Justin Dove

Photonic Boson Sampling in a Tunable Circuit

Computing Power of Quantum Mechanics There is much interest in developing quantum computers in order to perform certain tasks much faster than, or that are intractable for, a classical computer. A general quantum computer, however, requires the fabrication and operation a number of quantum logic devices (see the Perspective by Franson). Broome et al. (p. 794, published online 20 December) and Spring et al. (p. 798, published online 20 December) describe experiments in which single photons and quantum interference were used to perform a calculation (the permanent of a matrix) that is very difficult on a classical computer. Similar to random walks, quantum walks on a graph describe the movement of a walker on a set of predetermined paths; instead of flipping a coin to decide which way to go at each point, a quantum walker can take several paths at once. Childs et al. (p. 791) propose an architecture for a quantum computer, based on quantum walks of multiple interacting walkers. The system is capable of performing any quantum operation using a subset of its nodes, with the size of the subset scaling favorably with the complexity of the operation. Optical circuits are used to demonstrate a quantum-enhanced calculation. [Also see Perspective by Franson] Quantum computers are unnecessary for exponentially efficient computation or simulation if the Extended Church-Turing thesis is correct. The thesis would be strongly contradicted by physical devices that efficiently perform tasks believed to be intractable for classical computers. Such a task is boson sampling: sampling the output distributions of n bosons scattered by some passive, linear unitary process. We tested the central premise of boson sampling, experimentally verifying that three-photon scattering amplitudes are given by the permanents of submatrices generated from a unitary describing a six-mode integrated optical circuit. We find the protocol to be robust, working even with the unavoidable effects of photon loss, non-ideal sources, and imperfect detection. Scaling this to large numbers of photons should be a much simpler task than building a universal quantum computer.

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.

Phase-noise limitations on single-photon cross-phase modulation with differing group velocities

A framework is established for evaluating {\sc cphase} gates that use single-photon cross-phase modulation (XPM) originating from the Kerr nonlinearity. Prior work Phys. Rev. A {\bf 73,} 062305 (2006)], which assumed that the control and target pulses propagated at the same group velocity, showed that the causality-induced phase noise required by a non-instantaneous XPM response function precluded the possibility of high-fidelity

Phase-Noise Limitations on Nonlinear-Optical Quantum Computing

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Experimental bosonsampling in a photonic circuit

-radian conditional phase shifts. The framework presented herein incorporates the more realistic case of group-velocity disparity between the control and target pulses, as employed in existing XPM-based fiber-optical switches. Nevertheless, the causality-induced phase noise identified in [Phys. Rev. A {\bf 73,} 062305 (2006)] still rules out high-fidelity

BosonSampling with realistic single-photon sources

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Experimental BosonSampling in a tunable optical network

-radian conditional phase shifts. This is shown to be so for both a reasonable theoretical model for the XPM response function and for the experimentally-measured XPM response function of silica-core fiber.

Experimental Boson Sampling

We establish a framework for evaluating CPHASE gates that use single-photon Kerr nonlinearities in which one pulse overtakes another. We show that causality-induced phase noise precludes the possibility of high-fidelity π-radian conditional phase shifts.

Ultrabroadband Quantum-Secured Communication

The extended Church-Turing thesis posits that any computable function can be calculated efficiently by a probabilistic Turing machine. If this thesis held true, the global effort to build quantum computers might ultimately be unnecessary. The thesis would however be strongly contradicted by a physical device that efficiently performs a task believed to be intractable for classical computers. BosonSampling-the sampling from a distribution of n photons undergoing some linear-optical process-is a recently developed, and experimentally accessible example of such a task.

Floodlight Quantum Key Distribution: Breaking The One Photon Per Bit Barrier

Summary form only given. The extended Church-Turing thesis posits that any computable function can be calculated efficiently by a probabilistic Turing machine. If this thesis held true, the global effort to build quantum computers might ultimately be unnecessary. The thesis would however be strongly contradicted by a physical device that efficiently performs a task believed to be intractable for classical computers. BosonSampling - the sampling from a distribution of n photons undergoing some linear-optical process - is a recently developed, experimentally accessible example of such a task [1].Here we report an experimental verification of one key assumption of BosonSampling: that multi-photon interference amplitudes are given by the permanents of submatrices of a larger unitary describing the photonic circuit. We built a tunable photonic circuit consisting of a central 3x3 fiber beamsplitter (Fig. 1) and exploited orthogonal polarization modes to extend the network to 6x6 modes. We developed a direct characterization method [2] to obtain the unitary description of this network and compared theoretical interference patterns predicted from this unitary with an experimental signature obtained via non-classical interference of three single photons [3].Our results show good agreement with theory, and we can rule out an explanation of the observed interference via classical means. We conclude that small-scale BosonSampling can be performed in the presence of unavoidable optical loss, imperfect photon sources, and inefficient detection [3].To reach a regime of 20 to 30 photons, where BosonSampling experiments are expected to start outperforming modern computers, we need to precisely quantify the contributions of realistic noise created in current photon sources. To this end, we modeled the detrimental effects of spectral and photon-number impurity [4] of independently generated photon pairs on the expected multi-photon interference patterns. We tested our model by fully mapping out three-photon interference as a function of individual temporal delays. While a full-scale demonstration is still out of reach, our results promise that a scaling-up of BosonSampling to single-photon numbers reached in state-of-the-art quantum optics experiments is feasible.