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EP-4422076-B1 - ZZZ COUPLER FOR SUPERCONDUCTING QUBITS

EP4422076B1EP 4422076 B1EP4422076 B1EP 4422076B1EP-4422076-B1

Inventors

  • FERGUSON, DAVID GEORGE
  • PRZYBYSZ, Anthony Joseph
  • STRAND, Joel D.

Dates

Publication Date
20260513
Application Date
20180131

Claims (5)

  1. A method (250) for providing a ZZZ coupling among three qubits (12, 13, 14, 72, 74, 76, 202, 204, 206), comprising: Coupling (252) a first qubit (12, 72, 202) of the three qubits (12, 13, 14, 72, 74, 76, 202, 204, 206) to a second qubit (13, 74, 204) of the three qubits (12, 13, 14, 72, 74, 76, 202, 204, 206) via a first tunable coupler (16, 62, 212) utilizing galvanic Josephson mutual inductance; coupling (254) the second qubit (13, 74, 204) to a third qubit (14, 76, 206) of the three qubits (12, 13, 14, 72, 74, 76, 202, 204, 206) via a second tunable coupler (18, 64, 214) utilizing galvanic Josephson mutual inductance; coupling (256) the third qubit (14, 76, 206) to the first qubit (12, 72, 202) via a third tunable coupler (216) utilizing galvanic Josephson mutual inductance; coupling (258) the first qubit (12, 72, 202) to the second tunable coupler (18, 64, 214) via a fourth tunable coupler (222) such that a flux from the first qubit (12, 72, 202) is directed into a tuning loop of the second tunable coupler (18, 64, 214); coupling (206) the second qubit (13, 74, 204) to the third tunable coupler (216) via a fifth tunable coupler (224) such that a flux from the second qubit (13, 74, 204) is directed into a tuning loop of the third tunable coupler (216); and coupling the third qubit (14, 76, 206) to the first tunable coupler (16, 62, 212) via a sixth tunable coupler (226) such that a flux from the third qubit (14, 76, 206) is directed into a tuning loop of the first tunable coupler (16, 62, 212).
  2. The method (250) of claim 1, further comprising: providing a first control signal to the first tunable coupler (16, 62, 212) to tune a first coupling strength of the first tunable coupler (16, 62, 212); providing a second control signal to the second tunable coupler (18, 64, 214) to tune a second coupling strength of the second tunable coupler (18, 64, 214); providing a third control signal to the third tunable coupler (216) to tune a third coupling strength of the third tunable coupler (216); providing a fourth control signal to the fourth tunable coupler (222) to tune a fourth coupling strength of the fourth tunable coupler (222); providing a fifth control signal to the fifth tunable coupler (224) to tune a coupling strength of the fifth tunable coupler (224); and providing a sixth control signal to the sixth tunable coupler (226) to tune a coupling strength of the sixth tunable coupler (226).
  3. The method (250) of claim 2, wherein providing the fourth control signal to the fourth tunable coupler (222) comprises providing the fourth control signal to the fourth tunable coupler (222) such that the fourth coupling strength is non-zero, the fourth tunable coupler (222) coupling the first qubit to the second tunable coupler such that the second coupling strength is a function of the second control signal and a state of the first qubit (12,72,202).
  4. The method (250) of claim 2, wherein providing the fourth control signal to the fourth tunable coupler (222) comprises providing the fourth control signal to the fourth tunable coupler (222) such that the fourth coupling strength is zero, such that the second coupling strength is independent of the state of the first qubit (12, 72, 202).
  5. The method (250) of claim 4, wherein providing the fourth control signal to the fourth tunable coupler (222) comprises providing one half of a flux quantum to a compound Josephson function associated with the fourth tunable coupler (222).

Description

TECHNICAL FIELD This invention relates to quantum computing, and more particularly, to a coupler for coupling the Z basis states of three superconducting qubits. BACKGROUND A classical computer operates by processing binary bits of information that change state according to the laws of classical physics. These information bits can be modified by using simple logic gates such as AND and OR gates. The binary bits are physically created by a high or a low signal level occurring at the output of the logic gate to represent either a logical one (e.g., high voltage) or a logical zero (e.g., low voltage). A classical algorithm, such as one that multiplies two integers, can be decomposed into a long string of these simple logic gates. Like a classical computer, a quantum computer also has bits and gates. Instead of using logical ones and zeroes, a quantum bit ("qubit") uses quantum mechanics to occupy both possibilities simultaneously. This ability and other uniquely quantum mechanical features enable a quantum computer can solve certain problems exponentially faster than that of a classical computer. Quantum annealing is an alternate computing methodology that uses quantum effects to solve optimization problems. Quantum annealing operates by initializing qubits into a quantum-mechanical superposition of all possible qubit states, referred to as candidate states, with equal probability amplitudes. This is implemented by applying a strong transverse field Hamiltonian to the qubits. The computer then evolves following the time-dependent Schrödinger equation as the transverse field Hamiltonian is decreased and the problem Hamiltonian is turned on. In some variants of quantum annealing a driver Hamiltonian is applied at intermediate times. During this evolution, the probability amplitudes of all candidate states keep changing, realizing quantum parallelism. If the rates of change of the Hamiltonians are slow enough, the system stays close to the ground state of the instantaneous Hamiltonian. At the end of the evolution the transverse field is off, and the system is expected to have reached a ground or other lower energy state of the problem Hamiltonian, with high probability. The problem Hamiltonian typically encodes the solution of a constraint satisfaction or other optimization problem as the ground state of an associated Ising model. Thus, at the end of the evolution, the quantum annealing computing system generates the solution or an approximate solution to the target optimization problem. The article of J.Gosh and B.C.Sanders "Quantum simulation of macro and micro quantum phase transition from paramagnetism to frustrated magnetism with a superconducting circuit" , New Journal of Physics, Vol.18, No.3, March 7, 2016 considers developing a scalable scheme for simulating a quantum phase transition from paramagnetism to frustrated magnetism in a superconducting flux-qubit network, and shows how to characterize this system experimentally both macroscopically and microscopically. The macroscopic characterization of the quantum phase transition is based on the transition of the probability distribution for the spin-network net magnetic moment with this transition quantified by the difference between the Kullback-Leibler divergences of the distributions corresponding to the paramagnetic and frustrated magnetic phases with respect to the probability distribution at a given time during the transition. Microscopic characterization of the quantum phase transition is performed using the standard local-entanglement-witness approach. Simultaneous macro and micro characterizations of quantum phase transitions would serve to verify a quantum phase transition in two ways especially in the quantum realm for the classically intractable case of frustrated quantum magnetism. WO 2017/027733 A1 discloses further creating and using higher degree interactions between qubits. SUMMARY OF THE INVENTION In accordance with an aspect of the present invention, a method is provides a ZZZ coupling among three qubits. A first qubit of the three qubits is coupled to a second qubit of the three qubits via a first tunable coupler utilizing galvanic Josephson mutual inductance. The second qubit is coupled to a third qubit of the three qubits via a second tunable coupler utilizing galvanic Josephson mutual inductance. The third qubit is coupled to the first qubit via a third tunable coupler utilizing galvanic Josephson mutual inductance. The first qubit is coupled to the second tunable coupler via a fourth tunable coupler such that a flux from the first qubit is directed into a tuning loop of the second tunable coupler. The second qubit is coupled to the third tunable coupler via a fifth tunable coupler such that a flux from the second qubit is directed into a tuning loop of the third tunable coupler. The third qubit is coupled to the first tunable coupler via a sixth tunable coupler such that a flux from the third qubit is directed into a tuning loop of