Quantum transduction via generalized continuous-variable teleportation

Quantum transduction converts quantum states between different frequencies. Similarly, quantum teleportation transfers quantum states between different systems. While often appreciated for quantum communication between distant locations, teleportation can also operate between different frequencies. In terms of bosonic modes, quantum teleportation relies on two-mode squeezing – squeezed on the balanced Einstein-Podolsky-Rosen (EPR) quadratures – for entanglement and a balanced beamsplitter plus homodyne detection for Bell measurement. Since cross-frequency band entanglement is challenging to generate, we propose to enable transduction with teleportation, while only relying on a conventional transduction device and narrow-band entanglement. Our insight is that a transduction device performs a cross-frequency-band beamsplitter, almost what is needed in a Bell measurement, except that it is often not balanced. We resolve the issue by proposing a generalized teleportation protocol with arbitrarily unbalanced EPR quadratures and matched unbalanced beamsplitters. The protocol can enhance quantum transduction, using only off-the-shelf components – intraband beamsplitters, offline squeezing, homodyne detection and displacements. The proposed protocol achieves perfect transduction when applying to a transduction device with an arbitrarily low efficiency, at infinite squeezing. A minimum squeezing in the range of [1.195,4.343] decibels is needed to achieve quantum capacity enhancement.

💡 Research Summary

Quantum transduction – the task of converting quantum states between disparate frequency bands – is a critical enabling technology for linking quantum processors (e.g., microwave superconducting qubits) with long‑distance optical communication channels. Conventional transducers implement a direct frequency‑conversion beamsplitter with efficiency η, but they suffer from a trade‑off among conversion efficiency, pump‑induced heating, and bandwidth, leaving current devices far from the performance required for scalable quantum networks. Continuous‑variable (CV) quantum teleportation offers an alternative: by sharing an Einstein‑Podolsky‑Rosen (EPR) entangled pair, a quantum state can be transferred without physically moving the carrier. However, generating cross‑frequency EPR entanglement is as hard as building a high‑efficiency transducer, so prior teleportation‑based proposals have limited practicality.

The authors observe that a standard transduction device already implements a cross‑frequency beamsplitter, albeit generally unbalanced (splitting ratio η ≠ ½). By allowing the same imbalance in the preparation of the EPR resource, they construct a generalized CV teleportation protocol that removes the requirement of a balanced beamsplitter and balanced squeezing. The protocol proceeds as follows:

1. Resource preparation – Two single‑mode squeezed vacua are mixed on a beamsplitter of ratio η, producing a zero‑mean two‑mode Gaussian state |ψ⟩AB. The generalized EPR quadratures are
   (\hat q_{-,\eta}= \sqrt{1-\eta},\hat q_A - \sqrt{\eta},\hat q_B) and
   (\hat p_{+,\eta}= \sqrt{\eta},\hat p_A + \sqrt{1-\eta},\hat p_B).
   Their variances are reduced by the squeezing gain G, giving (\mathrm{var}(\hat q_{-,\eta})=\mathrm{var}(\hat p_{+,\eta})=1/G).

2. Bell‑type measurement – The unknown signal mode S is mixed with mode A on another beamsplitter of the same ratio η (the transducer itself). The resulting modes S′ and A′ are measured by homodyne detection of the momentum of S′ and the position of A′, yielding classical outcomes (\tilde p) and (\tilde q).

3. Feed‑forward correction – The measurement results are sent to the receiver, who displaces mode B by (\tilde q/\sqrt{\eta}) in the position quadrature and (-\tilde p/\sqrt{1-\eta}) in the momentum quadrature, producing the output mode B′.

Theorem 1 proves that in the limit of infinite squeezing (G → ∞) the output state exactly reproduces the input, i.e. (\hat\rho_{B’}=\hat\rho_S). For finite G the output suffers additive Gaussian noise with variances (1/(\eta G)) (position) and (1/