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Physical Review A

Counterfactual exchange of unknown quantum states

Zheng-Hong Li, M. Al-Amri, Xihua Yang, Muhammad Suhail Zubairy

Published Aug 8, 2019• 20 citations
Publisher

Abstract

We show that two unknown quantum states, either pure or mixed, can be bidirectionally ``teleported'' counterfactually. In the process of information exchange, there is no physical particle traveling between two communicators. Unlike conventional quantum teleportation, our protocol requires neither prearranged entangled pairs nor Bell measurements. We entangle and disentangle a photon and an atom through multiple measurements in a special sequence. These multiple measurements can smoothly but reversibly control system evolution. The total effect of multiple measurements can be considered as a unitary time evolution operator. Therefore, our protocol can be regarded as a counterfactual quantum swap gate. Based on our approach, all known conventional quantum gates can be counterfactualized.

DOI
10.1103/physreva.100.022110

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We show that two unknown quantum states, either pure or mixed, can be bidirectionally ``teleported'' counterfactually. In the process of information exchange, there is no physical particle traveling between two communicators. Unlike conventional quantum teleportation, our protocol requires neither prearranged entangled pairs nor Bell measurements. We entangle and disentangle a photon and an atom through multiple measurements in a special sequence. These multiple measurements can smoothly but reversibly control system evolution. The total effect of multiple measurements can be considered as a unitary time evolution operator. Therefore, our protocol can be regarded as a counterfactual quantum swap gate. Based on our approach, all known conventional quantum gates can be counterfactualized.
We propose a quasi-counterfactual quantum swap gate for exchanging Alice's unknown photon state and Bob's unknown atomic state under the condition that only Alice's photon may appear in the transmission channel between Alice and Bob, while the probability of the existence of photon in the transmission channel is controllable and can tend to zero. Unlike standard counterfactual quantum communication protocols, quantum states exchange in present scenario is achieved by multiple phase operations, rather than multiple measurements. The total effect of those operations can be considered as a unitary time evolution operator. Therefore, the communication fidelity and efficiency of our protocol are always one if system imperfection and channel noise are not considered. Compared to standard counterfactual communication protocols, our protocol is easy to implement. We also show that it can be easily converted to a standard counterfactual one.
We show that an unknown quantum state can be transferred with neither quantum nor classical particle traveling in the transmission channel. Our protocol does not require prearranged entangled photon pairs and Bell measurements. By utilizing quantum Zeno effect and counterfactuality, we can entangle and disentangle a photon and an atom by nonlocal interaction. It is shown that quantum information is completely transferred from an atom to photon due to controllable disentanglement processes. There is no need to cross-check the result via classical channels.
This paper proposes two new full-duplex quantum communication protocols to exchange classical or quantum information between two remote parties simultaneously without transferring a physical particle over the quantum channel. The first protocol, called quantum duplex coding, enables to exchange of a classical bit using a preshared maximally entangled pair of qubits by means of counterfactual disentanglement. The second protocol, called quantum telexchanging, enables to exchange an arbitrary unknown qubit without using preshared entanglement by means of counterfactual entanglement and disentanglement. We demonstrate the quantum duplex coding and quantum telexchanging by exploiting counterfactual electron-photon interaction gates and show that these quantum duplex communication protocols form full-duplex binary erasure channel and quantum erasure channels, respectively.
Quantum teleportation circumvents the uncertainty principle using dual channels: a quantum one consisting of previously-shared entanglement, and a classical one, together allowing the disembodied transport of an unknown quantum state over distance. It has recently been shown that a classical bit can be counterfactually communicated between two parties in empty space, “Alice” and “Bob”. Here, by using our “dual” version of the chained quantum Zeno effect to achieve a counterfactual CNOT gate, we propose a protocol for transporting an unknown qubit counterfactually, that is without any physical particles travelling between Alice and Bob—no classical channel and no previously-shared entanglement.
We uncover a new quantum paradox, where a simple question about two identical systems of nested interferometers reveals surprising answers on the whereabouts of a photon, when considering weak measurements. The paradox is reproduced using consistent histories. Our resolution of the paradox amounts to a proof of counterfactuality - communication without particle exchange - for our protocol for communicating an unknown qubit without exchanging any particles. We go on to propose a feasible implementation of this disembodied transport, estimating resources for beating the classical limit, except that, unlike teleportation, no previously-shared entanglement nor classical communication are required. Moreover, while counterfactual communication can be intuitively explained in terms of interaction-free measurement and the Zeno effect, we show that, surprisingly, neither is necessary - with direct implications in support of the reality of the wave-function.
We demonstrate quantum information can be transferred between two distant participants without any physical particles traveling between them. The key procedure of the counterfactual scheme is to entangle two nonlocal qubits with each other without interaction, so the scheme can also be used to generate nonlocal entanglement counterfactually. We here illustrate the scheme by using flying photon qubits and Rydberg atom qubits assisted by a mesoscopic atomic ensemble. Unlike the typical teleportation, the present scheme can transport an unknown qubit in a nondeterministic manner without prior entanglement sharing or classical communication between the two distant participants.