Ultrafast switching of telecom photon-number states

Abstract

A crucial component of photonic quantum information processing platforms is the ability to modulate, route, convert, and switch quantum states of light noiselessly with low insertion loss. For instance, a high-speed, low-loss optical switch is crucial for scaling quantum photonic systems that rely on measurement-based feed-forward approaches. Such a device will also ideally be capable of operating on photon-number states, which can act as non-Gaussian resources. Here, we demonstrate ultrafast all-optical switching of heralded photon-number states, of up to 6 photons, using the optical Kerr effect in a single-mode fiber. A local birefringence is created by a high-intensity pump pulse at a center wavelength of 1030 nm which overlaps temporally with the 1550 nm photons in the fiber. A switching efficiency of $>$99 % is reached with a resolution of 2.3 ps, an insertion loss of $2.27\pm0.08$ dB, and a signal-to-noise ratio of 32,000.

Figures (3)

• Figure 1: Experimental setup. A simplified experimental setup is shown in (a), where a pulsed Ytterbium (Yb) laser pumps an optical parametric amplifier (OPA) and provides the pump pulses necessary for ultrafast all-optical switching. Output pulses from the OPA undergo spontaneous parametric downconversion (SPDC) in a periodically poled potassium titanyl phosphase (ppKTP) crystal for photon-pair generation. The idler photons, measured at detector $D_i$, herald the detection of the switched (unswitched) signal photons at detector $D_S$ ($D_U$). Photons are detected by either superconducting nanowire single-photon detectors (SNSPDs) or transition-edge sensors (TESs). Extracting 2D timing histograms from the detection events measured by the SNSPDs, as shown in (b), allows us to filter the one-photon events (enclosed by a white dashed line corresponding to a 60 ps correlation window) from events with more than one photon. The SPDC source is operated in the low mean photon number regime, $\braket{n}=0.24\pm0.02$, for the single-photon switching demonstrations made with the SNSPDs. On the other hand, the source is operated in the high mean photon number regime, $\braket{n}=3.86\pm0.05$, for the switching of heralded number states with up to 6 photons, as measured by the TESs. A joint photon-number intensity measurement is shown in (c) for the latter case. The signal photon spectrum remains unchanged by the switch, as shown in (d). DM: dichroic mirror; SMF: single-mode fiber; PBS: polarizing beamsplitter; HWP: half-wave plate; QWP: quarter-wave plate; $\Delta\tau$: pump delay line.
• Figure 2: Characterizing the switch. The experimental and simulated (see Supplemental Material) switching efficiency, as a function of pump pulse energy and delay, are shown in (a) and (b), respectively. Slices from these 2D datasets are shown for (c) a constant pump pulse delay of $\Delta\tau=0$ and (d) a constant pump pulse energy of 8 nJ, where the black circles correspond to experimental data and red lines correspond to simulation results. Also shown in (c) are the measured pump noise counts per pulse, indicated by the blue circles and squares for the switched (V) and unswitched (H) ports, respectively. Note that error bars based on Poissonian statistics are smaller than the symbol size for all data points in (c) and (d). Here, SNSPDs are used for experimental measurements.
• Figure 3: Switching of heralded photon-number states. The measured probability, $P_{n_S,n_U}$, of detecting $n_S$ photons at $D_S$ and $n_U$ photons at $D_U$, heralded by measurement of $N$ photons at $D_i$, where $n_S+n_U=N$, is shown in (a)--(f) for $N=$ 1--6. Probabilities are shown as a function of pump pulse delay, where pump pulse energy is held constant at 8 nJ. Experimental data (denoted by markers with error bars to one standard deviation) are compared to simulation results (lines with area shaded below). Note that in some cases, error bars are smaller than the data points. TESs are used for all measurements presented here.