Free-space spectro-temporal and spatio-temporal conversion for pulsed light.

We present a new apparatus for converting between spectral and temporal representation of optical information, designed for operating with pulsed light sources. Every input pulse is converted into a pulse train in which the pulse intensities represent the spatial or temporal frequency spectrum of the original pulse. This method enables spectral measurements to be performed by following the temporal response of a single detector and, thus, is useful for real-time spectroscopy and imaging, and for spectral correlation measurements. The apparatus is based on multiple round-trips inside a 2f-cavity-like mirror arrangement in which the spectrum is spread on the back focal plane, and a small section of it is allowed to escape after each round-trip. Unlike existing methods, it relies neither on fibers nor on interference effects. It offers easy wavelength range tunability, and a prototype built achieves over 10% average efficiency in the near infrared (NIR). We demonstrate the application of the prototype for an efficient measurement of the joint spectrum of a non-degenerate bi-photon source in which one of the photons is in the NIR.

Cavity-Enhanced Room-Temperature Broadband Raman Memory.

Broadband quantum memories hold great promise as multiplexing elements in future photonic quantum information protocols. Alkali-vapor Raman memories combine high-bandwidth storage, on-demand readout, and operation at room temperature without collisional fluorescence noise. However, previous implementations have required large control pulse energies and have suffered from four-wave-mixing noise. Here, we present a Raman memory where the storage interaction is enhanced by a low-finesse birefringent cavity tuned into simultaneous resonance with the signal and control fields, dramatically reducing the energy required to drive the memory. By engineering antiresonance for the anti-Stokes field, we also suppress the four-wave-mixing noise and report the lowest unconditional noise floor yet achieved in a Raman-type warm vapor memory, (15±2)×10^{-3} photons per pulse, with a total efficiency of (9.5±0.5)%.

Optically induced rotation of an exciton spin in a semiconductor quantum dot.

We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.

Coherent optical writing and reading of the exciton spin state in single quantum dots.

We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one-to-one mapping between any point on the Poincaré sphere of the light polarization to a point on the Bloch sphere of the exciton spin.

Entangled photon pairs from semiconductor quantum dots.

Tomographic analysis demonstrates that the polarization state of pairs of photons emitted from a biexciton decay cascade becomes entangled when spectral filtering is applied. The measured density matrix of the photon pair satisfies the Peres criterion for entanglement by more than 3 standard deviations of the experimental uncertainty and violates Bell's inequality. We show that the spectral filtering erases the "which path" information contained in the photons' color and that the remanent information in the quantum dot degrees of freedom is negligible.