The technologies of quantum information and quantum control are rapidly improving, but full exploitation of their capabilities requires complete characterization and assessment of processes that occur within quantum devices. We present a method for characterizing, with arbitrarily high accuracy, any quantum optical process. Our protocol recovers complete knowledge of the process by studying, via homodyne tomography, its effect on a set of coherent states, that is, classical fields produced by common laser sources. We demonstrate the capability of our protocol by evaluating and experimentally verifying the effect of a test process on squeezed vacuum.

The photon creation and annihilation operators are cornerstones of the quantum description of the electromagnetic field. They signify the isomorphism of the optical Hilbert space to that of the harmonic oscillator and the bosonic nature of photons. We perform complete experimental characterization (quantum process tomography) of these operators. By measuring their effect on coherent states by means of homodyne tomography, we obtain their process tensor in the Fock basis, which explicitly shows the "raising" and "lowering" properties of these operators with respect to photon number states. This is the first experimental demonstration of complete tomography of nondeterministic quantum processes.

We report complete characterization of an optical memory based on electromagnetically induced transparency. We recover the superoperator associated with the memory, under two different working conditions, by means of a quantum process tomography technique that involves storage of coherent states and their characterization upon retrieval. In this way, we can predict the quantum state retrieved from the memory for any input, for example, the squeezed vacuum or the Fock state. We employ the acquired superoperator to verify the nonclassicality benchmark for the storage of a Gaussian distributed set of coherent states.

Here we show an ultra-low noise regime of operation in a simple quantum memory in warm 87 Rb atomic vapor. By modelling the quantum dynamics of four-level room temperature atoms, we achieve fidelities >90% for single-photon level polarization qubits, clearly surpassing any classical strategy exploiting the non-unitary memory efficiency. This is the first time such important threshold has been crossed with a room temperature device. Additionally we also show novel experimental techniques capable of producing fidelities close to unity. Our results demonstrate the potential of simple, resource-moderate experimental room temperature quantum devices.PACS numbers: 42.50. Ex, 42.50.Gy Robust and operational room temperature quantum devices are a fundamental cornerstone towards building quantum networks composed of a large number of lightmatter interfaces [1,2]. Such quantum networks will be the basis of the creation of quantum repeater networks [3] and measurement device independent quantum cryptography links [4,5]. Given the recent success in the creation of elementary playgrounds in which single photons interact with atoms in controlled low temperature environments [6][7][8][9][10], the next technological frontier is the design of interfaces where such phenomena can be performed without extra-cooling [11][12][13][14][15]. The big challenge for such room temperature operation is to defeat the inherent strong atomic motion, decoherence and a considerable amount of background photons present [16][17][18][19][20][21][22][23]. A pertinent metric of these effects is the SBR, defined as η/q, where η is the retrieved fraction of a single excitation stored in a quantum memory and q the average number of concurrently emitted photons due to background processes. Quantum memory setup and storage parameters optimization. Our experimental setup includes four aspects of utmost relevance in order to allow for high SBR and quantum memory fidelity at the single-photon level: a) Dual rail operation. We store pulses containing on average one qubit in warm 87 Rb vapor using electromagnetically induced transparency (EIT). Two independent control beams coherently prepare two volumes within a single 87 Rb vapor cell at 60 • C, containing Kr buffer gas, thus serving as the storage medium for each mode of a polarization qubit. We employed two externalcavity diode lasers phase-locked at 6.835 GHz. The probe field frequency is stabilized to the 5S 1/2 F = 1 → 5P 1/2 F = 1 transition at a wavelength of 795 nm (detuning ∆) while the control field interacts with the 5S 1/2 F = 2 → 5P 1/2 F = 1 transition. b) Control field suppression. Polarization elements supply 42 dB of control field attenuation (80% probe transmission) while two temperature-controlled etalon resonators (linewidths of 40 and 24 MHz) provide additional 102 dB. The total probe field transmission is 4.5% for all polarization inputs, exhibiting an effective, control/probe suppression ratio of 130 dB. c) Background/efficiency compromise. The storage efficiency and the number of ba...

An optical quantum memory is a stationary device that is capable of storing and recreating photonic qubits with a higher fidelity than any classical device. Thus far, these two requirements have been fulfilled for polarization qubits in systems based on cold atoms and cryogenically cooled crystals. Here, we report a room-temperature memory capable of storing arbitrary polarization qubits with a signal-to-background ratio higher than 1 and an average fidelity surpassing the classical benchmark for weak laser pulses containing 1.6 photons on average, without taking into account non-unitary operation. Our results demonstrate that a common vapor cell can reach the low background noise levels necessary for polarization qubit storage using single-photon level light, and propels atomic-vapor systems towards a level of functionality akin to other quantum information processing architectures.