Brent J. Yen

Minimum Rényi and Wehrl entropies at the output of bosonic channels

The minimum Renyi and Wehrl output entropies are found for bosonic channels in which the signal photons are either randomly displaced by a Gaussian distribution (classical-noise channel), or coupled to a thermal environment through lossy propagation (thermal-noise channel). It is shown that the Renyi output entropies of integer orders z{>=}2 and the Wehrl output entropy are minimized when the channel input is a coherent state.

Capacity of Bosonic Communications

The capacity C for transmitting classical information is investigated for Bosonic channels with isotropic Gaussian noise. For the pure‐loss channel—in which signal photons may be lost in propagation—the exact value of C is derived. The Holevo information of this channel is shown to be additive, and a “classical” encoding procedure employing coherent states is shown to achieve capacity. For active channel models—in which noise photons are injected from an external environment or the signal is amplified with unavoidable quantum noise—upper and lower bounds are obtained for the capacity. These bounds are asymptotically tight at low and high noise levels. Exact capacity results—given by the lower bounds—would follow from proving the conjecture that a coherent‐state input minimizes the output entropy from such channels.There are many possible gravitational applications of an effective approach to Quantum Field Theory (QFT) in curved space. We present a brief review of effective approach and discuss its impact for such relevant issues as the cosmological constant (CC) problem and inflation driven by vacuum quantum effects. Furthermore it is shown how one can impose significant theoretical constraints on a non-metric gravity using only theoretical effective field theory framework.

Symmetric M-ary phase discrimination using quantum-optical probe states

We present a theoretical study of minimum error probability discrimination, using quantum- optical probe states, of M optical phase shifts situated symmetrically on the unit circle. We assume ideal lossless conditions and full freedom for implementing quantum measurements and for probe state selection, subject only to a constraint on the average energy, i.e., photon number. In particular, the probe state is allowed to have any number of signal and ancillary modes, and to be pure or mixed. Our results are based on a simple criterion that partitions the set of pure probe states into equivalence classes with the same error probability performance. Under an energy constraint, we find the explicit form of the state that minimizes the error probability. This state is an unentangled but nonclassical single-mode state. The error performance of the optimal state is compared with several standard states in quantum optics. We also show that discrimination with zero error is possible only beyond a threshold energy of (M - 1)/2. For the M = 2 case, we show that the optimum performance is readily demonstrable with current technology. While transmission loss and detector inefficiencies lead to a nonzero erasure probability, the error rate conditional on no erasure is shown to remain the same as the optimal lossless error rate.

Optimal quantum states for image sensing in loss.

We consider a general image sensing framework that includes many quantum sensing problems by an appropriate choice of image set, prior probabilities, and cost function. For any such problem, in the presence of loss and a signal energy constraint, we show that a pure input state of light with the signal modes in a mixture of number states minimizes the cost among all ancilla-assisted parallel strategies. Lossy binary phase discrimination with a peak photon number constraint and general lossless image sensing are considered as examples.

Minimum Bosonic Channel Output Entropies

Coherent‐state inputs are conjectured to minimize the von Neumann entropies at the outputs of two Bosonic channels with thermal noise. Evidence in support of this conjecture is provided, including the fact that coherent‐state inputs minimize the integer‐order Renyi entropy and the Wehrl entropy at the outputs of these channels. A stronger conjecture—that output states resulting from coherent‐state inputs majorize the output states from other inputs—is also discussed.

Error models for long-distance qubit teleportation

A recent proposal for realizing long-distance, high-fidelity qubit teleportation is reviewed. This quantum communication architecture relies on an ultrabright source of polarization-entangled photons plus a pair of trapped-atom quantum memories, and it is compatible with long-distance transmission over standard telecommunication fiber. Models are developed for assessing the effects of amplitude, phase, and frequency errors in the entanglement source, as well as fiber loss and imperfect polarization restoration, on the throughput and fidelity of the system.

Quantum enhancement of a coherent ladar receiver using phase-sensitive amplification

We demonstrate a balanced-homodyne LADAR receiver employing a phase-sensitive amplifier (PSA) to raise the effective photon detection efficiency (PDE) to nearly 100%. Since typical LADAR receivers suffer from losses in the receive optical train that routinely limit overall PDE to less than 50% thus degrading SNR, PSA can provide significant improvement through amplification with noise figure near 0 dB. Receiver inefficiencies arise from sub-unity quantum efficiency, array fill factors, signal-local oscillator mixing efficiency (in coherent receivers), etc. The quantum-enhanced LADAR receiver described herein is employed in target discrimination scenarios as well as in imaging applications. We present results showing the improvement in detection performance achieved with a PSA, and discuss the performance advantage when compared to the use of a phase-insensitive amplifier, which cannot amplify noiselessly.

Quantum enhanced lidar resolution with multi-spatial-mode phase sensitive amplification

Phase-sensitive amplification (PSA) can enhance the signal-to-noise ratio (SNR) of an optical measurement suffering from detection inefficiency. Previously, we showed that this increased SNR improves LADAR-imaging spatial resolution when infinite spatial-bandwidth PSA is employed. Here, we evaluate the resolution enhancement for realistic, finite spatial-bandwidth amplification. PSA spatial bandwidth is characterized by numerically calculating the input and output spatial modes and their associated phase-sensitive gains under focused-beam pumping. We then compare the spatial resolution of a baseline homodyne-detection LADAR system with homodyne LADAR systems that have been augmented by pre-detection PSA with infinite or finite spatial bandwidth. The spatial resolution of each system is quantified by its ability to distinguish between the presence of 1 point target versus 2 closely-spaced point targets when minimum error-probability decisions are made from quantum limited measurements. At low (5-10 dB) SNR, we find that a PSA system with a 2.5kWatts pump focused to 25μm × 400μm achieves the same spatial resolution as a baseline system having 5.5 dB higher SNR. This SNR gain is very close to the 6 dB SNR improvement possible with ideal (infinite bandwidth, infinite gain) PSA at our simulated system detection efficiency (0.25). At higher SNRs, we have identified a novel regime in which finite spatial-bandwidth PSA outperforms its infinite spatial-bandwidth counterpart. We show that this performance crossover is due to the focused pump systems input-to-output spatial-mode transformation converting the LADAR measurement statistics from homodyne to heterodyne performance.

Information capacity of bosonic channels

The capacity C for transmitting classical information is investigated for noisy bosonic channel models. An exact result is obtained for the pure-loss case. Upper and lower bounds are established for channels with active noise sources

Error models and error mitigation for long-distance high-fidelity quantum secret sharing

A quantum communication architecture is being developed for long-distance, high-fidelity transmission and storage of Greenberger-Horne-Zeilinger states. This system uses an ultrabright narrowband source of polarization-entangled photons plus trapped-atom quantum memories, and it is compatible with long-distance transmission over standard telecommunication fiber. An error model for the preceding architecture is derived, and the use of quantum error correction or entanglement purification protocols to improve the performance of this quantum communication system is also discussed.