Session F32: Invited Session: Quantum Communication and Cryptography

Network-Centric Quantum Communications

Single-photon quantum communications (QC) offers ``future-proof'' cryptographic security rooted in the laws of physics. Today's quantum-secured communications cannot be compromised by unanticipated future technological advances. But to date, QC has only existed in point-to-point instantiations that have limited ability to address the cyber security challenges of our increasingly networked world. In my talk I will describe a fundamentally new paradigm of network-centric quantum communications (NQC) that leverages the network to bring scalable, QC-based security to user groups that may have no direct user-to-user QC connectivity. With QC links only between each of N users and a trusted network node, NQC brings quantum security to N$^{\mathrm{2}}$ user pairs, and to multi-user groups. I will describe a novel integrated photonics quantum smartcard (``QKarD'') and its operation in a multi-node NQC test bed. The QKarDs are used to implement the quantum cryptographic protocols of quantum identification, quantum key distribution and quantum secret splitting. I will explain how these cryptographic primitives are used to provide key management for encryption, authentication, and non-repudiation for user-to-user communications. My talk will conclude with a description of a recent demonstration that QC can meet both the security and quality-of-service (latency) requirements for electric grid control commands and data. These requirements cannot be met simultaneously with present-day cryptography.   

Spectrally Multiplexed Solid-State Memories for Quantum Repeaters

Quantum communication is currently limited to channel lengths on the order of 100 km. The possibility to overcome this limit hinges on using a quantum repeater that in turn relies on the heralded distribution of entangled photons across subsections of the whole communication channel, on the storage of entanglement (by quantum memories) at the end-nodes of each subsection, and on the swapping of entanglement established in neighbouring sections to the end-points of the total channel. Workable distribution rates can be obtained if multiplexing is used to overcome the low success probabilities of multi-photon operations required in such a repeater. To this end, quantum memory research has focused on photons arriving at different times at the memory (i.e. temporal multiplexing) and recall on demand via a variable storage time. However, quantum repeaters multiplexed with respect to other degrees-of-freedom, such as frequency (spectral multiplexing), can be utilized with memories having fixed storage times, supplemented with on-demand shifting in the degree-of-freedom of importance. In this talk we describe how to build a quantum repeater using qubits encoded into different spectral modes, and present experimental results showing readout on demand from a spectrally multiplexed quantum memory based on atomic frequency combs in a Ti:Tm:LiNbO$_3$ waveguide cooled to 3 K. Our measured fidelity of 0.95 $\pm$ 0.03 significantly violates the maximum fidelity of 0.67 achievable using a classical memory, confirming the validity of the spectral multiplexing approach. We anticipate that this will accelerate the development of quantum repeaters, linear optics quantum computing, and advanced quantum optics experiments. \\[4pt] In collaboration with E. Saglamyurek, H. Mallahzadeh, J.A. Slater, M.P. Hedges, D. Oblak, C. Simon and W. Tittel, Institute of Science and Technology, and Department of Physics \& Astronomy, University of Calgary; and M. George, R. Ricken and W. Sohler, Department of Physics -- Applied Physics, University of Paderborn, Germany.   

Probabilistic protocols in quantum information science: Use and abuse

Protocols in quantum information science often succeed with less than unit probability, but nonetheless perform useful tasks because success occurs often enough to make tolerable the overhead from having to perform the protocol several times. Any probabilistic protocol must be analyzed from the perspective of the resources required to make the protocol succeed. I present results from analyses of two probabilistic protocols: (i) nondeterministic (or immaculate) linear amplification, in which an input coherent state is amplified some of the time to a larger-amplitude coherent state, and (ii) probabilistic quantum metrology, in which one attempts to improve estimation of a parameter (or parameters) by post-selecting on a particular outcome. The analysis indicates that there is little to be gained from probabilistic protocols in these two situations.   

Toward Noiseless Amplification and Frequency Conversion

In this talk, I will review recent progress toward noiseless amplification and frequency conversion by four-wave mixing (FWM) processes in optical fibers. In these processes, one or two strong pump waves drive weak signal and idler waves (or photon wavepackets). Depending on the relative frequencies of the waves, FWM can amplify the signal (without frequency conversion) or frequency convert it (with or without amplification). These functions enable a variety of applications. Amplification with a noise figure of 1 dB (close to the quantum limit of 0 dB) has been demonstrated. So also has the frequency conversion of single photons. I will review these results in the contexts of conventional communication systems and quantum information science. A theme of current research is the encoding of information in different temporal eigenfunctions of arbitrary single-photon wavepackets. FWM driven by pulsed pumps provides the means to detect and manipulate information encoded in these eigenfunctions.