Quantum key distribution (QKD) protocols make it possible for two quantum parties to generate a secret shared key. Semiquantum key distribution (SQKD) protocols, such as "QKD with classical Bob" and "QKD with classical Alice" (that have both been proven robust), achieve this goal even if one of the parties is classical. However, existing SQKD protocols are not experimentally feasible with current technology. Here we suggest a new protocol, "Classical Alice with a controllable mirror", that can be experimentally implemented with current technology (using 4-level systems instead of qubits), and we prove it to be robust.Comment: 6 page

Entanglement is an important concept in quantum information, quantum communication, and quantum computing. We provide a geometrical analysis of entanglement and separability for all the rank-2 quantum mixed states: complete analysis for the bipartite states, and partial analysis for the multipartite states. For each rank-2 mixed state, we define its unique Bloch sphere, that is spanned by the eigenstates of its density matrix. We characterize those Bloch spheres into exactly five classes of entanglement and separability, give examples for each class, and prove that those are the only classes.Comment: 7 pages; 5 figure

A semiquantum key distribution (SQKD) protocol makes it possible for a quantum party and a classical party to generate a secret shared key. However, many existing SQKD protocols are not experimentally feasible in a secure way using current technology. An experimentally feasible SQKD protocol, "classical Alice with a controllable mirror" (the "Mirror protocol"), has recently been presented and proved completely robust, but it is more complicated than other SQKD protocols. Here we prove a simpler variant of the Mirror protocol (the "simplified Mirror protocol") to be completely non-robust by presenting two possible attacks against it. Our results show that the complexity of the Mirror protocol is at least partly necessary for achieving robustness.

Quantum Cryptography uses the counter-intuitive properties of Quantum Mechanics for performing cryptographic tasks in a secure and reliable way. The Quantum Key Distribution (QKD) protocol BB84 has been proven secure against several important types of attacks: collective attacks and joint attacks. Here we analyze the security of a modified BB84 protocol, for which information is sent only in the z basis while testing is done in both the z and the x bases, against collective attacks. The proof follows the framework of a previous paper [1], but it avoids a classical information-theoretical analysis and proves a fully composable security. We show that this modified BB84 protocol is as secure against collective attacks as the original BB84 protocol, and that it requires more bits for testing.2. In public-key cryptography [3], a public key (known to everyone) and a secret key (known only to the receiver) are used: the sender uses the public key for encrypting his or her message, and the receiver uses the secret key for decrypting the message. Examples of public-key ciphers include RSA [4] and elliptic curve cryptography.One of the main problems with current public-key cryptography is that its security is not formally proved. Moreover, its security relies on the computational hardness of specific computational problems, such as integer factorization and discrete logarithm (that can both be efficiently solved on a quantum computer, by using Shor's factorization algorithm [5]; therefore, if a scalable quantum computer is successfully built in the future, the security of many public-key ciphers, including RSA and elliptic curve cryptography, will be broken). Symmetric-key cryptography requires a secret key to be shared in advance between the sender and the receiver (in other words, if the sender and the receiver want to share a secret message, they must share a secret key beforehand). Moreover, no security proofs for many current symmetric-key ciphers, such as AES and DES, are known (even if one is allowed to rely on the computational hardness of problems), and unconditional security proofs against computationally-unlimited adversaries are impossible unless the secret key is used only once and is at least as long as the secret message [6]. The one-time pad (symmetric-key) cipher, that, given a message M and a secret key K of the same length, defines the encrypted message C to be C = M ⊕ K (and then decryption can be performed by computing M = C ⊕ K), is fully and unconditionally secure against any adversary [6] -namely, even if the adversary Eve intercepts the encrypted message C, she gains no information about the original message M (assuming that she has no information about the secret key K). This means that, for obtaining perfect secrecy, all that is needed is an efficient way for sharing a random secret key between the sender and the receiver; unfortunately, "classical key distribution" cannot be achieved in a fully secure way if the adversary can listen to all the communication between Alice and Bob.Quantum key dist...

The Quantum Key Distribution (QKD) protocol BB84 has been proven secure against several important types of attacks: the collective attacks and the joint attacks. Here we analyze the security of a modified BB84 protocol, for which information is sent only in the z basis while testing is done in both the z and the x bases, against collective attacks. The proof follows the framework of a previous paper [1], but it avoids the classical information-theoretical analysis that caused problems with composability. We show that this modified BB84 protocol is as secure against collective attacks as the original BB84 protocol, and that it requires more bits for testing.