Unknown quantum information cannot be perfectly copied (cloned). This statement is the bedrock of quantum technologies and quantum cryptography, including the seminal scheme of Wiesner's quantum money, which was the first quantum-cryptographic proposal. Surprisingly, to our knowledge, quantum money has not been tested experimentally yet. Here, we experimentally revisit the Wiesner idea, assuming a banknote to be an image encoded in the polarization states of single photons. We demonstrate that it is possible to use quantum states to prepare a banknote that cannot be ideally copied without making the owner aware of only unauthorized actions. We provide the security conditions for quantum money by investigating the physically-achievable limits on the fidelity of 1-to-2 copying of arbitrary sequences of qubits. These results can be applied as a security measure in quantum digital right management. 7 Although, various protocols of quantum money have already been proposed (see, e.g., refs 8-17), this interest cannot be compared with the immense popularity and applicability of QKD (see refs 18-20 as an example of recent and fundamental achievements). This is partially because there have not been, to our knowledge, any experimental realizations of quantum money performed yet. Here, we report not only an experimental implementation of quantum money but also an experimental attempt to its forgery using optimal cloning machines.Our experimental work basically describes one-by-one attacks on each single qubit. In the quantum money scheme, however, eavesdroppers, in principle, can access every qubit at once. So, they can globally access multiple qubits and can seek superior attacks using such global access. This could be a reason why there has not been a known representative work for the experiment of attacking quantum money, because this would need to treat numerous qubits and difficult global controls of their quantum states. The attacks presented in this work are less distinguished from quantum cloning itself or the attack for BB84 quantum key distribution. Thus, collective or coherent attacks on multiple qubits

Inspired by the recent experiment of Hamsen et al. [Phys. Rev. Lett. 118, 133604 (2017)], which demonstrated two-photon blockade in a driven nonlinear system (composed of a harmonic cavity with a driven atom), we show that two-photon blockade and other nonstandard types of photonblockade and photon-induced tunneling can be generated in a driven harmonic cavity without an atom or any other kind of nonlinearity, but instead coupled to a nonlinear (i.e., squeezed) reservoir. We also simulate these single-and two-photon effects with squeezed coherent states and displaced squeezed thermal states.

We show how the entanglement of two atoms, trapped in distant separate cavities, can be generated with arbitrarily high probability of success. The scheme proposed employs sudden excitation of the atoms proving that the weakly driven condition is not necessary to obtain the success rate close to unity. The modified scheme works properly even if each cavity contains many atoms interacting with the cavity modes. We also show that our method is robust against the spontaneous atomic decay.PACS numbers: 03.67. Hk, 03.67.Mn Many quantum information tasks require an entanglement, especially an entanglement shared by distant atoms can play a very important role in quantum information processing. This is due to the fact that atomic states are ideal for quantum information storage. Therefore, a variety of schemes for entanglement of distant atoms have been proposed recently [1,2,3,4,5,6,7,8,9]. The schemes employ also photonic states providing fast quantum information transfer over long distances. Most of the schemes describe two cavities, each containing one trapped atom. The photons leaking out from the cavities are mixed at a beam splitter and detected [3,4,5,6,7]. In those schemes, however, only two of the four Bell states can be used, and therefore, the success rate is less than 50% [10,11]. Moreover, the success rate is lowered by the spontaneous atomic emissions. In most of the schemes the population of the excited state is considerable [5,6,7] during the entangling operations and can therefore drastically lower the success rate as it has been proved in [12]. Only the proposal of Browne et al.[4] avoids all of the above problems. The whole operation is performed there in such a way that the population of the excited state is negligible thanks to the use of large detunings. Furthermore, the scheme uses the requirement of weak driving since a sudden excitation of the atoms limits the entanglement efficiency to 50% as is suggested in [4]. However, the condition makes it impossible to perform many operation requiring strong driving, and therefore, it can be difficult to use the entangled atoms in quantum computations. It is possible to change this condition by controlling laser intensity but then the entanglement operation time will be long.In this paper a scheme is presented that employs sudden excitation to entangle two atoms with high success probability. The main idea of the scheme is to use a protocol which first prepares each cavity in one photon state, next creates maximally entangled state of both cavity fields detecting one photon decay from the cavities and finally maps the entangled state onto two distant atoms. The strong driving condition makes it possible to use the entangled distant atoms in various quantum information tasks. The setup consists of two cavities, a 50-50 beam require also many qubits [13,14] thus each cavity can contain up to N atoms. Each atom is modeled by a threelevel Λ system with one excited state |2 and two ground states |0 and |1 . The energy level structure of the atom is shown ...

We propose a scheme to teleport an entangled state of two ⌳-type three-level atoms via photons. The teleportation protocol involves the local redundant encoding protecting the initial entangled state and allowing for repeating the detection until quantum information transfer is successful. We also show how to manipulate a state of many ⌳-type atoms trapped in a cavity.

A scheme for fine tuning of quantum operations to improve their performance is proposed. A quantum system in $\Lambda$ configuration with two-photon Raman transitions is considered without adiabatic elimination of the excited (intermediate) state. Conditional dynamics of the system is studied with focus on improving fidelity of quantum operations. In particular, the $\pi$ pulse and $\pi/2$ pulse quantum operations are considered. The dressed states for the atom-field system, with an atom driven on one transition by a classical field and on the other by a quantum cavity field, are found. A discrete set of detunings is given for which high fidelity of desired states is achieved. Analytical solutions for the quantum state amplitudes are found in the first order perturbation theory with respect to the cavity damping rate $\kappa$ and the spontaneous emission rate $\gamma$. Numerical solutions for higher values of $\kappa$ and $\gamma$ indicate a stabilizing role of spontaneous emission in the $\pi$ and $\pi/2$ pulse quantum operations. The idea can also be applied for excitation pulses of different shapes.Comment: 11 pages, 11 figures, published versio