Transformed composite sequences for improved qubit addressing

Abstract

Selective laser addressing of a single atom or atomic ion qubit can be improved using narrowband composite pulse sequences. We describe a Lie-algebraic technique to generalize known narrowband sequences and introduce new sequences related by dilation and rotation of sequence generators. Our method improves known narrowband sequences by decreasing both the pulse time and the residual error. Finally, we experimentally demonstrate these composite sequences using 40 Ca + ions trapped in a surface-electrode ion trap.PACS numbers: 03.67. Lx, 32.80.Qk, 37.10.Ty In ion trap quantum computers [1,2] and neutral atom optical lattices [3,4], single-qubit addressing typically requires focused lasers where the beam waist is smaller than the inter-atom separation. Closely spaced atoms are generally desirable to improve two-qubit coupling rates, often demanding inter-atom spacings approaching the diffraction limit. In practice, single-qubit addressing requires precise focal alignment and ultra-stable beam steering to prevent unwanted excitations on neighboring atoms, a significant challenge as the number of qubits increases [5]. Furthermore, achieving the tight focus required for single-ion addressing is often made difficult by geometric constraints and restricted optical access [6,7]. These factors combine to make single-qubit addressing a major challenge in many experimental systems.One method of improving single-qubit addressing applies an auxiliary field gradient to shift qubit transition frequencies, affording a degree of selective control [8,9]. A similar technique uses an intense laser to introduce a position-dependent AC Stark shift [10,11]. Recently, quantum control has been used in conjunction with frequency shifts to achieve addressing with inhomogeneous control fields [12]. A recent proposal has also examined spatial refocusing through precise laser positioning coupled with controlled phase shifts [13]. These methods frequently require time-consuming calibrations to remove systematic errors while adding to experimental complexity, limiting scalability. These techniques also make strong physical assumptions of the nature of the qubit, and are generally not translatable to other qubit technologies.In this article we demonstrate an alternative control scheme that replaces simple single-qubit gates with a narrowband composite sequence of laser pulses designed for local addressing [14,15]. These sequences allow the exclusive manipulation of a single qubit even when neighboring qubits are subjected to significant laser intensity. Such compensating sequences reduce systematic control errors at the cost of increasing the time required to produce gates [15]. Our main result is a new technique to generate fully-compensating narrowband sequences using Lie-algebraic transformations of other known sequences. We use numerical minimization to identify sequences with superior error correction properties and low operation times compared to the original sequence family. Further, we demonstrate the effectiveness of these sequences ...

Simulating the performance of a distance-3 surface code in a linear ion trap

“…Note that we have omitted errors introducing ion crosstalk during individual ion addressing. This is because systems can be built to minimize this effect and control sequences exist that can reduce the effect of such errors [109][110][111][112].…”

Single-ion addressing via trap potential modulation in global optical fields

“…Maintaining a high degree of optical isolation between such tightly spaced qubit locations is challenging. In practice, the atoms often are moved to larger separations prior to one-qubit gates [2,3], or a composite pulse sequence is used to compensate the effect of finite beam waist on neighboring qubits [4,5]. The first option isolates neighboring qubits from neighboring laser beams, but the required ion transport operations can be costly both in time and in atom motional excitation [6].…”

Heisenberg scaling of imaging resolution by coherent enhancement

“…This allows achievement of a given resolution much faster than is possible classically, useful in any situation where the available time for imaging is limited. The narrow excitation window as a function of control drive intensity also allows this technique to be used for site-selective addressing of one ion or other coherent target within an array [18][19][20], and allows straightforward generalization to imaging of multiple targets.…”