Quantum Computing in the NISQ era and beyond

Before moving on to the main focus of this article, I want to include a disclaimer to avoid misunderstanding. There are some very interesting and important aspects of quantum technology that I’m not really going to talk about, but I’ll mention them so you’ll know I’m not going to talk about them.

(1) Quantum-resistant cryptography. We foresee disruptive effects of quantum computers on how we protect our privacy — the public key cryptosystems that are in widespread use today will become obsolete in the years ahead because they can be easily broken by sufficiently powerful quantum computers. We should be thinking now about how we’re going to protect our privacy in the future. One possible way is to replace our existing cryptosystems with new ones which we’re confident are resistant to attacks by quantum computers [16].

(2) Quantum key distribution, quantum networks, and quantum repeaters. We might also use traveling qubits — most likely photons — to create shared keys that can be used for encryption and decryption. Quantum key distribution exploits the principle that you cannot eavesdrop on quantum communication without producing a detectable disturbance [17]. But we don’t yet have the technology to distribute quantum entanglement, and hence secret key, around the world; that’s another interesting technological challenge [18]. It is also notable that, aside from applications to key exchange, a global quantum network might be used for other purposes, like sharing information among quantum devices.

(3) Quantum randomness expansion. Unlike deterministic classical devices, quantum devices can generate intrinsic randomness. Remarkably, this feature of quantum physics can be exploited to expand a short random seed to a much longer string of certifiably random bits, even if one distrusts the equipment used for this purpose — it suffices to assume that spacelike separated parties are unable to communicate with one another [19], or to make suitable assumptions about (quantum) computational hardness [20]. Certifiable randomness has many potential applications, for example to secure communication protocols, unbiased statistical sampling, and Monte Carlo simulations.

(4) Quantum sensing. Quantum technology has advantages for some kinds of sensing.

Quantum systems can sense weak forces with better sensitivity and higher spatial resolution than other sensing technologies [21]; thus quantum sensing could have relatively near-term high-impact applications, to medicine for example.

Quantum communication, networking, randomness expansion, and sensing cannot be cleanly separated from quantum computing, because similar technological challenges are faced by all these quantum-information goals. Nevertheless, I’ll give short shrift to these topics here.