Alexei Ourjoumtsev

Generation of optical 'Schrödinger cats' from photon number states

Schrödinger’s cat is a Gedankenexperiment in quantum physics, in which an atomic decay triggers the death of the cat. Because quantum physics allow atoms to remain in superpositions of states, the classical cat would then be simultaneously dead and alive. By analogy, a ‘cat’ state of freely propagating light can be defined as a quantum superposition of well separated quasi-classical states—it is a classical light wave that simultaneously possesses two opposite phases. Such states play an important role in fundamental tests of quantum theory and in many quantum information processing tasks, including quantum computation, quantum teleportation and precision measurements. Recently, optical Schrödinger ‘kittens’ were prepared; however, they are too small for most of the aforementioned applications and increasing their size is experimentally challenging. Here we demonstrate, theoretically and experimentally, a protocol that allows the generation of arbitrarily large squeezed Schrödinger cat states, using homodyne detection and photon number states as resources. We implemented this protocol with light pulses containing two photons, producing a squeezed Schrödinger cat state with a negative Wigner function. This state clearly exhibits several quantum phase-space interference fringes between the ‘dead’ and ‘alive’ components, and is large enough to become useful for quantum information processing and experimental tests of quantum theory.

Increasing entanglement between Gaussian states by coherent photon subtraction.

We experimentally demonstrate that the entanglement between Gaussian entangled states can be increased by non-Gaussian operations. Coherent subtraction of single photons from Gaussian quadrature-entangled light pulses, created by a nondegenerate parametric amplifier, produces delocalized states with negative Wigner functions and complex structures more entangled than the initial states in terms of negativity. The experimental results are in very good agreement with the theoretical predictions.

Quantum homodyne tomography of a two-photon Fock state.

We present a continuous-variable experimental analysis of a two-photon Fock state of free-propagating light. This state is obtained from a pulsed nondegenerate parametric amplifier, which produces two intensity-correlated twin beams. Counting two photons in one beam projects the other beam in the desired two-photon Fock state, which is analyzed by using a pulsed homodyne detection. The Wigner function of the measured state is clearly negative. We developed a detailed analytic model which allows a fast and efficient analysis of the experimental results.

Femtosecond Ti:sapphire cryogenic amplifier with high gain and MHz repetition rate

We demonstrate high gain amplification of 160-femtosecond pulses in a compact double-pass cryogenic Ti:sapphire amplifier. The setup involves a negative GVD mirrors recompression stage, and operates with a repetition rate between 0.2 and 4 MHz with a continuous pump laser. Amplification factors as high as 17 and 320 nJ Fourier-limited pulses are obtained at a 800 kHz repetition rate.

Characterization of Non-Classical Photonic States Retrieved from a Cold Atomic Memory

We experimentally demonstrate, through precise characterization methods, a model of efficiently transferring a non-classical state prepared in a cold-atom memory onto a controlled light pulse, fulfilling all requirements for subsequent use in quantum information processing.