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.
The realization of an elementary quantum network that is intrinsically secure and operates over long distances requires the interconnection of several quantum modules performing different tasks. In this work we report the interconnection of four different quantum modules: (i) a random polarization qubit generator, (ii) a free-space quantum communication channel, (iii) an ultra-low noise portable quantum memory and (iv) a qubit decoder, in a functional elementary quantum network possessing all capabilities needed for quantum information distribution protocols. We create weak coherent pulses at the single photon level encoding polarization states |H , |V , |D , |A in a randomized sequence. The random qubits are sent over a free-space link and coupled into a dual rail room temperature quantum memory and after storage and retrieval are analyzed in a four detector polarization analysis akin to the requirements of the BB84 protocol. We also show ultra-low noise and fully-portable operation, paving the way towards memory assisted all-environment free space quantum cryptographic networks.
Any optical quantum information processing machine would be comprised of fully-characterized constituent devices for both single state manipulations and tasks involving the interaction between multiple quantum optical states. Ideally for the latter, would be an apparatus capable of deterministic optical phase shifts that operate on input quantum states with the action mediated solely by auxiliary signal fields. Here we present the complete experimental characterization of a system designed for optically controlled phase shifts acting on single-photon level probe coherent states. Our setup is based on a warm vapor of rubidium atoms under the conditions of electromagnetically induced transparency with its dispersion properties modified through the use of an optically triggered N-type Kerr non-linearity. We fully characterize the performance of our device by sending in a set of input probe states and measuring the corresponding output via time-domain homodyne tomography and subsequently performing the technique of coherent state quantum process tomography. This method provides us with the precise knowledge of how our optical phase shift will modify any arbitrary input quantum state engineered in the mode of the reconstruction.
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