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Physical Review A

Single-atom quantum gate for light

Klaus M. Gheri, Helmut Ritsch

Published Oct 1, 1997• 24 citations
Publisher

Abstract

We propose a scheme for an optical quantum gate, i.e., a mirror in a quantum superposition state of total transmission and reflection. The model consists of a high-$Q$ optical cavity strongly coupled to a single atom or ion in a quantum superposition state of two long lived internal states. By including spontaneous emission, dissipation, measurement back-action, and the time dependence of the atom-field coupling, we numerically simulate a system very close to experimental reality. The switching and quantum coherence properties are confirmed separately in two steps by using the gate directly and as an entrance beam splitter to an interferometer.

DOI
10.1103/physreva.56.3187

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We propose a scheme for an optical quantum gate, i.e., a mirror in a quantum superposition state of total transmission and reflection. The model consists of a high-QQQ optical cavity strongly coupled to a single atom or ion in a quantum superposition state of two long lived internal states. By including spontaneous emission, dissipation, measurement back-action, and the time dependence of the atom-field coupling, we numerically simulate a system very close to experimental reality. The switching and quantum coherence properties are confirmed separately in two steps by using the gate directly and as an entrance beam splitter to an interferometer.
A two-qubit quantum swap gate scheme based on coherent state qubit is proposed in cavity quantum electrodynamics system. Our scheme adopt the interaction between the Λ-type atomic ensemble and the two-mode optical cavity, and the swap gate can be realized directly only one step, so as to save a lot of quantum resources and reduce the number of single qubit rotation operations in the actual quantum information processing. It is found that under the condition of large detuning limit, the excited states of atoms are adiabatically eliminated, and the decoherence caused by the spontaneous emission of atoms is effectively suppressed. All atoms in the optical cavity are universally addressed, which reduces the experimental operation difficulty of the scheme. The fidelity of the quantum logic gate is maintained at a very high level when the photon loss of the cavity field is considered. Compared with the single-atom scheme, it is found that the atom-cavity interaction time required by the atomic ensemble scheme is inversely proportional to the number of atoms, so that the speed of the quantum logic gate can be increased by N times. Therefore, in principle, as long as enough atoms interacting with the optical field, an effective coupling of any atom-cavity field can be generated, which is conducive to selecting the appropriate quantum gate operation time in practical operation and effectively avoiding decoherence.
Within the framework of a two-dimensional microscopic, purely quantum mechanical model, we analyse the dynamics of single-photon wave packets interacting with optical elements (beam splitters, mirrors), modelled as systems of two-level atoms. That is, we utilize a two-dimensional cavity to simulate the quantum behaviour of simple optical components and networks made thereof. The field is quantized using the canonical procedure, and only the basis states with one unit of excitation are included. This, however, covers linear optical phenomena. The field is taken to interact with localized atoms through a dipole interaction. Using different configurations of atoms, and choosing their frequencies to be resonant or off-resonance, we can model mirrors, beam splitters, focusing devices and multicomponent systems. Thus we can model arbitrary linear networks of optical components. We show the time evolution of a photon wave packet in an interferometer as an example. As the state of the field is known at each instant, spectral properties and spatial coherence can immediately be obtained from the simulations. We also know the states of the two-level atoms constituting the components, which allows us to consider their quantum behaviour. Here the decay of an excited atom into the vacuum state of the electromagnetic field in the two-dimensional cavity is studied.
We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.
With the quantum Zeno dynamics, we present a scheme for realizing a quantum phase gate for two atoms trapped in two separate cavities connected by an optical fiber. Compared with the previous schemes, quantum Zeno dynamics shows different evolution properties in the atom–cavity–fiber system. The scheme needs only one step to perform the gate, which largely reduces the complexity of the experiment. We also numerically simulate the influence of photon leakage and atomic spontaneous emission on the fidelity of the phase gate. The result shows that our scheme is very robust against cavity decay and very promising for realization with current experimental technology.
Summary form only given. In quantum information processing, a qubit is realised as a superposition of atomic or ionic eigenstates, or as a superposition of the states of a photon. A coupling between these different types of qubits is necessary to perform quantum operations or quantum transmissions, e.g. in a quantum network where single atoms or ions are entangled with individual photons. To generate such an entanglement in a controlled way, a triggered source for single photons, a photon pistol, will be needed. Our scheme will lead to the emission of one photon into a single, predetermined mode of the radiation field in response to a trigger event. This photon is emitted from an atom strongly coupled to a high-finesse optical cavity, and the trigger is a classical light pulse. The device combines cavity-QED with an adiabatic transfer technique. We consider a single lambda-type three-level atom being placed inside an empty cavity.
A scheme is presented for realizing a two-qubit quantum SWAP gate for intracavity fields. In the scheme, a V\mathit{V}V-type atomic ensemble prepared in its ground state mediates the interaction between the two-cavity modes. Under certain conditions, the cavity-field degree of freedom is decoupled from the atomic degrees of freedom, and the effective coupling strength between the two-cavity modes scales up with N\sqrt{N}N​ (N\mathit{N}N is the number of effective atoms). The numerical simulation shows that the quantum SWAP gate may have a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity modes.