CDS (I) 2024 General Knowledge · Question 11 of 120

Correct — C, quantum computers. A qubit is short for 'quantum bit', and it is the unit of information a quantum computer works with, exactly as the bit is the unit a digital computer works with. It is 'two-valued' because a measurement on it can only ever come out as one of two outcomes, conventionally written 0 and 1; what makes it quantum is that before measurement it may sit in a superposition of those two values, and several qubits may be entangled with one another. The US Department of Energy's own explainer describes quantum computers as machines that use 'quantum bits, or qubits, that play a similar role to the bits in today's digital computers', which is the sense the question is testing.

• (a)classical computers — A classical computer works on the classical bit, which is strictly either 0 or 1 with nothing in between. The word 'qubit' was coined precisely to mark off the quantum unit from that classical one, so it does not describe classical machines.
• (b)classical cryptography — Classical cryptography encodes keys and messages in ordinary bits. There is a quantum counterpart — quantum key distribution — that does use quantum states, but the option says classical, and classical schemes involve no qubits.
• (d)lasers — A laser is a source of coherent light, described by the intensity, wavelength and coherence of its beam. Photons can be used to carry qubits in some quantum computers, but the laser itself is not defined in terms of qubits.

A qubit is the basic carrier of quantum information. Like a bit it yields one of two values when read out, but unlike a bit it can be prepared in a superposition of both values at once, and two or more qubits can be entangled so that their outcomes are correlated in ways no classical pair of bits can reproduce. Those two properties — superposition and entanglement — are what let a quantum computer explore many computational paths at once, and they are why n qubits span 2 to the power n basis states rather than storing n independent answers. The term was introduced by the physicist Benjamin Schumacher in his 1995 paper 'Quantum coding'.

The stem gives you the answer in its own first syllable — 'qu' in qubit is the same 'qu' as in quantum, and the only option carrying that word is quantum computers. The other three options are all classical technologies dressed up to look plausible. A useful check is to ask what the two values physically are: in a superconducting qubit they are two energy levels of a tiny circuit, in a trapped-ion qubit two electronic states of an ion, in a photonic qubit two polarisations of a single photon. None of those is a description of a laser or of a classical cipher.

• A qubit ('quantum bit') is the unit of quantum information; a measurement on it returns one of exactly two values, 0 or 1.
• Before measurement a qubit can occupy a superposition of both values, and qubits can be entangled with each other — neither is possible for a classical bit.
• The US Department of Energy describes quantum computers as using 'quantum bits, or qubits, that play a similar role to the bits in today's digital computers'.
• Physical qubits are built from superconducting circuits, trapped ions, or the polarisation states of single photons.

• Reading 'two-valued' as proof that a qubit is the same thing as a classical bit — a bit is only ever one of the two values, while a qubit can be in a superposition of both until measured.
• Assuming quantum key distribution makes 'classical cryptography' a correct option; the word 'classical' is what rules it out.
• Thinking that because lasers are used to control trapped-ion qubits, the qubit belongs to laser technology.

Asked as a one-line definition item — recognise the term and place it in the right technology; UPSC asked the same term in CSE Prelims 2022 in almost identical form.

• practice — not a real PYQ

  Which one of the following properties of a qubit has no counterpart in a classical bit?

  • (a)It can be copied freely
  • (b)It can exist in a superposition of two states
  • (c)It can be stored in a memory register
  • (d)It can be set to the value 1

  Answer(b) It can exist in a superposition of two states — superposition, along with entanglement, is what distinguishes a qubit from a bit.

• practice — not a real PYQ

  The security of quantum key distribution rests mainly on which one of the following?

  • (a)The difficulty of factorising large numbers
  • (b)The length of the encryption key used
  • (c)The fact that measuring a quantum state disturbs it
  • (d)The speed of the transmitting laser

  Answer(c) The fact that measuring a quantum state disturbs it — an eavesdropper's measurement leaves a detectable trace, unlike in classical key exchange whose security rests on computational hardness.