Bhaskar Roy Bardhan

Strong converse rates for classical communication over thermal and additive noise bosonic channels

We prove that several known upper bounds on the classical capacity of thermal and additive noise bosonic channels are actually strong converse rates. Our results strengthen the interpretation of these upper bounds, in the sense that we now know that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds these upper bounds. In order for these theorems to hold, we need to impose a maximum photon number constraint on the states input to the channel (the strong converse property need not hold if there is only a mean photon number constraint). Our rst theorem demonstrates that Koenig and Smith’s upper bound on the classical capacity of the thermal bosonic channel is a strong converse rate, and we prove this result by utilizing the structural decomposition of a thermal channel into a pure-loss channel followed by an amplier channel. Our second theorem demonstrates that Giovannetti et al.’s upper bound on the classical capacity of a thermal bosonic channel corresponds to a strong converse rate, and we prove this result by relating success probability to rate, the eective dimension of the output space, and the purity of the channel as measured by the R enyi collision entropy. Finally, we use similar techniques to prove that similar previously known upper bounds on the classical capacity of an additive noise bosonic channel correspond to strong converse rates.

Strong Converse for the Classical Capacity of Optical Quantum Communication Channels

We establish the classical capacity of optical quantum channels as a sharp transition between two regimes-one which is an error-free regime for communication rates below the capacity, and the other in which the probability of correctly decoding a classical message converges exponentially fast to zero if the communication rate exceeds the classical capacity. This result is obtained by proving a strong converse theorem for the classical capacity of all phase-insensitive bosonic Gaussian channels, a well-established model of optical quantum communication channels, such as lossy optical fibers, amplifier, and free-space communication. The theorem holds under a particular photon-number occupation constraint, which we describe in detail in this paper. Our result bolsters the understanding of the classical capacity of these channels and opens the path to applications, such as proving the security of noisy quantum storage models of cryptography with optical links.

Ultimate capacity of a linear time-invariant bosonic channel

Optical channels play an important role in the information era, in which establishing the ultimate rate for reliably transmitting communication is paramount. Building on recent breakthroughs, the authors derive the capacity of quantum channels including realistic processes, providing a direct link to practical broadband communications schemes.

Dynamical decoupling with tailored wave plates for long-distance communication using polarization qubits

We address the issue of dephasing effects in flying polarization qubits propagating through optical fiber by using the method of dynamical decoupling. The control pulses are implemented with half-wave plates suitably placed along the realistic lengths of the single-mode optical fiber. The effects of the finite widths of the wave plates on the polarization rotation are modeled using tailored refractive index profiles inside the wave plates. We show that dynamical decoupling is effective in preserving the input qubit state with the fidelity close to unity when the polarization qubit is subject to the random birefringent noise in the fiber, as well the rotational imperfections (flip-angle errors) due to the finite width of the wave plates.

Strong converse for the capacity of quantum Gaussian channels

We prove that a strong converse theorem holds for the classical capacity of all phase-insensitive bosonic Gaussian channels, when imposing a maximum photon number constraint on the inputs of the channel. This class is a natural extension of classical continuous Gaussian channels, and the well studied pure-loss, thermal, additive noise, and amplifier channels are all in this class of channels. The statement of the strong converse theorem is that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds the classical capacity. We prove this theorem by relating the success probability of any code with its rate of data transmission, the effective dimension of the channel output space, and the purity of the channel as quantified by the minimum output entropy. Our result bolsters the understanding of the classical capacity of these channels by establishing it as a sharp dividing line between possible and impossible communication rates over them.

Effects of Phase Fluctuations on the Sensitivity of NOON State in a Noisy Environment

We study effects of phase fluctuations on NOON state’s sensitivity in presence of realistic noise. Phase sensitivity is investigated with parity detection and calculated quantum Fisher information shows lower bound saturates quantum Cramer-Rao bound.

Ultimate capacity of linear time-invariant bosonic channels with additive Gaussian noise

Fiber-optic communications are moving to coherent detection in order to increase their spectral efficiency, i.e., their channel capacity per unit bandwidth. At power levels below the threshold for significant nonlinear effects, the channel model for such operation a linear time-invariant filter followed by additive Gaussian noise is one whose channel capacity is well known from Shannons noisy channel coding theorem. The fiber channel, however, is really a bosonic channel, meaning that its ultimate classical information capacity must be determined from quantum-mechanical analysis, viz. from the Holevo-Schumacher-Westmoreland (HSW) theorem. Based on recent results establishing the HSW capacity of a linear (lossy or amplifying) channel with additive Gaussian noise, we provide a general continuous-time result, namely the HSW capacity of a linear time-invariant (LTI) bosonic channel with additive Gaussian noise arising from a thermal environment. In particular, we treat quasi-monochromatic communication under an average power constraint through a channel comprised of a stable LTI filter that may be attenuating at all frequencies or amplifying at some frequencies and attenuating at others. Phase-insensitive additive Gaussian noise-associated with the continuous-time Langevin noise operator needed to preserve free-field commutator brackets is included at the filter output. We compare the resulting spectral efficiencies with corresponding results for heterodyne and homodyne detection over the same channel to assess the increased spectral efficiency that might be realized with optimum quantum reception.

Path-Symmetric States and Parity Detection in Quantum Optical Interferometry

In optical phase estimation, pure states that achieve maximal phase sensitivities with number counting, are symmetric with respect to a certain exchange of paths. We prove the optimality of photon-number parity detection for such path-symmetric states.

Quantum Entanglement For Target Detection And Probing Atmospheric Turbulence

We establish quantum entanglement as a direct measure if a target is present or not. We also provide a way to probe noisy atmosphere, estimating allowable range of noise for given entanglement to be preserved.

Dynamical Decoupling in Optical Fibers: Preserving Polarization Qubits from Birefringent Dephasing

We study preservation of polarization qubits in the polarization-maintaining fibers enhanced with dynamical decoupling sequence implemented in space instead of time. Such fibers maintain high fidelity with scalable waveplate implementations for specific input states.