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Finite-time quantum entanglement in propagating squeezed microwaves

Kirill G. Fedorov, Stefan Pogorzalek, Las Heras García, Urtzi, Sanz Ruiz, Mikel, Patrick Yard, Peter Eder, Michael Fischer, Jan Goetz, E. Xie, K. Inomata, Yasunobu Nakamura, Roberto Di Candia, E. Solano, Achim Marx, Frank Deppe, Rudolf Groß

Published Jan 1, 2018• 21 citations
OpenAlex

Abstract

Two-mode squeezing is a fascinating example of quantum entanglement manifested in cross-correlations of non-commuting observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their self-correlations. These properties make two-mode squeezed (TMS) states an ideal resource for applications in quantum communication. Here, we generate propagating microwave TMS states by a beam splitter distributing single mode squeezing emitted from distinct Josephson parametric amplifiers along two output paths. We experimentally study the fundamental dephasing process of quantum cross-correlations in continuous-variable propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain the insight into finite-time entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation.

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Two-mode squeezing is a fascinating example of quantum entanglement manifested in cross-correlations of non-commuting observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their self-correlations. These properties make two-mode squeezed (TMS) states an ideal resource for applications in quantum communication. Here, we generate propagating microwave TMS states by a beam splitter distributing single mode squeezing emitted from distinct Josephson parametric amplifiers along two output paths. We experimentally study the fundamental dephasing process of quantum cross-correlations in continuous-variable propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain the insight into finite-time entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation.
Two-mode squeezing is a fascinating example of quantum entanglement manifested in cross-correlations of non-commuting observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their self-correlations. These properties make two-mode squeezed (TMS) states an ideal resource for applications in quantum communication. Here, we generate propagating microwave TMS states by a beam splitter distributing single mode squeezing emitted from distinct Josephson parametric amplifiers along two output paths. We experimentally study the fundamental dephasing process of quantum cross-correlations in continuous-variable propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain the insight into finite-time entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation.
Two-mode squeezing is a fascinating example of quantum entanglement manifested in cross-correlations of incompatible observables between two subsystems. At the same time, these subsystems themselves may contain no quantum signatures in their self-correlations. These properties make two-mode squeezed (TMS) states an ideal resource for applications in quantum communication, quantum computation, and quantum illumination. Propagating microwave TMS states can be produced by a beam splitter distributing single mode squeezing emitted from Josephson parametric amplifiers (JPA) into two output paths. In this work, we experimentally quantify the dephasing process of quantum correlations in propagating TMS microwave states and accurately describe it with a theory model. In this way, we gain an insight into quantum entanglement limits and predict high fidelities for benchmark quantum communication protocols such as remote state preparation and quantum teleportation.
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.
This thesis presents the experimental achievements of (1) generating entangled-microwave fields propagating on two physically separate transmission lines and (2) verifying the entangled states with efficient measurements. Shared entanglement between two parties is an essential resource for quantum information processing and quantum communication protocols. Experimentally, entangled pairs of electromagnetic fields can be realized by distributing a squeezed vacuum over two separated modes. As a result, entanglement is revealed by the strong cross-correlations between specific quadratures of the two modes. Although it is possible to verify the presence of entanglement with low-efficiency quadrature measurements, higher detection efficiencies are desired for performing protocols that exploit entanglement with high fidelity.\nIn the microwave regime, Josephson parametric amplifiers (JPAs) fulfill the two major tasks mentioned above: JPAs prepare the required squeezed states to generate entanglement and enable us to perform efficient quadrature measurements. Therefore, for the purposes of entanglement generation and verification, ultralow-noise--frequency-tunable JPAs have been developed. Additionally, to increase the efficiency of entanglement generation, we integrate JPAs with two on-chip microwave passive components, a directional coupler and a quadrature hybrid, to form an entangler circuit. The two-mode entangled states are created at the two output modes of the entangler and are measured with a two-channel measurement apparatus where each of the two channels incorporates a JPA as a single-quadrature preamplifier. By employing this measurement scheme, the two measured quadratures of the two output modes can be chosen independently of each other, enabling a full characterization of the two-mode state. To definitively demonstrate the two-mode entanglement, I prove that the measured quadrature variances satisfy the inseparability criterion.
Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security.