Free-Space Quantum Communication with a Portable Quantum Memory

Mehdi Namazi, Giuseppe Vallone, Bertus Jordaan, Connor Goham, Reihaneh Shahrokhshahi, Paolo Villoresi, and Eden Figueroa

Phys. Rev. Applied 8, 064013 – Published 14 December 2017

Abstract

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 realization of a communication network functioning in a quantum regime, consisting of four different quantum modules: (i) a random polarization qubit generator, (ii) a free-space quantum-communication channel, (iii) an ultralow-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 $| H ⟩$ , $| V ⟩$ , $| D ⟩$ , and $| A ⟩$ 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 ultralow noise and fully portable operation, paving the way towards memory-assisted all-environment free-space quantum cryptographic networks.

Received 27 September 2016

DOI:https://doi.org/10.1103/PhysRevApplied.8.064013

Physics Subject Headings (PhySH)

Photonics Quantum communication Quantum cryptography Quantum information with atoms & light Quantum memories

Wireless communication networks

Quantum Information, Science & TechnologyAtomic, Molecular & Optical

Authors & Affiliations

Mehdi Namazi¹, Giuseppe Vallone², Bertus Jordaan¹, Connor Goham¹, Reihaneh Shahrokhshahi¹, Paolo Villoresi², and Eden Figueroa¹

¹Department of Physics and Astronomy, Stony Brook University, New York 11794-3800, USA
²Department of Information Engineering, University of Padova, Via Gradenigo 6b, 35131 Padova, Italy

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Vol. 8, Iss. 6 — December 2017

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Figure 1
Experimental setup for free-space quantum communication. In laboratory II, Alice creates a random sequence of four orthogonal qubits ( $| H ⟩, | V ⟩, | D ⟩, | A ⟩$ ). The 400-ns-long qubits are produced every $40 μ s$ . The qubits propagate in a free-space quantum-communication channel over a distance of approximately 20 m and are then directed into a dual-rail room-temperature rubidium vapor quantum memory in laboratory I. The control storage pulses are time optimized to the arrival of the qubits in front of the memory. In Bob’s site, a four-detector setup measures all possible bases at the exit of the memory to determine the quantum bit error rate (QBER). PBS, polarizing beam splitter; WP, wave plates; AOM, acousto-optical modulator; BD, beam displacer; GL, Glan-laser polarizer.
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Figure 2
Storage of a sequence of qubits. (a) A stream of polarization qubits with on average 3.5 photons propagates through a free-space quantum-communication channel of 20 m. In the quantum-memory site, the single-photon-level qubits are received and stored sequentially using timed control-field pulses. (b) Histograms for each of the polarization inputs after storage (dark blue) and background floor (light blue). Each histogram is presented in a $2 − μ s$ time interval (see dashed black divisions). The fidelities are estimated from the signal-to-background ratio.
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Figure 3
QBER evaluation of the long-distance communication setup plus memory. In Bob’s site, the polarization states are received and stored sequentially in a room-temperature quantum memory. We randomly choose one of the $Z$ and $X$ bases to measure the polarization state and then calculate the QBER over a region of interest equal to the input pulse width (red bars). We show histograms on the photon counts in each of the four polarizations. The first peak represents nonstored photons (leakage), while the second peak represents the retrieved photons. In an experiment with a high-input photon number, the obtained QBERs are less than 1%, as can be seen in the low counts corresponding to undesirable polarization detections.
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Figure 4
QBER evaluation for single-photon-level experiment. (a) The QBER is calculated in a 100-ns window (red bars). QBERs of 11.0% and 12.9% are, respectively, achieved for $Z$ and $X$ bases. At the single-photon level, undesirable polarization rotations remain absent, and noise in the orthogonal channel arises from control-field-induced nonlinear processes.
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Figure 5
Ultralow-noise quantum-memory operation. (a) Noise reduction by introducing an auxiliary field; the interaction between dark-state polaritons creates a background-free region. Retrieving the probe under these conditions results in a SBR of $> 25$ . The SBR is calculated using a 100-ns integration region at the peak of the retrieve signal and a minimized averaged background obtained in a $1 − μ s$ region centered around $5.2 μ s$ (divided by 10). The dark blue histogram includes the signal, background noise, and auxiliary field. The light blue histogram includes only the auxiliary field. (b) Quantum key distribution rate vs mean photon number and quantum bit error rate. The color bar represents the key rate. The line intersecting light blue and dark blue (negative key rate area) corresponds to the boundary for the positive key rate. The white dots indicate the regime of bare quantum memory and ultralow-noise memory regimes.
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Figure 6
(a) Prototype of a room-temperature portable quantum memory. Upper-right inset: Detail of one of the frequency filtering units, including the silica etalon, isolation oven, temperature control cold plate, and proportional integral differential temperature-regulation circuitry. Bottom-left inset: Detail of the interaction zone including the Rb cell, temperature control electronics, and three-layer magnetic shielding. (b) Storage of single-photon-level light pulses in the portable quantum memory.
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