Adaptive Detection of Arbitrarily Shaped Ultrashort Quantum Light States

A quantum state of light is the excitation of a particular spatiotemporal mode of the electromagnetic field. A precise control of the mode structure is therefore essential for processing, detecting, and using photonic states in novel quantum technologies. Here we demonstrate an adaptive scheme, combining techniques from the fields of ultrafast coherent control and quantum optics, for probing the arbitrary complex spectrotemporal profile of an ultrashort quantum light pulse. The ability to access the modal structure of a quantum light state could boost the capacity of current quantum information protocols.

Figure 1

(a) Conceptual scheme. A quantum light state is analyzed by a homodyne detector fed with a shaped local oscillator (LO). If the feedback loop is closed, the mode of the LO can be adaptively matched to that of the quantum state, based on measurement results; the latter can thus be fully characterized by measurements on the intense LO field. If the feedback loop is open, the quantum state can be probed in the arbitrarily defined mode of the LO. (b) Experimental setup. Definitions: LBO, lithium triborate crystal; BBO, $\beta$-barium borate crystal; APD, avalanche silicon photodiode; F, spectral filter; SMF, single-mode fiber; BHD, balanced homodyne detector; HT, high-transmission beam splitter.

Figure 2

Measuring the shape of a single photon. Experimental FROG traces (top panels), reconstructed spectral intensity and phase (middle panels), and temporal intensity profiles (bottom panels) for the optimal LO pulses matching (a) a frequency-dispersed single-photon (FWHM spectral width of about 9.4 nm); (b) a spectrally cropped single-photon (FWHM spectral width of about 6.0 nm); and (c) a spectrally and temporally two-peaked single-photon (note the expected $\pi$ phase jump in the center of the spectrum). The maximum value of the homodyne efficiency is about 60% for cases (a) and (b), whereas a slight degradation of the pump spatial mode is likely responsible for an efficiency of about 45% in case (c).

Figure 3

Probing a spectrally and temporally delocalized single photon. FROG-measured spectral intensity and phase profiles of the LO mode for (a) ${\varphi }_{\mathrm{LO}}=0$; (b) ${\varphi }_{\mathrm{LO}}=\pi$. The corresponding temporal intensity profiles are shown in (c) and (d), respectively. Note the clear double-peak structure of the LO in the ${\varphi }_{\mathrm{LO}}=0$ case and the nearly orthogonal temporal shape obtained in the ${\varphi }_{\mathrm{LO}}=\pi$ condition. (e) Variation of the measured single-photon homodyne efficiency $\eta$ as a function of the phase ${\varphi }_{\mathrm{LO}}$ between the two LO spectral peaks. Reconstructed Wigner functions of the detected state: a single photon in (f) for ${\varphi }_{\mathrm{LO}}=0$ and vacuum in (g) for ${\varphi }_{\mathrm{LO}}=\pi$.