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arXiv:0809.2133 · Physical Review A

High-speed quantum gates with cavity quantum electrodynamics

Chun‐Hsu Su, Andrew D. Greentree, William J. Munro, Kae Nemoto, Lloyd C. L. Hollenberg

Published Dec 23, 2008• 47 citations
arXivHTMLPDF

Abstract

Cavity quantum electrodynamic schemes for quantum gates are amongst the earliest quantum computing proposals. Despite continued progress and the recent demonstration of photon blockade, there are still issues with optimal coupling and gate operation involving high-quality cavities. Here we show that dynamic cavity control allows for scalable cavity-QED based quantum gates using the full cavity bandwidth. This technique allows an order of magnitude increase in operating speed, and two orders reduction in cavity $Q$, over passive systems. Our method exploits Stark shift--based $Q$ switching, and is ideally suited to solid-state integrated optical approaches to quantum computing.

DOI
10.1103/physreva.78.062336

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Cavity quantum electrodynamic schemes for quantum gates are amongst the earliest quantum computing proposals. Despite continued progress and the recent demonstration of photon blockade, there are still issues with optimal coupling and gate operation involving high-quality cavities. Here we show that dynamic cavity control allows for scalable cavity-QED based quantum gates using the full cavity bandwidth. This technique allows an order of magnitude increase in operating speed, and two orders reduction in cavity QQQ, over passive systems. Our method exploits Stark shift--based QQQ switching, and is ideally suited to solid-state integrated optical approaches to quantum computing.
Cavity quantum electrodynamics (cQED) enables deterministic controlled gates between neutral atom and photon qubits, such as the controlled phase flip (CPF) gate [1] , which is a key component for implementing quantum computation. The gate fidelity drops sharply when the length of an input-photon pulse is shorter than the inverse of the bandwidth of a cavity, because of the distortion of the output pulse. This limits the operational speed of the gates. One way to boost the limit of gate speed may be to reduce cavity length, since this increases the bandwidth of the cavity with keeping high cooperativity [2] .
Cavity QED, wherein a quantum emitter is coupled to electromagnetic cavity modes, is a powerful platform for implementing quantum sensors, memories, and networks. However, due to the fundamental trade-off between gate fidelity and execution time, as well as limited scalability, the use of cavity QED for quantum computation was overtaken by other architectures. Here, we introduce a new element into cavity QED—a free charged particle, acting as a flying qubit. Using free electrons as a specific example, we demonstrate that our approach enables ultrafast, deterministic, and universal discrete-variable quantum computation in a cavity-QED-based architecture, with potentially improved scalability. Our proposal hinges on a novel excitation blockade mechanism in a resonant interaction between a free-electron and a cavity polariton. This nonlinear interaction is faster by several orders of magnitude with respect to current photon-based cavity-QED gates, enjoys wide tunability and can demonstrate fidelities close to unity. Furthermore, our scheme is ubiquitous to any cavity nonlinearity, either due to light-matter coupling as in the Jaynes-Cummings model or due to photon-photon interactions as in a Kerr-type many-body system. In addition to promising advancements in cavity-QED quantum computation, our approach paves the way towards ultrafast and deterministic generation of highly entangled photonic graph states and is applicable to other quantum technologies involving cavity QED. Published by the American Physical Society 2024
Cavity quantum electrodynamics (QED) uses a cavity to engineer the mode structure of the vacuum electromagnetic field such as to enhance the interaction between light and matter. Exploiting these ideas in solid-state systems has lead to circuit QED which has emerged as a valuable tool to explore the rich physics of quantum optics and as a platform for quantum computation. Here we introduce a simple approach to further engineer the light-matter interaction in a driven cavity by controllably decoupling a qubit from the cavity's photon population, effectively cloaking the qubit from the cavity. This is realized by driving the qubit with an external tone tailored to destructively interfere with the cavity field, leaving the qubit to interact with a cavity which appears to be in the vacuum state. Our experiment demonstrates how qubit cloaking can be exploited to cancel the ac-Stark shift and measurement-induced dephasing, and to accelerate qubit readout. In addition to qubit readout, applications of this method include qubit logical operations and the preparation of non-classical cavity states in circuit QED and other cavity-based setups.
Quantum electrodynamics of excitons in a cavity is shown to be relevant to quantum operations. We present a theory of an integrable solid-state quantum controlled-phase gate for generating entanglement of two photons using a coupled nanodot-microcavity-fiber structure. A conditional phase shift of O(pi/10) is calculated to be the consequence of the giant optical nonlinearity keyed by the excitons in the cavities. Structural design and active control, such as electromagnetically induced transparency and pulse shaping, optimize the quantum efficiency of the gate operation.
The first realization of solid state quantum computer was demonstrated recently by using artificial atoms — transmons in superconducting resonator. Here, we propose a novel quantum computer based on quantum electrodynamic cavity coupling many quantum nodes of controlled atomic ensembles. The quantum computer contains quantum memory and processing nodes. For the first time, we find the optimal practically attainable parameters of the atoms and optical scheme of the computer for realization of the multimode quantum memory for the photonic qubits with efficiency close to 100%. Then we reveal self modes for reversible transfer of the qubits between the quantum memory node and the processing nodes. Also, we find a realization of iSWAP gate via direct coupling of two processing nodes with an operation rate accelerated proportionally to the number of atoms in the nodes. Collective dynamic and static blockade mechanisms are proposed for realization of $\sqrt {iSWAP} $ quantum gate in multi atomic ensembles. A large number of the two-qubit gates can be simultaneously realized that opens a possibility for parallel quantum processing in the proposed quantum computer.
A scheme is proposed to obtain three-qubit gates, such as quantum-phase gate and C2{C}^{2}C2-NOT gate operations, in a cavity QED system where highly detuned cavity-field modes interact with a four-level system in an inverted-Y configuration. The influence of the Stark shift is also included in such proposed gate operations. Since only the metastable lower levels are involved in the gate operations, the gates are not affected by the atomic decay rates. The potential application of such gates to realize Grover's algorithm is also discussed.