First-order Crosstalk Research Articles

First-order crosstalk mitigation in parallel quantum gates driven with multi-photon transitions

We demonstrate an order of magnitude reduction in the sensitivity to optical crosstalk for neighboring trapped-ion qubits during simultaneous single-qubit gates driven with individual addressing beams. Gates are implemented via two-photon Raman transitions, where crosstalk is mitigated by offsetting the drive frequencies for each qubit to avoid first-order crosstalk effects from inter-beam two-photon resonance. The technique is simple to implement, and we find that phase-dependent crosstalk due to optical interference is reduced on the most impacted neighbor from a maximal fractional rotation error of 0.185(4) without crosstalk mitigation to ≤0.006 with the mitigation strategy. Furthermore, we characterize first-order crosstalk in the two-qubit gate and avoid the resulting rotation errors for the arbitrary-axis Mølmer–Sørensen gate via a phase-agnostic composite gate. Finally, we demonstrate holistic system performance by constructing a composite CNOT gate using the improved single-qubit gates and phase-agnostic two-qubit gate. This work is done on the Quantum Scientific Computing Open User Testbed; however, our methods are widely applicable for individual addressing Raman gates and impose no significant overhead, enabling immediate improvement for quantum processors that incorporate this technique.

Crosstalk Suppression in Individually Addressed Two-Qubit Gates in a Trapped-Ion Quantum Computer.

Crosstalk between target and neighboring spectator qubits due to spillover of control signals represents a major error source limiting the fidelity of two-qubit entangling gates in quantum computers. We show that in our laser-driven trapped-ion system coherent crosstalk error can be modeled as residual Xσ[over ^]_{ϕ} interaction and can be actively canceled by single-qubit echoing pulses. We propose and demonstrate a crosstalk suppression scheme that eliminates all first-order crosstalk utilizing only local control of target qubits, as opposed to an existing scheme which requires control over all neighboring qubits. We report a two-qubit Bell state fidelity of 99.52(6)% with the echoing pulses applied after collective gates and 99.37(5)% with the echoing pulses applied to each gate in a five-ion chain. This scheme is widely applicable to other platforms with analogous interaction Hamiltonians.

Analysis of crosstalk in spectral beam combining of diode laser bar

Spectral beam combining (SBC) of laser diodes has been proven to be an effective method to improve beam brightness. The crosstalk between different emitters in the SBC system will significantly affect the beam-combining efficiency and output beam properties without effective control. The interfering factors of the crosstalk beam intensity in semiconductor laser SBC have been theoretically analyzed, and experiments have been built to carry out the SBC of a semiconductor laser bar with a high filling factor of 80%. It is found that by adding an inverted Kepler telescope, reducing the focal length of the transmission lens or increasing the spacing of the grating and the output coupling mirror, the crosstalk can be suppressed. In an experiment of beam combining with five emitters, the ratio of the first-order crosstalk peak energy to that of the central beam spot is about 0.34 when the spacing of the grating and the output coupling mirror is 50 mm and the focal length of the transmission lens is 200 mm. The ratio decreases to 0.035 when the spacing of the grating and the output coupling mirror is 200 mm. With the addition of an inverted Kepler telescope with a magnification of 2, the ratio decreases to 0.085. In addition, the ratio decreases to 0.07 when the focal length of the transmission lens is 80 mm.

Silicon non-blocking 4 × 4 optical switch with automated polarization adjustment

We demonstrate a polarization-insensitive silicon 4×4 optical switch based on Mach–Zehnder interferometer (MZI) switch elements. On-chip polarization controllers are integrated before the switch fabric to automatically adjust an arbitrary input polarization to the transverse electric mode. The 4×4 switch fabric is based on a dilated double-layer network architecture to completely cancel the first-order crosstalk. Thermo-optic phase shifters are integrated in the MZI switch elements and the polarization controllers for adjustment of the switching state and polarization, respectively. We develop a polarization control algorithm based on a gradient descent method for automated polarization control. The polarization recovery time is less than 4 ms, and the measured polarization-dependent loss is ∼2 dB. The scheme provides a new solution for realizing polarization-insensitive silicon optical switches.

Experimental demonstration of single-sideband, four-level pulse amplitude modulation/direct-detection degenerate mode-division multiplexing transmission based on multiple-input, multiple-output nonlinear equalizer.

We experimentally demonstrate a degenerate mode-division multiplexing transmission system with single-sideband Nyquist pulse-shaped pulse amplitude modulation-4 and direct detection techniques. Hilbert superposition cancellation (HSC) processing is applied to cancel the first-order cross talk at the receiver side. A least mean square Volterra-based multiple-input, multiple-output (MIMO) nonlinear equalizer (NLE) is used to compensate the second-order distortion. Experimental results show that compared with the MIMO linear equalizer without HSC processing, the MIMO NLE with HSC processing can help improve the bit error rate performance by almost 1 order of magnitude at the received optical power of $ - {10}\;{\rm dBm}$-10dBm.

Scalable Microring-Based Silicon Clos Switch Fabric With Switch-and-Select Stages

We propose and analyze a scalable microring-based Clos switch fabric architecture constructed with switch-and-select switching stages. A silicon 4 × 4 building block that was designed and fabricated through American Institute for Manufacturing Integrated Photonics is used for the proof-of-principle demonstration of a 16 × 16 Clos switch fabric. By fully blocking the first-order crosstalk, the 4 × 4 device is measured to show a crosstalk ratio in the range of –57 to –48.5 dB, enabling better than –39 dB crosstalk for the 16 × 16 switch. Our study shows that the three-stage Clos design enables up to a factor of 4 in the reduction of the number of switching cells compared to single-stage switch-and-select fabrics. We further explore the design space for both first-order and second-order switching elements using the foundry-validated parameters and how these factors impact the performance and scalability of the three-stage Clos switch. A detailed power penalty map is drawn for Clos switch fabrics with various scales, which reveals that the ultimate key limiting factor is the shuffle insertion loss. An optimized 32-port Clos switch fabric using foundry-enabled parameters is shown to have a less than 10-dB power penalty.

Ultralow-crosstalk, strictly non-blocking microring-based optical switch

We report on the first monolithically integrated microring-based optical switch in the switch-and-select architecture. The switch fabric delivers strictly non-blocking connectivity while completely canceling the first-order crosstalk. The 4×4 switching circuit consists of eight silicon microring-based spatial (de-)multiplexers interconnected by a Si/SiN dual-layer crossing-free central shuffle. Analysis of the on-state and off-state power transfer functions reveals the extinction ratios of individual ring resonators exceeding 25 dB, leading to switch crosstalk suppression of up to over 50 dB in the switch-and-select topology. Optical paths are assessed, showing losses as low as 0.1 dB per off-resonance ring and 0.5 dB per on-resonance ring. Photonic switching is actuated with integrated micro-heaters to give an ∼24 GHz passband. The fully packaged device is flip-chip bonded onto a printed circuit board breakout board with a UV-curved fiber array.

Lower-Bound on Blocking Probability of a Class of Crosstalk-Free Optical Cross-Connects (OXCs)

A combination of horizontal expansion and vertical stacking of optical Banyan (HVOB) is the general architecture for building Banyan-based optical cross-connects (OXCs), and the intrinsic crosstalk problem of optical signals is a major constraint in designing OXCs. In this paper, we analyze the blocking behavior of HVOB networks and develop the lower bound on blocking probability of a HVOB network that is free of first-order crosstalk in switching elements. The proposed lower-bound is significant because it provides network designers an effective tool to estimate the minimum blocking probability they can expect from a HVOB architecture regardless what kind of routing strategy to be adopted. Our lower bound can accurately depict the overall blocking behavior in terms of the minimum blocking probability in a HVOB network, as verified by extensive simulation based on a network simulator with both random routing and packing routing strategies. Surprisingly, the simulated and theoretical results show that our lower bound can be used to efficiently estimate the blocking probability of HVOB networks applying packing strategy. Thus, our analytical model can guide network designers to find the tradeoff among the number of planes (stacked copies), the number of SEs, the number of stages and blocking probability in a HVOB network applying packing strategy.

Influence of crosstalk on the scalability of WDM cross-connect networks

Intraband crosstalk contributions in three types of WDM cross-connect topologies based on the Benes architectures are identified. The crosstalk behaviours of the WDM networks are compared and discussed. It is demonstrated that the first-order crosstalk contributions are dominated by the crosstalk from the switching elements. The network scalability is analysed in terms of fibres, wavelengths and nodes. The results show that routing different wavelengths in each switching architecture in WDM cross-connect networks is a reasonable approach to achieve perfect performance in the presence of component crosstalk.

Design of photonic rearrangeable networks with zero first-order switching-element-crosstalk

The development of optical cross-connect architectures is a very important topic today. We consider here in particular the class of optical space-division switching fabrics configured as multistage structures built with 2/spl times/2 optical switching elements (SEs) and derived from a combination of vertical replication and horizontal expansion of Banyan networks. We determine the necessary and sufficient conditions for these matrices to be rearrangeably nonblocking and free of first-order crosstalk in SEs. This impairment is one of the major limitations in optical cross-connect performance. We focus on rearrangeable matrices since they have lower complexity than their strict-sense nonblocking counterparts. Given the current high cost of optical SEs, the rearrangeable solution looks attractive today.

Diagnosing crosstalk faults in a class of dilated blocking optical multistage interconnection networks

Optical Multistage Interconnection Networks (OMINs), which are constructed by photonic switches, have been studied extensively as an important interconnecting scheme for communication and parallel computing systems. A basic element of photonic switching network is a 2×2 directional coupler with two inputs and two outputs. A crosstalk occurs, when two signal channels interact with each other, caused by the improper control of voltage, temperature and polarization along with the aging of the material. In this paper, we will focus on a specific architecture called the Dilated Blocking Networks. The characteristic of dilated blocking networks, such as Dilated Omega Network (DON), is that only one possible path can be established between any input-output pair. The two-level diagnosis algorithm is proposed to satisfy the need of this architecture. The first-order crosstalk will be detected and located in the first level. In addition, the number of unused photonic switches grows in Θ( N 2) as the size of network increases in N× N during the diagnosing crosstalk algorithm. By this property, we speed up the test by paralleling two first-order crosstalk faults at the first-level algorithm (we call it as disjoint faults). The second-level algorithm will diagnose the second- or higher-order crosstalk as well as all crosstalks will be diagnosed. Finally, we apply the diagnosing algorithm to other dilated photonic isomorphic topologies, say, Dilated Baseline Network and Dilated Banyan Network.

Space-wavelength tradeoff in the design of nonblocking directional-coupler-based networks under crosstalk constraint

Directional couplers are electrooptical switching devices that are capable of switching multiple wavelength signals. Once the state of a coupler is set up, optical signals can pass through the coupler with the rate of terabits per second. A directional coupler-based photonic switching system, however, suffers from the intrinsic crosstalk problem. This shortcoming has been the most limiting factor in building a large switching network of this kind. It has been shown that by using more hardware (couplers) we can reduce the crosstalk inside a switching system. In this paper, we explore the wavelength approach to achieve the same goal. We show that if two wavelengths are available for sending data, the number of couplers suffering the first-order crosstalk can be reduced to zero while the total number of couplers remains the same as in a network without any crosstalk constraint.

Volume holographic global and local interconnecting patterns with maximal capacity and minimal first-order crosstalk

High capacity and compact optical interconnecting schemes utilizing volume holograms are analyzed. Local interconnections between the same number of input and output pixels and global interconnections between different numbers of input and output pixels are considered. Sampling grids, i.e., 2-D arrangements of input and output pixels, for such interconnecting patterns are presented.

Volume holographic interconnections with maximal capacity and minimal cross talk

Optical interconnections utilizing volume holography are described. Intrinsic cross-talk effects that limit the number of independent interconnections are identified and analyzed by applying coupled-wave analysis. Sampling grids for removing the first-order cross talk are presented, resulting in a system limited by second- and third-order cross talk only.