Efficient multi-step distillation of quantum states counteracting Gaussian decoherence without the need for quantum memories
Final Report Abstract
An important part of current and future quantum technologies requires quantum systems in quantum correlated states. An example is laser sensing with noise below the photon counting noise. This is already being realised in the worldwide network of gravitationalwave observatories on the basis of squeezed states of light. Another example is the one-way, device-independent quantum key distribution (QKD) for secure communication. A proof-ofconcept experiment based on two-mode squeezed light, the so-called Gaussian ‘Einstein-Podolsky-Rosen (EPR) entanglement’, was realised here. A third example is the envisioned measurement-based optical quantum computing on the basis of microscopic optical Schrödinger cat states with required amplitudes of α ≥ 2. In all applications, some of the photons are lost due to absorption and scattering and are not captured by the photo-electric detectors. The consequence is the reduction of the quantum advantage. The sensitivity in laser sensing degrades, quantum communication gets slower or is made impossible, and quantum computers loose their speed. Lost quantum correlations cannot be recovered deterministically. But if the correlated quantum uncertainties are turned into non-Gaussian ones, at least probabilistic distillation protocols can counteract decoherence including photon loss — but only if many probabilistic distillation steps are applied. The current problem is that the distillation efficiency degrades exponentially with the number of distillation steps, since close to perfect quantum memories do not exist. In this DFG project we focused on the distillation of quantum correlated states for measurement-based quantum information protocols. In these protocols, all states are physically processed in the same way as is the case with quantum key distribution. In these cases, the distillation can be performed by applying an emulated distillation to the measured data. The final data is indistinguishable from measurement data from states distilled prior to collection. With emulated distillation, no quantum memories are needed at all. For the first time, we have achieved the two-step distillation of squeezed states. The first step was physically accomplished by subtracting two photons from the lossy squeezed states using two superconducting nanowire single-photon counters. Our experimental innovation was the subsequent synchronised simultaneous detection of two non-commutating field quadratures. Based on this data, we arrived at a second, emulated, distillation step. The squeeze factor was 2.4 dB initially, then 2.8 dB after two-photon subtraction and 3.14 dB after the second distillation step. In a second experiment, we applied our technology to the generation and growth of microscopic Schrödinger cat states. We applied the same synchronised detection and emulated a two-step growth process and increased the state’s amplitude α from initially 1.1 to 1.7 to finally 2.6. This amplitude satisfies the requirement of optical quantum computation. The next important steps of our research are to extend the experiments to EPR entanglement and to analyse to what extent QKD and measurement-based quantum computers benefit from the emulated quantum-memory-free distillation.
Publications
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Experimental growth of Schrödinger kitten states, PhD thesis, Universität Hamburg
Julian Göttsch
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Multistep Two-Copy Distillation of Squeezed States via Two-Photon Subtraction. Physical Review Letters, 129(27).
Grebien, Stephan; Göttsch, Julian; Hage, Boris; Fiurášek, Jaromír & Schnabel, Roman
