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arXiv:2106.12272 · Physical Review Letters

Universal Unitary Transfer of Continuous-Variable Quantum States into a Few Qubits

Jacob Hastrup, Kimin Park, Jonatan Bohr Brask, Radim Filip, Ulrik Lund Andersen

Published Mar 15, 2022• 9 citations
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Abstract

We present a protocol for transferring arbitrary continuous-variable quantum states into a few discrete-variable qubits and back. The protocol is deterministic and utilizes only two-mode Rabi-type interactions that are readily available in trapped-ion and superconducting circuit platforms. The inevitable errors caused by transferring an infinite-dimensional state into a finite-dimensional register are suppressed exponentially with the number of qubits. Furthermore, the encoded states exhibit robustness against noise, such as dephasing and amplitude damping, acting on the qubits. Our protocol thus provides a powerful and flexible tool for discrete-continuous hybrid quantum systems.

DOI
10.1103/physrevlett.128.110503

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The transfer of quantum information between physical systems of a different nature is a central matter in quantum technologies. Particularly challenging is the transfer between discrete and continuous degrees of freedom of various harmonic oscillator systems. Here we implement a protocol for teleporting a continuous-variable optical qubit, encoded by means of low-amplitude coherent states, onto a discrete-variable, single-rail qubit-a superposition of the vacuum and single-photon optical states-via a hybrid entangled resource. We test our protocol on a one-dimensional manifold of the input qubit space and demonstrate the mapping onto the equator of the teleported qubit's Bloch sphere with an average fidelity of 0.83±0.04. Our work opens up the way to the wide application of quantum information processing techniques where discrete- and continuous-variable encodings are combined within the same optical circuit.
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We propose a deterministic hybrid continuous-variable-discrete-variable (CV-DV) scheme for the single-step transfer of an nnn-qubit W state encoded in photonic Schro¨\ddot{o}o¨dinger cat-state qubits within a circuit QED architecture. Logical qubits are encoded in even- and odd-parity cat states of bosonic modes, while effective Raman-type interactions between resonator pairs are mediated by a single superconducting flux qutrit operating in the dispersive regime. The protocol coherently transfers the multipartite W state in a single collective operation without populating higher excited atomic levels, thereby strongly suppressing decoherence. Numerical simulations based on the full Lindblad master equation, including realistic cavity dissipation, qutrit relaxation and dephasing, and inter-cavity crosstalk, show that a three-qubit cat-state W state can be transferred with a maximum fidelity of approximately 0.920.920.92. These results demonstrate the feasibility of scalable hybrid CV-DV entanglement transfer using current circuit QED technology.
Squeezed states of harmonic oscillators are a central resource for continuous-variable quantum sensing, computation, and communication. Here, we propose a method for the generation of very good approximations to highly squeezed vacuum states with low excess antisqueezing using only a few oscillator-qubit coupling gates through a Rabi-type interaction Hamiltonian. This interaction can be implemented with several different methods, which has previously been demonstrated in superconducting circuit and trapped-ion platforms. The protocol is compatible with other protocols manipulating quantum harmonic oscillators, thus facilitating scalable continuous-variable fault-tolerant quantum computation.