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Producing and measuring entanglement between two beams of microwave light

Emmanuel Flurin, Nicolas Roch, F. Mallet, Michel Devoret, Benjamin Huard

Published Apr 3, 2012• 0 citations
OpenAlex

Abstract

Pairs of entangled electromagnetic beams propagating on physically separated channels, the continuous variable version of Einstein-Podolsky-Rosen (EPR) states, constitute an essential resource in quantum information processing, communication and measurements. We have performed an interference experiment demonstrating the production and measurement of EPR states of microwave light using non-degenerate Josephson mixers. Driven by a pump tone, a first mixer generates a pair entangled beams out of vacuum quantum noise. A second mixer, driven by a phase shifted copy of the first pump tone, recombines and disentangles the two beams. We show that interference fringes of the intensity of the final output noise, referred to the input of the second mixer, pass under the level of vacuum for a range of phase shifts. The minimal measured noise intensity provides a quantitative measure of entanglement and yields a lower bound of 6 Mebits/s on the rate of entangled bits produced by the first mixer.

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Pairs of entangled electromagnetic beams propagating on physically separated channels, the continuous variable version of Einstein-Podolsky-Rosen (EPR) states, constitute an essential resource in quantum information processing, communication and measurements. We have performed an interference experiment demonstrating the production and measurement of EPR states of microwave light using non-degenerate Josephson mixers. Driven by a pump tone, a first mixer generates a pair entangled beams out of vacuum quantum noise. A second mixer, driven by a phase shifted copy of the first pump tone, recombines and disentangles the two beams. We show that interference fringes of the intensity of the final output noise, referred to the input of the second mixer, pass under the level of vacuum for a range of phase shifts. The minimal measured noise intensity provides a quantitative measure of entanglement and yields a lower bound of 6 Mebits/s on the rate of entangled bits produced by the first mixer.
Using a superconducting circuit, the Josephson mixer, we demonstrate the first experimental realization of spatially separated two-mode squeezed states of microwave light. Driven by a pump tone, a first Josephson mixer generates, out of quantum vacuum, a pair of entangled fields at different frequencies on separate transmission lines. A second mixer, driven by a π-phase shifted copy of the first pump tone, recombines and disentangles the two fields. The resulting output noise level is measured to be lower than for the vacuum state at the input of the second mixer, an unambiguous proof of entanglement. Moreover, the output noise level provides a direct, quantitative measure of entanglement, leading here to the demonstration of 6 Mebit · s(-1) (mega entangled bits per second) generated by the first mixer.
Path entanglement constitutes an essential resource in quantum information and communication protocols. Here, we demonstrate frequency-degenerate entanglement between continuous-variable quantum microwaves propagating along two spatially separated paths. We combine a squeezed and a vacuum state using a microwave beam splitter. Via correlation measurements, we detect and quantify the path entanglement contained in the beam splitter output state. Our experiments open the avenue to quantum teleportation, quantum communication, or quantum radar with continuous variables at microwave frequencies.
Einstein-Podolsky-Rosen (EPR) entanglement is a criterion that is more demanding than just certifying entanglement. We theoretically and experimentally analyze the low-resource generation of bipartite continuous-variable entanglement, as realized by mixing a squeezed mode with a vacuum mode at a balanced beam splitter, i.e., the generation of so-called vacuum-class entanglement. We find that in order to observe EPR entanglement the total optical loss must be smaller than 33.333.333.3 %. However, arbitrarily strong EPR entanglement is generally possible with this scheme. We realize continuous-wave squeezed light at 155015501550 nm with up to 9.99.99.9 dB of nonclassical noise reduction, which is the highest value at a telecom wavelength so far. Using two phase-controlled balanced homodyne detectors we observe an EPR covariance product of $0.502\ifmmode\pm\else\textpm\fi{}0.006<1$, where 111 is the critical value. We discuss the feasibility of strong Gaussian entanglement and its application for quantum key distribution in a short-distance fiber network.
We report on the generation of a continuous variable Einstein-Podolsky-Rosen (EPR) entanglement using an optical fiber interferometer. The Kerr nonlinearity in the fiber is exploited for the generation of two independent squeezed beams. These interfere at a beam splitter and EPR entanglement is obtained between the output beams. The correlation of the amplitude (phase) quadratures is measured to be 4.0+/-0.2 (4.0+/-0.4) dB below the quantum noise limit. The sum criterion for these squeezing variances 0.80+/-0.03<2 verifies the nonseparability of the state. The product of the inferred uncertainties for one beam (0.64+/-0.08) is well below the EPR limit of unity.
Abstract Mechanical systems facilitate the development of a hybrid quantum technology comprising electrical, optical, atomic and acoustic degrees of freedom1, and entanglement is essential to realize quantum-enabled devices. Continuous-variable entangled fields—known as Einstein–Podolsky–Rosen (EPR) states—are spatially separated two-mode squeezed states that can be used for quantum teleportation and quantum communication2. In the optical domain, EPR states are typically generated using nondegenerate optical amplifiers3, and at microwave frequencies Josephson circuits can serve as a nonlinear medium4–6. An outstanding goal is to deterministically generate and distribute entangled states with a mechanical oscillator, which requires a carefully arranged balance between excitation, cooling and dissipation in an ultralow noise environment. Here we observe stationary emission of path-entangled microwave radiation from a parametrically driven 30-micrometre-long silicon nanostring oscillator, squeezing the joint field operators of two thermal modes by 3.40 decibels below the vacuum level. The motion of this micromechanical system correlates up to 50 photons per second per hertz, giving rise to a quantum discord that is robust with respect to microwave noise7. Such generalized quantum correlations of separable states are important for quantum-enhanced detection8 and provide direct evidence of the non-classical nature of the mechanical oscillator without directly measuring its state9. This noninvasive measurement scheme allows to infer information about otherwise inaccessible objects, with potential implications for sensing, open-system dynamics and fundamental tests of quantum gravity. In the future, similar on-chip devices could be used to entangle subsystems on very different energy scales, such as microwave and optical photons.