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Efficient universal quantum channel simulation in a cloud quantum computer

Science China Press

The study of quantum channels is the fundamental field and promises wide range of applications, because any physical process can be represented as a quantum channel transforming an initial state into a final state. Performing quantum computing using quantum cloud will become a standard way in the future. IBM Q is an industry-first initiative to build commercially available universal quantum computers for business and science in cloud. Recently, a study demonstrated a new algorithm of constructing quantum channel experimentally using the universal IBM cloud quantum computer and study the properties of different qubit quantum channels.

Since Feynman proposed the idea of quantum computer and envisioned the possibility of efficiently simulating quantum systems, significant progress has been made in closed system quantum simulation.

Moreover, every practical quantum system is open system because of the inevitable coupling to the environment. Thus, quantum simulation of open system is an equally important and more general subject to explore. However, open quantum system simulation is still in the early stages of development, which remains largely unexplored. The quantum simulation of open system promises powerful applications in a class of physical problem, such as preparing various special state, thermalizing in spin-boson systems and complex many fermion-boson systems, studying non-equilibrium dynamics, and performing universal quantum computation.

Given the importance of the simulation of open quantum system, efficiently performing quantum channels which represent the most general quantum dynamics possible is critical. A straightforward way suggested by the Stinespring dilation theorem is to enlarge the system to include the environment, which can be regarded as a bigger closed quantum system. The cost of qubits resource and unitary operators resource make it inefficient in high-dimensional.

Here the researchers present a new method which can realize universal qubit channels deterministically with controlled NOT operations.

The approach is a total quantum algorithm without a classical random number in generator and realize four Kraus operators simultaneously.

This algorithm requires two qubits maximum as ancillary system to simulate the environment by performing the controlled operations on the single-qubit work system. Moreover, there are only single direction controlled operations from ancillary system to work qubit which are not dependent on the state of the single-qubit work system, making the method more general and scalable in higher dimension. The gate complexity is significant reduced compared with Stinespring dilation.

The researchers demonstrate the whole algorithm experimentally using the universal IBM cloud quantum computer and study the properties of different qubit quantum channels. They illustrate the quantum capacity of the general qubit quantum channels, which quantifies the amount of quantum information that can be protected. The behaviour of quantum capacity in different channels reveal which types of noise processes can support information transmission, and which types are too destructive to protect information. There is a general agreement between the theoretical predictions and the experiments, which strongly supported our method. For the large system, the improvement of performance of their method is significant compared with Stinespring dilation. Furthermore, they explore the universal qubit quantum channel properties and calculate the quantum capacity of different channels. By realizing arbitrary qubit channel, this work provides a universal way to explore various properties of quantum channel and novel prospect of quantum communication.


This work was supported by the National Basic Research Program of China under Grant No. 2015CB921002, the National Natural Science Foundation of China under Grants No. 11774197 and No. 61727801; and the National Natural Science Foundation of China (Grants Nos. 11175094 and 91221205)

See the article: S. J. Wei, T. Xin, and G. L. Long, Efficient universal quantum channel simulation in IBM's cloud quantum computer, Sci. China-Phys. Mech. Astron. 61, 070311 (2018),

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