Ultrafast Time-Division Demultiplexing of Polarization-Entangled Photons

Maximizing the information transmission rate through quantum channels is essential for practical implementation of quantum communication. Time-division multiplexing is an approach for which the ultimate rate requires the ability to manipulate and detect single photons on ultrafast time scales while preserving their quantum correlations. Here we demonstrate the demultiplexing of a train of pulsed single photons using time-to-frequency conversion while preserving their polarization entanglement with a partner photon. Our technique converts a pulse train with 2.69 ps spacing to a frequency comb with 307 GHz spacing which may be resolved using diffraction techniques. Our work enables ultrafast multiplexing of quantum information with commercially available single-photon detectors.

Figure 1

Time-to-frequency conversion concept and experimental setup. (a) A train of temporally narrow polarized photonic signals $A$–$C$ are converted into a comb of spectrally narrow and correspondingly polarized photons with a central frequency dependent on their time of arrival. The different frequency modes may then be demultiplexed using diffraction techniques. (b) Two $\alpha$-BBO crystals and a series of wave plates prepared a train of pump pulses 2.69 ps apart, which were then used to create a pulse sequence of polarization-entangled states through SPDC. The single photons were chirped in single-mode fiber and combined with an antichirped strong escort pulse using a dichroic mirror. This beam was then focused in two 10-mm BiBO crystals arranged in a Sagnac configuration for polarization-maintaining sum-frequency generation (PM-SFG). The polarizations of the output photons were measured, and the three signals were then separated with a diffraction grating and coupled to detectors $D_{A-C}$. A removable mirror to $D_{\text{in}}$ enabled measurement of the input state.

Figure 2

Up-converted single-photon spectra for each temporal mode. We prepared the pump to maximize the count rate in each of the three temporal modes and measured the spectra shown (with background subtraction). The time delay between the modes maps each to a distinct central wavelength and the spectral bandwidth is compressed by a factor of 20 relative to the input.

Figure 3

Demultiplexing two orthogonal states. With the pump prepared in modes $A$ and $B$ to either produce (i) orthogonal separable states or (ii) orthogonal maximally entangled states, the density matrices measured before time-to-frequency conversion (left) appear the same, with negligible coherences. After demultiplexing, the experimentally reconstructed output density matrices (right) are revealed to describe vastly different quantum states, which are separable in case (i) but show a high degree of entanglement in case (ii).

Figure 4

Demultiplexing three entangled states. The pump was prepared to produce a train of three maximally entangled states. Weak coherences are seen in the density matrix measured before time-to-frequency conversion (left), with a calculated tangle of 0.21. After being demultiplexed, all three experimentally reconstructed density matrices show much stronger coherence and larger entanglement, with tangles of 0.40, 0.72, and 0.58 in modes $A$–$C$, respectively.