High-fidelity squeezing gate for continuous-variable quantum light field

Measurement-based quantum computation offers a particularly promising avenue for engineering a universal quantum computer. The difficulty of this approach resides in the upfront generation of cluster state. However, many... [ view full abstract ]

Measurement-based quantum computation offers a particularly promising avenue for engineering a universal quantum computer. The difficulty of this approach resides in the upfront generation of cluster state. However, many improvements have recently emerged, amongst which one benchmark experiment demonstrated a highly scalable cluster state comprised of one million temporal modes of an optical field. These experimental progresses highlighted the advantage of continuous-variable (CV) quantum computation over its discrete-variable counterpart. Nevertheless, scaling up quantum operations over continuous variables remains a bottleneck which demands quantum operations being implemented in a fault tolerant way. Intrinsic restriction arises from the in principle finite squeezing used to generate cluster states which induces noise during repeated application of local measurements and thus, reduces the fidelity of quantum operations. Although recent proposal shows that -20.5dB initial squeezing suffices to enact fault-tolerant quantum computation employing a qubit-based error-correcting code, it yet poses a formidable challenge beyond currently accessible experimental techniques.

In this work, we construct and experimentally demonstrate a high fidelity quantum squeezing gate (SG) which is a prototype example of a measurement-based quantum computer. A squeezing gate works by using an ancillary squeezed state to produce an output state that is a squeezed version of the input. A filter function with an inverse Gaussian profile is incorporated into the feedforward line as illustrated in Fig. 1, leading to an enhancement in precision  of the dual homodyne measurement and therefore combats efficiently the correlation degradation due to loss and noise introduced during feedforward. A significantly higher fidelity is therefore obtainable (Fig. 2), at an expense of a finite success probability. We observed a squeezing fidelity of 98.49 for a target squeezing of -2.3dB with only modest amount of ancilla squeezing (-6dB), which would otherwise require -20.5dB initial squeezing using a conventional deterministic SG (Fig. 3). Remarkably, with our SG, an arbitrary input state can always be squeezed to the same level of the initial ancilla squeezing with unity fidelity, implying that quantum correlation is fully conserved. We emphasize that our SG scheme can be extended to realize other Gaussian operations with significantly higher fidelity than that is achievable using a conventional deterministic setup. We thus envisage this new SG diagram to be of great interest in CV fault-tolerant quantum computation as well as other protocols involving measurement and feedforward or feedback controls.