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arXiv:1008.1523 · Physical Review A

Long-lived qubit from three spin- 12 atoms

Rui Han, Niels Lörch, Jun Suzuki, Berthold‐Georg Englert

Published Jul 19, 2011• 1 citation
arXivHTMLPDF

Abstract

A system of three spin-$\frac{1}{2}$ atoms allows the construction of a reference-frame-free (RFF) qubit in the subspace with total angular momentum $j=1/2$. The RFF qubit stays coherent perfectly as long as the spins of the three atoms are affected homogeneously. The inhomogeneous evolution of the atoms causes decoherence, but this decoherence can be suppressed efficiently by applying a bias magnetic field of modest strength perpendicular to the plane of the atoms. The resulting lifetime of the RFF qubit can be many days, making RFF qubits of this kind promising candidates for quantum information storage units. Specifically, we examine the situation of three ${}^{6}\text{Li}$ atoms trapped in a ${\text{CO}}_{2}$-laser-generated optical lattice and find that, with conservatively estimated parameters, a stored qubit maintains a fidelity of 0.9999 for two hours.

DOI
10.1103/physreva.84.012322

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Summary form only given. Long-term storage of quantum information is a long standing goal since decades. It is particularly motivated by the fact that quantum network implementations strongly rely on quantum memories for light [1]. Long storage time and high efficiency are the main two figures of merit for a qubit memory. However, although quantum memories have been implemented in a large variety of physical systems, it turns out to be challenging to simultaneously fulfill those two main requirements.In our setup, a single 87Rb atom is trapped in a two-dimensional optical lattice inside a high-finesse cavity. Based on seminal work [2], the photonic polarization qubit a|L) + ß|R) described in the left/right polarization basis is mapped onto the atomic state |F = 2,= -1) + ß|F = 2, mf = +1) via a stimulated Raman adiabatic passage (STIRAP) as depicted in step I) of Fig. 1: The readout efficiency of this quantum memory protocol is basically constant for our typical coherence time. This is in contrast to systems based on atomic ensemble where decoherence affects the collective excitation reemission. However, fluctuations of the magnetic field deteriorate the coherence of the superposition. Indeed, the two levels we use are split by the Zeeman effect and thus the free evolution of the system will accumulate a random phase increasing with time. Typically, the coherence information of the qubit is lost after 200μs.
Long-lived and high-fidelity memory for a photonic polarization qubit (PPQ) is crucial for constructing quantum networks. We present a millisecond storage system based on electromagnetically induced transparency, in which a moderate magnetic field is applied on a cold-atom cloud to lift Zeeman degeneracy and, thus, the PPQ states are stored as two magnetic-field-insensitive spin waves. Especially, the influence of magnetic-field-sensitive spin waves on the storage performances is almost totally avoided. The measured average fidelities of the polarization states are 98.6% at 200 μs and 78.4% at 4.5 ms, respectively.
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Quantum memories that are capable of storing multiple spatial modes offer advantages in speed and robustness when incorporated into quantum networks. When it comes to spatial degrees of freedom, orbital angular momentum (OAM) modes have received widespread attention since they enable encoding with inherent infinite number of dimensions. Although the faithful storage of OAM qubits or qutrits has been realized in previous works, the achieved lifetimes are still on the order of a few microseconds as limited by the spatially dependent decoherence. We here demonstrate a long-lived quantum memory for OAM qutrits by suppressing the decoherence in the transverse and longitude direction simultaneously; the achieved fidelity beats the quantum-classical criteria after a storage time of 400 μs, which is 2 orders of magnitude longer than earlier works. The present work is promising for establishing high-dimensional quantum networks.
Quantum systems promise to revolutionize information processing science and technology [1-3]. The preservation of quantum coherence, the defining property of qubits, fundamentally constrains the performance of quantum information processing with quantum memories [4]. While trapped atomic ions theoretically support million-year coherence based on spontaneous emission [5-7], experimental demonstrations have reached far less, only about an hour [8-13]. Here we combine clock-state qubits with decoherence-free subspace (DFS) encoding to achieve coherence exceeding ten hours. Using correlation-based phase tracking in 171Yb+ ion pairs sympathetically cooled by 138Ba+ ion, we demonstrate this without magnetic shielding or enhanced microwave phase stabilization that previously limited coherence times. DFS encoding references the qubit phase to the inter-ion energy difference to reject microwave phase noise and common-mode magnetic fluctuations, while clock states provide environmental insensitivity. Throughout measurements extended to 1600 seconds, we observe minimal coherence decay, with exponential fits yielding a coherence time of (3.77 +/- 1.09) x 10^4 seconds. Our results establish DFS encoding as a form of passive error correction that eliminates technical noise constraints, unlocking the million-year coherence potential of atomic ions for scalable quantum information processing.
The effectiveness of decoherence suppression schemes is explored using quantum bits (qubits) stored in Li np Rydberg states. Following laser excitation, pulsed electric fields coherently control the electronic spin-orbit coupling, facilitating qubit creation, manipulation, and measurement. Spin-orbit coupling creates an approximate decoherence-free subspace for extending qubit storage times. However, sequences of fast NOT operations are found to be substantially more effective for preserving coherence.