Robust storage qubits in ultracold polar molecules

Abstract

Quantum states with long-lived coherence are essential for quantum computation, simulation and metrology. The nuclear spin states of ultracold molecules prepared in the singlet rovibrational ground state are an excellent candidate for encoding and storing quantum information. However, it is important to understand all sources of decoherence for these qubits, and then eliminate them, in order to reach the longest possible coherence times. Here, we fully characterise the dominant mechanisms for decoherence of a storage qubit in an optically trapped ultracold gas of RbCs molecules using high-resolution Ramsey spectroscopy. Guided by a detailed understanding of the hyperfine structure of the molecule, we tune the magnetic field to where a pair of hyperfine states have the same magnetic moment. These states form a qubit, which is insensitive to variations in magnetic field. Our experiments reveal an unexpected differential tensor light shift between the states, caused by weak mixing of rotational states. We demonstrate how this light shift can be eliminated by setting the angle between the linearly polarised trap light and the applied magnetic field to a magic angle of arccos (1/ √ 3) ≈ 55 • . This leads to a coherence time exceeding 6.9 s (90% confidence level). Our results unlock the potential of ultracold molecules as a platform for quantum computation.

Electric-field-controlled cold dipolar collisions between trapped CH$_3$F molecules

“…We want to note that finer step sizes only provide improvements to the accuracy in the low single-digit percent range. We calculate the loss cross-section due to dipolar-relaxation for five control fields E c = [0.5, 1.0, 1.5, 2.0, 2.5] kV/cm and 14 different relative velocities, covering the entire relative-velocity distribution in our trap (v rel = [2,4,7,10,12,14,17,21,25,30,34,38,42,46] m/s).…”

“…Polar molecules offer research opportunities that are not shared by other particles such as atoms [1]. The strong and long-range electric dipole-dipole interaction in particular can affect quantum-chemical reaction pathways [2][3][4][5][6], can induce large-scale correlations and novel phases in molecular quantum gases [7][8][9][10][11], and can be the basis for a robust quantum-computing architecture [12][13][14][15][16][17]. Towards these applications, closed-shell symmetric-top molecules stand out as an ideal platform due to their simple rotational energy-level structure, favorable matrix elements for cycling transitions [18], and linear response to an electric field [19].…”

Quantum computation in a hybrid array of molecules and Rydberg atoms

“…This high-dimensional space could be used to encode error-corrected qubits [11]. Coherence times of several seconds have been demonstrated for hyperfine states [12,13] and hundreds of milliseconds for rotational states [14] of molecules. Single molecules have been trapped in optical tweezers [15][16][17], and a tweezer array of molecules is a particularly attractive platform for quantum computing.…”

Seconds-scale coherence on nuclear spin transitions of ultracold polar molecules in 3D optical lattices

“…Pairs of nuclear spin states in the same rotational state, which are insensitive to external fields and free from decoherence due to DDI, can be used as storage qubits. In two recent experiments, nuclear spin coherence times of 0.7 s for 23 Na 40 K molecules [33] and 5.6 s for 87 Rb 133 Cs molecules [34] have been reported. However, these experiments were performed with bulk samples which are subject to significant two-body losses due to two-molecule complex formation [35][36][37][38] even though both of them are chemically stable.…”

“…The origin of this polarizability anisotropy in J = 0 is the nuclear electric quadrupole interaction which can induce coupling between J = 0 to J = 2 in presence of light fields [33,34]. This leads to a small anisotropic polarizability term on top of the background isotropic polarizability α 0 .…”