Indefinite Causal Order in a Quantum Switch.

@article{Goswami2018IndefiniteCO,
  title={Indefinite Causal Order in a Quantum Switch.},
  author={Kaumudibikash Goswami and Christina Giarmatzi and Michael Kewming and Fabio Costa and Cyril Branciard and Jacquiline Romero and A. G. White},
  journal={Physical review letters},
  year={2018},
  volume={121 9},
  pages={
          090503
        }
}
Quantum mechanics allows events to happen with no definite causal order: this can be verified by measuring a causal witness, in the same way that an entanglement witness verifies entanglement. Here, we realize a photonic quantum switch, where two operations A[over ^] and B[over ^] act in a quantum superposition of their two possible orders. The operations are on the transverse spatial mode of the photons; polarization coherently controls their order. Our implementation ensures that the… 

Figures from this paper

Superposition of causal orders for quantum discrimination of quantum processes

We address the superposition of causal orders in the quantum switch as a convenient framework for quantum process discrimination in the presence of noise in qubit systems, using Bayes strategy. We

Classical communication through quantum causal structures

It is found that a one-way communication protocol through an arbitrary process cannot outperform a causally separable process, i.e., the authors can send at most one bit per qubit.

Experimental entanglement of temporal order

The study of causal relations has recently been applied to the quantum realm, leading to the discovery that not all physical processes have a definite causal structure. While indefinite causal

Indefinite causal order enables perfect quantum communication with zero capacity channels

Quantum mechanics is compatible with scenarios where the relative order between two events can be indefinite. Here we show that two independent instances of a noisy process can behave as a perfect

Indefinite causal order for quantum metrology with quantum thermal noise

Causal order as a resource for quantum communication

In theories of communication, it is usually presumed that the involved parties perform actions in a fixed causal order. However, practical and fundamental reasons can induce uncertainties in the

Quantifying superpositions of quantum evolutions

  • M. N. Bera
  • Physics, Philosophy
    Physical Review A
  • 2019
Quantum mechanics allows coherent superposition between different states of matter. This quality is responsible for major non-classical phenomena that occur in quantum systems. Beyond states,

Experimental implementation of arbitrary entangled operations

A scheme to realize arbitrary entangled operations based on a coherent superposition of local operations that can reduce the complexity or required resources of the quantum circuits and provide insights to investigate properties of entangled operations is proposed.

Experiments on quantum causality

The authors will walk through the basic theory of indefinite causal order and focus on experiments that rely on a physically realizable indefinite causal ordered process—the quantum switch.
...

References

SHOWING 1-10 OF 35 REFERENCES

Experimental superposition of orders of quantum gates

This work experimentally demonstrates the creation of the required superposition of gate orders by using additional degrees of freedom of the photons encoding qubits, which could allow quantum algorithms to be implemented with an efficiency unlikely to be achieved on a fixed-gate-order quantum computer.

Experimental verification of an indefinite causal order

Researchers report the first decisive demonstration of a process with an indefinite causal order, and experimentally determine a causal witness, demonstrating by almost 7 SDs that the experimentally implemented process does not have a definite causal order.

Quantum correlations with no causal order

It is shown that in a classical limit causal order always arises, which suggests that space-time may emerge from a more fundamental structure in a quantum-to-classical transition.

Witnessing causal nonseparability

This work shows that a recently demonstrated resource for quantum computation—the quantum switch—is a genuine example of 'indefinite causal order', and introduces a new tool—the causal witness—which can detect the causal nonseparability of any quantum resource that is incompatible with a definite causal order.

Computational advantage from quantum-controlled ordering of gates.

It is shown that this model of quantum computing is physically realizable, by proposing an interferometric setup that can implement such a quantum control of the order between the gates, and that this new resource provides a reduction in computational complexity.

Enhanced Communication with the Assistance of Indefinite Causal Order.

This Letter shows that two identical copies of a completely depolarizing channel become able to transmit information when they are combined in a quantum superposition of two alternative orders, counter to the intuition that if two communication channels are identical, using them in different orders should not make any difference.

Witnesses of causal nonseparability: an introduction and a few case studies

This work presents a shorter introduction to the concept of witnesses of causal nonseparability, and focuses on some explicit examples—by considering in particular different noise models for the quantum switch—to show how to construct and use such witnesses in practice.

Theoretical framework for quantum networks

We present a framework to treat quantum networks and all possible transformations thereof, including as special cases all possible manipulations of quantum states, measurements, and channels, such

Causal and causally separable processes

The idea that events are equipped with a partial causal order is central to our understanding of physics in the tested regimes: given two pointlike events A and B, either A is in the causal past of

Quantum superposition of the order of parties as a communication resource

It is shown that quantum superpositions of the direction of communication between parties can also serve as a resource to improve the probability of success and provide the first semi-device-independent certification of the absence of a definite order of communication.