Ions operate in an ultra-high vacuum and cryogenic environment inside traps like Penning traps or Paul traps, where electric fields create potentials that confine ions. Below are the photonic components you can typically find in an ion-trapped setup.
■ Lasers: Ion cooling, preparation (ex: optical pumping), reading out, and gate operation.
■ Modulators (ex: AOD, AOM, EOD, DMD, MEMS mirror): Control and direct laser beam at the ions for individual addressing.
■ Optics: Magnify the ions and capture the scattering light for detection.
■ PMT: Measure the fluorescence or lack thereof to indicate the ion’s state.
■ Camera: Image the array or indicate the number of ions in an array, as well as their positioning.
Trapped ion state detection distills down to a histogram that is formed over a measurement time, ranging from several hundred microseconds to a few milliseconds, where photon counts are collected from ions that scatter (bright state) and from ions that do not scatter (dark state). A threshold value of photon counts can be used to distinguish between the bright and dark states with high accuracy .
Figure 1. Trapped ions experimental setup
Figure 2. Illustration of photon collection histograms for state detection of a trapped ion
To realize an FTQC that has the ability to solve hard problems, a large number of qubits will be needed. This is where scaling poses some challenges especially for trapped ions. It’s difficult to keep adding ions in a trap. However, there are two proposed scaling architectures for trapped ions that are currently being pursued.
■ Quantum charge-coupled device (QCCD): Chip with a large number of interconnected ion traps where ions are shuttled around either to a region of logic operations or memory region (store quantum information) .
■ Network of trapped ion modules: Small ion trapped modules are configured into a network using optical fibers, photonic interconnects, and optical crossconnect switches .
Integration is another factor that is often considered when scaling up trapped ions. A trapped ion setup is made up of hundreds of components such as lasers, fibers, bulk optics, cameras, and detectors to name a few. When thinking of building a quantum computer of hundreds of qubits, not all components scale easily such as bulk optics. For ion fluorescence detection, some trapped ion setups use a high-NA UV objective with a large field of view (FOV), but as you scale the objective diameter or FOV ratio, blind spots are produced.
There have been great strides in research for trapped ion integration such as fabricating waveguides, grating couplers , and detectors such as single-photon avalanche diode (SPAD)  and superconducting nanowire single-photon detector (SNSPD) . Integrating on-chip state readout is attractive for scaling and high collection efficiency .
Neutral atoms operate in a magneto-optical trap (MOT) or ultra-high vacuum cell, and the atoms are confined in arrays with optical tweezers. Below are the photonic components you can typically find in a neutral atoms setup.
■ Lasers: Optical trapping, atom rearrangement, cooling, optical pumping, and gate operations.
o LCOS-SLM: Generate optical trap sites.
o AOD: Sort atoms.
o AOM & EOM: Pulse duration, laser intensity.
o CCD camera: Image the array.
o EMCCD/sCMOS/qCMOS: Measure neutral atom’s fluorescence or lack thereof to indicate the atom’s state.
Neutral atom state detection is very similar to trapped ion state detection, where a histogram is constructed over a measurement time, on the order of tens of milliseconds, to distinguish between two states using a threshold derived from the photon counts collected .
Figure 3. Neutral atom experimental setup
Figure 4. Illustration of the histogram of photon counts collected from a single neutral atom site
Quantum key distribution (QKD) is one of the active areas of research and development within quantum communications. QKD is a hardware-based approach that enables two authorized parties, Alice (sender) and Bob (receiver), to establish a secret key at a distance . The secret key is exchanged via photons, and if an eavesdropper were listening in, it would disturb the photon in a measureable way (ex. polarization of photon would change). There are different schemes within QKD such as discrete-variable QKD (DV-QKD), which uses single-photon detectors, and continuous-variable QKD (CV-QKD), which uses homodyne or heterodyne detectors (ex: matched pair of photodiodes) .
In DV-QKD the secret key can be generated by a source such as a highly attenuated coherent laser, single-photon source, or entangled photon source . Once a photon travels to the end of a fiber channel, it is detected by a single-photon detector such as a single-photon avalanche diode (SPAD) or a superconducting nanowire single-photon detector (SNSPD).
One of the main challenges of building DV-QKD land networks is the distance limit due to the optical fiber loss. Detector characteristics such as the dark count rate (DCR) for the receiving single-photon detector play a role in setting the maximum transmission distance for quantum information due to the fiber losses.
Figure 5. Illustration of two different QKD schemes: discrete-variable QKD (DV-QKD) and continuous-variable QKD (CV-QKD)
Klea Dhimitri is an applications engineer out of Hamamatsu’s office in Bridgewater, NJ, where she focuses on product offerings for emerging quantum technology applications that utilize photonics. Her expertise includes photodetectors such as photomultiplier tubes (PMT), SPPC (SPAD), MPPC (SiPM), photodiodes, and avalanche photodiodes (APD), as well as their role in quantum applications. Klea leads Hamamatsu's efforts in bringing our R&D from Japan together with researchers and early adopters in North America to provide a range of photonic solutions, from detectors to light modulators to cameras, for the current and future quantum landscape. She also manages Hamamatsu Corporation’s engagement and activities in North American quantum hubs like Chicago Quantum Exchange (CQE). In her spare time, she enjoys endurance-based sports such as running and biking, and is currently training to run her first marathon in the fall.
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