Free space propagation and conventional optical systems such as lenses and mirrors all perform spatial unitary transforms. However, the subset of transforms available through these conventional systems is limited in scope. We present here a unitary programmable mode converter (UPMC) capable of performing any spatial unitary transform of the light field. It is based on a succession of reflections on programmable deformable mirrors and free space propagation. We first show theoretically that a UPMC without limitations on resources can perform perfectly any transform. We then build an experimental implementation of the UPMC and show that, even when limited to three reflections on an array of 12 pixels, the UPMC is capable of performing single mode tranforms with an efficiency greater than 80% for the first four modes of the transverse electromagnetic basis.

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Using electromagnetically induced transparency (EIT), it is possible to delay and store light in atomic ensembles. Theoretical modeling and recent experiments have suggested that the EIT storage mechanism can be used as a memory for quantum information. We present experiments that quantify the noise performance of an EIT system for conjugate amplitude and phase quadratures. It is shown that our EIT system adds excess noise to the delayed light that has not hitherto been predicted by published theoretical modeling. In analogy with other continuous-variable quantum information systems, the performance of our EIT system is characterized in terms of conditional variance and signal transfer. DOI: 10.1103/PhysRevLett.97.183601 PACS numbers: 42.50.Gy, 03.67.ÿa Following theoretical proposals [1], electromagnetically induced transparency (EIT) [2] has become the subject of much interest for controlled atomic storage of quantum states of light. Indeed, the delay and storage of optical qubits in an atomic medium via EIT has recently been shown allowing, in principle, the synchronization of quantum information processing systems [3,4]. Earlier works with classical signals in a vapor cell [5] and cold atoms [6] have shown large signal delay with group velocities as low as 17 ms ÿ1 . Storage of classical pulses has also been shown for atomic vapor cells [7,8], cold atomic clouds [9], and solid state systems [10,11] (although it should be noted that alternative interpretations of such pulse storage experiments have also been published [12,13] ). One experiment [14] has even shown the transmission of a squeezed state through an EIT system in a vapor cell under the conditions of very small delay. While these experiments are all excellent demonstrations of EIT, to the best of our knowledge, no attempt has been made to experimentally quantify the efficacy of EIT for continuousvariable quantum information systems.Quantum-theoretical treatments of delay and storage via EIT, in the presence of decoherences, have suggested that no excess noise is added to the delayed light [15][16][17][18]. These works show that the degradation of a quantum state in an EIT system results from-(i) the finite transparency window and (ii) a degradation in the transparency induced by ground state dephasing. The implication is that, within the EIT window and for small ground state dephasing, quantum states of light can be delayed and preserved in an EIT medium. In this Letter, we present experimental results that examine the quantum noise performance of an EIT system for conjugate amplitude and phase quadratures that are measured at sideband frequencies (!) around the optical carrier. Since much work on EIT is motivated by quantum information processing, we evaluate the performance of the EIT system using well-established criteria for continuous-variable (CV) quantum state measurements. In analogy with quantum teleportation and nondemolition experiments where states are transferred from an input to an output, we utilize the conditional variance and si...

The traversal of an elliptically polarized optical field through a thermal vapor cell can give rise to a rotation of its polarization axis. This process, known as polarization self-rotation ͑PSR͒, has been suggested as a mechanism for producing squeezed light at atomic transition wavelengths. We show results of the characterization of PSR in isotopically enhanced rubidium-87 cells, performed in two independent laboratories. We observed that, contrary to earlier work, the presence of atomic noise in the thermal vapor overwhelms the observation of squeezing. We present a theory that contains atomic noise terms and show that a null result in squeezing is consistent with this theory.

We demonstrate the possibility of surpassing the quantum noise limit for simultaneous multi-axis spatial displacement measurements that have zero mean values. The requisite resources for these measurements are squeezed light beams with exotic transverse mode profiles. We show that, in principle, lossless combination of these modes can be achieved using the non-degenerate Gouy phase shift of optical resonators. When the combined squeezed beams are measured with quadrant detectors, we experimentally demonstrate a simultaneous reduction in the transverse x-and ydisplacement fluctuations of 2.2 dB and 3.1 dB below the quantum noise limit.