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Register a new userCounterfactual quantum erasure: spooky action without entanglement
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Authors
Hatim Salih
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We combine the eyebrow-raising quantum phenomena of erasure and counterfactuality for the first time, proposing a simple yet unusual quantum eraser: A distant Bob can decide to erase which-path information from Alices photon, dramatically restoring interference, without previously-shared entanglement, and without Alices photon ever leaving her lab.
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In science, one observes correlations and invents theoretical models that describe them. In all sciences, besides quantum physics, all correlations are described by either of two mechanisms. Either a first event influences a second one by sending some information encoded in bosons or molecules or other physical carriers, depending on the particular science. Or the correlated events have some common causes in their common past. Interestingly, quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This new kind of cause reveals itself, e.g., in correlations that violate Bell inequalities (hence cannot be described by common causes) between space-like separated events (hence cannot be described by classical communication). Einstein branded it as spooky action at a distance. A real spooky action at a distance would require a faster than light influence defined in some hypothetical universally privileged reference frame. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test during more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed 2-photon interferences well above the Bell inequality threshold. Taking advantage of the Earths rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of this spooky influence. For instance, if such a privileged reference frame exists and is such that the Earths speed in this frame is less than 10^-3 that of the speed of light, then the speed of this spooky influence would have to exceed that of light by at least 4 orders of magnitude.
We call a probabilistic theory complete if it cannot be further refined by no-signaling hidden-variable models, and name a theory spooky if every equivalent hidden-variable model violates Shimonys Outcome Independence. We prove that a complete theory is spooky if and only if it admits a pure steering state in the sense of Schrodinger. Finally we show that steering of complementary states leads to a Schrodingers cat-like paradox.
Entanglement is widely believed to lie at the heart of the advantages offered by a quantum computer. This belief is supported by the discovery that a noiseless (pure) state quantum computer must generate a large amount of entanglement in order to offer any speed up over a classical computer. However, deterministic quantum computation with one pure qubit (DQC1), which employs noisy (mixed) states, is an efficient model that generates at most a marginal amount of entanglement. Although this model cannot implement any arbitrary algorithm it can efficiently solve a range of problems of significant importance to the scientific community. Here we experimentally implement a first-order case of a key DQC1 algorithm and explicitly characterise the non-classical correlations generated. Our results show that while there is no entanglement the algorithm does give rise to other non-classical correlations, which we quantify using the quantum discord - a stronger measure of non-classical correlations that includes entanglement as a subset. Our results suggest that discord could replace entanglement as a necessary resource for a quantum computational speed-up. Furthermore, DQC1 is far less resource intensive than universal quantum computing and our implementation in a scalable architecture highlights the model as a practical short-term goal.
The phenomenon of quantum erasure has long intrigued physicists, but has surprisingly found limited practical application. Here, we propose an erasure-based protocol for quantum key distribution (QKD) that promises inherent security against detector attacks.
Quantum channels, which break entanglement, incompatibility, or nonlocality, are not useful for entanglement-based, one-sided device-independent, or device-independent quantum information processing, respectively. Here, we show that such breaking channels are related to certain temporal quantum correlations, i.e., temporal separability, channel unsteerability, temporal unsteerability, and macrorealism. More specifically, we first define the steerability-breaking channel, which is conceptually similar to the entanglement and nonlocality-breaking channels and prove that it is identical to the incompatibility-breaking channel. Similar to the hierarchy relations of the temporal and spatial quantum correlations, the hierarchy of non-breaking channels is discussed. We then introduce the concept of the channels which break temporal correlations, explain how they are related to the standard breaking channels, and prove the following results: (1) A certain measure of temporal nonseparability can be used to quantify a non-entanglement-breaking channel in the sense that the measure is a memory monotone under the framework of the resource theory of the quantum memory. (2) A non-steerability-breaking channel can be certified with channel steering because the steerability-breaking channel is equivalent to the incompatibility-breaking channel. (3) The temporal steerability and non-macrorealism can, respectively, distinguish the steerability-breaking and the nonlocality-breaking unital channel from their corresponding non-breaking channels. Finally, a two-dimensional depolarizing channel is experimentally implemented as a proof-of-principle example to compare the temporal quantum correlations with non-breaking channels.
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