To build a quantum computer, one needs quantum bits (qubits) that can be fabricated in large numbers and that can be coupled to each other to create entangled quantum states. Superconducting qubits are attractive candidates. In his Perspective, [Mooij][1] highlights the demonstration of two coupled superconducting qubits by [ McDermott et al .][2] The state of both qubits could be determined after performing quantum operations by means of microwave pulses. Quantum oscillations between the excited states of one or the other qubit were observed with this simultaneous state measurement. The approach can, in principle, be used to build a large quantum computer.
[1]: http://www.sciencemag.org/cgi/content/full/307/5713/1210
[2]: http://www.sciencemag.org/cgi/content/short/307/5713/1299
To build a quantum computer, one needs quantum bits (qubits) that can be fabricated in large numbers and that can be coupled to each other to create entangled quantum states. Superconducting qubits are attractive candidates. In his Perspective, Mooij highlights the demonstration of two coupled superconducting qubits by McDermott et al . The state of both qubits could be determined after performing quantum operations by means of microwave pulses. Quantum oscillations between the excited states of one or the other qubit were observed with this simultaneous state measurement. The approach can, in principle, be used to build a large quantum computer.
Quantum computation is a completely new and different paradigm for how to store and process information. It offers the possibility of exponential computational advantage for certain types of hard problems, but the hardware for implementing quantum algorithms is still at an early stage. The basic idea of quantum computing is to replace the ordinary binary bits of conventional computing with their quantum equivalent, or qubit. A qubit can be any quantum system whose states can be prepared and controlled, provided that we can then fabricate them in large quantities and then couple them to one another in prescribed ways. While there are several candidate technologies for building a quantum computer, one of the most promising is superconducting quantum circuits, operated at cryogenic temperatures approaching 0.01 K.
Few technological ideas in recent memory have stirred as much excitement—or skepticism—as quantum computing. The promise is genuine: for certain computational tasks, a quantum machine can do in seconds what would take a classical supercomputer longer than the age of the universe. At the heart of realising that promise is the qubit, the quantum analogue of a classical bit, and among the many physical systems that can host a qubit, superconducting circuits have quietly become the frontrunner. They can be fabricated in the same cleanrooms used to make conventional chips, controlled with ordinary microwave electronics, and scaled up in ways that other approaches cannot yet match. This thesis traces the full arc from first principles to working code. We start with the quantum mechanics a reader needs—superposition, entanglement, measurement—and build from therethrough the physics of superconductivity to the engineering of real qubit circuits. A key character in that story is the Josephson junction: a thin sandwich of metal and insulator whose quirky nonlinear behaviour is what makes the whole enterprise of superconducting qubits possible. A major focus of this work is the surface code, a topological quantum error-correcting code widely regarded as the most promising approach for achieving fault-tolerant quantum computation. We provide a rigorous analysis of the surface code’s lattice structure, stabilizer formalism, syndrome extraction, and decoding algorithms—in particular the minimum-weight perfect matching (MWPM) algorithm. A full Python/Qiskit implementation of a distance-3 surface code is presented, including qubit initialization, syndrome measurement circuits, error simulation, and decoding logic. We close by looking ahead. Modular processors, 3D circuit integration, and the long road to fault-tolerant machines are all on the horizon, and we try to give an honest assessment of where things stand and what the real obstacles are. Throughout, the goal has been to bridge theory and practice—to be precise enough to be useful, but grounded enough to stay connected to what is actually happening in laboratories today.
If quantum computers are ever built, they might need only minutes to perform tasks that would take today's fastest supercomputer millions of years. Quantum bits, or qubits, which store and process information, can assume the values of 1 and 0 simultaneously. That means 100 qubits can calculate in just one step a computation that ordinary bits would need to repeat 2100 times. Confused or intrigued? Check out the University of Oxford's Centre for Quantum Computation Web site (http://www.qubit.org/). Besides serving experts, “we try to maintain information which will interest people who simply wonder what quantum computing is all about,” says one of the Web masters, Benjamin Simon.
We have seen how information can be encoded onto a quantum system by selecting the state in which it is prepared. Retrieving the information is achieved by performing a measurement, and the optimal measurement in any given situation is usually a generalized measurement. In between preparation and measurement, the information resides in the quantum state of the system, which evolves in a manner determined by the Hamiltonian. The associated unitary transformation may usefully be viewed as quantum information processing; if we can engineer an appropriate Hamiltonian then we can use the quantum evolution to assist in performing computational tasks. Our objective in quantum information processing is to implement a desired unitary transformation. Typically this will mean coupling together a number, perhaps a large number, of qubits and thereby generating highly entangled states. It is fortunate, although by no means obvious, that we can realize any desired multiqubit unitary transformation as a product of a small selection of simple transformations and, moreover, that each of these need only act on a single qubit or on a pair of qubits. The situation is reminiscent of digital electronics, in which logic operations are decomposed into actions on a small number of bits. If we can realize and control a very large number of such operations in a single device then we have a computer. Similar control of a large number of qubits will constitute a quantum computer. It is the revolutionary potential of quantum computers, more than any other single factor, that has fuelled the recent explosion of interest in our subject. We shall examine the remarkable properties of quantum computers in the next chapter. In digital electronics, we represent bit values by voltages: the logical value 1 is a high voltage (typically +5 V) and 0 is the ground voltage (0 V). The voltage bits are coupled and manipulated by transistor-based devices, or gates. The simplest gates act on only one bit or combine two bits to generate a single new bit, the value of which is determined by the two input bits. For a single bit, with value 0 or 1, the only possible operations are the identity (which does not require a gate) and the bit flip.
Quantum computing holds the promise of a novel form of information processing in which data resides in quantum states. By exploiting superpositions and entanglement among these states, quantum logic operations are expected to far surpass conventional digital logic in certain classes of problems. To realize this prospect, we employ superconducting circuits at millikelvin temperatures. The nonlinearity of a Josephson tunnel junction enables a two-level quantum system, or qubit, with excitation energy in the microwave regime. Key recent advances in such devices are the demonstration of robust and simple multi-qubit gates, the integration of superconducting qubits into arrays of four or more, and gate fidelities reaching 99%. [1,2,3]
There is great interest in exploiting the unique behavior of quantum systems for computation, but building real devices has proved difficult. One approach is to use electron spin to represent the qubits (quantum bits) in a computational device. In their Perspective, [Glazman and Ashoori][1] discuss results reported in the same issue by [ Craig et al .][2] on using quantum dots as quantum logic elements. The dots act as artificial atoms, with the number of electrons on each subject to control. The electron spins in a pair of quantum dots interact with each other, and the Kondo effect enables detection of the state of the spin pair. This ability to control and detect the interaction between the spins may one day find use in quantum computation.
[1]: http://www.sciencemag.org/cgi/content/full/304/5670/524
[2]: http://www.sciencemag.org/cgi/content/short/304/5670/565