Quantum repeaters and computation by a single module: Remote nondestructive parity measurement

Abstract

We introduce a simple photonic probing scheme of remote nondestructive parity measurement (RNPM) on a pair of matter qubits. The protocol works as a single module for key operations such as entanglement generation, Bell measurement, and parity check measurement, which are sufficient not only for building up a quantum repeater but also for equipping it with entanglement distillation. Moreover, the RNPM protocol can also be used for generating cluster states toward measurement-based quantum computation.

Fundamental rate-loss trade-off for the quantum internet

“…This class of QKD protocols leads to a square root improvement in the secret key rate over conventional QKD schemes (without quantum repeaters) bounded by the TGW bound. Nonetheless, it is implementable21 without the need of matter quantum memories or quantum error correction, which is in a striking contrast to quantum repeaters18222324252627282930313233343536. In particular, those intercity QKD protocols are modifications of the measurement-device-independent QKD39 (mdiQKD), and all of them use a single (untrusted) intermediate node C in the middle of communicators Alice and Bob.…”

“…Actually, there are many quantum repeater schemes18222324252627282930313233343536, depending on the assumed devices of the repeater nodes { C 1 , C 2 , …, C n }. For instance, a protocol assumes repeater nodes equipped with atomic-ensemble quantum memories as well as optical devices1823.…”

“…For example, the point-to-point quantum communication over 1,000 km needs18 to take almost one century to provide just one secret bit or one ebit for Alice and Bob under the use of a typical standard telecom optical fibre with loss of about 0.2 dB km −1 . Therefore, for the request from far distant Alice and Bob, the quantum internet necessitates long-distance quantum communication schemes utilizing intermediate nodes, such as intercity quantum key distribution (QKD) protocols192021 and quantum repeaters18222324252627282930313233343536. In particular, these schemes would be in greater demand for the quantum internet than the point-to-point quantum communication, analogously to the current Internet, where a client communicates with a far distant client via repeater nodes routinely and even unconsciously.…”

“…Moreover, thanks to inheriting the generality of the TGW bound, in contrast to Pirandola's bounds37 applied only to teleportation stretchable quantum channel networks, our bounds can be estimated and applied to any situation that can be regarded as the quantum internet as Kimble has considered1, including the simulation of the quantum many-body systems as well as purely quantum communication tasks. As a non-trivial example to imply this, we present upper bounds on the performance of any Duan–Lukin–Cirac–Zoller (DLCZ)-type quantum repeater protocol18232430 by considering not only loss of optical channels but also time-dependent decay of matter quantum memories. These bounds conclude that the coherence time of the matter quantum memories should be, at least, larger than 100 μs for enjoying the blessing of the DLCZ-type quantum repeaters even if they are equipped with any single-shot quantum error correction, as well as any entanglement distillation.…”

Aggregating quantum repeaters for the quantum internet

“…This network, called quantum internet, could accomplish tasks that are intractable in the realm of classical information processing, and it serves opportunities and challenges across a range of intellectual and technical frontiers, including quantum communication, computation, metrology, and simulation [1]. So far, the main interest in the quantum internet has been its realization [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. But, it must be one of the most fundamental trials from a theoretical perspective to grasp the full potential of the quantum internet.…”