Showing papers by "Jeffrey H. Shapiro published in 2018"

Distributed quantum sensing using continuous-variable multipartite entanglement

Abstract: Distributed quantum sensing uses quantum correlations between multiple sensors to enhance the measurement of unknown parameters beyond the limits of unentangled systems. We describe a sensing scheme that uses continuous-variable multipartite entanglement to enhance distributed sensing of field-quadrature displacement. By dividing a squeezed-vacuum state between multiple homodyne-sensor nodes using a lossless beam-splitter array, we obtain a root-mean-square (rms) estimation error that scales inversely with the number of nodes (Heisenberg scaling), whereas the rms error of a distributed sensor that does not exploit entanglement is inversely proportional to the square root of the number of nodes (standard quantum limit scaling). Our sensor's scaling advantage is destroyed by loss, but it nevertheless retains an rms-error advantage in settings in which there is moderate loss. Our distributed sensing scheme can be used to calibrate continuous-variable quantum key distribution networks, to perform multiple-sensor cold-atom temperature measurements, and to do distributed interferometric phase sensing.

Qudit-Basis Universal Quantum Computation Using χ^{(2)} Interactions.

Abstract: We prove that universal quantum computation can be realized---using only linear optics and ${\ensuremath{\chi}}^{(2)}$ (three-wave mixing) interactions---in any ($n+1$)-dimensional qudit basis of the $n$-pump-photon subspace. First, we exhibit a strictly universal gate set for the qubit basis in the one-pump-photon subspace. Next, we demonstrate qutrit-basis universality by proving that ${\ensuremath{\chi}}^{(2)}$ Hamiltonians and photon-number operators generate the full $\mathfrak{u}(3)$ Lie algebra in the two-pump-photon subspace, and showing how the qutrit controlled-$Z$ gate can be implemented with only linear optics and ${\ensuremath{\chi}}^{(2)}$ interactions. We then use proof by induction to obtain our general qudit result. Our induction proof relies on coherent photon injection or subtraction, a technique enabled by ${\ensuremath{\chi}}^{(2)}$ interaction between the encoding modes and ancillary modes. Finally, we show that coherent photon injection is more than a conceptual tool, in that it offers a route to preparing high-photon-number Fock states from single-photon Fock states.

Quantum-optimal detection of one-versus-two incoherent optical sources with arbitrary separation

Abstract: We analyze the fundamental quantum limit of the resolution of an optical imaging system from the perspective of the detection problem of deciding whether the optical field in the image plane is generated by one incoherent on-axis source with brightness $$\epsilon$$ or by two $$\epsilon {\mathrm{/}}2$$ -brightness incoherent sources that are symmetrically disposed about the optical axis. Using the exact thermal-state model of the field, we derive the quantum Chernoff bound for the detection problem, which specifies the optimum rate of decay of the error probability with increasing number of collected photons that is allowed by quantum mechanics. We then show that recently proposed linear-optic schemes approach the quantum Chernoff bound—the method of binary spatial-mode demultiplexing (B-SPADE) is quantum-optimal for all values of separation, while a method using image inversion interferometry (SLIVER) is near-optimal for sub-Rayleigh separations. We then simplify our model using a low-brightness approximation that is very accurate for optical microscopy and astronomy, derive quantum Chernoff bounds conditional on the number of photons detected, and show the optimality of our schemes in this conditional detection paradigm. For comparison, we analytically demonstrate the superior scaling of the Chernoff bound for our schemes with source separation relative to that of spatially resolved direct imaging. Our schemes have the advantages over the quantum-optimal (Helstrom) measurement in that they do not involve joint measurements over multiple modes, and that they do not require the angular separation for the two-source hypothesis to be given a priori and can offer that information as a bonus in the event of a successful detection. Two schemes to distinguish between emitters can nearly reach the best possible resolution more easily than the known optimal method. The accuracy with which two light sources can be identified is normally taken from the Rayleigh criterion, which is a classical calculation. By performing quantum measurements the classical limit can be beaten, but the protocol necessary to achieve the optimal result involves difficult-to-perform joint measurements and knowledge of the source separation. Xiao-Ming Lu from Hangzhou Dianzi University and the National University of Singapore, with colleagues from the USA, has shown that two previously-introduced measurement schemes called B-SPADE and SLIVER can achieve nearly the same accuracy while being much simpler to perform. Potential applications of the result include identifying binary stars and identifying separate emitters in medical fluorescence imaging.

Distributed Quantum Sensing Using Continuous-Variable Multipartite Entanglement

Abstract: Continuous-variable multipartite entanglement, obtained from dividing a squeezed state between multiple nodes, is shown to enhance distributed field-quadrature displacement sensing. In the lossless case, its performance has Heisenberg scaling in the number of nodes.

Experimental quantum key distribution at 1.3 gigabit-per-second secret-key rate over a 10 dB loss channel

Abstract: Quantum key distribution (QKD) enables unconditionally secure communication ensured by the laws of physics, opening a promising route to security infrastructure for the coming age of quantum computers. QKD's demonstrated secret-key rates (SKRs), however, fall far short of the gigabit-per-second rates of classical communication, hindering QKD's widespread deployment. QKD's low SKRs are largely due to existing single-photon-based protocols' vulnerability to channel loss. Floodlight QKD (FL-QKD) boosts SKR by transmitting many photons per encoding, while offering security against collective attacks. Here, we report an FL-QKD experiment operating at a 1.3 Gbit s−1 SKR over a 10 dB loss channel. To the best of our knowledge, this is the first QKD demonstration that achieves a gigabit-per-second-class SKR, representing a critical advance toward high-rate QKD at metropolitan-area distances.

Security-proof framework for two-way Gaussian quantum-key-distribution protocols

Abstract: Two-way Gaussian protocols have the potential to increase quantum key distribution (QKD) protocols' secret-key rates by orders of magnitudes [Phys. Rev. A 94, 012322 (2016)]. Security proofs for two-way protocols, however, are underdeveloped at present. In this paper, we establish a security proof framework for the general coherent attack on two-way Gaussian protocols in the asymptotic regime. We first prove that coherent-attack security can be reduced to collective-attack security for all two-way QKD protocols. Next, we identify two different constraints that each provide intrusion parameters which bound an eavesdropper's coherent-attack information gain for any two-way Gaussian QKD protocol. Finally, we apply our results to two such protocols.

Experimental Quantum Key Distribution at 1.3 Gbit/s Secret-Key Rate over a 10-dB-Loss Channel

Abstract: We demonstrate quantum key distribution at 1.3-Gbit/s secret-key rates over a 10-dB-loss channel. By transmitting many photons per bit with multi-mode encoding our protocol overcomes channel loss to achieve gigabit-per-second rates without compromising security.

High-visibility interference between two heralded pure-state photons without spectral filtering

Abstract: We measure 90% visibility in Hong-Ou-Mandel interference between two independent heralded single photons from pulsed spontaneous parametric downconversion with a Gaussian spectral phase-matching profile, and study its dependence on the pump spectral properties.