What is Deterministic Secure Quantum Communication (DSQC)?

Deterministic Secure Quantum Communication (DSQC) is a class of quantum communication protocols designed to achieve the deterministic transmission of secret messages through a quantum channel, without the prior creation of a secret key. This differentiates it from Quantum Key Distribution (QKD) protocols, which focus on generating a shared secret key to be used for encrypting classical messages. In DSQC, the secret message itself is encoded onto quantum states and transmitted to read the message directly after performing certain operations.

 Differences from Quantum Key Distribution (QKD)

The difference between DSQC and QKD lies in their objectives. QKD generates a secret key between two parties, Alice and Bob, which can be used with classical encryption methods to securely transmit information. The security of QKD relies on the principles of quantum mechanics, such as any attempt to eavesdrop on the quantum transmission will inescapably cause disturbances. After the quantum transmission, Alice and Bob use a classical channel to compare their measurement results to estimate the error rate, which reveals the presence of an eavesdropper. If the error rate is below a convinced edge, they can distil a secure key from the remaining bits.

DSQC aims for the direct transmission of a secret message. The secret information is encoded directly onto quantum states, and Bob can decode the message without an intermediary secret key exchange. While some DSQC protocols still involve classical communication channels for support purposes, the idea is to bypass the key generation step and transmit the secret message itself using quantum principles.

Requirements for Secure DSQC Protocols

Secure DSQC protocol preferably satisfies the following three critical requirements:

1. Eavesdropper Detection: Before Alice and Bob communicate to encode the secret message onto quantum states, they have a mechanism to detect the presence of any eavesdropper (Eve). This involves some form of preliminary quantum transmission and error checking to ensure the quantum channel’s integrity.
2. Direct Message Reading: Bob read the secret message directly from the received quantum states without additional classical channels with Alice to exchange a secret key and ensure the protocol’s security. This direct decoding is a defining characteristic of DSQC.
3. Information Security: In any attack performed by Eve, no useful information is transmitted; the secret message should be stolen. The protocol must be strong against several eavesdropping strategies that Eve might employ.

Examples of DSQC Protocols

Some DSQC protocols have been proposed based on quantum resources and encoding techniques. One of the early DSQC protocols, proposed by Beige et al. in 2002, utilized the exchange of single photons, where each photon transmitted one bit of the secret message.

Another example involves a protocol

• where Bob prepares a sequence of single photons all in the state  |+⟩ = 1/2(|0⟩ + |1⟩) and sends this sequence to Alice.
• Receiving the sequence, Alice generates a random string M with a length equal to the number of photons Bob sent.
• Then, Alice applies a phase shift  𝜓i ∈ {0, 𝜋∕2, 𝜋, 3𝜋∕2} to each photon in the sequence S based on the i^th bit of her secret message M and a chance bit she chooses.

 For example, she might use the following encoding: for a secret bit ‘0’,

• she applies a phase shift of 0 or 𝜋, and for a secret bit ‘1’, she applies 𝜋∕2 or  3𝜋∕2, chosen randomly.
• After encoding, Alice sends the modified photon sequence back to Bob. Bob then measures each received photon in the rectilinear basis |0⟩, |1⟩ and obtains a measurement outcome.
• By analysing the statistics of these outcomes and comparing them with the initial state preparation, Bob can extract Alice’s secret message with the help of transmitted classical information bits via a classical channel.

Other DSQC protocols are proposed based on GHZ states and the rearrangement of single photons. Some protocols even aim for deterministic secure quantum communication without entangled states.

Security in DSQC

The security of DSQC protocols, similar to QKD, depends on the principles of quantum mechanics. Any attempt by an eavesdropper to intercept or measure the quantum states carrying the secret message will, with high probability, disturb these states in a way that can be detected by Alice and Bob. Alice and Bob can estimate the level of disturbance in the quantum channel and, consequently, determine if an eavesdropper is present. If the disturbance, they abort the communication to ensure security.

The no-cloning theorem in quantum mechanics states that an unknown quantum state cannot be perfectly copied. This prevents an eavesdropper from simply making a copy of the transmitted quantum states to learn the secret message without introducing any disturbance that could be detected.

The security analysis of DSQC protocols can be complicated than that of QKD. Since the message itself is being transmitted directly, the eavesdropper’s goal is to learn the message without necessarily a detectable level of disturbance, or to operate the message without being noticed. Therefore, careful design and difficult security proofs are important for any applied DSQC protocol.

Role of Classical Channels in DSQC

While the primary goal of DSQC is direct quantum communication of a secret message, classical communication channels often play a supporting role. For instance, in the protocol where Bob sends |+⟩ states to Alice, after Alice encodes her message and sends the photons back, Bob uses a classical channel to communicate with Alice to control the suitable operations for decoding the message. Classical channels are also used for announcing measurement bases in  protocols and performing checks to detect eavesdropping. The critical difference remains that the secret information is encoded and transmitted quantum mechanically, and the receiver can deterministically obtain the message, potentially with the classical communication that does not involve sharing a pre-existing secret key for decryption.

Entanglement in DSQC

Some DSQC protocols, like those based on GHZ states, utilise quantum entanglement as a resource. Entangled particles show strong connections, even when separated by large distances, and these connections can be broken for secure communication. However, as mentioned earlier, there are also DSQC protocols to achieve secure communication using single photons in mixed states or without relying on entangled states. The quantum resources depend on the specific design of the protocol and the desired security and efficiency trade-offs.

Advantages

• Direct Secret Message Transmission: DSQC offers the potential for transmitting secret information directly without the need for a prior key distribution phase, which can shorten communication in convinced situations.
• Deterministic Communication: Unlike some probabilistic quantum communication protocols, DSQC for the deterministic recovery of the secret message by the receiver.

Limitations

• Protocol Complexity: Designing and implementing secure and well-organized DSQC protocols can be complex.
• Channel Requirements: DSQC protocols require hi-fi quantum channels with low noise levels to ensure reliable transmission and security.
• Practical Implementation Challenges: Implementing the quantum technologies, such as single-photon sources and detectors with high efficiency, can be challenging.
• Security Proofs: Difficult security proofs for DSQC protocols against all possible quantum opponents can be complicated and are an active area of research.
• Need for Classical Assistance: DSQC protocols still require some form of classical communication, which needs to be authenticated to prevent operation by an eavesdropper.

Relation to Other Quantum Communication Protocols

DSQC is related to other branches of quantum cryptography, including:

• Quantum Key Distribution (QKD): QKD focuses on key generation, while DSQC focuses on direct message transmission. Some researchers consider DSQC as a balancing approach to QKD for achieving secure communication.
• Quantum Secret Sharing (QSS): QSS protocols allow a secret quantum state to be encoded into some shares, such that only authorized subsets of shareholders can reconstruct the original secret. While QSS deals with sharing secrets, DSQC deals with transmitting them directly.
• Deterministic Secure Quantum Communication Protocols (DSQC): The term itself encompasses various protocols for deterministic and secure direct communication.

Deterministic Secure Quantum Communication represents an attractive and powerful approach to quantum cryptography, offering the prospect of directly transmitting secret messages with security rooted in the laws of quantum mechanics. While still facing practical and theoretical challenges, ongoing research continues to discover the possibilities and limitations of DSQC,  to provide secure communication methods for the future.