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arXiv:1509.05682 · Physical Review A

Quantum controlled- Z gate for weakly interacting qubits

Michal Mičuda, Robert Stárek, Ivo Straka, Martina Miková, Miloslav Dušek, Miroslav Ježek, Radim Filip, Jaromı́r Fiurášek

Published Aug 19, 2015• 4 citations
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Abstract

We propose and experimentally demonstrate a scheme for the implementation of a maximally entangling quantum controlled-$Z$ gate between two weakly interacting systems. We conditionally enhance the interqubit coupling by quantum interference. Both before and after the interqubit interaction, one of the qubits is coherently coupled to an auxiliary quantum system, and finally it is projected back onto qubit subspace. We experimentally verify the practical feasibility of this technique by using a linear optical setup with weak interferometric coupling between single-photon qubits. Our procedure is universally applicable to a wide range of physical platforms including hybrid systems such as atomic clouds or optomechanical oscillators coupled to light.

DOI
10.1103/physreva.92.022341

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•Feb 19, 2013•0 citations
We propose and experimentally demonstrate a scheme for the implementation of a maximally entangling quantum controlled-ZZZ gate between two weakly interacting systems. We conditionally enhance the interqubit coupling by quantum interference. Both before and after the interqubit interaction, one of the qubits is coherently coupled to an auxiliary quantum system, and finally it is projected back onto qubit subspace. We experimentally verify the practical feasibility of this technique by using a linear optical setup with weak interferometric coupling between single-photon qubits. Our procedure is universally applicable to a wide range of physical platforms including hybrid systems such as atomic clouds or optomechanical oscillators coupled to light.
We theoretically investigate and experimentally demonstrate a procedure for conditional control and enhancement of an interferometric coupling between two qubits encoded into states of bosonic particles. Our procedure combines local coupling of one of the particles to an auxiliary mode and single-qubit quantum filtering. We experimentally verify the proposed procedure using a linear optical setup where qubits are encoded into quantum states of single photons and coupled at a beam splitter with a fixed transmittance. With our protocol, we implement a range of different effective transmittances, demonstrate both enhancement and reduction of the coupling strength, and observe dependence of two-photon bunching on the effective transmittance. To make our analysis complete, we also theoretically investigate a more general scheme where each particle is coupled to a separate auxiliary mode and show that this latter scheme enables us to achieve higher implementation probability. We show that our approach can be extended also to other kinds of qubit-qubit interactions.
One of the strengths of quantum information theory is that it can treat quantum states without referring to their particular physical representation. In principle, quantum states can be therefore fully swapped between various quantum systems by their mutual interaction and this quantum state transfer is crucial for many quantum communication and information processing tasks. In practice, however, the achievable interaction time and strength are often limited by decoherence. Here we propose and experimentally demonstrate a procedure for faithful quantum state transfer between two weakly interacting qubits. Our scheme enables a probabilistic yet perfect unidirectional transfer of an arbitrary unknown state of a source qubit onto a target qubit prepared initially in a known state. The transfer is achieved by a combination of a suitable measurement of the source qubit and quantum filtering on the target qubit depending on the outcome of measurement on the source qubit. We experimentally verify feasibility and robustness of the transfer using a linear optical setup with qubits encoded into polarization states of single photons.
We present a scheme for direct implementation of an N-qubit tunable controlled phase gate for spin qubits in quantum dots coupled to optical cavities, resorting to spin selective dipole coupling interaction and linear optical manipulation. In the scheme, we first design a quantum entangler device to transform the operated physical qubits, which are represented by the states of the electron spins, into non-maximal hybrid entangled states with a certain probability of success via the introduction of auxiliary circularly polarized photons. Then we show that, based on this quantum entangler operation, a spin-based N-qubit tunable controlled phase gate can be probabilistically achieved by the interference and coincidence detection of polarized photons. This might lead to a useful step toward the construction of more efficient quantum circuits and quantum algorithms in solid-state qubits.
Quantum-dot hybrid qubits exploit an extended charge-noise sweet spot that suppresses dephasing and has enabled the experimental achievement of high-fidelity single-qubit gates. However, current proposals for two-qubit gates require tuning the qubits away from their sweet spots. Here, we propose a two-hybrid-qubit coupling scheme, based on exchange interactions, that allows the qubits to remain at their sweet spots at all times. The interaction is controlled via the interqubit tunnel coupling. By simulating such gates in the presence of realistic quasistatic and 1/f1/f1/f charge noise, we show that our scheme should enable controlled-ZZZ gates of length $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$, and zero-controlled-not gates of length $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$, both with fidelities >99.9>99.9%>99.9.
Hybrid qubits have recently drawn intense attention in quantum computing. We here propose a method to implement a universal controlled-phase gate of two hybrid qubits via two three-dimensional (3D) microwave cavities coupled to a superconducting flux qutrit. For the gate considered here, the control qubit is a microwave photonic qubit (particle-like qubit), whose two logic states are encoded by the vacuum state and the single-photon state of a cavity, while the target qubit is a cat-state qubit (wave-like qubit), whose two logic states are encoded by the two orthogonal cat states of the other cavity. During the gate operation, the qutrit remains in the ground state; therefore, decoherence from the qutrit is greatly suppressed. The gate realization is quite simple, because only a single basic operation is employed and neither classical pulse nor measurement is used. Our numerical simulations demonstrate that with current circuit quantum electrodynamics technology, this gate can be realized with a high fidelity. The generality of this proposal allows implementing the proposed gate in a wide range of physical systems, such as two 1D or 3D microwave or optical cavities coupled to a natural or artificial three-level atom. Finally, this proposal can be applied to create a novel entangled state between a particle-like photonic qubit and a wave-like cat-state qubit.
We investigate methods related to achieving interaction between photonic qubits. The first general approach to achieve this is to transfer photonic qubits to solid state systems, where the qubit interactions can then take place. The other approach is to enhance the strength of the photon-photon interaction. -- We first robustly study methods related to the first approach: in particular we focus on how to convert the photonic quantised excitation into an atomic excitation. The Stark-shift-chirped rapid adiabatic passage (SCRAP) technique in a three level Λ-type system is a coherent population transfer (CPT) technique similar to Stimulated Raman Adiabatic Passage (STIRAP), which in itself is closely related to electro-magnetically induced transparency (EIT). SCRAP has been shown to perform CPT with a high delity for a range of different detunings making SCRAP far more robust when there is large inhomogeneous broadenings present in the ensemble. We make use of optimum control techniques in order to optimise the standard SCRAP pulses so as to minimise the decrease in delity brought on by inhomogeneous broadenings of the transitions. Our result is that we can improve the average delity of population transfer over a wide range of detunings for both the ground to excited state detuning and the ground to target state detuning (two-photon detuning). -- Finally we consider the enhancement of photon-photon interactions and the implementation of an optical quantum controlled not (CNOT) gate via a cavity-QED setup. We explore the Nitrogen-vacancy centre in diamond as a suitable four-level tripod system in which to generate the cross-Kerr nonlinearity required to facilitate the strong interaction between the two fields (control and target) held in a cavity. We show that with an ultra-high quality factor cavity, and only a single NV centre strongly coupled to the trapped light, a suffciently large interaction can be generated between the two fields to obtain a conditional phase shift (CPS) in excess of the required π for the successful implementation of a CNOT gate. We also show that it is possible to use this system to create an entangled state of two macroscopically distinguishable states, that is a Schrödinger cat state, by using two weak coherent fields as input and making a measurement on the second field.