Lasers shape the world of quantum technologies

Rajesh Uppal January 6, 2023 Photonics, Quantum Comments Off on Lasers shape the world of quantum technologies 134 Views

The quest for quantum computing supremacy is a geopolitical priority for Europe, China, Canada, Australia and the United States. The quantum computing market was valued at $472m earlier in 2021 and is expected to reach $1.7bn by 2026, according to a Markets and Markets projection. Boston Consulting Group’s 2018 report that estimates a quantum computing market of nearing $60 billion in 2035, which would grow further to $295 billion in 2050, which explains why nations, corporates and startups alike are all jockeying for first position. Advantage gained by acquiring the first computer that renders all other computers obsolete would be enormous and bestow economic, military and public health advantages to the winner.

Governments in Canada, China, Europe, and North America are devoting multi-billion-dollar programs to develop quantum technologies, and commercial investment is flowing as well. The global investment in Quantum technologies is estimated to be in tune of $20 Billion.

Quantum technology (QT) applies quantum mechanical properties such as quantum entanglement, quantum superposition, and No-cloning theorem to quantum systems such as atoms, ions, electrons, photons, or molecules. Quantum bit is the basic unit of quantum information. Whereas in a classical system, a bit is either in one state or the another. However, quantum qubits can exist in large number of states simultaneously, a property called Superposition.

Quantum effects open up numerous possibilities in sensing, computing, and cryptography. Quantum computers shall bring power of massive parallel processing, the equivalent of a supercomputer to a single chip. They can consider different possible solutions to a problem simultaneously, quickly converge on the correct solution without check each possibility individually. This dramatically speed up certain calculations, such as number factoring. Quantum communication refers to a quantum information exchange that uses photons as quantum information carriers over optical fibre or free-space channels

Quantum Cryptography or Quantum key distribution (QKD) employs single or entangled photons to generate shared secret key between the parties that is perfectly secure. The security is guaranteed by Heisenberg’s uncertainty principle. This ensures that any attempts to intercept and measure quantum transmissions, will introduce an anomalously high error rate in the transmissions and therefore will be detectable.

Quantum Sensing exploit high sensitivity of quantum systems to external disturbances to develop highly sensitive sensors. They can measure Quantities such as time, magnetic and electrical fields, inertial forces, temperature, and many others. They employ quantum systems such as NV centers, atomic vapors, Rydberg atoms, and trapped ions

Laser, a product of the first quantum revolution, is an enabling technology for quantum technologies. Its application is by no means limited to purely optical quantum technologies. Lasers are rather found in the majority of quantum setups.

Laser light sources are at the heart of quantum networks, because photons are the natural carriers of quantum states over large distances. They enable applications like quantum key distribution and the future interconnection of quantum computers, but lasers are also essential components in many quantum computers, quantum sensors, and optical clocks. In essence, the incredible control over all degrees of freedom of the light emanating from a laser, often at the quantum limit, is a prime tool to initialize, manipulate, and read out other quantum systems.

Besides the application-determined and physics-driven demand for control of all laser parameters, there are many technological challenges to be met by light sources for quantum technologies. Mode-hop-free tuning, large tuning ranges, pointing and power stability, reliability, compactness, low power consumption, low cost of ownership, and remote control are among the many features that are essential.

Diode lasers meet many of these requirements extremely well. They are intrinsically small devices that convert electric energy into laser light with unparalleled efficiency. Most of the other requirements have to be met by careful engineering of the complete system consisting of driving and control electronics and the laser head itself.

Quantum Computing

Architectures that employ lasers to manipulate qubits encoded in the internal states of single ions or atoms are leading the race to build a universal quantum computer, and they compare favorably in many aspects with the solid-state systems being developed by Google and IBM. For these laser-driven atomic qubits, quantum logic gates are created through precisely tuned laser pulses that perform the coherent control. A quantum algorithm consists of a sequence of gates, where each acts on one or more qubits.

The fidelity of each logic gate, broadly defined as the probability of reaching a target quantum state, is limited by how well the parameters of each laser pulse can be controlled. The fidelity must remain above a certain threshold to prevent a calculation from becoming infeasible due to gate errors. It follows that very high-purity and low-noise optical sources are necessary for quantum computing with ions and atoms.

The few basic building blocks needed for quantum computing, are miniature vacuum packages to store atoms, and lasers that are stable to nine or 10 digits of accuracy, said Chris Monroe of the U.S. National Quantum Initiative and founder of IonQ, For scaling up quantum technology, he sees a strong need for an engineering phase. That is one reason to start a company in which physicists, engineers, and software staff work together on quantum technology.

Qubits based on single trapped atoms come in two varieties, defined as optical or hyperfine. With optical qubits, the levels are chosen such that the upper level is extremely long-lived, with a lifetime on the order of 1 s, resulting in a transition linewidth of ~1 Hz. This lifetime sets an upper limit to the coherence time of the atom- laser interaction that, if maximized, allows completion of the required number of logic gates before the system loses coherence. This requires laser sources with extremely narrow emission linewidths of about 1 Hz to perform coherent control with optical qubits. Typically, out-of-the-box laser linewidths range from ~100 kHz to several megahertz, depending on the laser technology. Therefore, significant linewidth reduction is required through stabilization to high-finesse, low-drift, and vibration-insensitive optical reference cavities. Using this approach, single- and two-qubit entangling gates have been demonstrated with fidelities >99.9% with ⁴⁰Ca+ ions using a Ti:sapphire laser operating at 729 nm with a 1-Hz linewidth

Lasers in Quantum Communications

The timing, frequency, spatial structure, and even orbital angular momentum of single photons can likewise be used to encode quantum information, each with their specific advantages and disadvantages. Some of these degrees of freedom can even be used to go beyond qubits, which live in a two-level system, and encode qudits, a generalization of the qubit to additional dimensions. The phase and amplitude of pulses of light are also employed in quantum communication.

Every property of a laser—that is, its wavelength, linewidth, power, polarization, temporal, and spatial beam profile—is an important control parameter in quantum technologies. In quantum communication, this is reflected in the many different encodings for quantum information. The polarization of a single photon can carry a quantum bit (qubit) just as its temporal shape can be in the form of a time-bin qubit.

Another example is the narrow-linewidth laser diode Cobolt NLD 405 or 785 nm which are used to generate entangled pairs of photons via down-conversion in nonlinear-optical crystal.

Quantum Sensors

Current technologies lack the accuracy required to help you navigate in such circumstances but ‘Positioning, Navigation, and Timing’, or PNT for short, is one of the key technologies being developed as research into Quantum Technologies progresses.

Quantum technologies concentrate on the use of precisely stabilised particles or atoms, whereby knowing the properties of these atoms help us improve measurement accuracies of time and space. In order to be able to interact with these atoms, they first need to be slowed down, or ‘cooled’, so that they can be examined more thoroughly. For both cooling atoms and examining them, highly coherent light is used, such as a diode-pumped solid-state (DPSS) laser. In quantum applications, the narrower the linewidth of the source, the better the signal that can be expected from the atoms. It is also important to choose wavelengths that are relevant to the atom to be trapped.

One class of quantum sensors are scanning-probe magnetometers based on singe NV- defects in diamond tips which act as local optically addressable quantum sensors. These systems enable measuring magnetic fields with spatial resolutions on the nanometer scale. Applications for this class of magnetometers are microwave current imaging, characterization of electronics, and studying new materials like multiferroics and antiferromagnets. The realization of quantum-sensor based experimental setups and products relies on the availability of state-of-the art components like specialized diamond tips, fast low-noise electronics, and high-performance lasers. The Cobolt 06-01 Series of modulated laser diodes, eg the 06-MLD 515 nm and 633 nm lasers are well suited and popular for spin-initialization and read out owing to their fast (< 2.5 ns), deep (> 60 dB), and precise intensity modulation capabilities via live TTL control, high intensity stability and good Gaussian beam profile. The Cobolt Samba™ 532 nm and Cobolt Mambo™ 594 nm lasers with double-path acousto-optic modulators (AOM) are other lasers popular for this class of applications.

Research groups employ the widely tunable single-frequency cw laser C-WAVE to characterize new candidates for quantum centers, like Si-V, Ge-V, Sn-V and Pb-V color centers in diamond, quantum dots, single molecules, or Rydberg states of plasmon-exciton polaritons to name a few. The C-WAVE is also used to test the quality of artificially grown structures with color centers designed for quantum applications. Key characteristics of the C-WAVE are the wide spectral coverage in the visible and NIR (450 nm – 3.5 µm), narrow linewidth (< 1 MHz), mode-hop-free tunablility, high output power of several hundret milliwatts, and its nearly perfect Gaussian beam profile.

Atom interferometers that combine acceleration and rotation measurements are also emerging for quantum inertial navigation systems, in which long-term accuracy is guaranteed by an absolute atomic reference. An atom interferometer measurement sequence starts by preparing a cold atom cloud of about 5 µK through laser cooling in a magneto-optical trap. After the trap is turned off, the atoms are in free fall and interact with three laser pulses from a pair of vertically counter-propagating beams.

In analogy with the optical Mach-Zehnder interferometer, the pulses form the beamsplitters and combiners for the atomic wave functions. These matter-wave optics are implemented with stimulated Raman transitions, as employed in quantum computing systems using hyperfine qubits. The counter-propagating laser beams serve as an optical ruler that measures the position of the atoms as they fall, and the precision of the optical ruler is limited by the relative phase noise of the two frequencies. So, exceptionally low-phase-noise laser systems are critical for acceleration and rotation measurements of the highest sensitivity.

In addition to providing an optical ruler for acceleration measurements, atom interferometers require laser light at multiple optical frequencies for laser cooling, quantum state preparation, and detection of the atomic population at the end of the interferometer sequence.

For an atom interferometer based on 87Rb, these frequencies are separated by ~1 GHz at 780 nm. The laser system must switch between these frequency configurations on a timescale that is short compared to the total measurement sequence (~100 ms). High optical powers are necessary to enable the use of large beams for the optical ruler to ensure that the free-falling atoms see negligible wavefront curvature during free fall. Finally, the laser system must be compact and robust enough to operate on a transportable platform. Laser systems that combine these requirements have leveraged laser diode-based systems, frequency-doubled fiber lasers, and Ti:sapphire solid-state systems. These have already been deployed in high-sensitivity atom interferometers on moving platforms.

Laser Quantum industry

M Squared built a laser with extreme phase stability for qubits with 99.99% fidelity, said Graeme Malcolm from M Squared Lasers. Still, more qubits are needed and so are more laser systems. Based on market research from Tractica, Malcolm expects the market for lasers in quantum systems to go beyond $1 billion within the next five years. Citing Bill Gates, he noted the limits of current technology: If the sheer volume of mankind’s data grows steadily, we will run out of appropriate computing capacity. Further standardization of laser technology will be a key issue for the development of very small, inexpensive lasers for quantum computing.

Laser company TOPTICA Photonics AG (Munich, Germany), which has its origins in laser cooling and spectroscopy of atomic species, is now the major provider of laser systems for all areas of quantum technologies: quantum communication, quantum computing, quantum simulation, quantum metrology, and quantum sensing.