Physicists in Germany and the US want to take a principle from nature — that information stored on a quantum bit, or qubit, cannot be cloned — and apply it to the problem of credit card fraud. Collaborators at the Max Planck Institute of Quantum Optics, Harvard University and California Institute of Technology think quantum tokens could be an answer. The identity of the owner of the tokens is encoded on photons transmitted via an optical fiber or in the nuclear spin state of memory, and only the bank stores a full classical description of these quantum states. The idea of “quantum money” was first posited in 1970 by Columbia University graduate student Stephen J. Wiesner. The properties of quantum information that make it suitable for preventing forgery are also hard to achieve in the real world because of the fragility of the qubits’ physical carriers, which could be individual nuclei. Illustration of a quantum bill. Courtesy of ©Background by vektorportal.com. Collage by F. Pastawski. Noise, decoherence and operational imperfections are other challenges that need to be overcome. Professors Ignacio Cirac of Max Planck Institute of Quantum Optics (MPQ) and Mikhail Lukin of Harvard have focused on improving storage quality and on providing protocols to accommodate these real-world imperfections. They propose devising a token verification process between the bank and the vendor that allows for a certain amount of quantum bit failure, but not enough to compromise the security of the system. The physicists theorize that “no more than 83 percent of the secret digits may be duplicated correctly by a counterfeiter,” said MPQ’s Dr. Fernando Pastawski. In order to reach the timescales necessary for relevant applications, good qubit memories are needed, he said. “We have recently achieved storage times of one second for single qubits at room temperature, which is a big step, but not yet sufficient.” The proposed quantum token could serve as a basis for quantum money, which can change hands several times, or even quantum credit cards that cannot be forged. ”I expect to live to see such applications become commercially available,” Pastawski said. “However, quantum memory technology still needs to mature for such protocols to become viable.” The study appeared in the Proceedings of the National Academy of Science (doi: 10.1073/pnas.1203552109). For more information, visit: www.mpq.mpg.de