Grzegorz Chimczak

Teleportation with insurance of an entangled atomic state via cavity decay

We propose a scheme to teleport an entangled state of two {lambda}-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 {lambda}-type atoms trapped in a cavity.

Experimental quantum forgery of quantum optical money

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.Quantum money could protect currency against cryptographic attacksTraditional cash is being gradually replaced by digital payments and transactions with digital currency, such as bitcoin. Digital transactions are currently protected by cryptographic protocols. However, these protocols are potentially susceptible to attacks using quantum factoring algorithm. If quantum factoring is implemented on quantum computers, the resulting breach of security could make today’s digital currency obsolete. It is possible, however, to replace classical digital money with so-called quantum money, i.e., sequences of quantum bits copy-protected by their quantum nature. This paper reports on the experimental quantum optical implementation of a quantum money protocol, which was experimentally tested regarding its resistance to quantum counterfeiting based on the best physically-possible copying of individual unknown quantum bits.

High-fidelity atomic-state teleportation protocol with non-maximally-entangled states

We propose a protocol of the long-distance atomic state teleportation via cavity decay, which allows for high-fidelity teleportation even with currently available optical cavities. The protocol is based on the scheme proposed by Bose \emph{et al.} [Phys. Rev. Lett. {\textbf{83}}, 5158 (1999)] but with one important modification: it employs non-maximally-entangled states instead of maximally entangled states.

Efficient generation of distant atom entanglement via cavity decay

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 spontaneous atomic decay.

Direct method for measuring and witnessing quantum entanglement of arbitrary two-qubit states through Hong-Ou-Mandel interference

We describe a direct method for experimental determination of the negativity of an arbitrary two-qubit state with 11 measurements performed on multiple copies of the two-qubit system. Our method is based on the experimentally accessible sequences of singlet projections performed on up to four qubit pairs. In particular, our method permits the application of the Peres-Horodecki separability criterion to an arbitrary two-qubit state. We explicitly demonstrate that measuring entanglement in terms of negativity requires three measurements more than detecting two-qubit entanglement. The reported minimal set of interferometric measurements provides a complete description of bipartite quantum entanglement in terms of two-photon interference. This set is smaller than the set of 15 measurements needed to perform a complete quantum state tomography of an arbitrary two-qubit system. Finally, we demonstrate that the set of 9 Makhlins invariants needed to express the negativity can be measured by performing 13 multicopy projections. We demonstrate that these invariants are both a useful theoretical concept for designing specialized quantum interferometers and that their direct measurement within the framework of linear optics does not require performing complete quantum state tomography.

Fine tuning of quantum operations performed via Raman transitions

A scheme for fine tuning of quantum operations to improve their performance is proposed. A quantum system in

Quantum Entanglement and Teleportation of Quantum-Dot States in Microcavities

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The effect of a non-zero spontaneous decay rate on teleportation

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

High fidelity state mapping performed in a V-type level structure via stimulated Raman transition

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Creating a switchable optical cavity with controllable quantum-state mapping between two modes

pulse and