Medium Transport; Low Wing; 2 Turboprop Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 23
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic
Human Factors
Aircraft Y was climbing off of SLC direct RWF; approximately 50 miles west of OCS; Rock Springs Wyoming.  Aircraft Y was climbing out of FL330 for FL370.  Aircraft X was enroute from DEN direct to BOI level at FL340.  I recognized a potential conflict between the two aircraft and turned Aircraft X 15 (fifteen) degrees left for traffic.  Aircraft X acknowledged the 15 left turn.  A few minutes later the conflict alert flashed and I noticed Aircraft X aircraft had turned right instead of left.  I then issued a clearance to Aircraft X to turn 20 left and then descended the aircraft to FL330 and told him to expedite his descent.  Aircraft X acknowledged the new turn and the descent and stated he had the traffic on TCAS and the traffic was in sight.  I advised Aircraft X that the first turn had been issued to the left not to the right.  After he was clear of the Aircraft Y I resumed Aircraft X's course to BOI and climbed him back up to FL340.Upon review of the tapes of the incident; it was found that Aircraft X read back the initial 15 degree left turn but had apparently turned 15 right instead.  The closest proximity was approximately 4 miles and 800 feet.Apparently one pilot readback the clearance and the other pilot turned the aircraft the wrong direction.  Better cockpit communication would seem to be warranted.  From the controller's standpoint; watch each aircraft that they comply with each clearance given; but that is problematic when you have a busy sector.  We work under the expectations that our clearances will be complied with expeditiously and correctly.  When I noticed the mistake from the pilot; it took me a moment to realize what had happened.  In the future I could expect less from the pilots and be prepared for errors and how to correct them.
ZLC Controller reports of airborne conflict because pilot turned in opposite direction as instructed and readback.