacuum fluctuations entering the open port of an interferometer or detection setup set the shot-noise limit. Injecting squeezed vacuum into that port replaces these fluctuations with noise-reduced ones, lowering the noise floor in the squeezed quadrature without increasing optical power.

This matters most where power cannot simply be raised - power-sensitive samples, power-limited interferometers, or measurements already operating at their damage or saturation threshold. The improvement is bounded by total optical loss, including detection: with 10 dB of source squeezing, the loss budget of the full chain determines how much of that advantage survives at the detector.

Quantum sensing uses quantum states of light or matter to measure physical quantities - displacement, phase, magnetic fields, time - with sensitivity beyond what classical approaches allow with the same resources. In optical quantum sensing, squeezed light is one of the most mature techniques: it reduces quantum noise below the shot-noise limit in interferometric and spectroscopic measurements.

The performance of an optical quantum sensor depends on the full chain: the squeezed light source, the optical loss budget, and the detection. High quantum efficiency balanced detection is required at the end of the chain, since detection loss directly removes the quantum advantage.

Dark noise clearance is the margin between a detector’s electronic (dark) noise floor and the shot noise level at a given local oscillator power. It determines whether a measurement is limited by quantum noise or by the detector’s electronics. Noisy Labs balanced detectors provide a dark noise clearance of more than 20 dB in the MHz frequency range; at 20 dB clearance, electronic noise contributes at the 1% level to the measured noise power.

A shot-noise-limited balanced detector is one whose output noise, under normal operating conditions, is dominated by the shot noise of the detected light rather than by electronic noise or uncancelled classical noise. Reaching this regime requires sufficient dark noise clearance and high common-mode rejection. The practical test is power scaling: a shot-noise-limited detector shows a 3 dB noise increase when the local oscillator power is doubled.

The choice is set by wavelength. Silicon photodiodes cover roughly 400-1030 nm, with typical operation in the 600-900 nm region, and offer high quantum efficiency with low dark current at room temperature - no thermoelectric cooling required. Above approximately 1 µm, silicon becomes transparent and InGaAs is the standard photodiode material, covering the 1064 nm and 1550 nm wavelengths common in quantum optics and telecom-band experiments. Noisy Labs offers HQE balanced detectors in both material systems.