Spectral Shearing of Quantum Light Pulses by Electro-Optic Phase Modulation

Frequency conversion of nonclassical light enables robust encoding of quantum information based upon spectral multiplexing that is particularly well-suited to integrated-optics platforms. Here we present an intrinsically deterministic linear-optics approach to spectral shearing of quantum light pulses and show it preserves the wave-packet coherence and quantum nature of light. The technique is based upon an electro-optic Doppler shift to implement frequency shear of heralded single-photon wave packets by $\pm 200\mathrm{GHz}$, which can be scaled to an arbitrary shift. These results demonstrate a reconfigurable method to controlling the spectral-temporal mode structure of quantum light that could achieve unitary operation.

Figure 1

Experimental setup for single-photon spectral shearing by electro-optic phase modulation. Photon pairs produced by parametric down-conversion (PDC), pumped by second-harmonic generation (SHG) of a pulse train from a titanium sapphire laser (Ti:Sapph), are separated at a polarizing beam splitter (PBS). The herald photon (dashed lines) can be directed to a single-photon counting module (SPCM H) to trigger the single-photon spectrometer, $S(\omega )$, and $g_h^{(2)}(0)$ measurements at a fiber beam splitter (FBS), or directed to a FBS after passing through an interference filter (F) that can be tuned and adjustable time delay ($\tau$) to undergo two-photon interference with the signal photon (solid line). The EOM is driven by a sinusoidal rf field that is locked to a weak sample of the laser pulse train monitored by a photodiode (PD). Inset: Schematic illustrating a linear temporal phase applied to a pulsed mode of amplitude $|\psi (t)|$ (red) by synchronization of sinusoidal EOM driving voltage $V(t)$ (blue) with the optical pulse train.

Figure 2

Heralded single-photon spectra (data points) and Gaussian fits (solid lines) for the original pulse (black), positive (blue) and negative (red) linear temporal phase resulting in a spectral shear of $\mathrm{\Omega }/2\pi =\pm 200\mathrm{GHz}$. Uncertainties in coincidence counts are calculated assuming Poisson statistics and the uncertainty in wavelength is below the symbol size.

Figure 3

Two-photon interference between spectrally sheared single-photon wave packets and a reference single photon derived from the idler mode of the SPDC. Coincidence counts between detectors $A$ and $B$ after subtracting background events obtained by blocking the signal and reference paths are plotted against delay $\tau$ between the signal and reference. The three plots correspond to different spectral shear and reference pulses: (a) maximum blueshifted single photon and matched reference, (b) maximum redshifted single photon and reference matched to blueshifted photon, and (c) maximum redshifted single photon and matched reference. Error bars assume Poisson count statistics. We report two-photon interference visibilities obtained from inverted Gaussians (solid red) fitted to the data. Insets show spectra for the reference (dashed black) and sheared (solid green) single photons.