Coherent Protocols

I-Chao Chen, Tzonelih Hwang, Chuanming Li

On the basis of the modified four-coherent-state post-selection quantum key distribution protocol (Namiki and Hirano 2006 Preprint quant-ph/0608144v1), two 1-out-of-2 quantum oblivious transfer (QOT 1 2 ) protocols are proposed.The first proposed protocol (called the receiver-based QOT 1 2 protocol) requires the coherent states to be prepared by the receiver, whereas the second protocol (called the sender-based QOT 1 2 protocol) allows the coherent states to be generated by the sender.The main advantages of the proposed protocols are that (i) no quantum bit commitment schemes and the assumption of quantum memory are needed; (ii) less communication cost between participants is required, i.e. the receiver-based QOT 1 2 protocol requires only one quantum communication and one classical communication and the sender-based QOT 12 protocol requires only one quantum communication between participants during protocol execution; and (iii) the utilization of quantum states is very efficient, wherein the receiver-based and the sender-based QOT 1 2 protocols use only two coherent pulses and one coherent pulse respectively for sending the sender's two messages.

Mark M. Wilde

JR Minkel

Mark M. Wilde

This chapter demonstrates the power of both coherent communication from Chapter 7 and the particular protocol for entanglement-assisted classical coding from the previous chapter. Recall that coherent dense coding is a version of the dense coding protocol in which the sender and receiver perform all of its steps coherently.1 Since our protocol for entanglement-assisted classical coding from the previous chapter is really just a glorified dense coding protocol, the sender and receiver can perform each of its steps coherently, generating a protocol for entanglement-assisted coherent coding. Then, by exploiting the fact that two coherent bits are equivalent to a qubit and an ebit, we obtain a protocol for entanglement-assisted quantum coding that consumes far less entanglement than a naive strategy would in order to accomplish this task. We next combine this entanglement-assisted quantum coding protocol with entanglement distribution (Section 6.2.1) and obtain a protocol for quantum communication at a rate equal to the channel's coherent information (Section 13.5). This sequence of steps demonstrates an alternate proof of the direct part of the quantum capacity theorem stated in Chapter 24. Entanglement-assisted classical communication is one generalization of superdense coding, in which the noiseless qubit channel becomes an arbitrary noisy quantum channel while the noiseless ebits remain noiseless. Another generalization of super-dense coding is a protocol named noisy super-dense coding , in which the shared entanglement becomes a shared noisy state ρ AB and the noiseless qubit channels remain noiseless. Interestingly, the protocol that we employ in this chapter for noisy super-dense coding is essentially equivalent to the protocol from the previous chapter for entanglement-assisted classical communication, with some slight modifications to account for the different setting. We can also construct a coherent version of noisy super-dense coding, leading to a protocol that we name coherent state transfer . Coherent state transfer accomplishes not only the task of generating coherent communication between Alice and Bob, but it also allows Alice to transfer her share of the state ρ AB to Bob.

Kurt A. Jacobs, Xiaoting Wang, Howard M. Wiseman

We show that when the speed of control is bounded, there is a widely applicable minimal-time control problem for which a coherent feedback protocol is optimal, and is faster than all measurement-based feedback protocols, where the latter are defined in a strict sense. The superiority of the coherent protocol is due to the fact that it can exploit a geodesic path in Hilbert space, a path that measurement-based protocols cannot follow.

Ford Long Wong