[Paper Review] Prospects for Strong Cavity Quantum Electrodynamics with Superconducting Circuits

S. M. Girvin, Ren-Shou Huang+3 more|arXiv (Cornell University)|Oct 28, 2003

Quantum Information and Cryptography6 references3 citations

TL;DR

This paper proposes a superconducting circuit architecture using one-dimensional transmission line resonators to achieve the strong coupling regime in cavity quantum electrodynamics (cQED), enabling high-fidelity qubit control, quantum non-demolition (QND) readout of multiple qubits, and tunable two-qubit entanglement over centimeter-scale distances. The key result is that vacuum Rabi splitting exceeds decoherence rates, enabling robust quantum operations and potential for scalable quantum computation.

ABSTRACT

We propose a realizable architecture using one-dimensional transmission line resonators to reach the strong coupling limit of cavity quantum electrodynamics in superconducting electrical circuits. The vacuum Rabi frequency for the coupling of cavity photons to quantized excitations of an adjacent electrical circuit (qubit) can easily exceed the damping rates of both the cavity and the qubit. This architecture is attractive for quantum computing and control, since it provides strong inhibition of spontaneous emission, potentially leading to greatly enhanced qubit lifetimes, allows high-fidelity quantum non-demolition measurements of the state of multiple qubits, and has a natural mechanism for entanglement of qubits separated by centimeter distances. In addition it would allow production of microwave photon states of fundamental importance for quantum communication.

Motivation & Objective

• To realize the strong coupling regime in cavity quantum electrodynamics (cQED) using superconducting circuits.
• To enable high-fidelity quantum non-demolition (QND) measurement of multiple qubits in a single shot.
• To achieve tunable, long-distance entanglement of qubits via virtual photon exchange in a resonator.
• To suppress spontaneous emission and enhance qubit coherence through strong coupling to a high-quality-factor cavity.
• To enable generation and detection of microwave photon states for quantum communication.

Proposed method

• Use one-dimensional (1D) transmission line resonators to confine zero-point energy in a small effective volume, enhancing the vacuum Rabi coupling strength.
• Implement Cooper pair boxes as artificial atoms with large electric dipole moments, coupled capacitively to the resonator's center conductor.
• Achieve strong coupling by ensuring the vacuum Rabi frequency $ g $ exceeds both the cavity decay rate $ \kappa $ and the qubit decay rate $ \gamma $.
• Utilize dispersive coupling by detuning qubits from the cavity resonance to enable QND readout via measurable shifts in cavity transmission.
• Implement two-qubit gates via virtual photon exchange when qubits are resonant with each other but detuned from the cavity, governed by the effective Hamiltonian $ H_2 \propto (g^2/\Delta)(\sigma_i^+\sigma_j^- + \sigma_i^-\sigma_j^+) $.
• Enable single-shot, multi-qubit readout by engineering distinct cavity pull values $ \pm g_1^2/\Delta_1 \pm g_2^2/\Delta_2 $ for different qubit states.

Experimental results

Research questions

• RQ1 Can superconducting circuits in a 1D transmission line resonator architecture achieve the strong coupling regime of cQED?
• RQ2 Can strong coupling suppress spontaneous emission and extend qubit coherence times?
• RQ3 Can high-fidelity, single-shot quantum non-demolition (QND) readout be achieved for multiple qubits?
• RQ4 Can tunable, long-distance entanglement of qubits be realized via virtual photon exchange in a cavity?
• RQ5 Can this architecture generate and detect microwave photon states useful for quantum communication?

Key findings

• The vacuum Rabi frequency $ g $ can exceed both the cavity decay rate $ \kappa $ and the qubit decay rate $ \gamma $, satisfying the strong coupling condition $ g \gg \kappa, \gamma $.
• Qubit lifetimes are significantly enhanced due to strong inhibition of spontaneous emission into the cavity mode.
• High-fidelity quantum non-demolition (QND) readout of multiple qubits is possible in a single shot by measuring distinct cavity frequency shifts depending on the collective qubit state.
• Tunable two-qubit entanglement is achieved via virtual photon exchange, with a gate time of approximately $ 50\,\text{ns} $ for a $ \sqrt{i\text{SWAP}} $ operation.
• The number of operations per cavity decay time is estimated at $ N_{\text{op}} \sim 1200 $, indicating high coherence and gate fidelity potential.
• The system enables single-photon detection and source functionality, as single-photon emission or absorption causes a large, measurable change in cavity transmission.