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Rinton Press eBooks

Optical quantum information processing: Experiments with CW laser light

Hans‐A. Bachor, Ping Koy Lam, Warwick P. Bowen, Roman Schnabel, Shayne Bennetts, B. C. Buchler, Ulrik L. Andersen, Timothy C. Ralph

Published Jan 1, 2001• 0 citations
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Abstract

Experiments and devices based on continuous waves (CW) optical beams can provide solutions to optical quantum information processing which are analogous to those carried out with single photon entangled states. We report on our experimental achievements and goals for using squeezed light and entangled beams for quantum nondemolition measurements, dense coding, teleportation and quantum cryptography.

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Experiments and devices based on continuous waves (CW) optical beams can provide solutions to optical quantum information processing which are analogous to those carried out with single photon entangled states. We report on our experimental achievements and goals for using squeezed light and entangled beams for quantum nondemolition measurements, dense coding, teleportation and quantum cryptography.
This paper presents our experimental work on preparing and manipulating quantum entangled states in optical quantum information processing systems. The main contents are: (1) Using continuous wave(CW) laser beam to pump nonlinear crystals we successfully prepared two-photon polarization entangled states with high brightness and entanglement degree, and it is tunable in entanglement degree and we use this source to produce one kind of important mixed states——Werner states with tunable entanglement fraction in it. (2) We realized the 1→2 qubits Buek-Hillery cloning machine with linear optical devices and showed that the fidelities between the two output qubits and the original qubit are both 5/6 for arbitrary input pure states. (3) We presented a method to test the maximal violation of the CHSH inequality for non-maximally entangled pure states and experimentally realized it with our entangled state source. (4) We perform an all-or-nothing-type Kochen-Specker experiment to verify whether non-contextual hidden variables (NCHV) or quantum mechanics is right. The results strongly agree with quantum mechanics.
Summary form only given. In the quantum optics group at Caltech, we are attempting to lay the foundations for quantum information science by way of advances on several fronts in optical physics. Within the setting of cavity QED, single atoms are strongly coupled to the field of a high finesse optical cavity at the single photon level, with current work directed toward trapping and localization of atoms inside the cavity. Although there are daunting technical problems to overcome, a principal scientific objective is the creation of quantum networks to implement fundamental quantum communication protocols and for distributed quantum computation. Beyond quantum information processing with internal atomic states and photons serving as qubits, we are also investigating algorithms for continuous quantum variables. A recent example is our realization of quantum teleportation for the quadrature amplitudes of a beam of light. The experiment utilizes squeezed-state entanglement to achieve unconditional quantum teleportation.
We propose a pulse and continuous wave (CW) hybrid architecture of continuous-variable measurement-based optical quantum computation utilizing the strengths of both pulsed and CW light. In this architecture, input and ancillary non-Gaussian quantum states necessary for fault-tolerance and universality are generated with pulsed light, whereas quantum processors including continuous-variable cluster states and homodyne measurement systems are operated with CW light. This architecture is expected to enable both generation of quantum states with shorter optical wavepackets for ultrafast computation and low-loss manipulation and measurement of these states. In this study, as a proof-of-principle, an ultrafast homodyne measurement using a CW local oscillator was performed on single-photon states generated with pulsed light. The measured single-photon state's temporal width was around 70 ps and the value of the Wigner function at the origin was W(0,0)=−0.153±0.003W(0,0) = -0.153\pm0.003W(0,0)=−0.153±0.003, which is highly non-classical. This will be a core technology for high-speed optical quantum information processing.
Quantum correlations and entanglement are an important addition to the conventional properties of continuous laser beams. They allow us to avoid, at least in principle, the limitations imposed by quantum noise and they open new opportunities for the transfer and processing of quantum information. This manuscript reviews the state of art in the creation, detection of quantum correlations and entanglement, shows the analogy to the techniques based on single photons. It summarizes some of the recent advances in optical sensing and quantum information processing.
We have experimentally realized the scheme initially proposed as quantum dense coding with continuous variables [Ban, J. Opt. B: Quantum Semiclassical Opt. 1, L9 (1999); Braunstein and Kimble, Phys. Rev. A 61, 042302 (2000)]. In our experiment, a pair of EPR (Einstein-Podolsky-Rosen) beams is generated from two independent squeezed vacua. After adding a two-quadrature signal to one of the EPR beams, two squeezed beams that contain the signal were recovered. Although our squeezing level is not sufficient to demonstrate the channel capacity gain over the Holevo limit of a single-mode channel without entanglement, our channel is superior to conventional channels such as coherent and squeezing channels. In addition, the optical addition and subtraction processes demonstrated are elementary operations of universal quantum information processing on continuous variables.