Chirp and Compress: Toward Single-Cycle Biphotons

The Letter describes a technique for the spontaneous generation of chirped time-energy entangled photons. Transmitting either photon through an appropriate dispersive medium results in a temporally compressed, transform limited biphoton. At maximum bandwidth, the biphoton is a single cycle in length, with a waveform that has the same characteristic shape as a classical single-cycle pulse.

Figure 1

Quasi-Phase-Matching. The up-down arrows show the direction of the ${\overline{a}}_z$ axis of the domains of the quasi-phase-matched crystal. The poling period is chosen so that the signal frequency is phase matched for red emission at the left end of the crystal and for blue emission at the right end of the crystal.

Figure 2

Calculated spectrum of spontaneously generated chirped photons for a 2 cm long QPM crystal of ${\mathrm{LiNbO}}_3$ pumped at $0.42\mu \mathrm{m}$.

Figure 3

(a) Frequency Summing as an ultrafast correlator. Signal and idler photons sum to produce monochromatic photons at frequency ${\omega }_p$. The envelope of the biphoton wave function is obtained by varying the time delay $\tau$. (b) Homodyning the generated sum frequency with the pump beam and varying the time delay $\tau$ yield the biphoton wave function.

Figure 4

Chirped biphoton waveform and its envelope.

Figure 5

Calculated normalized sum power as a function of delay. (a) The QPM crystal is uniformly poled to phase match at $0.6\mu \mathrm{m}$ with a bandwidth of $2.6{\mathrm{cm}}^{-1}$. (b) The poling period is chirped to produce a bandwidth of $122{\mathrm{cm}}^{-1}$, and the wave packet is quadratically compressed.

Figure 6

(a) Normalized sum power and (b) homodyne current for a single-cycle pulse in ${\mathrm{LiNbO}}_3$. Dispersion is corrected as per Eq. (5).