Philip Walther

One-time programs, computer programs which self-destruct after being run only once, are a powerful building block in cryptography and would allow for new forms of secure software distribution. However, ideal one-time programs have been proved to be unachievable using either classical or quantum resources. Here we relax the definition of one-time programs to allow some probability of error in the output and show that quantum mechanics offers security advantages over purely classical resources. We introduce a scheme for encoding probabilistic one-time programs as quantum states with prescribed measurement settings, explore their security, and experimentally demonstrate various one-time programs using measurements on single-photon states. These include classical logic gates, a program to solve Yao’s millionaires problem, and a one-time delegation of a digital signature. By combining quantum and classical technology, we demonstrate that quantum techniques can enhance computing capabilities even before full-scale quantum computers are available.

Despite the range of security guarantees offered by the laws of quantum theory, quantum-cryptographic schemes all face the same practical challenge: participants must ensure that the messages they receive originate from the right person. This authentication step is typically achieved by integrating classical methods, assumed hard for quantum computers, into quantum systems. Here, we propose an alternative solution to replace a central link of the authentication chain with a quantum commitment scheme that is information-theoretically secure against attackers holding a perfect quantum memory of thousands of qubits. In this way, all other parts of the authentication rely only on the fundamental and widely accepted conjecture that P/=NP. Our protocol can be implemented with all standard sources such as down-conversion and highly-attenuated laser states. We however employ a single-photon source emitting directly in the telecom C-band to show how its performance may be boosted by several orders of magnitude, even at moderate channel loss. Over a deployed optical fiber link, we achieve 128-bit security by detecting 10^9 photons, allowing for 66 secure re-uses of the public-private key pair.

The use of quantum systems allows for new insights which promise to revolutionize information processing. Quantum cryptography, especially key distribution, has become one of the most prominent applications of quantum technology. However, this task still requires users to be capable of performing quantum operations, such as state preparation or measurements in multiple bases. A natural question, therefore, is can users' technological requirements be reduced? In this work, we experimentally demonstrate a novel quantum key distribution protocol where users are fully classical and quantum operations are only performed by an untrusted third party acting as a server. We derive an information theoretic proof of security for our protocol along with an experimental demonstration.