Zheshen Zhang

A 24 km fiber-based discretely signaled continuous variable quantum key distribution system.

We report a continuous variable key distribution system that achieves a final secure key rate of 3.45 kilobits/s over a distance of 24.2 km of optical fiber. The protocol uses discrete signaling and post-selection to improve reconciliation speed and quantifies security by means of quantum state tomography. Polarization multiplexing and a frequency translation scheme permit transmission of a continuous wave local oscillator and suppression of noise from guided acoustic wave Brillouin scattering by more than 27 dB.

Unconditional Security of Time-energy Entanglement Quantum Key Distribution using Dual-basis Interferometry

High-dimensional quantum key distribution (HDQKD) offers the possibility of high secure-key rate with high photon-information efficiency. We consider HDQKD based on the time-energy entanglement produced by spontaneous parametric down-conversion and show that it is secure against collective attacks. Its security rests upon visibility data-obtained from Franson and conjugate-Franson interferometers-that probe photon-pair frequency correlations and arrival-time correlations. From these measurements, an upper bound can be established on the eavesdroppers Holevo information by translating the Gaussian-state security analysis for continuous-variable quantum key distribution so that it applies to our protocol. We show that visibility data from just the Franson interferometer provides a weaker, but nonetheless useful, secure-key rate lower bound. To handle multiple-pair emissions, we incorporate the decoy-state approach into our protocol. Our results show that over a 200-km transmission distance in optical fiber, time-energy entanglement HDQKD could permit a 700-bit/sec secure-key rate and a photon information efficiency of 2 secure-key bits per photon coincidence in the key-generation phase using receivers with a 15% system efficiency.

Full-band quantum-dynamical theory of saturation and four-wave mixing in graphene

The linear and nonlinear optical response of graphene are studied within a quantum-mechanical, full-band, steady-state density-matrix model. This nonpurtabative method predicts the saturatable absorption and saturable four-wave mixing of graphene. The model includes τ(1) and τ(2) time constants that denote carrier relaxation and quantum decoherence, respectively. Fits to existing experimental data yield τ(2) < 1 fs due to carrier-carrier scattering. τ(1) is found to be on the timescale from 250 fs to 550 fs, showing agreement with experimental data obtained by differential transmission measurements.

Optimum mixed-state discrimination for noisy entanglement-enhanced sensing

We propose a structured receiver for optimum mixed-state discrimination in quantum illumination target detection, paving the way for entanglement-enhanced minimum-error-probability sensing in an entanglement-breaking environment.

High-resolution measurement of fiber length by using a mode-locked fiber laser configuration

A simple method to precisely measure fiber length has been experimentally demonstrated by using a mode-locked fiber laser configuration. Since the transit time in a cavity is exactly proportional to the cavity length, it is easy to obtain the fiber length from the generation of mode-locked pulses in the fiber laser with a long-range nonlinear optical loop mirror that includes the measured fiber. Our new method has a large measurement range, over hundreds of kilometers, and a high resolution, of the order of centimeters, as well as no measurement dead zone.

Floodlight quantum key distribution: A practical route to gigabit-per-second secret-key rates

The channel loss incurred in long-distance transmission places a significant burden on quantum key distribution (QKD) systems: they must defeat a passive eavesdropper who detects all the light lost in the quantum channel and does so without disturbing the light that reaches the intended destination. The current QKD implementation with the highest long-distance secret-key rate meets this challenge by transmitting no more than one photon per bit [Opt. Express 21, 24550-24565 (2013)]. As a result, it cannot achieve the Gbps secret-key rate needed for one-time pad encryption of large data files unless an impractically large amount of multiplexing is employed. We introduce floodlight QKD (FL-QKD), which floods the quantum channel with a high number of photons per bit distributed over a much greater number of optical modes. FL-QKD offers security against the optimum frequency-domain collective attack by transmitting less than one photon per mode and using photon-coincidence channel monitoring, and it is completely immune to passive eavesdropping. More importantly, FL-QKD is capable of a 2 Gbps secret-key rate over a 50 km fiber link, without any multiplexing, using available equipment, i.e., no new technology need be developed. FL-QKD achieves this extraordinary secret-key rate by virtue of its unprecedented secret-key efficiency, in bits per channel use, which exceeds those of state-of-the-art systems by two orders of magnitude.

Efficient generation and characterization of spectrally factorable biphotons

Spectrally unentangled biphotons with high single-spatiotemporal-mode purity are highly desirable for many quantum information processing tasks. We generate biphotons with an inferred heralded-state spectral purity of 99%, the highest to date without any spectral filtering, by pulsed spontaneous parametric downconversion in a custom-fabricated periodically-poled KTiOPO4 crystal under extended Gaussian phase-matching conditions. To efficiently characterize the joint spectral intensity of the generated biphotons at high spectral resolution, we employ a commercially available dispersion compensation module (DCM) with a dispersion equivalent to 100 km of standard optical fiber and with an insertion loss of only 2.8 dB. Compared with the typical method of using two temperature-stabilized equal-length fibers that incurs an insertion loss of 20 dB per fiber, the DCM approach achieves high spectral resolution in a much shorter measurement time. Because the dispersion amount and center wavelengths of DCMs can be easily customized, spectral characterization in a wide range of quantum photonic applications should benefit significantly from this technique.

Security of a discretely signaled continuous variable quantum key distribution protocol for high rate systems.

We propose a continuous variable based quantum key distribution protocol that makes use of discretely signaled coherent light and reverse error reconciliation. We present a rigorous security proof against collective attacks with realistic lossy, noisy quantum channels, imperfect detector efficiency, and detector electronic noise. This protocol is promising for convenient, high-speed operation at link distances up to 50 km with the use of post-selection.

Floodlight quantum key distribution: Demonstrating a framework for high-rate secure communication

Floodlight quantum key distribution (FL-QKD) is a radically different QKD paradigm that can achieve Gbit/s secret-key rates over metropolitan area distances without multiplexing [Phys. Rev. A 94, 012322 (2016)]. It is a two-way protocol that transmits many photons per bit duration and employs a high-gain optical amplifier, neither of which can be utilized by existing QKD protocols to mitigate channel loss. FL-QKD uses an optical bandwidth that is substantially larger than the modulation rate and performs decoding with a unique broadband homodyne receiver. Essential to FL-QKD is Alices injection of photons from a photon-pair source--in addition to the light used for key generation--into the light she sends to Bob. This injection enables Alice and Bob to quantify Eves intrusion and thus secure FL-QKD against collective attacks. Our proof-of-concept experiment included 10 dB propagation loss--equivalent to 50 km of low-loss fiber--and achieved a 55 Mbit/s secret-key rate (SKR) for a 100 Mbit/s modulation rate, as compared to the state-of-the-art systems 1 Mbit/s SKR for a 1 Gbit/s modulation rate [Opt. Express 21, 24550-24565 (2013)], representing ~500-fold and ~50-fold improvements in secret-key efficiency (SKE) (bits per channel use) and SKR (bits per second), respectively.

Entanglement-enhanced lidars for simultaneous range and velocity measurements

Lidar is a well known optical technology for measuring a targets range and radial velocity. We describe two lidar systems that use entanglement between transmitted signals and retained idlers to obtain significant quantum enhancements in simultaneous measurement of these parameters. The first entanglement-enhanced lidar circumvents the Arthurs-Kelly uncertainty relation for simultaneous measurement of range and radial velocity from detection of a single photon returned from the target. This performance presumes there is no extraneous (background) light, but is robust to the roundtrip loss incurred by the signal photons. The second entanglement-enhanced lidar---which requires a lossless, noiseless environment---realizes Heisenberg-limited accuracies for both its range and radial-velocity measurements, i.e., their root-mean-square estimation errors are both proportional to