Long-Lived Qubit Memory Using Atomic Ions

Abstract

We demonstrate experimentally a robust quantum memory using a magnetic-field-independent hyperfine transition in 9 Be + atomic ion qubits at a magnetic field B ≃ 0.01194 T. We observe that the single physical qubit memory coherence time is greater than 10 seconds, an improvement of approximately five orders of magnitude from previous experiments with 9 Be + . We also observe long coherence times of decoherence-free subspace logical qubits comprising two entangled physical qubits and discuss the merits of each type of qubit.PACS numbers: 03.67. Pp, 32.60.+i, 03.65.Yz, 03.67.Mn Scalable quantum information processing (QIP) requires physical systems capable of reliably storing coherent superpositions for periods over which quantum error correction can be implemented [1]. Moreover, suppressing memory error rates to very low levels allows for simpler error-correcting algorithms [2,3]. In many current atomic ion QIP experiments, a dominant source of memory error is decoherence induced by fluctuating ambient magnetic fields [4,5]. To address this problem, we investigate creating long-lived qubit memories using a first-order magnetic-field-independent hyperfine transition and logical qubits of a decoherence-free subspace [6].Atomic systems have proven themselves as good candidates for quantum information storage through their use in highly stable atomic clocks [7]. Here, the principle of using first-order magnetic-field-independent transitions is well established. A typical clock transition |F, m F = 0 ↔ |F ′ , m F ′ = 0 between hyperfine states of angular momentum F and F ′ in alkali atoms has no linear Zeeman shift at zero magnetic field, and coherence times exceeding 10 minutes have been observed [8]. Unfortunately, magnetic sublevels in each hyperfine manifold are degenerate at zero magnetic field. This makes it more advantageous to operate at a nonzero field in order to spectrally resolve the levels, thereby inducing a linear field dependence of the transition frequency. However, field-independent transitions between hyperfine states also exist at nonzero magnetic field. In the context of atomic clocks, coherence times exceeding 10 minutes have been observed in 9 Be + ions at a magnetic field B = 0.8194 T [9].In neutral-atom systems suitable for QIP, fieldindependent transitions at nonzero magnetic field have been investigated in rubidium [10,11]. The radio-frequency (RF)/microwave two-photon stimulatedRaman hyperfine transition |F = 1, m F = −1 ↔ |F ′ = 2, m F ′ = 1 is field-independent at approximately 3.23 × 10 −4 T , and coherence times of 2.8 s have been observed [11]. In these and the clock experiments, transitions were driven by microwave fields on large numbers of atoms. Using microwaves, it may be difficult to localize the fields well enough to drive individual qubits unless a means (e.g., a magnetic-field gradient or Stark-shift gradient) is employed to provide spectral selection [12,13], a technique that has the additional overhead of keeping track of the phases induced by these shifts. With transitio...

Optimization of segmented linear Paul traps and transport of stored particles

“…Starting with two-qubit gate operations [2,3], long lived two-qubit entanglement [4,5,6], teleportation experiments [7,8], and different sorts of multi-qubit entangled states [9,10,4,11], the record for qubit-entanglement is currently presented in a 6-qubit cat state and a 8-qubit W-state [12,13]. Future improvement is expected using the technique of segmented linear Paul traps which allow to shuttle ions from a "processor" unit to a "memory" section [14].…”

Measurement-induced entanglement for excitation stored in remote atomic ensembles

“…More recently, entanglement has been recognized as a critical resource for accomplishing tasks that are otherwise impossible in the classical domain [1]. Spectacular advances have been made in the generation of quantum entanglement for diverse physical systems [1, 2], including entanglement stored for many seconds in trapped ions for distances on the millimeter scale [10,11], long-lived entanglement of macroscopic quantum spins persisting for milliseconds on the centimeter scale [12], and remote entanglement carried by photon pairs over distances of tens of kilometers of optical fibers [13].For applications in quantum information science, entanglement can be created deterministically by way of precise control of quantum dynamics for a physical system, or probabilistically by way of quantum interference in a suitable measurement with random instances of success. In the latter case, it is essential that success be heralded unambiguously so that the resulting entangled state is available for subsequent utilization.…”

“…More recently, entanglement has been recognized as a critical resource for accomplishing tasks that are otherwise impossible in the classical domain [1]. Spectacular advances have been made in the generation of quantum entanglement for diverse physical systems [1, 2], including entanglement stored for many seconds in trapped ions for distances on the millimeter scale [10,11], long-lived entanglement of macroscopic quantum spins persisting for milliseconds on the centimeter scale [12], and remote entanglement carried by photon pairs over distances of tens of kilometers of optical fibers [13].…”

Trapped Ions Make Impeccable Qubits

“…In the last decade, researchers have demonstrated trapped-ion qubits with long coherence times [6], highfidelity state preparation and readout [7], and singleand two-qubit logic gate operations with low error rates [3,8]. Yet each of these properties was demonstrated individually in different systems.…”