Photo detectors capable to detect single photons are crucial for further development of quantum optics, quantum information and quantum communication. For many application the detector has not only to reach superior parameters of quantum efficiency, speed, low noise etc but has to possess highly non-classical properties such as photon number resolving (PNR) capability. In general, single PNR detector is able to distinguish between N and N+1 photons in the incident light signal at least up to some maximum high enough photon number. The perfect resolution is out of reach of the current technology and even approximate PNR detection represents a serious challenge.

We have developed a balanced eight-port PNR multichannel detector based on an optical-fiber time-multiplexed device with the unique total optical transmittance reaching 93% and a pair of avalanche photodiodes in Geiger regime [1].

The balanced operation means that input signal is divided into the output eight temporal detection channels with more less equal probability. This is achieved even for imperfect unbalanced fiber splitters used in the time-multiplexed device. We have shown high-speed photon counting at the rates of 100 kHz with the total detection efficiency exceeding 50%. The multiplex based design is convenient and robust but suffers from crosstalks and trade-off between maximum detectable photon number and maximum possible repetition rate. Other PNR techniques have to be devised for high-speed applications.

Balanced homodyne detection represents another feasible way how to measure the photon content and even optical phase of the incident light pulse. Mixing with reference coherent mode-matched local oscillator (LO) pulse with variable phase gives rise to an interference and it reveals the amplitude and phase structure of the measured quantum state of the pulse. Basically the same trick is used in holography but in spatial domain.

I have built and tested two different time-domain homodyne detectors. First based on a transimpedance amplifier with bandwidth from DC to 40 MHz [2]. It gives up to 12 dB signal-to-noise ratio for 40 MHz repetition rate 150 fs pulse train with average LO power around 10 mW (almost 10^9 photons per pulse). Further, I have utilized the original Amptek-based charge-sensitive design with some minor changes, particularly better bias filtering and balancing. The detector exceeds 30 dB signal-to-noise ratio for several tens micro W of LO power and works very stable at 25 dB for 20 micro W LO with repetition rate of 0.8 MHz and 5 ps pulse length (100 mio photons per pulse). The bandwidth spans an interval from approximately 100 Hz to 2 MHz at least. The detector works at 1.6 MHz repetition rate without any change in performance.

The latter time-domain 30 dB homodyne detector is used in Quantum information group of prof. Ulrik L. Andersen at Department of Physics of Technical University of Denmark for preparation of free-running non-classical states of light, phase super-resolution, quantum computing with optical Schrodinger cat states and for other applications.