Joint field and solid-state qubits

Solid state superconducting devices coupled to coplanar transmission lines offer an exquisite architecture for quantum optical phenomena probing as well as for quantum computation implementation, being the object of intense theoretical and experimental investigation lately. In appropriate conditions the transmission line radiation modes can get strongly coupled to a superconducting device with only two levels -for that reason called artificial atom or qubit. Employing this system we propose a hybrid two-quantum bit gate encoding involving quantum electromagnetic field qubit states prepared in a coplanar transmission line capacitively coupled to a single charge qubit. Since dissipative effects are more drastic in the solid state qubit than in the field one, it can be employed for storage of information, whose efficiency against the action of an ohmic bath show that this encoding can be readily implemented with present day technology. We extend the investigation to generate entanglement between several solid state qubits and the field qubit through the action of external classical magnetic pulses. See at http://xxx.lanl.gov/abs/1110.1355

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Conditions for Monogamy of Entanglement of Formation

We present a set of conditions that an arbitrary pure tripartite state must satisfy in order that the Entanglement of Formation (EOF) becomes a monogamic measure. The monogamic relation derived is deeply connected to the discrepancy between classical and quantum correlations, embodying the possible negative signal of the EOF tangle whenever the quantum correlation is larger than the classical one. The estimated limits bound the possible amount of quantum conditional entropy in the system. We present general analytical results for a three- qubit system and give a simple numerical algorithm to calculate the EOF tangle for a system with dimension 2 × 2 × N. Those results are employed to elucidate features of the distribution of entanglement during a dynamical evolution. See more at http://xxx.lanl.gov/abs/1110.1054

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Locally Inaccessible Information and Entanglement of Formation

Quantum discord (QD) measures the fraction of the pairwise mutual information that is locally inaccessible, in a multipartite system. Fundamental aspects related to two important measures in quantum information theory the Entanglement of Formation (EOF) and the conditional entropy, can be understood in terms of the distribution of this form of Local Inaccessible Information (LII). As such, the EOF for an arbitrarily mixed bipartite system AB can be related to the gain or loss of LII due to the extra knowledge that a purifying ancillary system E has on the pair AB. Similarly, a clear meaning of the negativity of the conditional entropy for AB is given. We exemplify by showing that these relations elucidate important and yet not well understood quantum features, such as the bipartite entanglement sudden death and the distinction between EOF and QD for quantifying quantum correlation. For that we introduce the concept of LII flow which quantifies the LII shared in multipartite system when a sequential local measurements are performed. To appear in New Journal of Physics. See preprint at http://xxx.lanl.gov/abs/1106.0289

Depiction of flow of Local Inaccessible Information departing from measurements in E (blue arrows) and concentrating in E (red arrows). The net result of these two flows is the difference between the EOF and the QD for AB when measurements are made on B.

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Entanglement irreversibility from quantum discord and quantum deficit

We relate the problem of irreversibility of entanglement with the recently defined measures of quantum correlation – quantum discord and one-way quantum deficit. We show that the entangle- ment of formation is always strictly larger than the coherent information and the entanglement cost is also larger in most cases. We prove irreversibility of entanglement under LOCC for a family of entangled states. This family is a generalization of the maximally correlated states for which we also give an analytic expression for the distillable entanglement, the relative entropy of entanglement, the distillable secret key and the quantum discord. Phys. Rev. Lett. 107, 020502 (2011) , also in http://xxx.lanl.gov/abs/1007.0228

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Quantum measurement and control in optomechanical systems

During the week of 9-13 of May prof. Gerard Milburn is visiting us and is delivering a series of lectures from 9 to 12 on

“Quantum measurement and control in optomechanical systems“

Prof. Gerard J. Milburn

The Univerity of Queensland, Brisbane, Australia

Director of  the “Centre of Excellence in Engineered Quantum Systems”

The emerging field of quantum optomechanics combines quantum optics and new fabrication techniques to control the quantum state of macroscopic mechanical resonators. This now provides a new approach for controlling the mutual interaction between light and mesoscopic structures, which is one of the eminent goals in quantum information science and of importance for fundamental experiments at the quantum-classical boundary. I will give an overview of this new field and discuss some specific models. These include a scheme to conditionally prepare a macroscopic mechanical resonator in an energy eigenstate by measurement, single photon optomechanics, and quantum entanglement in optomechanical networks.

Date and time: 09 to 11/05/2011 at 14:00 and colloquium in 12/05/2011 at 16:00.

Place: Seminar room DFMC/IFGW/UNICAMP and colloquium hall IFGW

More information please go to the course page

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Generation of Superposition States and Charge-Qubit Relaxation Probing in a Circuit

a) Schematic setup, with the central transmission line (resonator) capacitively coupled to the source and drain, and capacitively coupled to a SQUID-type qubit. (b) Variable energy levels of the qubit (0 < n < 1), with an external classical magnetic flux Φx(t). The resonator field is always blue detuned from that transition. J. Phys. B: At. Mol. Opt. Phys. 44 (2011) 135503. Also in http://xxx.lanl.gov/abs/1104.5189

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Entanglement, quantum discord, and the power of the quantum computer

Schematic illustration of the Quantum Computation with only a single pure qubit protocol (DQC1). A represents the pure qubit, B the maximally mixed state, obtained through maximal entanglement with the environment E. From left to right, initially B is entangled with E (purple bar). The protocol is then executed and A, although not directly entangled with B, gets entangled with the pair BE as the Quantum Discord between A and B increases. Phys. Rev. A 84, 012313 (2011). See also in arXiv:1006.2460

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Phase transition, entanglement and squeezing in a triple-well condensate

Occupational Husimi representation (left) and phase distribution (right) of the ground state for N = 30, Ω = −1, μ = 0 and self-collision parameter values: (a) χ=0, (b) χ=2 and (c) χ=3. In (b) we observe the squeezing in phase while the distribution in number broadens. The fragmentation of the phase in (c) characterizes a Quantum Phase Transition.  See more in Europhysics Letters 90, 10014 (2010).

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