Ultrafast Switching of Photonic Entanglement

To deploy and operate a quantum network which utilizes existing telecommunications infrastructure, it is necessary to be able to route entangled photons at high speeds, with minimal loss and signal-band noise, and—most importantly—without disturbing the photons’ quantum state. Here we present a switch which fulfills these requirements and characterize its performance at the single photon level. Furthermore, because this type of switch couples the temporal and spatial degrees of freedom, it provides an important new tool with which to encode multiple-qubit states in a single photon. As a proof-of-principle demonstration of this capability, we demultiplex a single quantum channel from a dual-channel, time-division-multiplexed entangled photon stream, effectively performing a controlled-bit-flip on a two-qubit subspace of a five-qubit, two-photon state.

Figure 1

(a) Entangled photon-pair source and switching test apparatus. Nondegenerate pairs (${\lambda }_{\mathrm{signal}}=1303.5\mathrm{nm}$ and ${\lambda }_{\mathrm{idler}}=1306.5\mathrm{nm}$) are generated in 500 m of standard single-mode fiber (SMF-28) and separated using a double-pass grating filter (DGF). Circ, circulator; FM, Faraday mirror; FPC, fiber polarization controller; HWP (QWP), half- (quarter-) wave plate; PBS, polarizing beam-splitter; PDD, polarization-dependent delay; WDM, wavelength-division multiplexer. (b) The single-photon switch. The length of the intraloop SMF-28 is directly proportional to the switching window. An $L_{100}\equiv 100\mathrm{m}$ loop (shown) results in a $\approx 200\mathrm{ps}$ switching window. (c) Reconstructed density matrix of the unswitched state for $L_{100}$ (fidelity to a maximally entangled state, $F=99.5\%\pm 0.2\%$). (d) Density matrix of the switched state for $L_{100}$ ($F=99.4\%\pm 0.4\%$). Similar reconstructions for the 500-m switch (not shown) yielded $F=99.5\%\pm 0.2\%$ (unswitched) and $F=99.2\%\pm 0.2\%$ (switched).

Figure 2

(a) Single-photon switching contrast. Plot shows switching probability versus pump energy for $L_{500}$ and $L_{100}$ (detector dark counts subtracted). (b) Temporal extent of the switching window, as measured using single photons. Plot shows single-photon counts versus relative delay between the single-photon and the pump pulses (dark counts subtracted).

Figure 3

(a) Diagram of the 5 degrees of freedom in a multiplexed entangled photon stream, which can be demultiplexed by applying the controlled switch operation (shown). (b) Arrangement of detected quantum information channels for the five-qubit state $|\Phi ⟩$. Tracing over the temporal qubit and projecting into spatial mode $|T^i⟩$, we reconstruct a highly mixed density matrix with $F=58.9\%\pm 0.5\%$. (c) Arrangement of quantum information channels after active switching to output $T$ (i.e., demultiplexing), which should produce the state $|{\Phi }^{\prime }⟩$. By projecting into spatial mode $|T^i⟩$ we recover the maximally entangled state $|{\psi }_1⟩$ ($F=98.6\%\pm 0.7\%$).