Dynamical oscillator-cavity model for quantum memories

Abstract

We propose a dynamical approach to quantum memories using an oscillator-cavity model. This overcomes the known difficulties of achieving high quantum input-output fidelity with long storage times compared to the input signal duration. We use a generic model of the memory response, which is applicable to any linear storage medium ranging from a superconducting device to an atomic medium. The temporal switching or gating of the device may either be through a control field changing the coupling, or through a variable detuning approach, as in more recent quantum memory experiments. An exact calculation of the temporal memory response to an external input is carried out. This shows that there is a mode-matching criterion which determines the optimum input and output pulse time evolution. This optimum pulse shape can be modified by changing the gate characteristics. In addition, there is a critical coupling between the atoms and the cavity that allows high fidelity in the presence of long storage times. The quantum fidelity is calculated both for the coherent state protocol, and for a completely arbitrary input state with a bounded total photon number. We show how a dynamical quantum memory can surpass the relevant classical memory bound, while retaining long storage times.

System Identification for Passive Linear Quantum Systems

“…Such an optimal pulse depends on the system's parameters, which therefore should be identified as accurately as possible. Note that several similar architectures for quantum memory have been proposed for instance in an inhomogeneously broadened ensemble of atoms or nitrogenvacancy centers in diamond [31], [32], [33], nano-mechanical oscillators [34], or a general linear network [35], all of which are modeled by passive linear systems. We should emphasize that the passivity property is essential, as in general an active system violates the energy balance and does not realize a perfect state transfer.…”

“…In this paper, we focus on the class of passive linear quantum system [21], [22], [23], [24], which serves as a device for several applications in quantum information technology, such as entanglement generation [25], [26], [27], [28], [29], quantum memory [30], [31], [32], [33], [34], [35], and linear quantum computing [36]. Analyzing this important class of systems provides the foundation for the general case, but it has a clear interest in its own right in the context of estimation, as described later in this section.…”

Theory of noise suppression inΛ-type quantum memories by means of a cavity

“…Cavity-enhanced Λ-memories have been analysed previously [32][33][34]. To compare with those works, we consider the case that there is no four-wave mixing (equivalent to the limit δ → ∞ so that C a = 0), and we assume the signal field to be tuned to the empty-cavity resonance with k s L = 0 mod(2π).…”

“…Working with atomic ensembles provides a collective enhancement of the atom-light coupling strength, such that the strong coupling regime of cavity QED is not required. The efficiency of cavity-enhanced light storage has been studied theoretically [32][33][34], though without considering fourwave mixing noise. Experimental achievements include a cavity-enhanced DLCZ-type 'emissive' quantum memory in cold atoms [35], and a demonstration of motional narrowing in a cavity-enhanced DLCZ setting using warm atoms [36], along with our own recent implementation of cavity-enhanced Raman storage in warm Cs vapour [37].…”

Dynamical Gaussian state transfer with quantum-error-correcting architecture

“…Candidates for a memory are largely divided into two categories: discrete variable systems such as an atom with distinct energy levels [1,2,3,4,5,6,7,8,9,10,11,12,13] and continuous variable systems such as an opto-mechanical oscillator with a vibration mode [14,15,16,17,18,19,20,21,22]. Remarkably, some experimental demonstrations of quantum state transfer have been reported [1,8,16,18,19].…”