Robust entanglement buffers based on SWAP interactions

Abstract

Quantum entanglement is the essential resource for quantum communication and distributed information processing in a quantum network. However, the remote generation over a network suffers from inevitable transmission loss and other technical difficulties. This paper introduces the concept of entanglement buffers as a potential primitive for preparing long-distance entanglement. We investigate the filling of entanglement buffers with either one Bell state or a stream of Bell states via SWAP interactions. We illustrate their resilience to imperfect interactions, noise, and losses, making the buffers suitable for a realistic quantum network scenario. Additionally, larger entanglement buffers can always enhance these benefits.

Figures (17)

• Figure 1: Sketch of a multi-pair entanglement buffer caching Bell-pair entanglement through a sequence of partial swap operations between the subsequent qubit pairs of the buffer and either (a) a single Bell pair over one or more repetitions or (b) a sequence of Bell pair copies. Entanglement is shared between two distant parties $A$ (upper part) and $B$ (lower part).
• Figure 2: Entanglement cached from a single Bell pair in a 1-pair buffer by means of (a) a partial SWAP unitary and (b) a partial iSWAP of varying angles $\alpha$. Both buffer qubits are initialized in a pure state $\cos\theta\hbox{$| 0 \rangle$}+\sin\theta\hbox{$| 1 \rangle$}$ of varying $\theta$. The entanglement is measured in ebits of logarithmic negativity.
• Figure 3: (a) Entanglement buffering from a single caching unitary of SWAP type with a single Bell pair for buffer sizes up to $k=4$. We plot the cached $E$ ebits as a function of the swap angle. (b) Number $n$ of repeated caching operations required to reach $E=1$ as a function of buffer size $k$, given fixed weak swap angles $\alpha$. We plot $n$ in terms of the total operation time $\propto nk\alpha$, which grows like $\sqrt{k}$ (dashed line). All buffer qubits are initialized in $|0\rangle$.
• Figure 4: (a) Steady-state entanglement $E_\infty$ (ebits) cached from a sequence of Bell pairs in a 1-pair buffer, using a caching unitary of varying interaction parameters $\alpha,\beta$. (b) Entanglement $E$ accumulated in a 1-pair buffer over $n$ subsequent iSWAP caching steps ($\beta=\alpha$) with independent source pairs, comparing various $\alpha$-values.
• Figure 5: Steady-state entanglement $E_\infty$ cached by multi-copy protocols with varying interaction parameters $\alpha,\beta$, for (a) a $2$-pair and (b) a $3$-pair buffer. The solid line marks the parameter range (right of it) for which the buffer contains more than 1 ebit.

Theorems & Definitions (2)

• Proposition 1
• proof