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.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.

arXiv:1008.1523Physical Review AJul 19, 20111 citation

How to build a long-lived qubit

Nathalie P. de Leon

Nature PhysicsOct 1, 20252 citations

Long-lived and multiplexed atom-photon entanglement interface with feed-forward-controlled readouts

Shengzhi Wang, Minjie Wang, Yafei Wen +4 more

Abstract Quantum interfaces (QIs) that generate entanglement between photonic and spin-wave (atomic memory) qubits are basic building block for quantum repeaters. Realizing ensemble-based repeaters in practice requires quantum memory providing long lifetimes and multimode capacity. Significant progress has been achieved on these separate goals. The remaining challenge is to combine the two attributes into a single QI. Here, by establishing spatial multimode, magnetic-field-insensitive and long-wavelength spin-wave storage in laser-cooled atoms inside a phase-passively-stabilized polarization interferometer, we constructed a multiplexed QI that stores up to three long-lived spin-wave qubits. Using a feed-forward-controlled system, we demonstrated that a multiplexed QI gives rise to a 3-fold increase in the atom–photon (photon–photon) entanglement-generation probability compared with single-mode QIs. For our multiplexed QI, the measured Bell parameter is 2.51±0.01 combined with a memory lifetime of up to 1 ms. This work represents a key step forward in realizing fiber-based long-distance quantum communications.Abstract Quantum interfaces (QIs) that generate entanglement between photonic and spin-wave (atomic memory) qubits are basic building block for quantum repeaters. Realizing ensemble-based repeaters in practice requires quantum memory providing long lifetimes and multimode capacity. Significant progress has been achieved on these separate goals. The remaining challenge is to combine the two attributes into a single QI. Here, by establishing spatial multimode, magnetic-field-insensitive and long-wavelength spin-wave storage in laser-cooled atoms inside a phase-passively-stabilized polarization interferometer, we constructed a multiplexed QI that stores up to three long-lived spin-wave qubits. Using a feed-forward-controlled system, we demonstrated that a multiplexed QI gives rise to a 3-fold increase in the atom–photon (photon–photon) entanglement-generation probability compared with single-mode QIs. For our multiplexed QI, the measured Bell parameter is 2.51±0.01 combined with a memory lifetime of up to 1 ms. This work represents a key step forward in realizing fiber-based long-distance quantum communications.

arXiv:2006.05631Communications PhysicsJul 22, 202123 citations

Entanglement of Spin-Pair Qubits with Intrinsic Dephasing Times Exceeding a Minute

Hans P. Bartling, M. H. Abobeih, Benjamin Pingault +7 more

Understanding and protecting the coherence of individual quantum systems is a central challenge in quantum science and technology.Over the past decades, a rich variety of methods to extend coherence have been developed.A complementary approach is to look for naturally occurring systems that are inherently protected against decoherence.Here, we show that pairs of identical nuclear spins in solids form intrinsically long-lived qubits.We study three carbon-13 pairs in diamond and realize high-fidelity measurements of their quantum states using a single nitrogen-vacancy center in their vicinity.We then reveal that the spin pairs are robust to external perturbations due to a combination of three phenomena: a decoherence-free subspace, a clock transition, and a variant on motional narrowing.The resulting inhomogeneous dephasing time is T Ã 2 ¼ 1.9ð3Þ min, the longest reported for individually controlled qubits.Finally, we develop complete control and realize an entangled state between two spin pairs through projective parity measurements.These long-lived qubits are abundantly present in diamond and other solids and provide new opportunities for ancilla-enhanced quantum sensing and for robust memory qubits for quantum networks.Understanding and protecting the coherence of individual quantum systems is a central challenge in quantum science and technology.Over the past decades, a rich variety of methods to extend coherence have been developed.A complementary approach is to look for naturally occurring systems that are inherently protected against decoherence.Here, we show that pairs of identical nuclear spins in solids form intrinsically long-lived qubits.We study three carbon-13 pairs in diamond and realize high-fidelity measurements of their quantum states using a single nitrogen-vacancy center in their vicinity.We then reveal that the spin pairs are robust to external perturbations due to a combination of three phenomena: a decoherence-free subspace, a clock transition, and a variant on motional narrowing.The resulting inhomogeneous dephasing time is T Ã 2 ¼ 1.9ð3Þ min, the longest reported for individually controlled qubits.Finally, we develop complete control and realize an entangled state between two spin pairs through projective parity measurements.These long-lived qubits are abundantly present in diamond and other solids and provide new opportunities for ancilla-enhanced quantum sensing and for robust memory qubits for quantum networks.

arXiv:2103.07961Physical Review XMar 14, 202256 citations

Controlling Three Atomic Qubits

Hartmut Häffner, Mark Riebe, Ferdinand Schmidt‐Kaler +9 more

Mar 1, 20060 citations

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.